Optimal trigger threshold with the bolus-tracking technique for the renal CT angiography protocol

Optimal trigger threshold with the bolus-tracking technique for the renal CT angiography protocol | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Optimal trigger threshold with the bolus-tracking technique for the renal CT angiography protocol Masashi Asano, Yoshifumi Noda, Nobuyuki Kawai, Tetsuro Kaga, Shingo Omata, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6840100/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 25 Aug, 2025 Read the published version in Abdominal Radiology → Version 1 posted 9 You are reading this latest preprint version Abstract Objectives This study aimed to investigate the optimal trigger threshold for the renal CT angiography (CTA) protocol with the bolus-tracking technique. Methods This retrospective study included patients who were suspected of renal diseases or candidate kidney transplant donors who underwent renal CTA from July 2019 to August 2021. Renal arterial (RA) phase scanning was initiated using either one of the following two trigger thresholds: 50 HU (50 HU group; n = 42) and 100 HU (100 HU group; n = 63). A radiologist measured CT attenuations of the vasculature and renal parenchyma on RA phase images. Two radiologists classified RA phase images into appropriate RA-, late RA-, or corticomedullary (CM)-timing. Unpaired t-tests and Fisher’s exact tests were conducted to assess differences in the CT attenuations and the proportion of categorical classifications between the two groups. Results Overall, 105 patients (mean age: 59.8 years; 60 men) were included. CT attenuations of the aorta and renal arteries were comparable between the two groups ( P = .38–.95). CT attenuations of the renal cortex and renal vein were higher in the 100-HU group than in the 50-HU group ( P = .006–.04). The proportion of the appropriate RA-timing was higher, whereas that of the CM-timing was lower in the 50-HU group than in the 100-HU group (appropriate RA-, late RA-, and CM-timing: 78.6% vs. 50.8%, 19.0% vs. 34.9%, and 2.4% vs. 14.3%; P = .01 for reviewer 1 and 64.3% vs. 27.0%, 33.3% vs. 63.5%, and 2.4% vs. 9.5%; P < .001 for reviewer 2, respectively). Conclusion The trigger threshold of 50 HU provided a higher probability of obtaining appropriate RA-timing images than that of 100 HU in the renal CTA protocol with the bolus-tracking technique. Renal CT Angiography Bolus-Tracking Trigger Threshold Image Timing Optimization Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The renal CT angiography (CTA) protocol (composed of the renal arterial [RA] and corticomedullary [CM] phases) has been employed for identifying arterial disorders at the RA phase and renal vein and tumor vascularity at the CM phase ( 1 – 5 ). The RA phase enables improved visualization of renal artery stenosis, aneurysms, dissections, and arteriovenous malformation or fistula ( 4 , 5 ). Furthermore, detailed assessments of vascular anatomy and anomalies in the RA phase, which are crucial for the preoperative evaluation of kidney transplant donors, enable surgery to be performed without complications ( 2 ). The RA phase images are obtained approximately 20–44 s after initiating contrast material administration ( 5 – 8 ). However, previous studies revealed that several factors, such as the site of vascular access, contrast injection rate, and contrast volume, affected the appropriate scan timing ( 6 , 7 , 9 ). Further, the hemodynamic status plays an important role in scan timing because worsened hemodynamic status causes the circulation of contrast material to slow, thereby delaying contrast bolus arrival and peak arterial enhancement ( 10 ). Thus, the bolus-tracking technique is recommended to identify the appropriate scan timing corresponding to the individual hemodynamic status ( 9 ). Previous studies have utilized either 50 HU, 100 HU, or 150 HU as a trigger threshold to obtain renal CTA ( 1 , 3 , 9 , 11 , 12 ). However, no studies have yet investigated the optimal trigger threshold for the bolus-tracking technique to obtain the appropriate RA phase images. We hypothesized that the trigger threshold of 50 HU would be more appropriate in renal CTA compared to that of 100 HU or 150 HU because the renal venous return is generally very fast, and we frequently encounter excess enhancement in the renal veins even at the RA phase in cases scanned with the trigger threshold of 100 HU. Therefore, this study aimed to determine the optimal trigger threshold to obtain an appropriate RA phase in the renal CTA protocol by comparing 50 HU and 100 HU as the trigger threshold for the bolus-tracking technique. Materials and Methods Patients Our institutional review board approved this retrospective study and waived written informed consent due to the retrospective nature of the study. This study included 117 consecutive patients who were suspected of having renal diseases or candidate renal transplant donors and who underwent renal CTA from July 2019 to August 2021. This study excluded 12 patients who underwent dual-energy scanning. After excluding these patients, this study included 105 patients, consisting of 42 patients scanned using a trigger threshold of 50 HU (50-HU group) and 63 patients scanned with a trigger threshold of 100 HU (100-HU group) (Fig. 1 ). Detailed patients’ information was obtained from the medical records to assess the demographic data of the patients. CT Scan Protocol and Contrast Material Injection A fast kilovoltage-switching dual-energy CT scanner (Revolution CT; GE Healthcare, Milwaukee, WI, USA) was used in the single-energy scan mode. The CT imaging parameters were as follows: X-ray tube voltage, 120 kilovolt peak (kVp); noise index, 7.0 at 5-mm slice collimation; tube current, variable; detector configuration, 80 detectors with 0.625-mm section thickness; beam collimation, 80 mm; rotation time, 0.35 s; pitch, 0.508:1; scan field-of-view, large body; and display field-of-view, 40 cm. An adaptive statistical iterative reconstruction-Veo (ASiR-V; GE Healthcare) of 40% with 5-mm section thickness with no gap was used to reconstruct CT raw data. The contrast material, containing 370 mg of iopamidol per milliliter, was intravenously injected with a fixed injection speed of 4 mL/s and an injection duration of 25 s. All patients underwent RA phase scanning using either one of the following two trigger thresholds: 50 HU and 100 HU. A circle with a diameter of 15–20 mm was placed as a region of interest (ROI) in the abdominal aorta at the first lumbar vertebral body level. Real-time fluoroscopic monitoring scans (120 kVp, 10 mA) were started 10 s after contrast administration. Diagnostic CT scanning was initiated with an additional delay of 5 s and 25 s for RA and CM phases, respectively, after a bolus-tracking program (SmartPrep; GE Healthcare) detected a bolus-tracking trigger, 50 HU or 100 HU, in the abdominal aorta. The time from contrast administration to the bolus-tracking trigger (bolus-tracking time), the time from the bolus-tracking trigger to the RA phase scan initiation (scan delay), and the time from contrast administration to the RA phase scan initiation (time to RA scan) were recorded using the bolus-tracking program in each patient. Quantitative Image Analysis A radiologist (____, with 10 years of post-training experience in interpreting body CT images) measured the CT attenuations of the abdominal aorta, renal artery, renal vein, inferior vena cava, and renal cortex and medulla on the axial RA phase images using a circler ROI. The CT attenuation of the abdominal aorta was measured with an ROI of approximately 100 mm 2 , encompassing as much of the vascular lumen as possible, avoiding vascular walls, calcification, thrombus, and artifacts. Similarly, the CT attenuations of the right and left renal arteries and renal veins was measured with an ROI of approximately 25 mm 2 , avoiding vascular walls, calcification, and artifacts. The CT attenuations of the right and left renal cortex and medulla were measured with an ROI of approximately 25 mm 2 , carefully avoiding the vessels, cysts, and artifacts. The CT attenuation of the inferior vena cava at the renal vein level was measured with an ROI of approximately 50 mm 2 , avoiding the vascular walls and artifacts. Qualitative Image Analysis Two radiologists (____ and ____, with 10 and 11 years of post-training experience in interpreting body CT images, respectively), who were blinded from the two trigger thresholds, classified RA phase images into appropriate RA-, late RA-, or CM-timing categories. Appropriate RA-timing is defined by maximal RA, mild renal cortical, and scarce renal venous enhancements. Late RA-timing is characterized by greater renal cortical enhancement compared with appropriate RA-timing and mild renal venous enhancement. CM-timing is defined by high renal cortical enhancement, clear corticomedullary contrast, and high renal venous enhancement. Statistical Analysis Statistical analyses were conducted using commercially available software (IBM SPSS Statistics for Windows, version 24.0; SPSS Inc., Chicago, IL). The unpaired t-test was used to assess the differences in patients’ age, body weight, body mass index, bolus-tracking time, scan delay, time to RA scan, and CT attenuations between the two groups. Fischer’s exact test was conducted to compare the differences in patients’ sex and the three categorical classifications of RA phase images between the two groups. Weighted ĸ statistics were used for assessing interobserver variability in the qualitative analyses. Kappa values of up to 0.20, 0.21–0.40, 0.41–0.60, 0.61–0.80, and ≥ 0.81 were interpreted as slight, fair, moderate, substantial, and almost perfect agreements, respectively. A P -value of < .05 was considered statistically significant. Results Patients The final study population consisted of 105 patients (mean age: 59.8 years; age range: 22–80 years; mean body weight: 62.3 kg; mean body mass index: 23.1 kg/m 2 ), including 60 men (mean age: 59.5 years; age range: 22–77 years; mean body weight: 68.9 kg; and mean body mass index: 23.8 kg/m 2 ) and 45 women (mean age: 60.2 years; age range: 36–80 years; mean body weight: 53.4 kg; and mean body mass index: 22.1 kg/m 2 ). Table 1 summarizes patients’ demographics. No differences were observed in all patients’ demographics between the two groups ( P = .35–>.99). Regarding the scan timing-related parameters, no difference was found in the bolus-tracking time (16.0 s in the 50-HU group and 16.8 s in the 100-HU group; P = .14) and scan delay (5.5 s in the 50-HU group and 5.7 s in the 100-HU group; P = .10). The time to RA scan was 21.5 s in the 50-HU group and 22.4 s in the 100-HU group, with no difference between the two groups ( P = .07). Table 1 Patients’ Demographics and Information of Renal Arterial Scanning Timing Parameter 50 HU group 100 HU group P value Patients’ demographics Number of patients 42 63 N.A. Age (y) 61.2 ± 11.9 (36–78) 58.8 ± 13.5 (22–80) .35 Men:Women 24:18 36:27 > .99 Body weight (kg) 62.1 ± 13.2 (37–86) 62.4 ± 14.0 (35–104) .91 Body mass index (kg/m 2 ) 22.7 ± 3.7 (15.2–30.0) 23.2 ± 4.4 (14.9–44.7) .56 RA scanning timing Bolus-tracking time (s) 16.0 ± 2.3 (11.7–22.3) 16.8 ± 2.9 (11.7–23.8) .14 Scan delay (s) 5.5 ± 0.4 (5.4–7.0) 5.7 ± 0.6 (5.4–6.9) .10 Time to RA scan (s) 21.5 ± 2.3 (17.1–27.7) 22.4 ± 3.0 (18.3–30.7) .07 Note.– Data are means ± 1 standard deviation. Numbers in parentheses are ranges. N.A. = not applicable. RA = renal arterial. Quantitative Image Analysis Table 2 summarizes the quantitative parameters. We revealed no differences in the CT attenuations of the abdominal aorta ( P = .38), renal artery (right, P = .95 and left, P = .64), renal medulla (right, P = .10 and left, P = .18), and inferior vena cava ( P = .07) between the two groups. Conversely, CT attenuations of the renal cortex (right, P = .006; left, P = .04) and renal vein (right, P = .004; left, P = .02) were higher in the 100-HU group than in the 50-HU group. Table 2 CT Attenuations of Vasculature and Renal Parenchyma Anatomy 50 HU group 100 HU group P value Abdominal aorta 333.6 ± 62.0 (231.9–478) 344.9 ± 66.5 (160.6–509.5) .38 Renal artery Right 299.9 ± 58.4 (217.9–494.8) 300.6 ± 71.1 (148.5–484.1) .95 Left 293.0 ± 58.0 (159.1–432.1) 299.4 ± 71.6 (98.7–478.1) .64 Renal cortex Right 122.5 ± 38.7 (41.5–258.8) 141.9 ± 30.8 (83.7–251.1) .006 Left 124.2 ± 45.2 (37.4–287) 139.7 ± 28.1 (72.1–224.4) .04 Renal medulla Right 50.2 ± 9.2 (32.6–81.9) 53.4 ± 9.8 (34.0–77.7) .10 Left 51.6 ± 17.9 (35.1–146.7) 55.4 ± 10.1 (36.1–82.7) .18 Renal vein Right 90.2 ± 44.0 (33.5–253.7) 116.5 ± 43.1 (23.0–250.7) .004 Left 85.8 ± 47.5 (31.3–246.0) 104.7 ± 36.1 (36.7–216.4) .02 Inferior vena cava 59.9 ± 22.9 (34.9–152.5) 69.6 ± 29.4 (40.5–188.3) .07 Note.– Data are means ± 1 standard deviation. Numbers in parentheses are ranges. Qualitative Image Analysis Table 3 summarizes the qualitative parameters. Both reviewers revealed that the 50-HU group demonstrated a higher proportion of appropriate RA-timing and a lower proportion of CM-timing compared with the 100-HU group (appropriate RA-, late RA-, and CM-timing: 78.6% vs. 50.8%, 19.0% vs. 34.9%, and 2.4% vs. 14.3%; P = .01 for reviewer 1 and 64.3% vs. 27.0%, 33.3% vs. 63.5%, and 2.4% vs. 9.5%; P < .001 for reviewer 2, respectively) (Figs. 2 and 3 ). The ĸ value of 0.63 indicated substantial agreement between the two reviewers. Table 3 Qualitative Evaluation of Renal Arterial Scan Timing Appropriate RA Late RA CM P value Reviewer 1 50 HU group 78.6% (33/42) 19.0% (8/42) 2.4% (1/42) .01 100 HU group 50.8% (32/63) 34.9% (22/63) 14.3% (9/63) Reviewer 2 50 HU group 64.3% (27/42) 33.3% (14/42) 2.4% (1/42) < .001 100 HU group 27.0% (17/63) 63.5% (40/63) 9.5% (6/63) Note.– RA = renal arterial. CM = corticomedullary. Discussion The appropriate timing of the RA phase is crucial for assessing vascular diseases and abnormalities ( 4 , 5 ) as well as for the preoperative assessment of renal transplant donors ( 2 ). However, when present, contrast enhancement of the renal vein interferes with the detailed assessment of the renal artery and reconstruction of the volume-rendered and maximum intensity projection images ( 3 ). Kidneys are characterized by both rapid and intense arterial enhancement and prompt venous return after contrast administration. It results in overlapping enhancements of the renal arteries, parenchyma, and veins within a narrow time window. This hemodynamic feature of the kidneys complicates the acquisition of the appropriate RA phase images ( 11 ). Therefore, we hypothesized that advancing the trigger timing for renal CTA acquisition could help prevent image quality degradation due to unintentional renal venous enhancement. Our study revealed that a trigger threshold of 50 HU demonstrated a higher probability of obtaining appropriate RA phase images compared with that of 100 HU. Quantitative analysis revealed that the CT attenuations of the abdominal aorta, renal artery, renal medulla, and inferior vena cava were comparable between the two groups. Conversely, the 50-HU group demonstrated significantly lower CT attenuations of the renal cortex and renal veins compared with the 100-HU group. It may improve renal artery and vein separation. The lack of difference in the CT attenuation of the renal medulla may be associated with its inherently low contrast enhancement in the RA phase, which is unlikely to produce substantial differences in the CT attenuation values ( 13 ). Qualitative analysis revealed that a trigger threshold of 50 HU significantly reduced the proportion of late RA- and CM-timing in the RA phase and increased the proportion of appropriate RA-timing. Notably, scan timing-related parameters (bolus-tracking time, scan delay, and time to RA scan) demonstrated no statistically significant difference between the two groups, with only a 0.9 s difference in the time to RA scan. The 50-HU group exhibited a higher proportion of appropriate RA-timing compared with the 100-HU group in the present study despite this minimal time difference. This result indicates that even a slight difference in time to RA scan causes substantial differences in image quality due to rapid renal venous return. Previous studies utilized 50 HU ( 9 , 11 ), 100 HU ( 1 ), or 150 HU ( 3 , 12 ) as trigger thresholds for the RA phase imaging in renal CTA protocols. However, no study has directly compared these trigger thresholds to the point of their ability to capture the appropriate RA phase. A previous study reported peak enhancement of the renal arteries and veins at approximately 25–30 s and 45 s after contrast administration, respectively ( 1 ). This indicates that the peaks of renal arteriovenous enhancement are swapped in just 15 s. Furthermore, as previously mentioned, a difference of only 1 s makes a decisive difference in the quantitative and qualitative image quality. Therefore, differences in trigger thresholds demonstrated a significant effect on the RA image quality, and identifying the optimal trigger threshold is of great clinical significance. This approach may be extended to determine optimal trigger thresholds in various organs beyond the kidneys, and further research is warranted. Our study had several limitations. First, the sample size was small, which may have introduced selection bias. Second, the diagnostic ability was not assessed in this study. Third, only a bolus-tracking program from a single vendor was employed. Finally, our results are limited to the comparison between the trigger thresholds of 50 HU and 100 HU. In conclusion, the trigger threshold of 50 HU provided a higher probability of obtaining an appropriate RA phase in the renal CTA protocol with the bolus-tracking technique. This approach minimized renal venous enhancement; hence, this may improve image quality and have clinical benefits for the assessment of renal vascular conditions and renal lesions. Declarations Conflict of Interest Author disclosure of potential conflict of interest. No relevant conflicts of interest to disclose. Author Contribution M.A. and Y.N. wrote the main manuscript textT.K., S.O., Y.T., and A.I.: literature researchT.I., T.M., and N.K.: statistical analysis, prepared the figures and tablesA.E., H.I., H.K., and M.M.: study oversight and approved the final version of the manuscriptAll authors reviewed the manuscript. Data Availability The data that support the findings of this study are not openly available due to reasons of sensitivity and are available from the corresponding author upon reasonable request. Data are located in controlled access data storage at Gifu University. References Kanematsu M, Goshima S, Kawai N, Kondo H, Miyoshi T, Watanabe H, et al. Low-Iodine-Load and Low-Tube-Voltage CT Angiographic Imaging of the Kidney by Using Bolus Tracking with Saline Flushing. Radiology. 2015;275(3):832–40. Aghayev A, Gupta S, Dabiri BE, Steigner ML. Vascular imaging in renal donors. Cardiovasc Diagn Ther. 2019;9(Suppl 1):S116-s30. Catalá V, Martí T, Diaz JM, Cordeiro E, Samaniego J, Rosales A, et al. Use of multidetector CT in presurgical evaluation of potential kidney transplant recipients. Radiographics. 2010;30(2):517–31. Kawashima A, Sandler CM, Ernst RD, Tamm EP, Goldman SM, Fishman EK. CT evaluation of renovascular disease. Radiographics. 2000;20(5):1321–40. Urban BA, Ratner LE, Fishman EK. Three-dimensional volume-rendered CT angiography of the renal arteries and veins: normal anatomy, variants, and clinical applications. Radiographics. 2001;21(2):373 – 86; questionnaire 549 – 55. Tsuge Y, Kanematsu M, Goshima S, Kondo H, Hoshi H, Yokoyama R, et al. Optimal scan delays for multiphasic renal multidetector row computed tomography performed with fixed injection duration of contrast medium. J Comput Assist Tomogr. 2009;33(1):101–5. Kaatee R, Van Leeuwen MS, De Lange EE, Wilting JE, Beek FJ, Beutler JJ, et al. Spiral CT angiography of the renal arteries: should a scan delay based on a test bolus injection or a fixed scan delay be used to obtain maximum enhancement of the vessels? J Comput Assist Tomogr. 1998;22(4):541–7. Kawamoto S, Montgomery RA, Lawler LP, Horton KM, Fishman EK. Multi-detector row CT evaluation of living renal donors prior to laparoscopic nephrectomy. Radiographics. 2004;24(2):453–66. Saade C, Deeb IA, Mohamad M, Al-Mohiy H, El-Merhi F. Contrast medium administration and image acquisition parameters in renal CT angiography: what radiologists need to know. Diagn Interv Radiol. 2016;22(2):116–24. Bae KT, Heiken JP. Scan and contrast administration principles of MDCT. Eur Radiol. 2005;15 Suppl 5:E46-59. Goshima S, Kanematsu M, Nishibori H, Kondo H, Tsuge Y, Yokoyama R, et al. Multi-detector row CT of the kidney: optimizing scan delays for bolus tracking techniques of arterial, corticomedullary, and nephrographic phases. Eur J Radiol. 2007;63(3):420–6. Murphy DJ, Aghayev A, Steigner ML. Vascular CT and MRI: a practical guide to imaging protocols. Insights Imaging. 2018;9(2):215–36. Yuh BI, Cohan RH. Different phases of renal enhancement: role in detecting and characterizing renal masses during helical CT. AJR Am J Roentgenol. 1999;173(3):747–55. Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6840100","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":468673018,"identity":"85ddeeee-9a6d-47dc-809d-3c8cef8b4f41","order_by":0,"name":"Masashi Asano","email":"","orcid":"","institution":"Gifu University","correspondingAuthor":false,"prefix":"","firstName":"Masashi","middleName":"","lastName":"Asano","suffix":""},{"id":468673019,"identity":"4c22b5ed-aaae-4114-91ba-40fca45a160b","order_by":1,"name":"Yoshifumi 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University","correspondingAuthor":false,"prefix":"","firstName":"Hiroki","middleName":"","lastName":"Kato","suffix":""},{"id":468673030,"identity":"809f9dc7-73e5-48e9-9859-cb29dcd77653","order_by":12,"name":"Masayuki Matsuo","email":"","orcid":"","institution":"Gifu University","correspondingAuthor":false,"prefix":"","firstName":"Masayuki","middleName":"","lastName":"Matsuo","suffix":""}],"badges":[],"createdAt":"2025-06-07 02:23:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6840100/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6840100/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00261-025-05158-6","type":"published","date":"2025-08-25T15:57:20+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":84423024,"identity":"18f08cb9-3a8b-4488-b8c9-c19b4d91c37d","added_by":"auto","created_at":"2025-06-11 18:41:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":13722,"visible":true,"origin":"","legend":"\u003cp\u003eFlow diagram illustrating the process of enrollment of the study population.\u003c/p\u003e","description":"","filename":"Onlinefigure1.png","url":"https://assets-eu.researchsquare.com/files/rs-6840100/v1/e4d533cf9a139c6d5fdb3758.png"},{"id":84423027,"identity":"1b965de1-bbbf-456e-98ac-206d896915c9","added_by":"auto","created_at":"2025-06-11 18:41:22","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":93965,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative images demonstrating qualitative timing categories of RA phase in CT angiography. (a) Early RA-timing, defined by maximal RA, mild renal cortical, and scarce renal venous enhancements, (b) late RA-timing, showing greater renal cortical enhancement compared with appropriate RA-timing and mild renal venous enhancement, and (c) CM-timing, with high renal cortical enhancement, clear corticomedullary contrast, and high renal venous enhancement.\u003c/p\u003e","description":"","filename":"Onlinefigure2.png","url":"https://assets-eu.researchsquare.com/files/rs-6840100/v1/f52f82a620c8dc4cf0e846b0.png"},{"id":84423030,"identity":"b8cd22ec-f2a0-4526-92ef-b46dc180ca24","added_by":"auto","created_at":"2025-06-11 18:41:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":138971,"visible":true,"origin":"","legend":"\u003cp\u003eA 72-year-old woman with a right renal artery aneurysm (arrow). (a) Axial and (b) coronal slab maximum intensity projection (MIP) images at the renal arterial phase acquired with a trigger threshold of 50 HU, corresponding to appropriate RA-timing. The renal cortical enhancement is mild, and the renal artery and renal vein are clearly distinguishable and could be separately identified.\u003c/p\u003e","description":"","filename":"Onlinefigure3.png","url":"https://assets-eu.researchsquare.com/files/rs-6840100/v1/10e697ee37ed4cc620ddbf65.png"},{"id":84423038,"identity":"16d63b45-002a-46c7-a881-bbd8f87caf0b","added_by":"auto","created_at":"2025-06-11 18:41:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":135660,"visible":true,"origin":"","legend":"\u003cp\u003eA 43-year-old man with a left renal hemorrhagic cyst. (a) Axial and (b) coronal slab MIP images at the renal arterial phase acquired with a trigger threshold of 100 HU, corresponding to CM-timing. Differentiation between the renal artery (arrow) and the renal vein (arrowhead) is difficult, particularly on the coronal slab MIP image.\u003c/p\u003e","description":"","filename":"Onlinefigure4.png","url":"https://assets-eu.researchsquare.com/files/rs-6840100/v1/430069460ef13d92c4cc16fb.png"},{"id":90344955,"identity":"5a654389-858b-49e8-b33d-3d93a42e7fab","added_by":"auto","created_at":"2025-09-01 16:08:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1265618,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6840100/v1/20637a2d-cf04-45c1-9620-753f8b6672f5.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Optimal trigger threshold with the bolus-tracking technique for the renal CT angiography protocol","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe renal CT angiography (CTA) protocol (composed of the renal arterial [RA] and corticomedullary [CM] phases) has been employed for identifying arterial disorders at the RA phase and renal vein and tumor vascularity at the CM phase (\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e). The RA phase enables improved visualization of renal artery stenosis, aneurysms, dissections, and arteriovenous malformation or fistula (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e). Furthermore, detailed assessments of vascular anatomy and anomalies in the RA phase, which are crucial for the preoperative evaluation of kidney transplant donors, enable surgery to be performed without complications (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe RA phase images are obtained approximately 20\u0026ndash;44 s after initiating contrast material administration (\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e). However, previous studies revealed that several factors, such as the site of vascular access, contrast injection rate, and contrast volume, affected the appropriate scan timing (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e). Further, the hemodynamic status plays an important role in scan timing because worsened hemodynamic status causes the circulation of contrast material to slow, thereby delaying contrast bolus arrival and peak arterial enhancement (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThus, the bolus-tracking technique is recommended to identify the appropriate scan timing corresponding to the individual hemodynamic status (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e). Previous studies have utilized either 50 HU, 100 HU, or 150 HU as a trigger threshold to obtain renal CTA (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e). However, no studies have yet investigated the optimal trigger threshold for the bolus-tracking technique to obtain the appropriate RA phase images. We hypothesized that the trigger threshold of 50 HU would be more appropriate in renal CTA compared to that of 100 HU or 150 HU because the renal venous return is generally very fast, and we frequently encounter excess enhancement in the renal veins even at the RA phase in cases scanned with the trigger threshold of 100 HU. Therefore, this study aimed to determine the optimal trigger threshold to obtain an appropriate RA phase in the renal CTA protocol by comparing 50 HU and 100 HU as the trigger threshold for the bolus-tracking technique.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003ePatients\u003c/h2\u003e \u003cp\u003e Our institutional review board approved this retrospective study and waived written informed consent due to the retrospective nature of the study. This study included 117 consecutive patients who were suspected of having renal diseases or candidate renal transplant donors and who underwent renal CTA from July 2019 to August 2021. This study excluded 12 patients who underwent dual-energy scanning. After excluding these patients, this study included 105 patients, consisting of 42 patients scanned using a trigger threshold of 50 HU (50-HU group) and 63 patients scanned with a trigger threshold of 100 HU (100-HU group) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Detailed patients\u0026rsquo; information was obtained from the medical records to assess the demographic data of the patients.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCT Scan Protocol and Contrast Material Injection\u003c/h3\u003e\n\u003cp\u003eA fast kilovoltage-switching dual-energy CT scanner (Revolution CT; GE Healthcare, Milwaukee, WI, USA) was used in the single-energy scan mode. The CT imaging parameters were as follows: X-ray tube voltage, 120 kilovolt peak (kVp); noise index, 7.0 at 5-mm slice collimation; tube current, variable; detector configuration, 80 detectors with 0.625-mm section thickness; beam collimation, 80 mm; rotation time, 0.35 s; pitch, 0.508:1; scan field-of-view, large body; and display field-of-view, 40 cm. An adaptive statistical iterative reconstruction-Veo (ASiR-V; GE Healthcare) of 40% with 5-mm section thickness with no gap was used to reconstruct CT raw data.\u003c/p\u003e \u003cp\u003eThe contrast material, containing 370 mg of iopamidol per milliliter, was intravenously injected with a fixed injection speed of 4 mL/s and an injection duration of 25 s. All patients underwent RA phase scanning using either one of the following two trigger thresholds: 50 HU and 100 HU. A circle with a diameter of 15\u0026ndash;20 mm was placed as a region of interest (ROI) in the abdominal aorta at the first lumbar vertebral body level. Real-time fluoroscopic monitoring scans (120 kVp, 10 mA) were started 10 s after contrast administration. Diagnostic CT scanning was initiated with an additional delay of 5 s and 25 s for RA and CM phases, respectively, after a bolus-tracking program (SmartPrep; GE Healthcare) detected a bolus-tracking trigger, 50 HU or 100 HU, in the abdominal aorta.\u003c/p\u003e \u003cp\u003eThe time from contrast administration to the bolus-tracking trigger (bolus-tracking time), the time from the bolus-tracking trigger to the RA phase scan initiation (scan delay), and the time from contrast administration to the RA phase scan initiation (time to RA scan) were recorded using the bolus-tracking program in each patient.\u003c/p\u003e\n\u003ch3\u003eQuantitative Image Analysis\u003c/h3\u003e\n\u003cp\u003eA radiologist (____, with 10 years of post-training experience in interpreting body CT images) measured the CT attenuations of the abdominal aorta, renal artery, renal vein, inferior vena cava, and renal cortex and medulla on the axial RA phase images using a circler ROI. The CT attenuation of the abdominal aorta was measured with an ROI of approximately 100 mm\u003csup\u003e2\u003c/sup\u003e, encompassing as much of the vascular lumen as possible, avoiding vascular walls, calcification, thrombus, and artifacts. Similarly, the CT attenuations of the right and left renal arteries and renal veins was measured with an ROI of approximately 25 mm\u003csup\u003e2\u003c/sup\u003e, avoiding vascular walls, calcification, and artifacts. The CT attenuations of the right and left renal cortex and medulla were measured with an ROI of approximately 25 mm\u003csup\u003e2\u003c/sup\u003e, carefully avoiding the vessels, cysts, and artifacts. The CT attenuation of the inferior vena cava at the renal vein level was measured with an ROI of approximately 50 mm\u003csup\u003e2\u003c/sup\u003e, avoiding the vascular walls and artifacts.\u003c/p\u003e\n\u003ch3\u003eQualitative Image Analysis\u003c/h3\u003e\n\u003cp\u003eTwo radiologists (____ and ____, with 10 and 11 years of post-training experience in interpreting body CT images, respectively), who were blinded from the two trigger thresholds, classified RA phase images into appropriate RA-, late RA-, or CM-timing categories. Appropriate RA-timing is defined by maximal RA, mild renal cortical, and scarce renal venous enhancements. Late RA-timing is characterized by greater renal cortical enhancement compared with appropriate RA-timing and mild renal venous enhancement. CM-timing is defined by high renal cortical enhancement, clear corticomedullary contrast, and high renal venous enhancement.\u003c/p\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eStatistical Analysis\u003c/h2\u003e \u003cp\u003eStatistical analyses were conducted using commercially available software (IBM SPSS Statistics for Windows, version 24.0; SPSS Inc., Chicago, IL). The unpaired t-test was used to assess the differences in patients\u0026rsquo; age, body weight, body mass index, bolus-tracking time, scan delay, time to RA scan, and CT attenuations between the two groups. Fischer\u0026rsquo;s exact test was conducted to compare the differences in patients\u0026rsquo; sex and the three categorical classifications of RA phase images between the two groups. Weighted \u003cem\u003eĸ\u003c/em\u003e statistics were used for assessing interobserver variability in the qualitative analyses. Kappa values of up to 0.20, 0.21\u0026ndash;0.40, 0.41\u0026ndash;0.60, 0.61\u0026ndash;0.80, and \u0026ge;\u0026thinsp;0.81 were interpreted as slight, fair, moderate, substantial, and almost perfect agreements, respectively. A \u003cem\u003eP\u003c/em\u003e-value of \u0026lt;\u0026thinsp;.05 was considered statistically significant.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003ePatients\u003c/h2\u003e \u003cp\u003eThe final study population consisted of 105 patients (mean age: 59.8 years; age range: 22\u0026ndash;80 years; mean body weight: 62.3 kg; mean body mass index: 23.1 kg/m\u003csup\u003e2\u003c/sup\u003e), including 60 men (mean age: 59.5 years; age range: 22\u0026ndash;77 years; mean body weight: 68.9 kg; and mean body mass index: 23.8 kg/m\u003csup\u003e2\u003c/sup\u003e) and 45 women (mean age: 60.2 years; age range: 36\u0026ndash;80 years; mean body weight: 53.4 kg; and mean body mass index: 22.1 kg/m\u003csup\u003e2\u003c/sup\u003e). Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes patients\u0026rsquo; demographics. No differences were observed in all patients\u0026rsquo; demographics between the two groups (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.35\u0026ndash;\u0026gt;.99). Regarding the scan timing-related parameters, no difference was found in the bolus-tracking time (16.0 s in the 50-HU group and 16.8 s in the 100-HU group; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.14) and scan delay (5.5 s in the 50-HU group and 5.7 s in the 100-HU group; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.10). The time to RA scan was 21.5 s in the 50-HU group and 22.4 s in the 100-HU group, with no difference between the two groups (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.07).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePatients\u0026rsquo; Demographics and Information of Renal Arterial Scanning Timing\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eParameter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50 HU group\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 HU group\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003ePatients\u0026rsquo; demographics\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNumber of patients\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAge (y)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e61.2\u0026thinsp;\u0026plusmn;\u0026thinsp;11.9 (36\u0026ndash;78)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e58.8\u0026thinsp;\u0026plusmn;\u0026thinsp;13.5 (22\u0026ndash;80)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.35\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMen:Women\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e24:18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e36:27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;.99\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBody weight (kg)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e62.1\u0026thinsp;\u0026plusmn;\u0026thinsp;13.2 (37\u0026ndash;86)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e62.4\u0026thinsp;\u0026plusmn;\u0026thinsp;14.0 (35\u0026ndash;104)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.91\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBody mass index (kg/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e22.7\u0026thinsp;\u0026plusmn;\u0026thinsp;3.7 (15.2\u0026ndash;30.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e23.2\u0026thinsp;\u0026plusmn;\u0026thinsp;4.4 (14.9\u0026ndash;44.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.56\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003eRA scanning timing\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBolus-tracking time (s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.0\u0026thinsp;\u0026plusmn;\u0026thinsp;2.3 (11.7\u0026ndash;22.3)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16.8\u0026thinsp;\u0026plusmn;\u0026thinsp;2.9 (11.7\u0026ndash;23.8)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eScan delay (s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 (5.4\u0026ndash;7.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6 (5.4\u0026ndash;6.9)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTime to RA scan (s)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e21.5\u0026thinsp;\u0026plusmn;\u0026thinsp;2.3 (17.1\u0026ndash;27.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e22.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.0 (18.3\u0026ndash;30.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eNote.\u0026ndash; Data are means\u0026thinsp;\u0026plusmn;\u0026thinsp;1 standard deviation. Numbers in parentheses are ranges.\u003c/td\u003e\u003c/tr\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eN.A. = not applicable. RA\u0026thinsp;=\u0026thinsp;renal arterial.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eQuantitative Image Analysis\u003c/h3\u003e\n\u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e summarizes the quantitative parameters. We revealed no differences in the CT attenuations of the abdominal aorta (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.38), renal artery (right, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.95 and left, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.64), renal medulla (right, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.10 and left, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.18), and inferior vena cava (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.07) between the two groups. Conversely, CT attenuations of the renal cortex (right, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.006; left, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.04) and renal vein (right, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.004; left, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.02) were higher in the 100-HU group than in the 50-HU group.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCT Attenuations of Vasculature and Renal Parenchyma\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAnatomy\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50 HU group\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 HU group\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAbdominal aorta\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e333.6\u0026thinsp;\u0026plusmn;\u0026thinsp;62.0 (231.9\u0026ndash;478)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e344.9\u0026thinsp;\u0026plusmn;\u0026thinsp;66.5 (160.6\u0026ndash;509.5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRenal artery\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRight\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e299.9\u0026thinsp;\u0026plusmn;\u0026thinsp;58.4 (217.9\u0026ndash;494.8)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e300.6\u0026thinsp;\u0026plusmn;\u0026thinsp;71.1 (148.5\u0026ndash;484.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.95\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeft\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e293.0\u0026thinsp;\u0026plusmn;\u0026thinsp;58.0 (159.1\u0026ndash;432.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e299.4\u0026thinsp;\u0026plusmn;\u0026thinsp;71.6 (98.7\u0026ndash;478.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRenal cortex\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRight\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e122.5\u0026thinsp;\u0026plusmn;\u0026thinsp;38.7 (41.5\u0026ndash;258.8)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e141.9\u0026thinsp;\u0026plusmn;\u0026thinsp;30.8 (83.7\u0026ndash;251.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.006\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeft\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e124.2\u0026thinsp;\u0026plusmn;\u0026thinsp;45.2 (37.4\u0026ndash;287)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e139.7\u0026thinsp;\u0026plusmn;\u0026thinsp;28.1 (72.1\u0026ndash;224.4)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRenal medulla\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRight\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50.2\u0026thinsp;\u0026plusmn;\u0026thinsp;9.2 (32.6\u0026ndash;81.9)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e53.4\u0026thinsp;\u0026plusmn;\u0026thinsp;9.8 (34.0\u0026ndash;77.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeft\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e51.6\u0026thinsp;\u0026plusmn;\u0026thinsp;17.9 (35.1\u0026ndash;146.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e55.4\u0026thinsp;\u0026plusmn;\u0026thinsp;10.1 (36.1\u0026ndash;82.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRenal vein\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRight\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e90.2\u0026thinsp;\u0026plusmn;\u0026thinsp;44.0 (33.5\u0026ndash;253.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e116.5\u0026thinsp;\u0026plusmn;\u0026thinsp;43.1 (23.0\u0026ndash;250.7)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.004\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeft\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e85.8\u0026thinsp;\u0026plusmn;\u0026thinsp;47.5 (31.3\u0026ndash;246.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e104.7\u0026thinsp;\u0026plusmn;\u0026thinsp;36.1 (36.7\u0026ndash;216.4)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInferior vena cava\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e59.9\u0026thinsp;\u0026plusmn;\u0026thinsp;22.9 (34.9\u0026ndash;152.5)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e69.6\u0026thinsp;\u0026plusmn;\u0026thinsp;29.4 (40.5\u0026ndash;188.3)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eNote.\u0026ndash; Data are means\u0026thinsp;\u0026plusmn;\u0026thinsp;1 standard deviation. Numbers in parentheses are ranges.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eQualitative Image Analysis\u003c/h2\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e summarizes the qualitative parameters. Both reviewers revealed that the 50-HU group demonstrated a higher proportion of appropriate RA-timing and a lower proportion of CM-timing compared with the 100-HU group (appropriate RA-, late RA-, and CM-timing: 78.6% vs. 50.8%, 19.0% vs. 34.9%, and 2.4% vs. 14.3%; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.01 for reviewer 1 and 64.3% vs. 27.0%, 33.3% vs. 63.5%, and 2.4% vs. 9.5%; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;.001 for reviewer 2, respectively) (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The \u003cem\u003eĸ\u003c/em\u003e value of 0.63 indicated substantial agreement between the two reviewers.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eQualitative Evaluation of Renal Arterial Scan Timing\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAppropriate RA\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLate RA\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCM\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eReviewer 1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50 HU group\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e78.6%\u003c/p\u003e \u003cp\u003e(33/42)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19.0%\u003c/p\u003e \u003cp\u003e(8/42)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.4%\u003c/p\u003e \u003cp\u003e(1/42)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e100 HU group\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50.8%\u003c/p\u003e \u003cp\u003e(32/63)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e34.9%\u003c/p\u003e \u003cp\u003e(22/63)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14.3%\u003c/p\u003e \u003cp\u003e(9/63)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eReviewer 2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50 HU group\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e64.3%\u003c/p\u003e \u003cp\u003e(27/42)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e33.3%\u003c/p\u003e \u003cp\u003e(14/42)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.4%\u003c/p\u003e \u003cp\u003e(1/42)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e100 HU group\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e27.0%\u003c/p\u003e \u003cp\u003e(17/63)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e63.5%\u003c/p\u003e \u003cp\u003e(40/63)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.5%\u003c/p\u003e \u003cp\u003e(6/63)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003eNote.\u0026ndash; RA\u0026thinsp;=\u0026thinsp;renal arterial. CM\u0026thinsp;=\u0026thinsp;corticomedullary.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe appropriate timing of the RA phase is crucial for assessing vascular diseases and abnormalities (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e) as well as for the preoperative assessment of renal transplant donors (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e). However, when present, contrast enhancement of the renal vein interferes with the detailed assessment of the renal artery and reconstruction of the volume-rendered and maximum intensity projection images (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e). Kidneys are characterized by both rapid and intense arterial enhancement and prompt venous return after contrast administration. It results in overlapping enhancements of the renal arteries, parenchyma, and veins within a narrow time window. This hemodynamic feature of the kidneys complicates the acquisition of the appropriate RA phase images (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e). Therefore, we hypothesized that advancing the trigger timing for renal CTA acquisition could help prevent image quality degradation due to unintentional renal venous enhancement. Our study revealed that a trigger threshold of 50 HU demonstrated a higher probability of obtaining appropriate RA phase images compared with that of 100 HU.\u003c/p\u003e \u003cp\u003eQuantitative analysis revealed that the CT attenuations of the abdominal aorta, renal artery, renal medulla, and inferior vena cava were comparable between the two groups. Conversely, the 50-HU group demonstrated significantly lower CT attenuations of the renal cortex and renal veins compared with the 100-HU group. It may improve renal artery and vein separation. The lack of difference in the CT attenuation of the renal medulla may be associated with its inherently low contrast enhancement in the RA phase, which is unlikely to produce substantial differences in the CT attenuation values (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eQualitative analysis revealed that a trigger threshold of 50 HU significantly reduced the proportion of late RA- and CM-timing in the RA phase and increased the proportion of appropriate RA-timing. Notably, scan timing-related parameters (bolus-tracking time, scan delay, and time to RA scan) demonstrated no statistically significant difference between the two groups, with only a 0.9 s difference in the time to RA scan. The 50-HU group exhibited a higher proportion of appropriate RA-timing compared with the 100-HU group in the present study despite this minimal time difference. This result indicates that even a slight difference in time to RA scan causes substantial differences in image quality due to rapid renal venous return.\u003c/p\u003e \u003cp\u003ePrevious studies utilized 50 HU (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e), 100 HU (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e), or 150 HU (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e) as trigger thresholds for the RA phase imaging in renal CTA protocols. However, no study has directly compared these trigger thresholds to the point of their ability to capture the appropriate RA phase. A previous study reported peak enhancement of the renal arteries and veins at approximately 25\u0026ndash;30 s and 45 s after contrast administration, respectively (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e). This indicates that the peaks of renal arteriovenous enhancement are swapped in just 15 s. Furthermore, as previously mentioned, a difference of only 1 s makes a decisive difference in the quantitative and qualitative image quality. Therefore, differences in trigger thresholds demonstrated a significant effect on the RA image quality, and identifying the optimal trigger threshold is of great clinical significance. This approach may be extended to determine optimal trigger thresholds in various organs beyond the kidneys, and further research is warranted.\u003c/p\u003e \u003cp\u003eOur study had several limitations. First, the sample size was small, which may have introduced selection bias. Second, the diagnostic ability was not assessed in this study. Third, only a bolus-tracking program from a single vendor was employed. Finally, our results are limited to the comparison between the trigger thresholds of 50 HU and 100 HU.\u003c/p\u003e \u003cp\u003eIn conclusion, the trigger threshold of 50 HU provided a higher probability of obtaining an appropriate RA phase in the renal CTA protocol with the bolus-tracking technique. This approach minimized renal venous enhancement; hence, this may improve image quality and have clinical benefits for the assessment of renal vascular conditions and renal lesions.\u003c/p\u003e"},{"header":"Declarations","content":" \u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eAuthor disclosure of potential conflict of interest. No relevant conflicts of interest to disclose.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM.A. and Y.N. wrote the main manuscript textT.K., S.O., Y.T., and A.I.: literature researchT.I., T.M., and N.K.: statistical analysis, prepared the figures and tablesA.E., H.I., H.K., and M.M.: study oversight and approved the final version of the manuscriptAll authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data that support the findings of this study are not openly available due to reasons of sensitivity and are available from the corresponding author upon reasonable request. Data are located in controlled access data storage at Gifu University.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKanematsu M, Goshima S, Kawai N, Kondo H, Miyoshi T, Watanabe H, et al. Low-Iodine-Load and Low-Tube-Voltage CT Angiographic Imaging of the Kidney by Using Bolus Tracking with Saline Flushing. Radiology. 2015;275(3):832\u0026ndash;40.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAghayev A, Gupta S, Dabiri BE, Steigner ML. Vascular imaging in renal donors. Cardiovasc Diagn Ther. 2019;9(Suppl 1):S116-s30.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCatal\u0026aacute; V, Mart\u0026iacute; T, Diaz JM, Cordeiro E, Samaniego J, Rosales A, et al. Use of multidetector CT in presurgical evaluation of potential kidney transplant recipients. Radiographics. 2010;30(2):517\u0026ndash;31.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKawashima A, Sandler CM, Ernst RD, Tamm EP, Goldman SM, Fishman EK. CT evaluation of renovascular disease. Radiographics. 2000;20(5):1321\u0026ndash;40.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eUrban BA, Ratner LE, Fishman EK. Three-dimensional volume-rendered CT angiography of the renal arteries and veins: normal anatomy, variants, and clinical applications. Radiographics. 2001;21(2):373\u0026thinsp;\u0026ndash;\u0026thinsp;86; questionnaire 549\u0026thinsp;\u0026ndash;\u0026thinsp;55.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTsuge Y, Kanematsu M, Goshima S, Kondo H, Hoshi H, Yokoyama R, et al. Optimal scan delays for multiphasic renal multidetector row computed tomography performed with fixed injection duration of contrast medium. J Comput Assist Tomogr. 2009;33(1):101\u0026ndash;5.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKaatee R, Van Leeuwen MS, De Lange EE, Wilting JE, Beek FJ, Beutler JJ, et al. Spiral CT angiography of the renal arteries: should a scan delay based on a test bolus injection or a fixed scan delay be used to obtain maximum enhancement of the vessels? J Comput Assist Tomogr. 1998;22(4):541\u0026ndash;7.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKawamoto S, Montgomery RA, Lawler LP, Horton KM, Fishman EK. Multi-detector row CT evaluation of living renal donors prior to laparoscopic nephrectomy. Radiographics. 2004;24(2):453\u0026ndash;66.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSaade C, Deeb IA, Mohamad M, Al-Mohiy H, El-Merhi F. Contrast medium administration and image acquisition parameters in renal CT angiography: what radiologists need to know. Diagn Interv Radiol. 2016;22(2):116\u0026ndash;24.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBae KT, Heiken JP. Scan and contrast administration principles of MDCT. Eur Radiol. 2005;15 Suppl 5:E46-59.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGoshima S, Kanematsu M, Nishibori H, Kondo H, Tsuge Y, Yokoyama R, et al. Multi-detector row CT of the kidney: optimizing scan delays for bolus tracking techniques of arterial, corticomedullary, and nephrographic phases. Eur J Radiol. 2007;63(3):420\u0026ndash;6.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMurphy DJ, Aghayev A, Steigner ML. Vascular CT and MRI: a practical guide to imaging protocols. Insights Imaging. 2018;9(2):215\u0026ndash;36.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYuh BI, Cohan RH. Different phases of renal enhancement: role in detecting and characterizing renal masses during helical CT. AJR Am J Roentgenol. 1999;173(3):747\u0026ndash;55.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"abdominal-radiology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aima","sideBox":"Learn more about [Abdominal Radiology](http://link.springer.com/journal/261)","snPcode":"261","submissionUrl":"https://submission.springernature.com/new-submission/261/3","title":"Abdominal Radiology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Renal CT Angiography, Bolus-Tracking, Trigger Threshold, Image Timing Optimization","lastPublishedDoi":"10.21203/rs.3.rs-6840100/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6840100/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eObjectives\u003c/h2\u003e \u003cp\u003eThis study aimed to investigate the optimal trigger threshold for the renal CT angiography (CTA) protocol with the bolus-tracking technique.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eThis retrospective study included patients who were suspected of renal diseases or candidate kidney transplant donors who underwent renal CTA from July 2019 to August 2021. Renal arterial (RA) phase scanning was initiated using either one of the following two trigger thresholds: 50 HU (50 HU group; \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;42) and 100 HU (100 HU group; \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;63). A radiologist measured CT attenuations of the vasculature and renal parenchyma on RA phase images. Two radiologists classified RA phase images into appropriate RA-, late RA-, or corticomedullary (CM)-timing. Unpaired t-tests and Fisher\u0026rsquo;s exact tests were conducted to assess differences in the CT attenuations and the proportion of categorical classifications between the two groups.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eOverall, 105 patients (mean age: 59.8 years; 60 men) were included. CT attenuations of the aorta and renal arteries were comparable between the two groups (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.38\u0026ndash;.95). CT attenuations of the renal cortex and renal vein were higher in the 100-HU group than in the 50-HU group (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.006\u0026ndash;.04). The proportion of the appropriate RA-timing was higher, whereas that of the CM-timing was lower in the 50-HU group than in the 100-HU group (appropriate RA-, late RA-, and CM-timing: 78.6% vs. 50.8%, 19.0% vs. 34.9%, and 2.4% vs. 14.3%; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.01 for reviewer 1 and 64.3% vs. 27.0%, 33.3% vs. 63.5%, and 2.4% vs. 9.5%; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;.001 for reviewer 2, respectively).\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eThe trigger threshold of 50 HU provided a higher probability of obtaining appropriate RA-timing images than that of 100 HU in the renal CTA protocol with the bolus-tracking technique.\u003c/p\u003e","manuscriptTitle":"Optimal trigger threshold with the bolus-tracking technique for the renal CT angiography protocol","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-11 18:41:18","doi":"10.21203/rs.3.rs-6840100/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-07-06T16:12:01+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-05T22:23:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"22459769034675753405257970585232686427","date":"2025-06-30T04:02:01+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-06-26T16:45:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"123110342649784764291627432997925210943","date":"2025-06-18T19:55:56+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-10T21:03:21+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-07T02:27:31+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-07T02:26:58+00:00","index":"","fulltext":""},{"type":"submitted","content":"Abdominal Radiology","date":"2025-06-07T02:11:59+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"abdominal-radiology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"aima","sideBox":"Learn more about [Abdominal Radiology](http://link.springer.com/journal/261)","snPcode":"261","submissionUrl":"https://submission.springernature.com/new-submission/261/3","title":"Abdominal Radiology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d07790c3-1826-4462-ad9b-156c9d803bce","owner":[],"postedDate":"June 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-09-01T16:04:24+00:00","versionOfRecord":{"articleIdentity":"rs-6840100","link":"https://doi.org/10.1007/s00261-025-05158-6","journal":{"identity":"abdominal-radiology","isVorOnly":false,"title":"Abdominal Radiology"},"publishedOn":"2025-08-25 15:57:20","publishedOnDateReadable":"August 25th, 2025"},"versionCreatedAt":"2025-06-11 18:41:18","video":"","vorDoi":"10.1007/s00261-025-05158-6","vorDoiUrl":"https://doi.org/10.1007/s00261-025-05158-6","workflowStages":[]},"version":"v1","identity":"rs-6840100","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6840100","identity":"rs-6840100","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}