The correlation between stiffness and viscoelasticity of the myometrium in adenomyosis: a prospective study

The uterus is a viscoelastic substance and is not purely elastic ( 1 ). The inner myometrium, also known as the uterine junction zone , is the inner one-third of the myometrium and is of great significance in regulating the growth, differentiation, and physiologic contraction of the endometrium. The uterine junction zone consists of a dense area and a transitional area. The dense area is closely related to low signal. Therefore T2-weighted (T2W) of magnetic resonance image (MRI) shows irregular low signal bands ( 2 ). Meanwhile, the smooth muscle cells and fibers contained in the uterine middle and outer myometrium are different from those in the inner myometrium and show a higher signal in T2-weighted imaging (T2WI) than does the inner myometrium ( 3 ), Therefore, the inner, middle, and outer myometrium of the uterus differ in terms of stiffness, viscoelasticity, and water. Shear wave elastography (SWE) can obtain Young’s modulus (E), which is a material property, correlated to stiffness (k), and reflects a structural property. The stiffness of a component depends on both the material’s Young’s modulus and the geometry of the component, so we use E to describe the stiffness of anatomical structures to aid in their detection and characterization ( 4 ). Shear wave dispersion (SWD) analyzes the speed of different shear wave components depending on their frequency via the shear wave dispersion slope (SWDS), which can be used to assess tissue viscoelasticity ( 5 ). Super microvascular imaging (SMI) can clearly display blood flow signals in the lesion tissue ( 6 ). To improve accuracy in the diagnosis adenomyosis (AM), we employed a combination of MRI and ultrasound in previous research ( 7 , 8 ). The T2 value is one of the fundamental tissue characteristics and is sensitive to the free water content in the tissue, depending on the type of tissue and the surrounding environment, with an increase in T2 principally reflecting tissue edema ( 9 ). AM is typically diagnosed when ectopic glands and stroma are observed in the myometrium surrounded by hyperplasic and hypertrophic smooth muscle cells in a hyperfasciculate trabecular pattern. The histology involves extensive fibrosis and increased microvascularization. These changes in the tissue ultrastructure may modify the stiffness of the myometrium, which may be detected by SWE and SWD ( 10 , 11 ). Therefore, determining the correlation between stiffness, viscoelasticity, blood flow, and water content of each myometrial segment may ensure the accurate diagnosis of AM. This study aimed to analyze the correlation between the stiffness, viscoelasticity, and water content of myometrium and lesion of uterus in AM and to determine their efficacy in diagnosing AM. We present this article in accordance with the STARD reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-349/rc ).

This prospective study (clinical trial registration No. ChiCTR2300077978) was approved by the Institutional Review Board of Shengjing Hospital of China Medical University (No. 2023PS1273K) and was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. From October 25 to December 20, 2023, patients with AM confirmed by MRI in Shengjing Hospital of China Medical University were prospectively enrolled. Meanwhile, women without any clinical symptoms and no lesions in the myometrium were recruited as the control group ( Figure 1 ). Finally, 58 women were recruited for study participation and provided informed consent. The patients were recruited into one of two groups: the AM and normal myometrium groups. Flowchart of study participants included and excluded in the study. IUD, intrauterine device; MRI, magnetic resonance imaging; SWD, shear wave dispersion; SWE, shear wave elastography. The inclusion criteria were as follows: (I) age 18–50 years old; (II) suspected AM due to related clinical symptoms (menstrual pain, menorrhagia, dysmenorrhea, and chronic pelvic pain); (III) AM confirmed by MRI examination 3–5 days before menstruation; (IV) no other benign uterine conditions such as myomas or polyps; (V) no other malignant gynecological diseases or serious heart and lung diseases; and (VI) voluntarily informed consent and compliance with program requirements. Meanwhile, the exclusion criteria were as follows: (I) menopausal women; (II) pregnant women; (III) use of intrauterine devices or hormonal contraception; (IV) previous surgery for AM or myoma or minimally invasive treatment for menorrhagia; and (V) a lack of no complete ultrasound and MRI data. SWE and SWD images were obtained via on Aplioi900 (Toshiba Medical Systems, Otawara, Japan) by two registered medical diagnostic sonographers with 2 years of experience. The uterus was imaged with a 3- to 11-MHz transabdominal probe, with representative still and dynamic grayscale, color Doppler, and microflow images being stored. In the SWE mode, a region of interest (ROI) (diameter 2 mm) was placed in the intrauterine, middle myometrium, extrauterine segments to obtain the Young’s modulus (E) and SWDS according to the anatomical location and target tissue thickness. The focal area included three ROIs, with the larger one covering the entire lesion and the smaller two being 2 mm in diameter, and their mean value was calculated. The size of the ROI was carefully selected to measure a sufficient area of soft tissue/lesion in a site restricted to the uterus/lesion. E and SWDS were measured by the SWE and SWD software on the ultrasound machine and were displayed on the image. The reliability of the measurement results mainly depends on the quality of the elastic color map overlaid on the grayscale image and the standard deviation of each measurement result. If there are insufficient color pixels in the ROI or if the standard deviation is greater than 30%, the measurement is considered invalid. In order to obtain accurate measurement results, the uterus must be parallel to the propagation wave line (the myometrium cannot be measured at a depth greater than 7 cm) ( 12 ). At least 5 values were measured for each patient at the same point before the mean was calculated. The microvascular ratio was measured by SMI software on the Aplio i900 system. First, ultrasound was used to determine the location of the lesion, and then the SMI mode was applied to appropriately adjust the size of the sampling frame and display it on the image ( 13 ). The blood flow ratio was obtained by tracing the lesions of AM as the ROI. As the reference standard, contrast-enhanced pelvic MRI was performed on a 3.00T Signa HDxt device (GE HealthCare, Chicago, IL, USA), and MR images were stored and reviewed on a Picture Archiving and Communication System (PACS) (McKesson Radiology, San Francisco, CA, USA). The MR examination was reviewed by two board-certified radiologists (each with 5 years of experience and training in abdominal examination) who were unaware of the ultrasound examination and clinical results ( 4 ). The presence, type, and location of any uterine pathology were noted. Through use of display software, an ROI (diameter of 2 mm) was placed on each of the intrauterine, middle myometrial, and extrauterine segments on T2WI to obtain the T2 value. At least 5 values were measured for each patient at the same point before the mean was calculated. All lesion areas were traced freely, and T2 values were obtained ( Figure 2 ). Method used to locate and measure the lesion and each myometrial segment. (A) A 29-year-old woman in whom a transabdominal two-dimensional gray scale-ultrasound image showed significant adenomyosis (axial and retroverted position) in the posterior wall of the uterus. (B) The SWE image was generated via QuadView and is composed of an elastic map, propagation map, two-dimensional reference map, and shear wave dispersion map. The ROI was carefully selected to obtain the E and SWDS of the inner, middle, and outer myometrium and lesion. (C) The microvascular flow ratio of the lesion of adenomyosis in the posterior wall of the uterus in a 45-year-old woman. The yellow ROI was carefully selected to obtain the ratio of micro-blood flow in the adenomyosis lesion. (D,E) A 42-year-old woman with T2WI had significant adenomyosis in the anterior uterine wall (retroversion of the uterus). The red ROIs were selected to obtain the T2 value of the normal inner, middle, and outer myometrium of the posterior wall. (F) The lesion area of anterior wall was traced freely (the red ROI), and the T2 value of anterior wall lesion was obtained. Ave, average; E, Young’s modulus; ROI, region of interest; SD, standard deviation; SMI, super microvascular imaging; SWDS, shear wave dispersion slope; SWE, shear wave elastography; T2WI, T2-weighted imaging. Statistical analyses were performed with IBM 24.0 (IBM Corp., Armonk, NY, USA) for Windows. The Kolmogorov-Smirnov test was used to determine the distribution of the data. For baseline characteristics, the results are expressed as the mean ± standard deviation (SD); for continuous normally distributed data, they are expressed as the median and interquartile range; and for continuous nonnormally distributed data, they are expressed as the number and percentage of patients in each group (for categorical data). Stiffness (E), viscoelasticity (SWDS), and water content (T2 value) were analyzed on per patient and per anatomical site basis, in recognition that multiple measurements within a single uterus are not independent observations. Continuous variables were analyzed via t -tests and one-way analysis of variance. The correlations between stiffness (E), viscoelasticity (SWDS), and water content (T2 value) were analyzed via Spearman correlation analysis. Multivariate logistic regression analysis showed that the stiffness and viscoelasticity of each myometrium were effective in the diagnosis of AM. A two-tailed P value <0.05 was considered to be statistically significant.

Multiple comparisons between the normal uterine group and the AM group are provided in Table 1 . The mean age of the participants with a normal myometrium was 33.4±1.2 years, which was lower than that of those with AM (38.2±1.6 years). The volume of the uterus (long diameter, anterior-posterior diameter, and transverse diameter) in patients with AM was significantly larger (199.80±28.23 cm 3 ) than that in the healthy controls (68.82±3.90 cm 3 ). Data are presented as the mean ± standard deviation. BMI, body mass index. The stiffness (E value) of each myometrium was significantly different both in the normal uterine group and the AM group (P<0.001) ( Table 2 ). However, there was no significant difference in viscoelasticity (SWDS value) of the different myometrial segments in both the normal uterine group and the AM group (P=0.662). In the AM group, the water content (T2 value) of each myometrium and lesion was not significantly different (P=0.165). Data are presented as the mean ± standard deviation. P<0.05 indicates a statistically significant difference. E, Young’s modulus; SWDS, shear wave dispersion slope. In the AM group, the stiffness, viscoelasticity, and water content of the inner myometrium were significantly correlated (P<0.01) ( Table 3 ). There was a correlation between the stiffness and water content of the middle and outer myometrium (P0.05). Water content was significantly correlated with stiffness, viscoelasticity, and microvascular flow in lesions (P<0.01). *, P<0.05; **, P<0.01. E, Young’s modulus; SMI, super microvascular imaging; SWDS, shear wave dispersion slope. In both the AM group and the normal uterine group, uterine volume (P=0.010), E value of the inner muscle (P=0.003), and E value of the medium muscle (P=0.048) were correlated with the diagnosis of AM ( Table 4 ). There was no significant correlation between age, BMI, SWDS, and the diagnosis of AM (P>0.05). P<0.05 indicates a statistically significant difference. BMI, body mass index; CI, confidence interval; E, Young’s modulus; OR, odds ratio; SWDS, shear wave dispersion slope. The receiver operating characteristic was used to determine the efficacy of different variables in diagnosing AM. The E value was effective in diagnosing AM. A SWE cutoff point of 20.642 KPa could diagnose AM with a sensitivity of 73%, a specificity of 100%, and an area under the curve (AUC) of 0.853 [95% confidence interval (CI): 0.693–1.000; P<0.003] ( Table 5 and Figure 3 ). † , under the nonparametric assumption; ‡ , null hypothesis: true area =0.5. E, Young’s modulus; SWDS, shear wave dispersion slope. Receiver operating characteristic curves for the efficacy of the MRI T2WI (blue), SWE (green), and SWD (yellow) models in diagnosing adenomyosis. MRI, magnetic resonance imaging; ROC, receiver operating characteristic; SWD, shear wave dispersion; SWE, shear wave elastography; T2WI, T2-weighted imaging.

This study showed that there were significant differences in E and T2 value between the inner, middle, and outer myometrium, indicating that the stiffness and water content of these segments vary. In addition, the E of each myometrial segment and the lesion in AM was negatively correlated with the T2 value, indicating that the less water content there is in each portion of the uterus, the more rigid the uterus. AM can be diagnosed with high sensitivity and specificity by SWE, greater than those of SWD and MRI. This observation is clinically relevant for several reasons. Abnormal contractions and excessive peristalsis of the uterus in patients with AM, especially in the intrauterine myometrium (uterine junction zone), are thought to cause pain, spasms, and poor fertility due to altered sperm transport ( 14 , 15 ). Muscle stiffness, contractile force, and shear wave propagation are related, which provides a pathway for using SWE in accurate diagnosis ( 4 ). Research indicates that the higher the signal of AM at T2WI is, the less fibrous are the tissue components, resulting in a less reflective interface of ultrasound, lower echo on ultrasound images, smaller quantized gray value, more abundant blood flow, and a higher pixel area ratio of blood flow signal ( 16 - 18 ). The results of our study are consistent these findings, with the stiffness, viscoelasticity, microflow ratio, and water content of each myometrial segment and lesion of AM being co-related to a certain extent. Similarly, Pongpunprut et al. ( 10 ) found that the stiffness of AM measured by SWV was significantly higher than that of normal myometrium and that SWE can be used as an alternative diagnostic tool for distinguishing between a normal myometrium and AM ( 19 ). It should be noted that the Pongpunprut et al. ( 10 ) study used pathological diagnosis as the gold standard, while in our study, we used MRI and clinical data to diagnose AM. Moreover, SWE was expressed in meter per second; however meter per second and kilopascals may differ. In addition, SWV has often used as a parameter in our previous studies ( 4 , 10 , 20 ), while our study used Young’s modulus (E) to directly assess tissue stiffness. SWE and SWD are novel techniques in objective quantitative ultrasound imaging and have been extensively studied in liver fibrosis ( 21 - 23 ) and thyroid ( 24 ) and breast neoplasms ( 25 , 26 ) over the past decade. In all instances, the pathologic condition tends to be firmer than the normal visceral parenchyma. In contrast, obstetric and gynecologic implementations have been limited, with only a few published studies on the SWE of AM ( 27 ). Pongpunprut et al. and Acar et al. ( 10 , 19 ) demonstrated that in patients with AM, the myometrial stiffness assessed with SWE technology is significantly high. The difference is that these SWE values were numerically higher than those in our study. However, few studies on the relationship between the stiffness and viscoelasticity of the inner, middle, and outer myometrium in the non-focal area of AM and the various myometrial segments of the normal uterus have been conducted. As predicted, there were statistically significant differences in the stiffness of each myometrial segment and the lesion (P<0.001), while there were no statistically significant differences in the stiffness of each myometrial segment between the normal uterus and lesion of AM. In the context of elastography studies, most previous studies, such as that by Săsăran et al. , have used transvaginal probes for imaging ( 11 , 28 , 29 ). Transvaginal probes typically provide higher resolution images due to their closer proximity to the target tissues. However, some patients may find the transvaginal probe uncomfortable or invasive and have a limited range. In contrast, our study used transabdominal probes for elastography. This method has several advantages. For one, it is noninvasive, relatively comfortable for patients, and is applicable in a wide range of clinical settings. For another, it is more be practicable and standardized and can be used in a wider range of organizations. Therefore, our study, including these probes, represents a more innovative and feasible approach in terms of patient comfort and image quality. The selection of our study population was based on strict MRI criteria. All enrolled women in the AM group underwent pelvic MRI, which was performed by a radiologist with expertise in gynecologic imaging. By analyzing the T2WI, we could obtain the water content of each myometrial segment and lesion for comparative analysis of stiffness and viscoelasticity. The MRI-based diagnosis of AM is supported by the literature and has high accuracy, along with a sensitivity of 77.5% and a specificity of 92.5% ( 30 ). Therefore, MRI is a tool that enables clinicians to diagnose AM in young women as an alternative to conventional histological diagnosis ( 7 ). In addition, the clinical data were exhaustive and detailed, and we prospectively recorded them using a structured questionnaire. The cutoff point of SWE at 20.642 KPa could diagnose AM with the sensitivity of 73%, a specificity of 100%, and an AUC of 0.853 (95% CI: 0.693–1.000) (P<0.003). Therefore, the stiffness of each myometrial segment and the lesion can diagnose AM. SWD provides a novel approach for the noninvasive evaluation of tissue viscoelasticity. SWDS involves the slope of pulse and SWE, which can reflect the tissue viscoelasticity ( 31 ). In a basic experiment, the viscoelasticity was positively correlated with the SWD ( 32 ). Moreover, Zhang et al. reported that shear wave propagation within the liver was highly dependent on both elasticity and viscosity and that liver viscosity could be used to diagnose fibrosis, necrosis, inflammatory activity, and steatosis ( 12 ). AM is a form of inflammation ( 33 , 34 ), and thus the viscoelasticity of each myometrial segment will be altered. However, we found there to be no significant difference in the viscoelasticity of each myometrial segment of the uterus and the lesions, and water content (T2 value) of each myometrium and lesion was not significantly different. This study employed a single-center design and did not conduct a comparison with the histopathological images directly corresponding to ultrasound images. Moreover, the sample size was small and needs to be further expanded. During the examination, the menstrual period was not controlled for, and the influence of hormones on the myometrium, especially the inner myometrium, could have led to changes in the E and SWDS. By analyzing the E, SWDS, and T2 value of the inner, middle, and outer myometrium, we found that the stiffness, viscoelasticity, and water content of each myometrial segment of the same uterus and between the lesion and each segment were different and could be used to achieve an accurate diagnosis of AM. In addition, through the preliminary analysis of the physical properties of each myometrial segment of the uterus, our study provides a certain physical basis for clarifying the pathogenesis of AM.

In this preliminary study, there were statistically significant differences in the stiffness and water content of the inner, medial, and outer myometrium in AM. Moreover, the stiffness, microvascular flow, and uterine volume of each myometrial and lesion were correlated with water content and can be used as diagnostic factors for AM. Through application of the SWE technique, AM can be accurately distinguished from a normal uterus. Therefore, our study provides a novel direction for developing the means to accurately diagnosing AM.

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