Enhanced detection of vascularization in rectal endometriosis: A comparative study of microvascular flow and power Doppler

This prospective study was conducted at a tertiary care hospital. Eligible participants were women diagnosed with deep endometriosis involving the rectum. All patients were referred to the Endometriosis Ultrasound Unit for ultrasound assessment as part of routine clinical evaluation between August 2024 and December 2024. Patients were recruited consecutively. All participants provided oral informed consent after receiving an explanation of the study. No incentive was offered for participating in this study. Patients were informed that the results obtained would not be taken into consideration for their clinical management. As the ultrasound examination was part of routine care, institutional review board approval was waived. Patients with a history of rectal surgery or those who were pregnant at the time of examination were excluded. Patients receiving medical therapy were not excluded; however, ongoing treatment was recorded. All women underwent transvaginal ultrasound examination following the scanning protocol recommended by the International Deep Endometriosis Analysis consensus[ 8 ]. All ultrasound examinations were performed by a single expert examiner (Alcazar JL). Endometriotic lesions involving the rectum were identified as hypoechoic lesions located in the anterior rectal wall with blurred margins ( Figure 1 ). Transvaginal ultrasound findings of endometriotic rectal nodules. A: Transvaginal ultrasound of an endometriotic rectal nodule; B: Another case of an endometriotic rectal nodule as observed by transvaginal ultrasound. First, the lesion was measured in three orthogonal planes. After identification and measurement, power Doppler was activated, the region of interest was adjusted to include the lesion, and lesion vascularization was assessed. Immediately after completion of the power Doppler evaluation, MV-Flow™ with LumiFlow™ was activated, and vascularization was reassessed. Power Doppler settings were as follows: Pulse repetition frequency 0.58 kHz, sensitivity 10, filter 3, and gain 50. MV-Flow™ settings were: Pulse repetition frequency 0.13 kHz, gain 50, and 3D quality set to “high2”. The same power Doppler and MV-Flow™ settings were used for all examinations. These settings were chosen according to previous studies in uterine pathologies, and they were considered adequate for this study[ 14 ]. All examinations were performed using a Samsung V8 ultrasound system (Samsung Medison Co., Ltd., Seoul, South Korea) equipped with an EV2-10A endovaginal transducer. Vascularization was assessed subjectively by the examiner using the vascular score proposed by the International Ovarian Tumor Analysis group for adnexal masses[ 18 ]: Score 1 (no vascularization), score 2 (minimal vascularization), score 3 (moderate vascularization), and score 4 (abundant vascularization). Reproducibility of the vascular score for both conventional power Doppler and MV-Flow™ was assessed by two additional examiners (Sanchez O and Pich Barroso DM), who independently evaluated recorded video clips from 10 cases. Considering the vascular score as a categorical variable, interobserver agreement was evaluated using the weighted kappa coefficient. Power calculation was not performed. Continuous variables were expressed as mean ± SD or as median (interquartile range), depending on data distribution. Data distribution was assessed using the Kolmogorov-Smirnov test. Categorical variables were expressed as n (%). Continuous variables were compared using the Student’s t test or the Mann-Whitney U test, as appropriate. Categorical variables were compared using the χ 2 test. A P < 0.05 was considered statistically significant. All analyses were performed using IBM SPSS software, version 25 (IBM Corp., Armonk, NY, United States). There is no previous data to allow a meaningful estimation. For this reason, we do consider this study as a preliminary exploratory study.

During the study period 39 women were offered to participate. After exclusions (six women had previous rectal surgery and three refused to participate), thirty women were ultimately included. The median age of the patients was 34.6 years, with a range of 29-43 years. All patients were premenopausal. Twelve women (40%) reported symptoms related to rectal endometriosis. Twenty-five women (83%) were receiving hormonal therapy (combined oral contraceptives or progestins). The median lesion length was 18 mm, ranging from 7 mm to 33 mm. Using conventional power Doppler, the vascular score was 1 in 87% of lesions ( n = 26) and 2 in 13% ( n = 4). No lesions demonstrated a vascular score of 3 or 4. In contrast, using MV-Flow™, the vascular score was 1 in 6% of cases ( n = 2), 2 in 27% ( n = 8), 3 in 40% ( n = 12), and 4 in 27% ( n = 8; Figure 2 ). Comparative assessment of rectal nodule vascularization using conventional power doppler and microvascular flow in four cases. A: Case 1: Power Doppler (vascular score 1) vs microvascular flow (vascular score 4); B: Case 2: Power Doppler (vascular score 1) vs microvascular flow (vascular score 4); C: Case 3: Power Doppler (vascular score 1) vs microvascular flow (vascular score 2); D: Case 4: Power Doppler (vascular score 1) vs microvascular flow (vascular score 2). When cases were grouped into scores 1-2 vs scores 3-4, a statistically significant difference was observed, with a substantially higher proportion of lesions classified as scores 3-4 when assessed using MV-Flow™ compared with conventional power Doppler (67% vs 0%, P < 0.01; Figure 3 ). Histogram depicting the distribution of cases according to vascular score using conventional power Doppler and microvascular flow. MV: Microvascular. Interobserver reproducibility was good for both conventional power Doppler (weighted kappa = 0.80; 95% confidence interval: 0.58-1.00) and MV-Flow™ (weighted kappa = 0.78; 95% confidence interval: 0.48-1.00).

In conclusion, this study demonstrates that many endometriotic nodules involving the rectum and sigmoid colon are substantially more vascularized than previously thought. Further studies are warranted to confirm these findings and to determine whether similar vascularization patterns are observed in endometriotic lesions at other anatomical locations, including the bladder, uterosacral ligaments, and abdominal wall.

In this study, we observed that most rectal endometriotic nodules exhibited moderate to high vascularization when assessed using MV-Flow™, in contrast to conventional power Doppler. Notably, our findings obtained with conventional power Doppler are consistent with those previously reported in the literature[ 19 , 20 ]. However, the results obtained with MV-Flow™ are particularly relevant, as they challenge the prevailing concept that deep endometriotic nodules are poorly vascularized[ 8 ]. Moreover, these findings support the ability of MV-Flow™ to visualize smaller vessels with slower blood flow, as has been demonstrated in other pathological conditions[ 14 - 17 ]. The main strength of our study lies in being the first to evaluate the vascularization of rectal endometriotic nodules using a relatively novel and more sensitive imaging technique for the assessment of tissue vascularity. An additional strength is the good interobserver reproducibility observed for both conventional power Doppler and MV-Flow™. Similar interobserver reproducibility of the International Ovarian Tumor Analysis vascular score has been reported in studies evaluating vascularization in endometrial and ovarian tumors[ 21 - 23 ]. To the best of our knowledge, no previous study has compared conventional power Doppler with MV-Flow™ in endometriotic lesions. So, we cannot compare our results with any other study. Several limitations of this study should be acknowledged. First, the sample size was small, and a priori sample size calculation was not performed; nevertheless, our results may serve as preliminary data for the design of future adequately powered studies. Second, this was a single-center study conducted using a single ultrasound system, which may limit the generalizability of the findings. Third, the analysis focused exclusively on rectal lesions; therefore, it remains unclear whether similar vascularization patterns would be observed in endometriotic lesions located at other anatomical sites, such as the bladder, uterosacral ligaments, rectovaginal septum, or parametrium. Furthermore, we acknowledge that potential selection bias might exist, since women with recurrent lesions were excluded. Despite these limitations, we believe that our findings may represent a shift in the current paradigm of vascular assessment in endometriosis, opening new avenues for research. Future studies should address whether lesion vascularization is associated with disease progression, response to medical therapy, or the risk of recurrence following surgical excision.

Pelvic endometriosis is a chronic, benign condition estimated to affect approximately 3%-10% of women[ 1 , 2 ]. The disease manifests in three distinct forms: Ovarian endometriosis, superficial endometriosis, and deep endometriosis[ 3 ]. Pelvic endometriosis may involve multiple pelvic sites and organs, with the rectum and sigmoid colon being among the most frequently affected locations[ 4 ]. The pathophysiology of endometriosis remains incompletely understood[ 5 ]. However, increasing evidence suggests that its development is driven by complex interactions among endocrine, immunological, pro-inflammatory, and pro-angiogenic mechanisms[ 6 ]. In particular, angiogenesis plays a pivotal role in the pathogenesis of the disease[ 7 ]. Transvaginal sonography is the first-line imaging modality for the diagnosis of pelvic and deep endometriosis[ 8 ]. Color and power Doppler ultrasound can be used to assess the vascularization of endometriotic lesions. Doppler studies have demonstrated that vascularization correlates with disease activity in ovarian endometriomas[ 9 ]. Traditionally, deep endometriosis nodules have been considered poorly vascularized, particularly in advanced fibrotic lesions[ 8 , 10 ]. However, more recent histopathological studies, supported by immunohistochemical evidence of vascular endothelial growth factor receptor positivity, suggest that deep endometriosis lesions are highly vascularized. This characteristic distinguishes them from normal endometrium and other forms of endometriosis[ 11 ]. Angiogenesis, vasculogenesis, and inosculation have been proposed as mechanisms underlying the development of microvasculature in these lesions, contributing to their progression and local aggressiveness[ 12 , 13 ]. The limitation of conventional power Doppler to detect microvasculature in endometriotic nodules might be explained by several factors. In these lesions, the vessel diameter is very small, the intravascular blood volume is minimal, and the number of erythrocytes is low. This may cause the generated signal to be lower than the equipment's detection threshold. In addition, in solid fibrotic tissues (such as many deep endometriosis nodules), the vessels are sparse, compressed and with intermittent flow, making even more difficult to detect flow. On the other hand, to eliminate artifacts due to tissue movement, the machine applies a wall filter that suppresses low-frequency signals. Very slow flow in microvessels generates very low frequency shifts. These shifts can overlap with those generated by tissue movement, so that the system cannot clearly distinguish between real slow blood flow and low-grade tissue movement. Microvascular (MV) flow imaging enables the visualization of small vessels with low-velocity blood flow. Compared with conventional Doppler techniques, MV-Flow™ uses a lower pulse repetition frequency and a higher frame rate. The increased frame rate allows for detailed, high-resolution imaging of both macrovascular and microvascular blood flow, while advanced adaptive filtering reduces motion artifacts and preserves directional flow detection. As a result, MV-Flow™ demonstrates greater sensitivity for the evaluation of low-velocity blood flow and improved performance in detecting small vessels[ 14 - 17 ]. The aim of this study was to compare the vascularization of rectal endometriotic nodules assessed using MV-Flow™ and conventional power Doppler ultrasound. We hypothesized that rectal endometriotic nodules would demonstrate higher vascularization when evaluated with MV-Flow™ compared with conventional power Doppler.