{"paper_id":"0c48e754-4c15-480f-b7cb-1f63148b02eb","body_text":"Ruaux et al. Insights into Imaging           (2024) 15:20 \nhttps://doi.org/10.1186/s13244-023-01588-2\nEDUCATIONAL REVIEW Open Access\n© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which \npermits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the \noriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or \nother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line \nto the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory \nregulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this \nlicence, visit http://creativecommons.org/licenses/by/4.0/.\nEndometriosis MR mimickers: \nT2-hypointense lesions\nEdouard Ruaux1, Wendaline M. VanBuren2, Stéphanie Nougaret3, Marie Gavrel1, Mathilde Charlot1, \nFlavia Grangeon1, Pierre‑Adrien Bolze4, Isabelle Thomassin‑Naggara5 and Pascal Rousset1*   \nAbstract \nEndometriosis is a common crippling disease in women of reproductive age. Magnetic resonance imaging (MRI) \nis considered the cornerstone radiological technique for both the diagnosis and management of endometriosis. \nWhile its sensitivity, especially in deep infiltrating endometriosis, is superior to that of ultrasonography, many sources \nof false‑positive results exist, leading to a lack of specificity. Hypointense lesions or pseudo‑lesions on T2‑weighted \nimages include anatomical variants, fibrous connective tissues, benign and malignant tumors, feces, surgical materi‑\nals, and post treatment scars which may mimic deep pelvic infiltrating endometriosis. False positives can have a major \nimpact on patient management, from diagnosis to medical or surgical treatment. This educational review aims to help \nthe radiologist acknowledge MRI criteria, pitfalls, and the differential diagnosis of deep pelvic infiltrating endometrio‑\nsis to reduce false‑positive results.\nCritical relevance statement\nMRI in deep infiltrating endometriosis has a 23% false‑positive rate, leading to misdiagnosis. T2‑hypointense lesions \nprimarily result from anatomical variations, fibrous connective tissue, benign and malignant tumors, feces, surgical \nmaterial, and post‑treatment scars.\nKey points\n• MRI in DIE has a 23% false‑positive rate, leading to potential misdiagnosis.\n• Anatomical variations, fibrous connective tissues, neoplasms, and surgical alterations are the main sources \nof T2‑hypointense mimickers.\n• Multisequence interpretation, morphologic assessment, and precise anatomic localization are crucial to prevent \noverdiagnosis.\n• Gadolinium injection is beneficial for assessing endometriosis differential diagnosis only in specific conditions.\nKeywords Endometriosis, Deep infiltrating endometriosis, Pelvic inflammatory disease, Genital diseases, Magnetic \nresonance imaging\n*Correspondence:\nPascal Rousset\npascal.rousset@chu‑lyon.fr\nFull list of author information is available at the end of the article\n\nPage 2 of 15Ruaux et al. Insights into Imaging           (2024) 15:20 \nGraphical Abstract\nBackground\nEndometriosis is a common disease of reproductive-age \nwomen related to endometrial-like tissue outside the \nuterine cavity with an estimated prevalence between 5 \nand 10% [1]. Three clinical phenotypes of endometrio -\nsis may coexist: ovarian cysts known as endometriomas, \nsuperficial peritoneal implants, and deep infiltrating \nendometriosis (DIE) [2]. DIE is defined as an extension \nof endometrial tissue-like below the peritoneal surface, \nwith the ability to invade adjacent structures, associated \nwith fibrosis and disruption of normal anatomy [2]. DIE \ncan affect almost any organ or structure, although most \nlesions are found in the pelvic region, especially the pos -\nterior compartment [3]. Patients usually present with \nchronic pelvic pain, severe dysmenorrhea, dyspareu -\nnia, dyschezia, dysuria, and infertility [4], depending on \nthe anatomic location and degree of infiltration, all of \nwhich can strongly affect quality of life [5]. This disease \nrepresents a public health issue, with a major impact on \nquality of life, highlighting the importance of an accu -\nrate and precise early diagnosis [6]. While transvaginal \nultrasonography can be a first-line imaging modality [7], \nmagnetic resonance imaging (MRI) is the cornerstone \nimaging technique in the evaluation of endometriosis, \nespecially for DIE, with an overall sensitivity of 94% [3]. \nDedicated DIE ultrasound protocols by experts have \nbeen shown to have a similar sensitivity for certain dis -\nease locations, predominantly the ovaries, uterosacral \nligaments (USLs), and bowel, but are not widely avail -\nable [8]. MRI allows exhaustive mapping of DIE lesions, \ndetermining the extent of disease and organ involvement, \nhelping gynecologists in the decision-making for a medi -\ncal or surgical management [9]. However, MRI may lack \nspecificity, leading to a 10% false positive rate overall [10], \nwhich increases to 23% in cases of DIE when compared \nto surgical findings [3]. In the dedicated recommended \nMRI protocol [11], DIE lesions are well-identified as \nT2-hypointense solid nodular or fibrotic thickening \nlesions, with potential associated microcystic or hemor -\nrhagic foci due to the presence of active ectopic glandular \ntissue [12]. However, other pelvic conditions including in \nparticular anatomic variations or infectious diseases, can \nexhibit T2-hypointense findings similar to DIE, which \nmay lead to misdiagnosis. Moreover, the challenge may \nbe heightened as the MRI pattern of DIE may also depend \non the ’age’ of the lesion as well as the degree of fibrosis. \nWhile medical history, symptoms, clinical examination, \nand the presence or absence of other pelvic endometri -\notic lesions on MRI can assist in diagnosing endometrio -\nsis, it is important to acknowledge that several challenges \n\nPage 3 of 15\nRuaux et al. Insights into Imaging           (2024) 15:20 \n \nand potential pitfalls still exist in achieving an accurate \ndiagnosis. Misdiagnosis of endometriosis at the initial \npresentation can result in improper medical or surgical \ntreatments and have significant psychological effects on \nthe patient. Additionally, mistaking other T2-hypoin -\ntense findings for endometriosis in confirmed cases can \noverestimate the disease’s extent and potentially lead to \ninappropriate decision-making and interventions.\nThis review aims to offer valuable tips for distin -\nguishing DIE from other pelvic conditions that exhibit \nT2-hypointense tissue-like findings. The approach \ninvolves interpreting the lesion signal across multiple \nsequences, conducting morphologic analysis, and ensur -\ning precise anatomic localization.\nT2‑hypointense lesions or condition‑like lesions \nmimicking DIE\nVarious conditions with T2-hypointense tissue-like \nformations can mimic DIE. These conditions typically \nexhibit features such as hypointense thickening, nodules, \nor infiltrating masses that invade pelvic structures or \norgans, leading to morphological changes and the loss of \nthe normal signal, particularly in the muscularis layer of \nthe affected organ (Table 1).\nAnatomical variations\nUterosacral ligaments\nThe uterosacral ligaments (USLs) originate from the \ntorus uterinus, located in the retrocervical area on the \nposterior surface of the cervix and upper vagina. They \nextend backward towards the sacrum, marking the upper \nboundaries of the posterior cul-de-sac, also referred \nto as the pouch of Douglas. It is a typical location of \nDIE, reported in a study as the second most frequently \ninvolved entity, following the ovaries [13]. MRI diagnos -\ntic performance for torus and USLs in endometriosis is \nexcellent [11]. However, MR imaging is highly sensi -\ntive with variable specificity (84%) [3]. Specificity may \nbe increased in combination with clinical examination \nand/or transvaginal sonography [14]. A recent MRI con -\nsensus lexicon on deep pelvic infiltrating endometriosis \nsuggests positive MR features of USL involvement [9]. \nThese features including nodular aspect in two different \nplanes, and/or retraction, and/or thickness > 5 mm, and/\nor hemorrhagic foci aid in precise and confident diag -\nnosis of USLs involvement in DIE. On the contrary, an \nasymmetrical aspect, and/or linear thickening ranging \nfrom 3 to 5  mm in thickness, and/or irregular margins \nand/or pseudo-nodular appearance (defined as present \nin only one plane) are considered equivocal and not spe -\ncific. These findings can be either a variant or attributed \nto other conditions, making the diagnosis less defini -\ntive [9]. Radiologists should be aware of the potentially \nnon-specific nature of T2-hypointense findings in the \nabsence of hemorrhagic implants or nodules.\nOn one hand, thickened appearance of USLs can be \nattributed to anatomical variants with asymmetries, var -\nying degrees of fibrous tissue, and regional veins (Fig.  1). \nIt is important to correlate these findings with physical \nexamination and/or ultrasonography. Furthermore, when \nevaluating the USLs, a past medical history of pelvic \ninflammatory disease (such as salpingitis or tubo-ovarian \nabscess) and intestinal diseases (like Crohn’s disease or \nprevious peritonitis) should be taken into consideration. \nThese conditions can involve the USLs and lead to post-\ninflammatory scar thickening (Fig. 2).\nRound ligaments\nRound ligaments are intra- and extraperitoneal fibro-\nmuscular structures that extend from the antero-cen -\ntral and antero-lateral pelvic compartments. On MRI, \nround ligaments are visualized as regular structures \nwith low signal intensity on both T1-weighted (T1-W) \nand T2-weighted (T2-W) sequences, extending from \nthe uterine horns to the inguinal canals to attach to the \nvestibule. Endometriosis involvement in the round liga -\nments is more commonly observed in the proximal third \nof the ligament, adjacent to the uterus. The right round \nligament is often more affected than the left due to ret -\nrograde implantation of endometrial tissue in the peri -\ntoneal cavity. In cases of a large lesion, it is frequently \nassociated with external adenomyosis [15]. More rarely, \nDIE involves the extra-pelvic segment within the canal of \nNuck [16].\nThe involvement of endometriosis in the round liga -\nments lacks a consensus definition. However, lesions are \ncommonly observed as nodular (> 1  cm) with irregular \nmargins and varying degrees of microcystic hemorrhagic \nfoci [ 9]. It is important to note that identifying endo -\nmetriosis in this location is challenging due to the lack \nof specificity caused by anatomical variations and the \nabsence of an accurate definition for physiological thick -\nness (Supplemental—Fig.  1). MRI may show tubular or \nserpiginous structures along thickened round ligaments \n(and USLs), that may also  show T1 bright spots  which \ncould potentially be attributed to the “entry slice phe -\nnomenon” artifact, a pitfall leading to overdiagnosis \n(Fig. 3) [17].\nUrachus\nImaging findings of urachal remnants typically manifest \nas T2-hypointense fibrotic tissue due to the presence \nof dense collagen deposition. However, a large fibrotic \nremnant can be mistaken for endometriosis of the blad -\nder wall, especially if the bladder is not adequately filled \n(empty or not full enough) during MR acquisition. \n\nPage 4 of 15Ruaux et al. Insights into Imaging           (2024) 15:20 \nTable 1 T2‑hypointense mimickers: MRI key features\nNature of the T2-hypointensity Structure involved and/or type of condition MRI key features\nAnatomical variation and pitfalls\nUterosacral ligaments Mostly pseudonodular and/or between 3 to 5 mm thickness, \nwithout hemorrhagic foci on T1 FS‑WI, use of multiple planes \nor multiplanar reconstruction on 3D T2‑WI\nPrevious history of pelvic surgery and/or upper genital infection\nRound ligaments Mostly pseudonodular and/or < 1 cm, use of multiple planes \nor multiplanar reconstruction on 3D T2‑WI, regular aspect with‑\nout hyperintense implant on T1 FS‑WI\nAnatomical variation: association with veinous structures (vari‑\ncosities)\nUrachus Mostly seen on moderately filled bladder, triangular aspect \non sagittal T2‑WI plane\nRespect of the muscular layer of the bladder, no hemorrhagic \nfoci on T1 FS‑WI\nUterine contraction Myometrial pseudonodular low signal intensity on T2‑WI \nat the level of the serosa\nPartial or complete resolution on different planes or repeated \nacquisition after a suitable interval\nFibrous tissue\nVesicouterine pouch\nCesarean scar Linear scar defect of variable thickness, sometimes pseudonodu‑\nlar, up to the pelvic wall\nIntra‑ or extra‑mural isthmocele + / − retained blood content\nAbsence of external adenomyosis, bladder wall invasion or hem‑\norrhagic foci on T1 FS‑WI\nPelvic wall\nRound ligaments ligamentoplasty Uterus anteversion, shortened round ligaments with a medial \ncourse and pseudonodular thickening up to their pelvic wall \ninsertion, no hemorrhagic foci on T1 FS‑WI\nDesmoid tumor* Intermediate signal intensity areas on T2‑WI, with high signal \nintensity on DWI, and intense contrast‑enhancement + / − fascial \ntail sign (inconsistent)\nVarying size (may be large), ill or well‑defined\nAbsence of microcystic structures on T2‑WI or hemorrhagic foci \non T1 FS‑WI\nInfectious conditions\nActinomycosis* Solid component masses in low to intermediate signal intensity \non T2‑WI\nNecrosis with moderate to high signal intensity on T1 FS‑WI \nand peripheral enhancement and/or micro‑abscess\nInfiltrating and inflammatory stranding pattern of other pelvic \nstructures/organs\nAlveolar echinococcosis* (extremely rare) Mostly infiltrating masses, high signal intensity microcystic \nchanges on T2‑WI\nNo hemorrhagic foci on T1 FS‑WI, calcifications may be seen \non CT\nCo‑existence of hepatic disease (multicystic infiltrative masses)\nPast history of pelvic infection or peritonitis USLs with mostly pseudonodular aspect < 5 mm, using other \nplanes or multiplanar reconstruction on 3D T2‑WI, without hem‑\norrhagic foci on T1 FS‑WI\nBenign tumors\nPelvic organs\nLeiomyomas* Rounded or oval well‑defined masses\nLow (or intermediate) signal intensity on T2‑WI without hemor‑\nrhagic foci on T1 FS‑WI\nExophytic growth may be seen without any retraction\n\nPage 5 of 15\nRuaux et al. Insights into Imaging           (2024) 15:20 \n \nConversely, this aspect is minimized when the bladder is \noverly distended. The specific anatomical location on the \nsagittal plane may suggest fibrotic thickening with low \nT2-W signal intensity. The morphological appearance \nof the urachus insertion, forming a triangular shape in \ncontinuity with the subperitoneal anatomical course, and \nrespect of the bladder muscularis layer intact on T2-W \nsequences aid in identifying this variant (Fig. 4).\nUterine contraction\nTransient myometrial contraction is a common physi -\nological phenomenon that can mimic pathological \nconditions such as focal or diffuse adenomyosis [18]. \nOn MRI, it appears as a T2-hypointense region within \nthe outer myometrium, potentially leading to bulg -\ning pseudo-thickening of the junctional zone, which \ncan be confused with internal adenomyosis or DIE \n(Supplemental –Fig.  2). In some cases, it can also be \nmisleading for external adenomyosis, presenting as \na pseudonodular T2-hypointense aspect at the level \nof the serosa (Fig.  5). The key finding to differentiate \nbetween these conditions is to compare T2-W images \nfrom different planes to assess for partial or complete \nresolution, as contractions typically improve or resolve \nbetween sequences. T2 cine-mode MRI is preferable \nfor evaluation. It is worth noting that while myometrial \n* Indicates conditions where gadolinium injection can enhance diagnostic accuracy\nTable 1 (continued)\nNature of the T2-hypointensity Structure involved and/or type of condition MRI key features\nMalignant tumors\nRectosigmoid\nColorectal carcinoma* Intrinsic endoluminal lesion with polypoid, semi‑circumferential \nor circumferential morphological aspect, mesorectum infiltration, \nand tumor deposits\nHigh signal intensity with high‑b values on DWI (and low ADC)\nSurgical material Ureteral meatus and parameters\nVesicoureteral reflux treatment Geometrical shaped structures at the ureterovesical junction \nor a little behind\nCommonly bilateral and symmetrical\nCollagen materials in low signal intensity on T2‑WI ± surrounding \ngranulomas\nMacroplastiques in iso or hyposignal on T1 FS‑WI\nHyperdense structures may be seen on CT\nUrethra\nPeriurethral incontinence treatment Bulking agent around or within the wall of the urethra in low \nsignal intensity on T2‑WI\nBulking agent in iso or hyposignal (or not seen) on T1 FS‑WI\nHyperdense structures may be seen on CT (around the urethra, \nunder the bladder)\nFeces Rectosigmoid Endoluminal digestive location on other planes or multiplanar \nreconstruction on 3D T2‑WI\nFeces‑like signal on T1 FS‑WI, gas with signal void in low signal \nintensity on T1‑WI\nFig. 1 Left utero‑sacral ligament (USL) varicosity in a 46‑year‑old woman with chronic catamenial pelvic pain. a Axial, (b) sagittal, and (c) coronal \nT2‑W MR images show a thickened and pseudonodular left USL (arrows) due to tubular and serpiginous T2‑hyperintense veinous structures. No \npelvic endometriosis was found at surgery; a pelvic venous congestion syndrome was then suggested\n\nPage 6 of 15Ruaux et al. Insights into Imaging           (2024) 15:20 \ncontraction is transient, it can be sustained for up to \n30 ± 45 min, in which case repeating a sequence at that \ntime may be necessary [19].\nFibrous tissue\nFibrous tissue comprises low-cellularity material in \ncombination with spindle, oval, or round cells resulting \nin collagen formation. Fibrosis typically demonstrates \nintermediate signal intensity on T1-WI and very low sig -\nnal intensity on T2-WI [20].\nPost-surgical scars\nCesarean scar\nUterine scar defects can occur in up to 50% of women \nwith infertility and prior cesarean section (C-section) \n[21]. Surgical interventions like C-section may result in \nFig. 2 Thickening of the utero‑sacral ligaments (USLs) in a 44‑year‑old woman with history of PID. Acute episode in 2020:a Axial T2‑W MR image \nshows irregular and pseudo‑nodular thickening of bilateral USLs (arrows). b Axial T1‑W fat‑suppressed contrast‑enhanced MR image shows \npyosalpinx (star) with thick‑walled fallopian tube and surrounding fat stranding. One year follow-up in 2021:c Axial and (d) sagittal T2‑W MR images \nshow persistence of pseudonodular thickening of the utero‑sacral ligaments (arrows) and the torus, without hemorrhagic foci (not shown)\nFig. 3 Right round ligament varicosity in the inguinal canal in a 33‑year‑old woman. a Axial T2‑W MR images show pseudocystic changes \nof the right round ligament in its inguinal course (arrows). Thickening appears regular, without any fluid around the ligament. b Axial fat‑suppressed \nT1‑W MR image reveals a few T1‑hyperintense foci (arrowheads) within the right round ligament, due to an “entry slice phenomenon artifact” . c Axial \nT1‑weighted fat‑suppressed contrast‑enhanced MR image shows homogeneous enhancement of tubular veins around the right round ligament \n(arrows)\n\nPage 7 of 15\nRuaux et al. Insights into Imaging           (2024) 15:20 \n \nfocal adhesions in the vesicouterine pouch, sometimes \ncausing complete obliteration of the anterior peritoneal \nspaces. Distinguishing between fibrous scar tissue and \nendometriosis can be difficult in these cases. Post-surgi -\ncal scars without endometriosis typically appear linear \nor pseudonodular without external adenomyosis, blad -\nder wall invasion, or hemorrhagic foci (Fig.  6). In some \ninstances, intra- or extra-mural isthmocele in the lower \nanterior uterine wall with retained blood content may be \nassociated and mistaken for endometriosis [22].\nLigamentoplasty\nSurgical procedures involving the uterine ligaments, \nsuch as ligamentoplasty of the round ligaments and/\nor uterosacral ligaments, can present challenges in MRI \ninterpretation. After retroversion and hysteropexy sur -\ngeries associated with Master Allen syndrome, round \nligament ligamentoplasty (which aims to shorten the \nligaments and antevert the uterus) can appear as a scar-\nlike, pseudo-nodular changes at their insertion to the \npelvic wall, slightly medial to their original course in the \ninguinal canal [23]. The kinking of the uterosacral liga -\nments may create a closure of the pouch of Douglas with \na pseudo-nodular aspect [24]. These findings are usually \nisolated, following the anatomical courses of the liga -\nments, and do not exhibit hemorrhagic foci on T1-WI \n(Supplemental –Fig. 3).\nBenign tumors\nDesmoid tumors\nDesmoid fibromatosis is a locally aggressive benign \ntumor that can occur within the abdominal wall, inter -\nnally in the abdomen and pelvis (often mesenteric), or \nextra-abdominal locations [25]. Most desmoid tumors are \nsporadic and have a predilection for women of reproduc -\ntive age, with a female-to-male ratio of 3:1 [26]. Sporadic \nlesions can affect surgical scars and have an unpredict -\nable natural course, which can involve rapid enlargement, \nspontaneous decrease in size, or resolution. Inheritance \nplays a role in up to 15% of desmoid fibromatosis tumors, \nFig. 4 Fibrous remnant of the urachus insertion in two distinct women of reproductive age. a Sagittal and (b) coronal T2‑W MR images show \na pseudonodular or triangular T2‑hypointense medial structure of the urinary bladder apex at the urachus insertion (arrows). Note the absence \nof abnormality of the urinary bladder muscular layer (arrowheads), or hemorrhagic foci on T1‑WI (no shown)\nFig. 5 Uterine contraction in a 19‑year‑old woman with chronic catamenial pelvic pain. a Sagittal T2‑W MR image shows a T2‑hypointense \nfocal thickening (arrow) of the outer myometrium on the back wall of the uterus. b Note the absence of abnormality of the myometrium and its \ncomplete resolution on additional T2‑W MR sequences repeated at the end of the exam\n\nPage 8 of 15Ruaux et al. Insights into Imaging           (2024) 15:20 \nwhich are associated with familial adenomatous polypo -\nsis–related syndromes as Gardner syndrome and Turcot \nsyndrome [27]. Distinguishing a desmoid tumor within \nthe anterior pelvic wall from abdominal wall endome -\ntriosis involving C-section scar tissues or laparoscopic \nport sites can be challenging. Both lesions may exhibit \nT2-hypointensity with irregular margins and an infiltrat -\ning pattern. Desmoid tumors typically display intermedi -\nate signal intensity on T2-WI due to increased cellularity, \nalong with high signal intensity on diffusion-weighted \nimaging (DWI) and intense contrast enhancement. A \n“fascial tail sign” may be present inconsistently, charac -\nterized by thickening and enhancement of the aponeu -\nrosis (Fig.  7) [28]. In contrast to intrabdominal DIE \nlesions, which often exhibit fibrosis and delayed enhance-\nment, wall endometriosis typically shows early and avid \nenhancement. Besides cyclic pain, key differentiat -\ning findings include the absence of hemorrhagic foci or \nmicrocystic structures in desmoid tumors. Obtaining tis -\nsue samples through US-guided percutaneous biopsies \ncan assist in achieving a definitive histological diagnosis, \nparticularly before or during minimally invasive treat -\nments like percutaneous cryotherapy.\nMyoma\nLeiomyomas, also known as uterine fibroids, can develop \nin various pelvic structures composed of muscular tis -\nsue, including the uterus, vagina, rectum, or urinary \nFig. 6 Anterior adhesions in the vesicouterine pouch after C‑section in a 40‑year‑old woman with medical history of pelvic endometriosis. a \nSagittal and (b) axial T2‑W MR images show a T2‑hypointense fibrous thickening of the vesicouterine pouch, with severe adhesions of the uterine \nbody (arrows). c Axial T1‑W fat‑suppressed MR image shows no hemorrhagic foci in the anterior subperitoneal space (arrow). Endometriosis surgical \nmanagement has been decided. During surgical procedure, pelvic anterior symphysis was proven with no obvious endometriosis lesion (confirmed \nwith negative biopsies at pathology)\nFig. 7 Sporadic pelvic wall desmoid tumor in a 30‑year‑old woman with history of cesarean section. a Axial T2‑W MR image shows an infiltrative \nmass of the right rectus muscle with heterogeneous T2 signal intensity varying from low (thin arrows) to intermediate (thick arrow) signal \nintensity areas. Anterior focal adhesions due to previous cesarean section are seen (arrowheads). b Axial diffusion‑weighted MR image shows high \nsignal intensity within the mass consistent with increased cellularity (arrows). c Axial T1‑W fat‑suppressed contrast‑enhanced MR image shows \nheterogeneous enhancement (arrows)\n\nPage 9 of 15\nRuaux et al. Insights into Imaging           (2024) 15:20 \n \nbladder (Supplemental –Fig.  4). They usually do not pre -\nsent a diagnostic dilemma, due to their T2-hypointense \nrounded morphology. However, small-sized leiomyomas \nwith poorly circumscribed margins or an extra-uterine \nlocation (especially in cases of prior morcellation) can be \nconfusing, particularly if there are areas of cystic degen -\neration that may resemble glands. On MRI, leiomyomas \ntypically appear as rounded or oval structures with a \nhomogeneous T2-hypointense signal within the muscu -\nlaris layer of the uterus, well-defined margins, and the \nabsence of hemorrhagic foci on T1-WI (Supplemental –\nFig. 5). The absence of extrinsic infiltration or any asso -\nciated retraction on T2-W sequences may help in the \ndifferential diagnosis.\nMalignant tumors\nColorectal carcinomas\nColorectal cancer is the second most common cancer in \nwomen [29]. Prevalence of colorectal carcinoma increases \nwith age. However, up to one-third of the patients under \n40-year-old have been reported to be linked to hereditary \nsyndromes, such as Lynch syndrome [30]. Nonspecific \nclinical findings, like rectal bleeding and rectal syndrome, \ncan be misleading during a physical examination. These \nsymptoms alone may not provide a clear indication of the \nunderlying cause, as clinical exam findings are often non-\nspecific and rectal bleeding may not always be present.\nRectosigmoid endometriosis has a distinct morpholog -\nical pattern different from colorectal cancer. Colorectal \ncancer typically presents with polypoid, circumferential, \nand/or semi-circumferential lesions that originate from \nthe mucosa and invade the inner layers. In contrast, rec -\ntosigmoid endometriosis rarely exhibits circumferen -\ntial growth pattern or mucosal invasion [12]. Instead, it \nbegins at the serosa and develops a specific “mushroom \ncap” sign over time, providing a highly specific indication \n[31]. Busard et  al.proposed a qualitative assessment of \nhigh b-value on DWI as a valuable, non-invasive tool to \nFig. 8 Rectal adenocarcinoma in a 33‑year‑old patient with chronic pelvic pain and rectal disorder with rectal bleeding, addressed for suspicion \nof endometriosis. a Sagittal, (b) coronal, and (c) axial T2‑W MR images show a T2‑hypointense focal wall thickening of the high rectum (thick arrows) \nwith a T2‑hypointense tumoral infiltration of the mesorectum (thin arrows). Note the absence of retrocervical deep infiltrating endometriosis. d \nAxial T1‑W fat‑suppressed contrast‑enhanced MR image shows a moderately enhanced tumor (arrows)\n\nPage 10 of 15Ruaux et al. Insights into Imaging           (2024) 15:20 \nfacilitate differentiation between endometriosis infiltrat -\ning the bowel and colorectal carcinoma [32]. They both \ndemonstrate low ADC (apparent diffusion coefficient) \nvalues. Colorectal carcinoma shows high signal intensity \non DWI due to high cellularity (true restricted diffusion), \nwhereas endometriosis displays hypointense signal inten-\nsity due to the “T2-blackout effect” of these lesions on \nDWI (Fig. 8).\nInfectious conditions\nActinomycosis\nActinomycosis is a chronic granulomatous disease and \nbacterial infection caused by Actinomyces species. Infec -\ntions of the female genital tract with Actinomyces rep -\nresent 20% of cases [33] and may be caused by surgery, \nperforation of the bowel, or foreign bodies, such as \nintrauterine devices (IUD) [34]. Actinomycosis asso -\nciated with an IUD typically affects the pelvic area and \naffected patients often present with chronic pelvic pain \nand insidious symptoms [35]. Pelvic actinomycosis can \nextend extensively, reaching a severity comparable to that \nof a frozen pelvis, which can resemble pelvic malignancy \nor endometriosis [36]. The intraabdominal extension typ-\nically occurs through contiguous spread, as the actinomy-\ncosis bacteria produce proteolytic enzymes that enable \ncrossing of normal anatomical barriers. This can result \nin an infiltrating retractile pattern with firm fibrotic tis -\nsue, and in some cases, the formation of abscesses and/\nor fistulas.\nPelvic actinomycosis shows prominent fibrotic tissue \nand inflammatory stranding, resulting in intermediate to \nlow signal intensity on T2-WI. The mass exhibits mild to \nFig. 9 Actinomycosis in a 36‑year‑old woman with chronic pelvic pain, rectal disorder. and dyspareunia. a Axial and (b) sagittal T2‑W MR \nimages show right subperitoneal infiltration with intermediate signal intensity, centered on the right utero‑sacral ligament (thin arrows) \nand the sacro‑recto‑genital septum up to the presacral space (star) from the first sacral vertebra to the sacrococcygeal junction. c Axial T2‑W \nMR image shows perirectal soft‑tissue infiltration (arrows) with intermediate T2‑signal intensity semicircumferential thickening of the upper \nand posterior rectum (arrowheads). d Axial T1‑W fat‑suppressed contrast‑enhanced subtracted MR image shows avid enhancement \nof the surrounding inflammatory infiltration with small abscesses in the presacral space (arrows). Past medical history of intrauterine device recent \nremoval was found\n\nPage 11 of 15\nRuaux et al. Insights into Imaging           (2024) 15:20 \n \nmarked enhancement, aiding in the differential diagno -\nsis (Fig.  9). High signal intensity components on T1-WI \ndue to hyperproteic content or free radicals are rare, \nwith mild intensity seen within necrotic areas. Surgical \nintervention should be avoided, and a CT-guided needle \nbiopsy is preferred for definitive microbiological diagno -\nsis prior to initiating medical treatment.\nAlveolar echinococcosis\nHuman echinococcosis is a parasitic disease, or zoono -\nsis, with endemic distribution in many parts of the world, \nincluding the Northern hemisphere. It is caused by Echi-\nnococcus granulosus that causes cystic echinococcosis \nand Echinococcus multilocularis that is the causative \nagent of alveolar echinococcosis [37]. The liver is the pre-\ndominant initial site of parasitic development. Peritoneal \nand pelvic tissue involvement in alveolar echinococcosis \nis very rare, occurring through direct extension or peri -\ntoneal dissemination. The fibrotic reaction in the host \ncan mimic DIE involvement of pelvic structures. MR \nkey findings of alveolar echinococcosis include hetero -\ngeneous infiltrating multivesicular masses with irregular \nmargins, along with T2-hypointense fibrotic components \n[38]. Small cystic and/or necrotic T2-hyperintense com -\nponents may be seen, but hemorrhagic foci are missing \non T1-W sequence. Necrosis may be seen in the center of \nthe lesions as areas of low to intermediate signal intensity \non T1-WI and heterogeneous signal intensity on T2-WI \n(Fig.  10). Calcifications may be seen in chronic pelvic \nfibrotic lesions on CT scan.\nAlveolar echinococcosis can resemble profuse and \nsevere DIE, but there are distinguishing features. While \nendometriosis causes distortion of the pelvic cavity with \nsolid lesions and fibrous tissue reaction, echinococcosis \nFig. 10 Pelvic alveolar echinococcosis in a 28‑year‑old woman with chronic pelvic pain, bladder disorder, and dyspareunia. a Sagittal and (b) \naxial T2‑W MR images show an infiltrative T2‑hypointense external myometrial infiltrative mass (thick arrows) with a few microcysts (arrowheads), \nand a contiguous infiltration of the bladder dome (thin arrows). c Axial T1‑W fat‑suppressed MR image shows several microcysts (arrowheads) \nwithout hemorrhagic foci in the extrinsic infiltrative uterine mass. d Axial T1‑W fat‑suppressed contrast‑enhanced subtracted MR image shows \ncentral necrosis (arrows) and irregular margin with a peripheral enhancement. History of liver alveolar echinococcosis infection in childhood \nwas then found\n\nPage 12 of 15Ruaux et al. Insights into Imaging           (2024) 15:20 \npresents as a multivesicular pattern with no substantial \nor faint long-lasting peripheral enhancement on contrast-\nenhanced images. Co-existence of multicystic masses in \nboth the pelvic and hepatic regions is pathognomonic for \nalveolar echinococcosis. In contrast, endometriosis does \nnot infiltrate liver parenchymal tissue in the same man -\nner and typically originates along the hepatic capsule \nrather than forming circumscribed masses within the \nliver.\nFeces\nDehydrated solid feces can sometimes be misleading \nwhen evaluating bowel endometriosis, as they appear \nas low signal intensity on T2-WI. Multiplanar analysis \nusing T2-W and 3D T1-W sequences helps for precise \nendoluminal location of feces, depicting a slightly het -\nerogeneous signal on T1-WI, in contrast to extrinsic \nfibrotic bowel involvement in DIE (Fig. 11). Large folds or \na wrinkled appearance of the rectum or sigmoid wall may \noccasionally be mistaken for endometriotic involvement, \nparticularly in the sagittal plane. However, pseudo-thick -\nening is typically present in only one plane with the same \nsignal as the contiguous intestinal wall. Recent European \nrecommendations [39] highly recommend bowel prepa -\nration and additional fasting prior to pelvic MRI in the \nevaluation of rectosigmoid endometriosis. The use of \nrectal opacification with sonographic gel and/or water \nis optional, with varying results reported for assessing \nthe pouch of Douglas and rectosigmoid endometriosis \naccording to different studies [40]. If there is uncertainty, \nFig. 11 Normal sigmoid colon filled with feces in a 35‑year‑old woman with chronic pelvic pain. No medical history. a Axial and (b) coronal \nT2‑W MR images show a T2‑hypointense “pseudo”‑thickening of the posterior sigmoid colon wall (arrows). Note the absence of retrocervical \ndeep infiltrating endometriosis (arrowheads). c Axial T1‑W fat‑suppressed MR image shows the absence of hyperintense foci nor abnormality \nof the sigmoid colon wall (arrow), with a more or less fecal content. Laparoscopy showed a normal recto‑sigmoid colon\nFig. 12 Unilateral right‑sided vesicoureteral reflux surgical bulking agent (Macroplastique®—polydimethylsiloxane injection) in a 26‑ year‑old \nwoman. a Sagittal and (b) axial T2‑W MR images show a T2‑hypointense unilateral right‑sided vesicoureteral reflux implant (arrows) \nat the ureterovesical junction. c Axial T1‑W fat‑suppressed MR image shows an ovoid geometrical shape in T1‑isosignal intensity (arrow) \n\nPage 13 of 15\nRuaux et al. Insights into Imaging           (2024) 15:20 \n \na dedicated transvaginal ultrasound for endometriosis \ncan be considered.\nSurgical material: vesicoureteral reflux \nand incontinence treatments\nThe endoscopic treatment of vesicoureteral reflux, primarily \nperformed in childhood, can be encountered in adult patients \nundergoing evaluation for endometriosis [41]. The presence \nof injected bulking agents or synthetic graft material at the \nureterovesical junction, or slightly behind in the pre-vesical \nterminal ureter, can potentially lead to a misdiagnosis of DIE \ninvolving the parametrium (Fig. 12). Implants, particularly \ncollagen materials, can exhibit low signal intensity on T2-W \nsequences, resembling endometrioid implants with sur -\nrounding tissue granulomas [42]. These implants are typically \nchallenging to visualize on T1-WI and fat-saturated T1-WI, \noften not visible or best depicted in isosignal. Imaging key \nfeatures such as bilateral and symmetric pattern, geometric \nshape, in the absence of distortion or extrinsic infiltration \nhelps in the correct diagnosis (Supplemental – Fig. 6). If there \nis uncertainty regarding the presence of surgical material, a \npelvic CT scan can be useful in visualizing calcifications and \nhyperdense foreign materials. Additionally, with the same \nMRI appearance as the implants mentioned above, peri-\nurethral injections for the treatment of incontinence in adult \nwomen has increased in recent years and should not be con-\nfounded for endometriosis [43] (Supplemental –Fig. 7).\nConclusion\nIn conclusion, the diagnosis of deep pelvic infiltrating \nendometriosis requires careful consideration of a wide \nrange of differential diagnosis on MRI. It is important \nto be aware of both pathological and non-pathological \nconditions that can mimic endometriosis. Among these, \ninjection of gadolinium may be useful to reach pre -\ncise diagnosis, but must remain justified, as systematic \ninjection is not recommended. While endometriosis is \nprevalent, it is crucial to appropriately communicate \nand consider alternative diagnosis. Incorrect diagnosis \ncan result in unnecessary medical and surgical interven -\ntions that may have long-term consequences. It is vital to \nunderstand the strengths and limitations of MRI in diag -\nnosing endometriosis to ensure accurate diagnosis and \nappropriate treatment decisions.\nAbbreviations\nADC  Apparent diffusion coefficient\nDIE  Deep infiltrating endometriosis\nDWI  Diffusion‑ weighted imaging\nFS T1‑WI  Fat‑suppressed T1‑weighted images\nMRI  Magnetic resonance imaging\nT1 T2‑WI  T1 T2‑weighted images\nUSL  Uterosacral ligament\nSupplementary Information\nThe online version contains supplementary material available at https:// doi. \norg/ 10. 1186/ s13244‑ 023‑ 01588‑2.\nAdditional file 1: Figure 1. Bilateral thickening of the round ligaments in \na 36‑year‑old woman. No medical history. Figure 2.  Uterine contraction \nin a 24‑year‑old woman, addressed for suspicion of endometriosis. Fig-\nure 3. Uterine retroversion surgery in a 38‑year‑old woman with anterior \npelvic pain. Figure 4. Urinary bladder leiomyoma in a 35‑year‑old woman \nwith chronic pelvic pain and bladder disorder. Figure 5. Vaginal leiomy‑\noma in a 42‑year‑old woman with dyspareunia and vaginal palpable mass. \nFigure 6. Bilateral vesicoureteral reflux surgical implants in a 32‑year‑old \nwoman. Figure 7. Urethral bulking agent (collagen) injection for the treat‑\nment of stress urinary incontinence in a 34‑year‑old woman.\nAuthors’ contributions\nER: writing — original draft, conceptualization, data curation. WV: review \nand editing. SN: review and editing. MG: data curation. MC: data curation. \nFG: conceptualization. PAB: data curation. ITN: writing — review and edition. \nPR: conceptualization, writing — review and editing, project administration, \nsupervision. All authors read and approved the final manuscript.\nFunding\nNot applicable.\nAvailability of data and materials\nThe data of cases in the manuscript are available from the corresponding \nauthor on reasonable request.\nDeclarations\nEthics approval and consent to participate\nAll procedures performed in studies involving human participants were in \naccordance with the ethical standards of the institutional and/or national \nresearch committee and with the 1964 Helsinki Declaration and its later \namendments or comparable ethical standards. The Institutional Review Board \nwaived the need to obtain informed consent.\nConsent for publication\nThe authors of this manuscript consent for publication.\nCompeting interests\nPascal Rousset reported consultant fees from Ziwig and EDAP TMS France and \nreported receiving lecture fees from Bracco and compensation for serving on \nthe board from Guerbet.\nStéphanie Nougaret is funded by the European Research Grant (ERC starting \ngrant) and Integrated Cancer Research Grant (SIRIC).\nIsabelle Thomassin‑Naggara reported receiving lecture fees from General \nElectric, Siemens, Canon, and GSK; lecture fees and compensation for serving \non the board from Guerbet; compensation for serving on the board from \nBayer; lecture fees from Incepto, ICAD, Fujifilm, and Hologic; and lecture fees \nand compensation for serving on the board from Bracco.\nAuthor details\n1 Department of Radiology, Hospices Civils de Lyon, Lyon Sud University \nHospital, Lyon 1 Claude Bernard University, EMR 3738, Pierre Bénite, France. \n2 Department of Radiology, Mayo Clinic, Rochester MN55905, USA. 3 Depart‑\nment of Radiology, Montpellier Cancer Institute, U1194, Montpellier University, \n34295 Montpellier, France. 4 Department of Gynecology and Obstetrics, \nHospices Civils de Lyon, Lyon Sud University Hospital, Lyon 1 Claude Bernard \nUniversity, EMR 3738, 69495 Pierre Bénite, France. 5 Department of Radiol‑\nogy, Service Imageries Radiologiques Et Interventionnelles Spécialisées, \nHôpital Tenon, Assistance Publique Hôpitaux de Paris, Sorbonne Université, \n75020 Paris, France. \nReceived: 21 July 2023   Accepted: 27 October 2023\n\n\nPage 14 of 15Ruaux et al. 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