Benign Disease of the Uterus

Abstract

Benign diseases of the uterus are common and typically present with disordered menstrual bleeding and pain. The first-line imaging investigation is pelvic ultrasound which readily demonstrates abnormal features within the myometrium and endometrium. Ultrasound examination demonstrates fibroids, adenomyosis, endometrial polyps and is also first-line imaging for endometriosis. MR examination is focused on addressing a particular question, e.g., assessment prior to fibroid embolization or mapping of endometriosis prior to surgical resection. Congenital anomalies of the uterus are important to define as intervention can lead to future successful pregnancy outcomes, e.g., resection of septum and imaging will also demonstrate associated renal anomalies. Optimal imaging sequences and planes allow the radiologist to accurately determine key characteristic features.

FormalPara Learning Objectives

• Recognize the normal appearances of the uterus and avoid benign pitfall appearances, e.g., myometrial contractions.

• Diagnose common benign disease processes of the uterus, e.g., fibroids, adenomyosis, endometriosis, and endometrial pathology.

• Understand how congenital anomalies of the uterus are classified.

FormalPara Key Points

• The diagnosis of DIE requires the presence of both morphological and signal intensity anomalies.

• Uterine leiomyomas use the FIGO classification system and are classified according to their location, to provide a uniform description to facilitate clinical care and research. It subdivides fibroids into submucosal, other (intramural and subserosal), and hybrid types.

• The Mullerian duct abnormality classification system is clinically orientated, based on anatomy. The external uterine contour and the uterine wall thickness—defined as the distance between the interostial line and a parallel line on the top of the fundus-are important considerations to appreciate.

13.1 Introduction

Benign diseases of the uterus are common and can be debilitating for patients with severe symptoms. Imaging is instrumental in diagnosing these conditions, ultrasound being the first-line investigation of choice. Correctly identifying congenital abnormalities of the uterus leads to optimal management, which in some cases can lead to a successful pregnancy outcome. Correct high-quality imaging performed optimally is therefore fundamental to patient management.

13.2 Modalities for Imaging the Uterus

13.2.1 Ultrasound

Pelvic ultrasound is the first-line examination in the investigation for gynecological symptoms both in pre- and post-menopausal patients [1]. Pelvic ultrasound is therefore the initial diagnostic test of choice for the investigation of symptoms that are due to benign diseases of the uterus. The most common of these are dysmenorrhea and menorrhagia. Ultrasound examination of the pelvis should include transvaginal examination (TVUS) which clearly demonstrates the uterus and its components, i.e., the myometrium, endometrium, and the myometrial/endometrial interface. The position of the uterus (anteverted, axial, or retroverted) should be assessed as well as uterine size (in longitudinal and transverse sections). The myometrium should be assessed for focal fibroids and diffuse heterogeneity. Heterogeneity of the myometrium and difficulty visualizing the myometrial and endometrial interface should raise the possibility of adenomyosis. The endometrial thickness should be measured as standard on the longitudinal section and the correlation with the pre-menopausal date in the cycle or post-menopausal status be made as routine.

13.2.2 MR/CT

MR imaging is utilized as second-line imaging following pelvic ultrasound with a focused question. When determining optimal fibroid treatment options, such as uterine artery embolization or MR-guided focused ultrasound surgery, MR imaging provides necessary pre-procedure anatomical and vascular supply detail. Similarly, MR imaging helps to plan the optimal surgical technique, such as open myomectomy versus hysteroscopic resection. In addition, MR imaging depicts the many different types of degeneration clearly, e.g., cystic, hyaline, or hemorrhagic, and may raise suspicious features for leiomyosarcoma which cannot be appreciated on pelvic ultrasound imaging.

MR imaging for endometriosis is required for surgical mapping of endometriosis patients prior to surgical resection. Subtle features of endometriosis, not apparent on US imaging, are often seen with MR imaging, such as thin endometriotic plaques and distortion.

The MR imaging protocol depends upon the study indication. Planning for uterine artery embolization or MR-guided focused ultrasound ablation, intravenous contrast administration is required. Most of the remaining indications for benign diseases of the uterus do not typically require intravenous contrast medium administration but require good preparation and technique for high-quality imaging interpretation. Patients should ideally be asked to empty their bladder at arrival for their appointment so that when their examination is started the bladder is not full or completely empty. This will help to decrease difficulty with movement artifact during the examination. An antiperistaltic agent, e.g., buscopan may be used, subject to contraindications, to also decrease movement artifact. The key sequences are multiplanar T2-weighted sequences in both sagittal and axial planes and then also T1-weighted sequences for the assessment of blood products. Dual-phase T1-weighted imaging (in-phase/out-of-phase) fat-saturated images allow for greater conspicuity of small areas of blood products in the assessment of endometriotic deposits and adenomyosis. A small field of view (FOV) axial oblique sequence perpendicular to the long axis of the uterus is required for optimal assessment of the endometrium and is also helpful for true assessment of the thickness of the junctional zone. This plane is also used to assess the fundal contour when suspicious of uterine anomaly sequence, which also requires evaluation of the upper abdominal in either a coronal or axial plane to visualize the kidneys fully to diagnose associated renal anomalies and agenesis.

There is no role for CT in the investigation of benign diseases of the uterus. However, benign diseases of the uterus are often incidentally identifiable on CT e.g. calcification of uterine fibroids. In addition, given the nature of the presentation of endometriosis with pelvic pain, it is important that the radiologist remains vigilant in the assessment of possible pathology during CT examinations for other requests.

13.3 Normal Anatomy

The flexion (angle between the longitudinal axis of the uterine fundus and cervix) and version (angle between the longitudinal axis of the cervix and vagina) are most commonly anteverted and anteflexed, but any of the four variants (anteverted and anteflexed, anteverted and retroflexed, retroverted and anteflexed, and retroverted and retroflexed) are considered normal. The size of the uterus is variable but typically between 6 and 9 cm in length. The pre-menopausal uterus demonstrates zonal anatomy (Fig. 13.1) from the central endometrial cavity (high signal intensity on T2-weighted MR imaging), inner myometrium junctional zone (low signal intensity on T2-weighted MR imaging) and the outer myometrium (higher signal intensity than the junctional zone on T2-weighted imaging). The outer serosal surface of the uterus is thin and of low signal intensity on T2-weighted imaging. On ultrasound examination, the endometrial cavity and myometrium are well demonstrated, and a thickened junctional zone can be seen as heterogeneity and difficulty in delineating the crisp endometrial margin with the myometrium. On MR imaging the zonal anatomy is best depicted on sagittal T2-weighted imaging. The normal thickness of the junctional zone on MR imaging is approximately 8 mm with >12 mm in keeping with adenomyosis. A pitfall is when there is uterine contraction which can cause “band-like” artifact and subjective increases in thickness of the junctional zone. It is helpful to review the localizer sequences which in transient uterine contraction will demonstrate a normal junctional zone thickness on another sequence in the same examination (Fig. 13.2).

Fig. 13.1

Sagittal T2 weighted image demonstrating normal zonal anatomy of the anteverted and anteflexed uterus. Endometrium (*), inner myometrium (junctional zone white arrow), and outer myometrium (black arrow)

Fig. 13.2

Sagittal T2 weighted image (a) and localizer image (b) The low signal “band-like” area (white arrow) extending from the endometrial and myometrial interface into the myometrium may be mistaken for adenomyosis but correlation to the localizer images demonstrates a transient appearance in keeping with myometrial contraction. MR imaging in this case was performed for the ovarian cyst

The endometrial thickness varies depending upon the menstrual cycle in the pre-menopausal uterus. During the proliferative phase, the endometrial thickness increases to become trilaminar in the mid-cycle which is seen clearly on ultrasound examination. The thickness in this phase is typically between 3 and 8 mm. In the latter secretory phase, the endometrium becomes more echogenic on ultrasound and increased in thickness to 8–12 mm. In the post-menopausal uterus, an endometrial thickness of >4 mm is used to guide further direct assessment of the endometrial cavity with hysteroscopy and sampling. The increased usage of hormone replacement therapy (HRT) tamoxifen has increased the referral of post-menopausal patients with endometrial thickness >4 mm, but this threshold remains for consideration of endometrial sampling to exclude a malignant cause. In addition to a decrease in endometrial thickness, the uterus decreases in size following menopause.

13.4 Benign Disease Processes

13.4.1 Endometriosis

Endometriosis is defined as ectopic functional endometrial glands and stroma outside of the uterus. The repeated bleeding of these areas causes fibrosis and anatomical distortion.

In recent years there has been increased awareness and support regarding the importance of earlier detection of endometriosis to avoid the delayed diagnoses of these patients who are typically in pain for many years prior to their ultimate diagnosis. This has led to increased imaging for pelvic pain and abnormal uterine bleeding at an earlier stage. First-line examination with ultrasound should ideally address four components as described from the IDEA (International Deep Endometriosis Analysis) group [2], namely: (1) routine examination of the uterus and adnexae (features for position of uterus, adenomyosis, and endometriomas); (2) evaluation of TVUS “soft-markers,” e.g., site-specific tenderness; (3) assessment of status of pouch of Douglas using real-time ultrasound-based “sliding sign” and; (4) assessment for deep infiltrating endometriosis (DIE). Involvement of the torus uterinus from endometriosis with plaque formation is an example of deep infiltrating endometriosis and can extend to involve the adjacent rectosigmoid colon (Fig. 13.3a, b). Similarly, involvement of the retrocervical region into the pouch of Douglas can cause immobility and therefore restricted sliding sign. The features of these deposits on ultrasound and restricted movement can be subtle and therefore proactive examination and assessment is required by an experienced practitioner. DIE nodules can be seen most typically at the torus uterinus, retrocervical area, uterovesical area, and uterosacral ligaments. DIE nodules on ultrasound are seen as hypoechoic areas and should be measured in three orthogonal planes.

Fig. 13.3

Sagittal T2 weighted image (a) and axial T2 weighted image (b) demonstrating low signal intensity stellate plaque extending from posterior aspect of torus uterinus (a white arrow) in keeping with deep infiltrating endometriosis with anatomical distortion and tethering of both ovaries and rectosigmoid colon (b white arrow)

MR imaging for endometriosis has also been optimized by clear guidelines from the ESUR [3]. MR imaging for endometriosis mapping of disease sites prior to surgical resection has improved surgical morbidity and led to improved patient outcomes. The importance of a multidisciplinary approach with the involvement of radiology, gynecology and colorectal or urological surgery when required helps to ensure optimal discussion of treatment options for these patients. The ESUR guidelines agreed that the diagnosis of DIE required the presence of both morphological and signal intensity anomalies. The signal intensity depends upon the age of the hemorrhage and therefore can have varying appearances [4]. The typical appearance involving the uterus is adhesions and DIE nodules. Adhesions are seen as low signal intensity plaques (similar to fibrosis) on the posterior aspect of the uterus at the torus uterinus or retrocervical region extending to the posterior compartment. Associated features of anatomical distortion and tethering are common. DIE nodules contain endometrial glands and stroma and in contradistinction to the adhesions which are low signal intensity on T1 and T2-weighted imaging these endometriotic deposits will typically demonstrate areas of focal high T1 signal intensity foci. Due to the multifocal nature of the disease, it is important to assess all pelvic compartments for endometriosis which is out of the scope for this chapter.

13.4.2 Adenomyosis

Adenomyosis is the presence of ectopic endometrial glandular cells within the myometrium. Adenomyosis may also be present in patients with leiomyomas or with endometriosis. In a recent study looking at the coexistence of leiomyomas, adenomyosis, and endometriosis and their risk for endometrial malignancy, >50% of patients with leiomyomas also had adenomyosis and half of the patients with endometriosis also had adenomyosis [5]. Differentiation of adenomyosis from leiomyomas is easier when adenomyosis is diffuse rather than focal but is very accurate on MR imaging. In focal adenomyosis there is less surrounding mass effect of the lesion relative to its size, e.g., distortion of the endometrial cavity for the size of the adenomyoma compared to leiomyomas, their outline is more indistinct, they appear more elliptical in shape compared to spherical leiomyomas and the adenomyoma contains typical key signal intensity characteristic with hyperintense foci on T2-weighted imaging and often striations out from the endometrial and myometrial junction (Fig. 13.4a). In diffuse adenomyosis, the thickness of the junctional zone >12 mm representing smooth muscle hyperplasia predicts diffuse adenomyosis with high accuracy (85%) [6]. In addition to the hyperintense foci on T2 weighted imaging, adenomyosis may also demonstrate high T1 signal intensity foci (in approximately 20% of cases) which represent small punctate hemorrhagic foci within ectopic endometrial tissue and has a 95% positive predictive value for adenomyosis. Cystic adenomyosis is less common and needs to be differentiated from cystic degeneration of a leiomyoma.

Fig. 13.4

Sagittal T2 weighted image (a) demonstrating thickening of the junctional zone and hyperintense focal punctate areas in keeping with extensive diffuse adenomyosis. Corresponding TVUS transverse section (b) of the uterus demonstrates the heterogeneity of the myometrium, indistinct endometrial and myometrial interface and focal small cystic areas

In comparison to MR imaging, which is highly accurate for diagnosis of adenomyosis, ultrasound appearances can be challenging in subtle cases such as mild diffuse adenomyosis. Given ultrasound is the first-line test it is important to be familiar with the appearances that raise suspicion for adenomyosis. The consensus statement from the morphological uterus assessment (MUSA) group [7] describes the key features on TVUS examination for adenomyosis as asymmetrical thickening of the myometrium (globular shaped uterus), presence of cystic areas within the myometrium, hyperechoic islands, fan-shaped shadowing, echogenic subendometrial lines and buds, translesional vascularity, irregular junctional zone and interrupted junctional zone (Fig. 13.4b).

13.4.3 Uterine Fibroids

Uterine fibroids (leiomyomas, myomas) are benign monoclonal tumors of uterine smooth muscle and are the single important indication for hysterectomy. Approximately 25% of women of reproductive age and over 70% of women by the time they reach menopause are symptomatic with uterine fibroids. Their growth is dependent on estrogen and progesterone, and they may enlarge with pregnancy and oral contraceptive use, and usually request during menopause. They are commonly multiple, and their size can vary greatly.

Ultrasound is usually the initial imaging test of choice for symptomatic patients. However, MRI provides a more accurate assessment of the location, number, and type of uterine fibroids and is often used for complex cases or to help decide optimal therapy [8, 9]. MRI is also helpful as a problem-solving tool to distinguish uterine fibroids from adenomyosis, myometrial contractions, and malignant disease entities such as leiomyosarcoma [10].

13.4.3.1 Imaging Features on Ultrasound

Both transabdominal and transvaginal ultrasounds are often needed to adequately evaluate the uterus. Large or subserosal pedunculated fibroids may be missed by transvaginal imaging alone, whereas transvaginal ultrasound is often best to adequately evaluate submucosal fibroids. On ultrasound, fibroids typically appear as solid masses which are hypoechoic compared to the normal myometrium. They are occasionally hyperechoic and may have some foci of calcification. When there are many fibroids, or the fibroids are large and extend out of the pelvis, accurate assessment and measurement by ultrasound may be difficult.

13.4.3.2 Imaging Features on MRI

MRI is the most accurate modality for determining the size, number, location, and cellular characteristics of fibroids. Most commonly, uterine fibroids are well-circumscribed and of low signal intensity on T2-weighted imaging compared to the surrounding myometrium. They are usually isointense on T1-weighted imaging and commonly enhance to the same or slightly less extent than the myometrium post-contrast administration.

Uterine leiomyomas are classified according to their location. The FIGO classification system (Fig. 13.5 and Table 13.1) was developed to provide a uniform description of location to “facilitate communication, clinical care and research” [11], and allows clinicians to determine the best treatment plan. Submucosal fibroids (FIGO 0, 1, and 2) are located beneath the mucosal lining: FIGO 0 are pedunculated intracavitary and attached to the endometrium by a stalk; FIGO 1 (Fig. 13.6) are ≥50% submucosal and <50% intramural, whereas FIGO 2 leiomyomas are <50% submucosal and ≥50% intramural. Differentiating FIGO 1 from FIGO 2 can be helpful to gynecologists during hysteroscopic resection as it provides a better understanding of the intramural extent. FIGO classifies all remaining leiomyomas that do not have a submucosal component as “other.” FIGO 3 leiomyomas (Fig. 13.6) are 100% intramural but may contact the endometrium with mass effect, but do not extend into the endometrial cavity. FIGO 4 leiomyomas (Fig. 13.6) are also 100% intramural but without any endometrial or serosal contact. Distinguishing FIGO 2 from FIGO 3 and 4 is important as the surgical approach is different, with FIGO 3 and 4 being removed via laparoscopy or laparotomy. Subserosal leiomyomas are divided into FIGO 5, 6, or 7 depending on the extent of subserosal involvement: FIGO 5 leiomyomas (Fig. 13.6) are ≥50% intramural and <50% subserosal, whereas FIGO 6 (Fig. 13.6) are <50% intramural and ≥50% subserosal. FIGO 7 leiomyomas are pedunculated without any intramural component. As they enlarge, they are at risk of torsion. Treatment options for subserosal fibroids usually include uterine artery embolization or myomectomy. Any extrauterine leiomyomas are classified as FIGO 8, including those arising from the cervix, broad ligament, or those parasitized in the pelvis. When a leiomyoma extends from the submucosal to the subserosal surface they are considered “hybrid” and denoted by two numbers (X-X), the first representing the submucosal component and the second representing the subserosal component. These are usually large and treatment options may include MR-guided focused ultrasound surgery, uterine artery embolization or hysterectomy. MRI is the preferred modality to assess for response post MR-guided focused ultrasound surgery or uterine artery embolization.

Fig. 13.5

FIGO fibroid subtypes. Submucosal fibroids (shown in red) include Type 0 (pedunculated intracavitary), Type 1 (≥ 50% submucosal), Type 2 (< 50% submucosal), and hybrid fibroids (here depicted as a Type 2–5 fibroid). Fibroids without submucosal components (shown in blue) include Type 3 (100% intramural fibroid with endometrial contact), Type 4 (100% intramural fibroid with no endometrial contact), Type 5 (≥ 50% intramural fibroid with subserosal component), Type 6 (< 50% intramural fibroid with subserosal component), Type 7 (pedunculated subserosal), and Type 8 (non-myometrial location, such as cervical, broad ligament, or parasitic fibroids) (Permission requested from Springer journals. Original Fig. 13.1 from Abdominal Radiology (2021) 46: 2146–2155. https://doi.org/10.1007/s00261-020-02882-z)

Table 13.1 FIGO fibroid classification system. Permission requested from Springer journals. Original Table 1 from Abdominal Radiology (2021) 46: 2146–2155. https://doi.org/10.1007/s00261-020-02882-z

Fig. 13.6

Coronal T2-weighted image depicting numerable uterine leiomyomas. They are classified as FIGO 1 (#1): ≥50% submucosal and < 50% intramural; FIGO 4 (#4): intramural without any serosal or endometrial contact; FIGO 5 (#5): ≥50% intramural and < 50% subserosal; FIGO 6 (#6): <50% intramural and ≥ 50% subserosal

There are many different forms of degeneration that can occur in uterine fibroids and are usually well depicted on MRI. The most common form is that of hyaline degeneration which occurs when the smooth muscle is replaced by fibrous connective tissue. Areas of very low signal intensity, sometimes speckled, are identified within the fibroid on T2-weighted imaging and there is usually less enhancement after administration of gadolinium compared to the remainder of the uterine fibroid.

The clinical presentation and symptoms of leiomyomas may overlap with those of a rare though aggressive malignant smooth muscle tumor, leiomyosarcoma [12]. The rate of tumor growth cannot differentiate benignity from malignancy, nor can specific serum markers such as lactate dehydrogenase [13] or CA-125 [14]. However, more recent studies have suggested that specific MR features such as intra-tumoral hemorrhage, ill-defined border with the myometrium and enhancing finger-like projections post-contrast are associated with leiomyosarcoma [10]. It is also suggested that diffusion weighted imaging (with a b value of 1000 s/mm²) and apparent diffusion coefficient mapping should also be used for the detection of leiomyosarcoma [15]. This differentiation is important, as although rare, leiomyosarcoma can have a devastating outcome.

13.4.4 Endometrial Pathology

Endometrial pathology is readily assessed with TVUS. The correlation with thickness of the expected appearance during the menstrual cycle is vital and if there is debate between normal appearances and pathology then further TVUS just shortly following menstruation when the endometrium should be at its thinnest can be helpful. Most endometrial polyps are seen in the postmenopausal patient group, and following ultrasound will undergo hysteroscopy and endometrial sampling.

Endometrial polyps are common causes of abnormal uterine bleeding. On ultrasound, these appear as a well-defined area within the endometrium and are typically homogeneous and isoechoic to the background endometrium. The ability to demonstrate a central feeding vessel on color doppler increases accuracy to >90% [16] (Fig. 13.7a). On MR imaging polyps are typically of intermediate T1 signal intensity but can be of heterogenous signal intensity on T2-weighted imaging as their size increases (Fig. 13.7b, c, d). The central fibrous core demonstrates low T2 signal intensity. Resection of the polyp is required to exclude malignancy or foci of atypical hyperplasia.

Fig. 13.7

TVUS transverse section (a) of the uterus demonstrates increased endometrial thickness (white arrow) with central vascularity. Hysteroscopy and subsequent pathology confirmed benign endometrial polyp. Corresponding MR examination sagittal T2 weighted image (b) and axial T2 weighted image (c) and T1 weighted image (d) demonstrate large central endometrial polyp (white arrow)

Endometrial hyperplasia is characterized by the proliferation of endometrial glands and is commonly seen in unopposed estrogen stimulation or in tamoxifen therapy. In postmenopausal patients, the TVUS appearances of a thickened endometrium >4 mm require further assessment with hysteroscopy and endometrial sampling. There are no definitive features on imaging currently which can differentiate benign endometrial hyperplasia from complex atypical hyperplasia or endometrial carcinoma and therefore a thickened endometrium should prompt cellular sampling.

Asherman’s syndrome is an inflammatory response causing adhesions within the endometrial cavity typically following previous intervention or from previous repeated inflammatory events. In severe cases, fibrous adhesions within the cavity can cause cavity obliteration. This can be a cause of infertility or pregnancy loss. On TVUS, adhesions are identified as echogenic bands extending transversely across the endometrium. MR imaging is more accurate for this diagnosis and demonstrates obliteration of the endometrial cavity and fibrous signal intensity. Hysterosalpingogram or sonohysterography, which distends the endometrial cavity, can be helpful in demonstrating the extent of involvement.

13.5 Mullerian Duct Anomalies of the Uterus

Mullerian duct anomalies (MDAs) are congenital disorders that arise from arrested development, incomplete fusion, or incomplete resorption of the mesonephric ducts. The Müllerian ducts undergo descent, fusion, and septum resorption to form the uterus, fallopian tubes, cervix, and upper two-third of the vagina. The ovaries and external genitalia/distal one-third of the vagina are spared because they originate from the primitive yolk sac and sinovaginal bud, respectively. MDAs are usually identified incidentally, and less commonly are identified as causes of infertility, endometriosis, recurrent miscarriages, or an obstructed reproductive tract. The prevalence of Mullerian duct anomalies in the general fertile population is 6.7%, versus 7.3% in the infertile population, and 13–17% in women with miscarriages [17].

The European Society of Human Reproduction and Embryology (ESHRE) and the European Society for Gynecological Endoscopy (ESGE) developed a clinically orientated classification system, based on anatomy [18]. US is commonly performed and may be diagnostic, especially when 3D US is used. MRI can be reserved for those cases in which the US is non-diagnostic or for complex cases. This system sorts the anomalies into classes based on increasing deviation from anatomical deviations (Fig. 13.8). Anomalies are classified into the following main classes, expressing uterine anatomical deviations deriving from the same embryological origin: U0, normal uterus; U1, dysmorphic uterus; U2, septate uterus; U3, bicorporeal/bicornuate uterus; U4, hemi-uterus; U5, aplastic uterus; U6, for unclassified cases. Uterine wall thickness (UWT) is an important parameter and a reference point for the definitions of dysmorphic T-shaped, septate, and bicorporeal uteri, and is defined as the distance between the tubal ostia (interostial line) and a parallel line on the top of the fundus [19] (Fig. 13.9).

Fig. 13.8

Schematic drawing of the ESHRE/ESGE classification system of uterine congenital anomalies from Ref. [18], dividing uterine anomalies into six classes

Fig. 13.9

Coronal 3D ultrasound views of the uterus depicting a normal uterus (a), a partial septate uterus (b), a complete septate uterus (c) and a bicornual uterus (d). Measurement 1 = uterine wall thickness: distance between tubal ostia and a parallel line on the top of the uterine fundus. Measurement 2 = internal midline indentation: distance between the tubal ostia and a parallel line on top of the indentation

13.5.1 Class U0

The normal uterus (U0) has either a straight or curved interostial line with an internal indentation ≤50% of the UWT at the fundal midline.

13.5.2 Class U1

Class U1 (dysmorphic uterus) has a normal uterine outline, but an abnormally shaped cavity (excluding septal abnormalities). An example is a T-shaped U1 which has thickened lateral walls. As with U0, the midline, fundal, inner indentation is <50% UWT.

13.5.3 Class U2

Class U2 uteri also have a normal outer contour, but there is abnormal resorption of the midline septum (either partial or complete) following normal Mullerian duct fusion. As such, for U2 cases there is midline, fundal, inner indentation is >50% of the UWT.

13.5.4 Class U3

Class U3 (bicorporeal) is due to abnormal fusion of the Mullerian ducts and has an abnormal outer contour with external indentation at the fundal midline >50% of the UWT. The extent to which the external fundal indentation divides the uterus above or to the level of the internal os defines partial or complete U3a vs U3b.

13.5.5 Class U

Class U4 category is unilateral uterine horn development, with associated incomplete (U4a) or absent (U4b) contralateral uterine horn remnant.

13.5.6 Class U5

In Class U5 there is uterine aplasia, with no fully developed or unilaterally developed uterus. There may be a functional rudimentary horn or horns (U5a) or no functioning rudimentary horns (U5b).

13.5.7 Class U6

This class is reserved for subtle or combined abnormalities that do not fit into classes 0-5.

13.6 Concluding Remarks

Ultrasound is usually the first imaging modality in the assessment of benign diseases of the uterus. MRI is an important adjunct, especially for patients with complicated congenital anatomy, for the detection of deep infiltrating endometriosis or pre-operative intervention.

Take-Home Message

Ultrasound examination is the first-line investigation for benign disease of the uterus. MR imaging is focused on a particular question, often for complex diagnoses or for surgical planning, and may require specific protocol as a result.