distance of these structures in relationship to the reference PC line (El Sayed et al. 2016; Kelvin et al. 2000) (Table 3).

By definition an organ descent is diagnosed if one or all of these reference structures descend below the PL line. Therefore a cystocele or a uterine descent is diagnosed if these structures descend below the PC line. The grading is based upon the largest perpendicular distance to the PCL measured in cm and taken during maximum straining. (Pannu et al. 2015; Woodfield et al. 2010; Yang et al. 1991; vKruyt et al. 1991; Lienemann et al. 2000a). Grading systems unfortunately only poorly correlate with the clinical findings and symptoms (Pannu et al. 2015; Pizzoferrato et al. 2014). The recommended and most frequently used MRI grading system is enlisted in Table 3 (El Sayed et al. 2016; Kelvin et al. 2000).

Defects in the cul-de-sac include peritoneoceles, enteroceles, and sigmoideoceles (Fig. 7). Clinically these are difficult to assess as a posterior bulge may present one of the latter above or the rectum. The normal depth of the rectouterine pouch is about 5 cm (Kuhn and Hollyock 1982). A peritoneocele is defined as herniation of peritoneum through the

Table 3  MR grading of organ prolapse (cystoceles, uterine prolapse, enteroceles); modified from El Sayed et al. (2016)

­rectovaginal septum and its extension beyond the upper third of the vagina (El Sayed et al. 2016; Kim 2011). In dynamic MRI a descent or widening of the pouch of Douglas below the PC line is considered to be pathological (El Sayed et al. 2016; Sprenger et al. 2000; Lienemann et al. 2000a, b). Small bowel loops and the sigmoid colon sliding down during straining constitute the diagnosis of an enterocele and sigmoidocele.

A rectocele presents mostly an anterior protrusion of the rectal wall. It is defined by the width of its outbulging beyond the expected anterior contour of the rectum (El Sayed et al. 2016). If its depth exceeds 2 cm the rectocele is considered pathological (El Sayed et al. 2016). Alternatively the distance from the anterior rectal wall to a reference line extending upwards from the anal canal has been used (Reiner et al. 2011). Of note, a considerable overlap between findings in normal volunteers and patients has been reported (Shorvon et al. 1989).

5\ Typical Findings

Traditionally the pelvic floor is divided into the three following compartments: (a) anterior with bladder and urethra, (b) middle with uterus and vagina, and (c) posterior containing the anorectum. However, functionally these are cooperating as units during defecation. Severity of symptom does not correlate well with the clinical grading of POP (Rogers and Fashokun 2016).

5.1\ Anterior Compartment

The bladder as a fluid-filled structure is hyperintense on T2-weighted images. Depending on the degree of filling the shape of the bladder can vary considerably, ranging from a more triangular to a round contour (Fig. 2b vs. Fig. 4a). At rest it is situated superior-posterior to the pubic symphysis (Fig. 2a). On midsagittal images an area of fat-equivalent signal intensity is seen between the pubic bone and the bladder (retropubic space; space of Retzius) which extends to the umbilicus. Abundant fat in the Retzius space is a sign of defect of the urethral suspension ligaments and indicative of level III defects. In females the urethra is normally not well delineated on the midsagittal images, but its typical target-like appearance can be easily visualized on axial images. Surrounding structures like small bowel loops are only partly depicted in the midsagittal plane, but adhesions between the dome of the bladder and bowel loops occur quite often. They can be diagnosed as the bowel loops stay attached to the upper bladder wall and do not glide freely as supposed while the patient is straining.

a

The pelvic floor, urethra, and bladder are exposed to the increasing intra-abdominal pressure during Valsalva maneuver. Widening of the pelvic floor and descent of up to 2 cm is a physiological finding (Lienemann et al. 2000a). A cystocele is present if the bladder neck or any part of the posterior wall of the bladder moves >1 cm below the PC line (El Sayed et al. 2016) (Fig. 2c). Both the proximal urethra and the bladder neck descend and rotate around the pubic bone, initially moving posterior-inferior.

A nonspecific finding in patients with involuntary loss of urine is funneling of the proximal urethra (Fig. 3b). An additional kinking of the urethra at the urethrovesical junction can occur in large cystoceles. Furthermore, due to the limited space provided by the urogenital hiatus a large cystocele can block the prolapse of other pelvic structures and thus mask a rectocele or enterocele.

Recurrence of a cystocele after retropubic or vaginal surgeries for stress incontinence can be clearly detected by functional MRI. In these patients the proximal urethra and the bladder neck maintain their normal position superior to the symphysis, but the posterior wall of the bladder bulges into the anterior vaginal wall (Fig. 8).

b

Fig. 8  A 67-year-old female patient after sacrocolpopexy with recurrence of a cystocele. Midsagittal static (a) and functional (b) T2-weighted MR images at rest (a) and during straining (b). (a) Static MR images demonstrate the synthetic material fixing the vaginal apex (V) to the promontory (arrows). B bladder, R rectum. (b) Typical

findings after sacrocolpopexy are the normal position of the bladder neck (asterisk) in contrast to the descent of the posterior wall of the bladder (arrow) below the PC line (white line). The vagina (V) is kept in place by the intact foreign material. R rectum, S small bowel

Finally in previous procedures for urinary sling material or injections of bulk-enhancing agents such as collagen, all of which are normally hypointense on MR images, should be carefully analyzed (Alt et al. 2014; Carr et al. 1996).

5.2\ Middle Compartment

In a normal anatomical setting the vagina, rectum, and posterior components of the levator ani muscle are seen in the same level (Fig. 2a). Therefore, the position of the vagina and uterus depends on the amount of filling of the rectal ampulla. If a vaginal or uterine descent is present, only after the rectum has been emptied both structures move ventrocaudally beyond the PC line (Fig. 2b, c). This is why the PC line itself might underestimate an organ descent in the middle compartment (Kelvin et al. 2000). The landmark to assess uterine prolapse is either the posterior fornix of the vagina or the cervix. In repeated, long-standing prolapse the vagina is often shortened and the vaginal wall may be thickened or even everted. In addition the pouch of Douglas is widened, thus facilitating the development of a peritoneocele or enterocele (Fig. 7). Elongated sacrouterine ligaments may be identified on coronal images. Defects of the endofascial fascia can only be assessed by indirect signs. However typical imaging findings as the drooping moustache sign or the saddleback sign even assist in allocating the level of defect (Bitti et al. 2014).

The pouch of Douglas normally represents the deepest peritoneal reflection within the intra-­ abdominal cavity and predisposes for an internal hernia that have been reported in 18–37% (Rogers and Fashokun 2016). These may contain fat (peritoneocele), small bowel loops (enterocele), or sigmoid colon (sigmoidocele). The criterion to diagnose an enterocele is the descent of small bowel loops below the PC line (Kelvin et al. 2000; Lienemann et al. 2000b). The hernia follows the course of the posterior vagina along the rectovaginal septum and causes posterior vaginal wall bulging. Widening of the rectovaginal space or deepening of the pouch of Douglas below the PC line without bowel loops is defined as a peritoneocele. Occasionally the herniation can be lat-

eral or anterior, in which case coronal images allow a correct assessment. Again a large enterocele can mask either a cystocele or a rectocele (Lienemann et al. 2000b). After sacrocolpopexy or uteropexy dynamic MRI is able to depict the foreign material and to demonstrate its intact function or course (Alt et al. 2014) (Fig. 8).

5.3\ Posterior Compartment

The rectum behaves like a flexible and expansible tube. It can fold on itself with a lateral or an anterior displacement and kinking. Most often a bulging of the anterior rectal wall, a rectocele, is noted (Pannu et al. 2000). In MR defecography a rectocele with a depth of 4 cm is considered as a pathological finding (El Sayed et al. 2016; Yoshioka et al. 1991). MR grading of rectoceles is enlisted in Table 4 (El Sayed et al. 2016). Nevertheless it should be emphasized that there is a considerable overlap between healthy volunteers and women with pelvic prolapse (Shorvon et al. 1989). The direction of a rectocele is either anterior (to the distal segment of the posterior vaginal wall) (Fig. 2b), lateral (Fig. 9), or dorsal (Fig. 3a). It is pivotal to include in the report if the rectocele empties completely during defecation or if gel remains trapped within the rectocele. Lateral rectoceles may be missed in the midsagittal images but clearly become apparent in the coronal images (Fig. 9).

Intussusceptions present mucosal or mural rectal wall invaginations which can be located anteriorly or posteriorly or can affect the whole circumference (Kim 2011). They usually begin at 6–8 cm above the anal canal (Kim 2011). In imaging they present as a circumscribed thickening of rectal mucosa and wall (Fig. 3a). Small intususceptions during defecation present a common normal finding seen in nearly 80% of healthy

Table 4  MR grading of rectoceles; modified from El Sayed et al. (2016)

Fig. 9  A 74-year-old female with a history of aggravated and incomplete defecation. Coronal T2-weighted MR image during maximal straining. Large lateral rectocele (arrows) to the left side, detectable only in the coronal images, missed in the midsagittal and axial images. A anal canal

volunteers (El Sayed et al. 2016; Woodfield et al. 2009). The differentiation between an intussusception and an internal rectal prolapse is not clearly determined and the two terms are often overlapping. A more prominent internal rectal prolapse is often seen as a V-shaped or double rectal wall on the midsagittal image (Fig. 10). Extrarectal intussusception either mucosal or full thickness through the anal canal is defined as rectal prolapse (Mortele 2007). Organ descent of the anterior or middle compartment may lead to compression of the anterior rectal wall. This finding can promote an internal rectal prolapse and might account for a stool entrapment and stool outlet obstruction (Fig. 2c).

The cross point formed by a line along the posterior border of the rectum and the central anal canal is the anorectal junction (Flusberg et al. 2011). It serves as a landmark to define rectal descent. Inferior displacement of the anorectal junction in relation to the PC line is classified grade 1 in 3–5 cm, and grade 2 in displacement of >5 cm below this line (El Sayed et al. 2016; Goh et al. 2000; Halligan et al. 1996).

Fig. 10  A 66-year-old female with fecal incontinence. Midsagittal T2-weighted MR images obtained during defecation. The arrows mark an internal rectal prolapse with folding of the rectal mucosa and rectal wall in the direction of the opened anal canal (asterisk). Moderate descensus of the bladder (B) and vagina (V)

5.4\ Levator Ani Muscle

On the midsagittal images the posterior aspect of the levator ani muscle is demonstrated. A ballooning of the levator muscle on axial images is typically found in patients with weakness of the pelvic floor muscles (Fig. 2d). In contrast, in normal volunteers the genital hiatus and puborectalis sling maintain the typical V-like shape under load (Sprenger et al. 2000). On axial images an ­asymmetric appearance of the puborectal part of the levator ani muscle can be seen after episiotomy.

During defecation an adequate relaxation of the puborectal sling with a resulting vertical orientation of the posterior levator ani and widening of the anorectal angle is visualized. If the levator ani muscle is still contracting during straining and even during defecation, this paradoxical finding is called dyssynergetic defecation, which can cause incomplete evacuation and stool outlet obstruction symptoms (Bolog and Weishaupt 2005).

Synonymous terms include anismus, dyskinetic puborectalis muscle, spastic pelvic floor syndrome, and pelvic floor synergia (Reiner et al. 2011). Delayed onset of defecation and atypical decrease of the anorectal angle (normal angle 90°–110° at rest) during straining are considered as typical imaging signs indicative of dyssynergetic evacuation. Other findings include impression of the puborectalis muscle or the anal sphincter in the posterior anorectal wall due to paradoxical sphincter contraction and lack of lowering of the pelvic floor during straining and defecation (Reiner et al. 2011). Reiner et al. reported findings of MR defecography in 48 patients suffering from chronic obstipation. Although impaired evacuation was seen in all of the 18 patients with dyssynergetic defecation, it rendered only low sensitivities and PPV. Half of these patients exhibited an abnormal anorectal angle and the majority (16/18 patients) demonstrated paradoxical sphincter contraction (Reiner et al. 2011). Associated findings were pelvic floor relaxation and pelvic organ prolapses. In the control group with obstipation a paradoxical sphincter contraction and abnormal anorectal angle were only rarely seen (Reiner et al. 2011). Prolonged evacuation time of 30 s or longer is typical for this condition and has been reported to yield PPV of 90% (Halligan et al. 2001).

6\ MRI of the Pelvic Floor

in Asymptomatic Females

Considerable overlap in normal and asymptomatic patients is an evident problem in assessing pelvic floor disorders. Findings often correlate poorly between clinical exam and imaging studies, as well as clinical symptoms of pelvic floor dysfunction (Pannu et al. 2015; Rogers and Fashokun 2016; Pizzoferrato et al. 2014). Particularly stage I prolapses of the anterior and posterior vaginal wall are so common in asymptomatic females that they may be considered within the range of normal findings (Mann 2014). There is still a debate on the definition of a clinically relevant prolapse (Mann 2014). Based on

expert panel consensus relevant pelvic organ prolapse as ICS-quantified POP (qPOP) stage II or greater is considered abnormal (Kenton and Mueller 2006). However, larger series showed that approximately half of normal patients even met these criteria. Swift et al. reported almost 50% out of 497 women undergoing routine gynecologic assessment to have POP of at least stage II. This was confirmed in a larger multicenter study of clinically asymptomatic females, where qPOP stages in the following distribution were reported: stage 0 in 24%, stage I in 38%, stage III in 35%, and stage III in 2% (Swift 2000; Swift et al. 2005). Although most studies include asymptomatic volunteers as a control group only few data are published exclusively defining the normal range of findings in asymptomatic individuals (Goh et al. 2000). Goh et al. examined 25 men and 25 women on a 1.0-T system in supine position with the volunteers at rest and during straining. All volunteers had to pass a detailed questionnaire. They measured the descent of the bladder base, cervix, and anorectal junction in relation to the PC line and calculated the pelvic floor hiatus area and perimeter as well as the anorectal angle and the levator plate angle (Goh et al. 2000). Lienemann et al. studied 20 nulliparous females with normal clinical examination and urodynamics (Lienemann et al. 2000a). MRI was performed in the supine position with opacification of vagina and rectum with ultrasound gel. Among the large number (29) of pelvic parameters analyzed, the physiologic position of the bladder base, posterior vaginal fornix, and anorectal junction in relation to the PC line and the normal width of the hiatus were defined. In their study the normal pelvic organs did not descent below the PCL. It is accepted that a rectocele of 2 cm and the descent of bladder base of 1 cm are within the normal range of findings during straining. The normal width of the levator hiatus was 4.7 cm at rest and 5.3 cm under pressure. Schreyer et al. also assessed ten asymptomatic nulliparous females (median 31 years) to define the normal range of organ movement at pelvic floor maneuvers (Schreyer et al. 2012). Using the pubococcygeal line as reference during straining the

anorectal junction moved about 2.5 cm, the bladder base moved 1.5 cm, and the uterovaginal junction showed a relative movement of 3.5 cm (Schreyer et al. 2012). The anorectal angle widened from approximately 93° to 110° with a relative change of 15° (Lienemann et al. 2000a).

7\ Value of MRI

Versus Conventional

Techniques

In the last decade MRI has become an important examination technique to assess complex pelvic floor disorders. Its evident advantage is comprehensive diagnostic information not only in respect of pelvic organ prolapse, but also of muscular floor weakness and associated muscular or fascial defects. It also provides functional information of the pelvic floor in patients suffering from chronic constipation (Pannu et al. 2015; El Sayed et al. 2016; Reiner et al. 2011). Findings assist in treatment stratification in multidisciplinary patient management and may impact on treatment including both urogynecologic and colorectal surgery techniques (El Sayed et al. 2016; Elshazly et al. 2010).

Imaging complements the physical exam that allows diagnosis of POP only by indirect signs and imaging may also reveal findings that are clinical occult but may be relevant for treatment planning (El Sayed et al. 2016; Comiter et al. 1999). Particularly differentiation of high rectoceles from enteroceles, and differentiation of the contents of enteroceles, is clinically important, as it will influence the type of surgery. However, due to the broad overlap of physiological and abnormal findings in patients with pelvic floor dysfunction imaging studies have to be interpreted­ in the context with the patient’s history,­ the clinical gynecological assessment, and other diagnostic studies, e.g., anorectal manometry, endosonography, or cystomanometry (Pannu et al. 2015; Law and Fielding 2008; Faccioli et al. 2010).

Colpocystoproctography combining voiding cystography, vaginal opacification, and defecography is regarded as the gold standard to assess

pelvic organ prolapse. According to the ACR appropriateness criteria both colpocystoproctography and MR defecography with rectal filling are rated with highest appropriateness to assess suspected pelvic organ prolapse (Pannu et al. 2015; Yang et al. 1991c; Faccioli et al. 2010). This X-ray technique is widely available, allows fluoroscopic information but is limited by considerable radiation exposure (Pannu et al. 2015; Goei and Kemerink 1990). In terms of patient comfort a survey in 60 patients undergoing both colpocystoproctography and MRI demonstrated that 90.7% of these women prefer MRI to fluoroscopy, thus yielding a high rate of acceptance (Sprenger et al. 2000).

Several studies compared MRI of the pelvic floor with conventional X-ray techniques. The majority of these studies concluded that functional MRI is at least equal to conventional fluoroscopic methods and can be superior in some aspects. An early study compared defecography and MRI of the pelvic floor in qualitative grading and measurements of anterior rectoceles in 14 patients. They stated that the potential of MRI regarding anterior rectoceles seems absent (Delemarre et al. 1994b). Limitations of this early study were the prone position of the patient and lack of rectal opacification. Lienemann et al. who had already used the recently recommended MRI technique favored dynamic MRI (Lienemann et al. 1997). In 5 asymptomatic volunteers and 44 female patients MRI was either identical (21 cases) or superior (18 cases) to dynamic fluoroscopy. In this study MRI of the pelvic floor was particularly helpful in the depiction of pathologies within the middle compartment and in revealing changes in the dominant type of prolapse (Healy et al. 1997c). They found a significant correlation of standard measurements of the anorectal configuration using MRI and evacuation proctography in women with constipation. In addition, functional MRI was able to show significant changes of muscular parameters in women with otherwise normal proctograms. In comparison to bead-chain cystourethrography/ colpocystorectography MRI correctly diagnosed the degree of bladder descent with a coefficient of determination of 0.81 and 0.85 in 32 women