floor at least two additional slice orientations perpendicular to each other allow better depiction of pelvic floor abnormalities (Lienemann 1998). The ESUR guidelines advise angulated transaxial and coronal planes as optional (El Sayed et al. 2016) (Table 1).

4\ MR Image Analysis

Image analysis should include the following aspects: bony pelvis, muscles and ligaments of the pelvic floor, and presence and degree of movement of organs and reference structures during evacuation. Static images provide detection and classification of structural abnormalities and dynamics are assessed to detect qualitatively and quantitatively assess the three compartments of the pelvic floor (El Sayed et al. 2016).

Measurements assist in quantifying the extent of pelvic floor organ prolapse and of pelvic floor relaxation. These MRI findings are optimally reported in a structured MRI report (El Sayed et al. 2016). Depending on the referring sites speciality-­focused MRI reports may render more specific information according to urologic, urogynecologic, and proctologic focus (El Sayed et al. 2016; Macura et al. 2006).

4.1\ Bony Pelvis

The bony pelvis and its inserting muscolofascial diaphragm, the pelvic floor are exposed to changing forces, which serve as inferior closure of the abdominal cavity and provides bladder and bowel control (Bitti et al. 2014). The pelvic bones as a surrounding frame protect and support the soft tissues and pelvic viscera (Retzky et al. 1996). A perpendicular relationship of the abdominal and pelvic cavity in a properly orientated bony pelvis, which directs the pressure towards the pubic symphysis and away from the pelvic floor, has been proposed (Retzky et al. 1996).

In MR pelvimetry a wider transverse inlet and a shorter obstetrical conjugate were associated with pelvic floor disorders (Handa et al. 2003). Incidental findings include Tarlov cysts, occult

stress fractures of the sacral bone, or coccygodynia. In the latter, bone edema as well as a surrounding small rim of fluid can be seen (Maigne et al. 2012). Configuration and mobility of the coccygeal bone may vary. Bo et al. found a ventrocranial movement of 8.1 mm during contraction­ and a caudodorsal movement of 3.7 mm during straining, but there were no statistical difference between continent and incontinent women (Bo et al. 2001).

4.2\ Pelvic Floor Muscles

and Ligaments

The pelvic floor is composed of three layers: the endopelvic fascia which is too thin to be directly depicted on MRI, the pelvic floor muscles, and the perineal membrane which can be visualized at imaging as the perineal body, a connective softtissue condensation at the insertion of the perineal muscles, and external sphincter (Bitti et al. 2014).

The muscular pelvic floor including the components of the levator ani muscles does not represent a simple linear plate or hammock which is ­interconnected between the bony structures, but a complex 3D structure (Hjartardottir et al. 1997). Postprocessing with volume rendering techniques which are still a field of ongoing research assists in understanding the complexity of its anatomy and function (Bitti et al. 2014). Linear measurements on 2D MR images can vary considerably. In their study Hoyte et al. measured the anterior-posterior dimension of the levator hiatus using slightly rotated images (Hoyte and Ratiu 2001). Calculated and measured values differed and showed up to 15% variation. This may be explained as most cuts on MR images are not completely perpendicular to the muscle, and therefore oblique measurements will overestimate the muscular thickness (Bo et al. 2001). Positioning of the patient within the MR scanner may also impact on the measurements. It is highly recommended to position the patient on the coronal localizer with both acetabular bones at the same level. Tilting of the pelvis in the vertical axis during straining should also be avoided to prevent asymmetries (Bump et al. 1996). Interobserver accuracy has to be considered, especially in thin structures of only a few millimeters in size (Carr et al. 1996). In

the literature a variety of parameters concerning pelvic floor muscles have been analysed: width of the levator hiatus on axial on coronal and sagittal images, thickness of the iliococcygeal portion of the levator ani muscle on coronal and axial images, range of movement of the levator ani muscle on coronal images, urogenital hiatus on axial, as well as surface of the levator ani muscle assessed on coronal images (Woodfield et al. 2010; Hoyte and Ratiu 2001; Fielding 2002; Goh et al. 2000; Lienemann et al. 2000a; Pannu et al. 2000; Goodrich et al. 1993b; Hjartardottir et al. 1997b; Singh et al. 2002; Healy et al. 1997b). In addition, several different angles have been proposed: the levator-­plate, levator-vagi- nal, and iliococcygeal angle (Goodrich et al. 1993; Hodroff et al. 2002; Goh et al. 2000; Healy et al. 1997b). The levator plate angle (LPA) is the angle between the posterior part of the levator ani muscle (iliococcygeal portion) as seen on the midsagittal image and the pubococcygeal reference line (PC line). In a similar way, the levator-vaginal angle is calculated by measuring the angle between the posterior portion of the levator ani plate and a line drawn through the horizontal axis of the upper third of the vagina (Singh et al. 2002). Another parameter to assess the orientation and slope of the iliococcygeal muscle is the angle between this muscle and the transverse plane of the pelvis on coronal images (Singh et al. 2002). However, measuring angles is challenging and limited by interand even intraobserver variability because of the often not completely even, but slightly curved, shape of the anatomical structures, e.g., the levator plate or the vaginal wall.

The shape of the various parts of the levator ani muscle reveals important additional information. Muscle defects with or without hernias are best seen on coronal images. A vertical orientation of the anococcygeal ligament on midsagittal­ images and a ballooning of the puborectal portion on axial images is indicative of pelvic floor weakness (Fig. 2c, d) (Bitti et al. 2014; Hoyte and Ratiu 2001). Normally the course of the anococcygeal ligament roughly parallels the PCL line at rest and during straining (Bitti et al. 2014).

Asymmetry or even complete loss of the right puborectal portion of the levator ani is a frequent finding in parous women after episiotomy. Intramuscular hematomas due to excessive ­straining

or a thickened coccygeal portion in patients with levator ani syndrome or extensive scars due to previous surgery are other common findings.

Support to the pelvic floor is provided not only by the muscles, but also by ligaments and connective tissues (Bitti et al. 2014). Tears within these ligaments have been reported to be the cause of rectoceles or uterine/vaginal descent (Bitti et al. 2014; Bazot et al. 2011; El Sayed et al. 2008).

On MRI the rectovaginal septum, anococcygeal ligament, and sacrouterine ligaments can be clearly visualized. The first two structures are best seen on midsagittal images, whereas the sacrouterine ligaments can be delineated on coronal images or with oblique angulation (Bazot et al. 2011). The rectovaginal septum is seen between the posterior wall of the vagina and the anterior wall of the rectum, which are both of intermediate to low signal intensity. Separation of these two structures by a small rim of high signal intensity on T2-weighted images may just indicate a deep pouch of Douglas.

The connective tissues consist of fascias comprising the arcus tendinous levator ani and fascia pelvis (Bitti et al. 2014). It also wraps around the bladder, vagina, and uterus and suspends these organs to the pelvis (Bitti et al. 2014). The fascia itself is too thin to be visualized on imaging; however indirect signs of fascial defects have been described in MRI. Central to the understanding of fascial tears is the concept of the three levels of fascial support of the vagina: Level I consists of the posterior fornix and cervix, level II of the middle third of the vagina, and level III of the lower third of the vagina (Bitti et al. 2014; Huddleston et al. 1995). Imaging features of endofascial defects differ depending on the level involved. In level I defects due to loss of support of the vaginal apex by the uterosacral ligaments the upper vagina may appear flat or curved on a transaxial plane. This is typically found in multiparous women and usually caused by detachment from the ischial spine (Bitti et al. 2014). On MR imaging this facial defect is characterized by the chevron sign, a bilateral distortion of the upper vagina (Bitti et al. 2014).

In level II endofascial defects the normal H shape of the vagina is lost and due to loss of fascial suspension the bladder is displaced posteriorly and gives rise to the saddle bag sign (Bitti et al. 2014; Macura et al. 2006) (Fig. 5). Besides

Fig. 5  Level II fascial defect. The normal H shape (arrow) of the vagina is seen in a. In contrast, in b displacement of the vagina and the saddle bag sign (arrow) is

found. This finding supports the finding of level II fascial defects. Other findings: thinning and widening of the levator ani muscle (short arrow) in b

these lateral (paravaginal) defects central fascial defects at this level are postulated to predispose for enteroceles (Bitti et al. 2014).

Level III is defined by the lower vagina, perineal membrane, and urethral suspension ligaments. Disruption or complete absence of urethral suspension ligaments can lead to enlargement of the Retzius space between pubic bone and urethra, the drooping moustache sign (El Sayed et al. 2008).

4.3\ Assessment of Pelvic Organ

Mobility: Reference Lines

To evaluate the range of movement of the organs of the pelvic floor many reference lines have been published. On general consensus, the ideal reference line system should accomplish the following criteria: (1) mark the level of the levator ani muscle as the main supporting structure of the entire pelvic floor; (2) be independent of tilting of the pelvis by using two or more well-defined bony landmarks; (3) describe the range of organ movement in at least two different imaging planes; and (4) provide the possibility to compare findings on MR images with the results of the clinical examination and clinical classification

systems. To date no single reference line or grading system meets all the above-mentioned criteria.

The most commonly used reference line is the pubococcygeal line (PC line) (Pannu et al. 2015; El Sayed et al. 2016; Yang et al. 1991b) (Figs. 2, 3, 4, and 6). This line is obtained on a midsagittal plane and connects the inferior aspect of the pubic symphysis to the last mobile coccygeal joint (Bertschinger et al. 2002). It is recommended by the ESUR as reference as it shows the lowest interand intraobserver variability (El Sayed et al. 2016). Three different variants of the PC line have been published in the literature. All PC lines are drawn on midsagittal images and start at the lower margin of the symphysis pubis. Apart from the above-described last coccygeal joint alternative second bony landmarks are either the first sacrococcygeal joint, or the point of insertion of the coccygeal portion of the levator ani (Vanbeckevoort et al. 1999; Healy et al. 1997a; Hjartardottir et al. 1997; Singh et al. 2001; Gufler et al. 1999).

The PCL serves as base for grading of POP. After defining the PCL the distance from each reference point within the three compartments is assessed perpendicularly to the PCL at

Fig. 6  H and M Lines to assess pelvic floor muscle insufficiency. PCL pubococcygeal line

Table 2  MR grading of pelvic floor descent

the superior border of the pubic bone and the distal­ sacral bone; the midpubic line extends through the longitudinal axis of the pubic bone (Lienemann 1998; Singh et al. 2002; Delemarre et al. 1994b). It represents approximately the level of the vaginal introitus and correlates with the clinical reference systems used in the quantification staging system of pelvic organ prolapsed (qPOP), where the hymen serves as a clinical reference (El Sayed et al. 2016; Bump et al. 1996; Singh et al. 2002).

rest and at maximum strain (El Sayed et al. 2016). Not only grading alone but also reporting the range of movement of the organs at rest and during straining are advised, as it provides more valuable information than grading alone (El Sayed et al. 2016).

The hiatus/muscle/organ (HMO) classification system is widely used to assess pelvic floor relaxation, which often coexists with POP but presents a different pathologic entity (Comiter et al. 1999). The reference lines in the HMO system are the H and M line (Fig. 6). The H line presents the length of the urogenital hiatus. It measures the distance between inferior symphysis pubis and puborectalis insertion. The M line is the perpendicular distance between the levator muscle plate and the PCL. Based on these reference lines pelvic relaxation is present in a symptomatic patient when the distance of the H line is >5 cm and the M line is >2 cm. A commonly used grading system is enlisted in Table 2 (El Sayed et al. 2016).

Numerous other reference lines of the pelvic floor have been published. The symphysiosacral line is obtained on midsagittal images between

4.4\ Definition of Pathological

Findings and Grading

Presence and extent of pelvic organ prolapse are analyzed in the cine mode display in the midsagittal plane by using points of reference for rest and during defecation. Optional display in a second plane allows detection of atypical rectoceles or enteroceles and facilitates the diagnosis of muscular defects (Lienemann 1998). Although the pelvic floor structures interact in a complex mode, abnormal descent and associated findings should be analyzed separately for each compartment. It is crucial to understand that complete emptying of the rectum maximizes detection of enteroceles and pelvic organ prolapse (Kelvin et al. 2000; Lienemann et al. 2000b).

Within the anterior compartment the bladder base or the most caudal part (mostly the dorsal wall) of the bladder is used as a landmark. In the middle compartment the anterior cervical lip or posterior fornix or after hysteroscopy the vaginal vault and in the posterior compartment the anorectal junction are used as references. Grading is then performed by measuring the perpendicular