MRI of Pelvic Floor Dysfunction - An Overview

Faiq Shaikh, M.D. is a dual fellowship-trained nuclear medicine physician & Informaticist, with a focus on translational research in the domains of Cancer imaging, Radiomics, Genomics, Informatics and Machine learning applications in Medicine. He has written more than 35 scientific articles and abstracts and 3 book chapters on related topics.

Introduction

Pelvic floor weakening is a common (occuring in half of women 50+) condition that leads to descent of the urinary bladder, uterovaginal vault, and rectum in the females, leading to urinary and fecal incontinence, and in extreme cases, pelvic organ prolapse.

Causes

Pelvic floor weakness is caused by a variety of factors, most of which increase the intra-abdominal pressure, such as pregnancy, multiparity, advanced age, menopause, obesity, connective tissue disorders, smoking, chronic obstructive pulmonary disease, etc. All these conditions lead to weakness of the pelvic musculature, ligaments, and fascia support result in descent of the pelvic floor organs.

Anatomy

The pelvic floor is divided into three compartments:

• Anterior compartment: contains the urinary bladder and urethra
• Middle compartment: contains the uterus, cervix, and vagina
• Posterior compartment: contains the rectum.

The structures in these compartments are supported by muscles, fascia, and ligaments anchoring them to the bony pelvis.

The endopelvic fascia is the most superior layer and covers the levator ani muscles and the pelvic viscera. Laterally, it forms the arcus tendineus. It attaches the cervix and vagina to the pelvic side wall as the parametrium and paracolpium. Posteriorly, the endopelvic fascia forms the rectovaginal fascia between the posterior vaginal wall and the rectum.

These fascial condensations are not well visualized on conventional MRI but their defects may be seen indirectly through secondary findings. These ligaments are not visualized on conventional MRI but may be visualized with an endovaginal coil which allows higher resolution and signal-to-noise ratio.

The levator ani muscles lie deep in relation to the endopelvic fascia and comprise of the puborectalis and the iliococcygeus muscles. Posteriorly and in the midline, the iliococcygeus condenses to form the levator plate. These are all well visualized on MRI. The perineal membrane lies inferior to the levator ani muscles and separates the vagina and rectum, which may be damaged during vaginal delivery when episiotomy is performed.

Pathophysiology

Pelvic floor relaxation is the weakness of the supporting muscles, fascia, and ligaments. This weakness progresses with age and may be related to hypoestrogenic states, such as menopause.

• occurs when the urinary bladder prolapses into the anterior vaginal wall, which may cause urinary incontinence.
• occurs due to weakness of the rectovaginal fascia, prolapsing rectum into the posterior vaginal wall, which may cause fecal incontinence.
• The parametrium and paracolpium weakness causes prolapse of the cervix and uterus.
• occurs when the small bowel prolapses through the rectovaginal fascia.

Accurate assessment of all compartments of the pelvic floor is necessary for surgical planning in order to minimize the risk of recurrence.

Diagnostics

Methods for the assessment of pelvic floor weakness include urodynamics, voiding cystourethrography, ultrasonography of the bladder neck and anal sphincter, fluoroscopic cystocolpodefecography, and MRI - which m is now the standard-of-care for preoperative planning for pelvic floor dysfunction, although it’s still not used for routine assessment.

Magnetic resonance imaging

MRI visualizes all three compartments of the pelvic floor and the pelvic support muscles and organs. We perform dynamic MRI of the pelvic floor with the patient in the supine or lateral decubitus positions. Conversely, MRI defecography or fluoroscopic cystocolpodefecography are performed in the sitting position, which is closer to the physiologic state. MR defecography is not superior to dynamic supine MRI for depiction of clinically relevant bladder descent and rectoceles. Overall, MRI accurately detects enteroceles and its contents when compared with fluoroscopic cystodefecography.

The preferred MRI pelvis protocols include: Ultrafast, large-field-of-view, T2-weighted sequences such as single-shot fast spin-echo (SSFSE), and half-Fourier acquisition turbo spin-echo (HASTE). After the dynamic examination is completed, small-field-of-view (20–24 cm) T2-weighted axial fast spin-echo (FSE) or axial turbo spin-echo (TSE) sequences are acquired to obtain high-resolution images of the muscles and fascia of the pelvic floor. The entire examination is typically completed in 20 minutes. This exam is performed with a torso phased-array coil wrapped around the pelvis. Endovaginal coil may be used to improve the spatial resolution of the pelvic ligaments, but it is invasive and can be uncomfortable.

MRI visualizes the uterus, cervix, and rectovaginal space. Ultrasonic gel may be administered into the vagina and rectum for better visualization. Also, incompletely voiding the urinary bladder improves visualization of the bladder and anterior vaginal wall prolapse.

For patients with a rectocele, patient is imaged after having evacuated the rectal contents. Chronic constipation and perineal hernias show as ballooning of the iliococcygeus muscle. The level of the pelvic floor is demarcated radiologically on the midsagittal image using the pubococcygeal line (from the most inferior portion of the pubic symphysis to the last horizontal sacrococcygeal joint). The levator plate should be parallel to the pubococcygeal line in normal cases.
The H line (5 cm) extends from the inferior symphysis pubis to the posterior anorectal junction on the midsagittal image and depicts the levator hiatus. The M line (2 cm) goes perpendicular from the pubococcygeal line to the most distal aspect of the H line and depicts the descent of the levator hiatus from the pubococcygeal line. Pelvic floor prolapse causes sloping of the levator plate and increasing length of the H and M lines, indicating widening and descent of the levator hiatus.

The T2-weighted axial images of the pelvic floor should be analyzed for signal intensity, symmetry, thickness, and fraying of the pelvic floor muscles. Bladder neck at strain should be less than 1 cm away from the pubococcygeal line. Descent of the bladder neck below the pubococcygeal line depicts the prolapse of the urinary bladder through the anterior vaginal wall resulting in a cystocele. Descent of the bladder neck during strain results in clockwise rotational descent of the bladder neck and proximal urethra. Distortion of the periurethral and paraurethral ligaments is seen in stress urinary incontinence. The normal butterfly shape of the vagina may also be altered by weakening of the paravaginal ligaments as it is displaced posteriorly. Prolapse of the middle compartment is associated with the vaginal apical prolapse and damage to the paracolpium seen in post-hysterectomy patients.  On midsagittal MR images, descent of the uterus, cervix and vagina usually suggests disruption of the uterosacral or cardinal ligaments and elongated H and M lines. Pelvic organ prolapse increases the urogenital hiatus in the levator muscles. Caudal angle of more than 10° between the levator plate and the pubococcygeal line on midsagittal image is a sign of pelvic floor weakness.

On the midsagittal image, rectocele is identified by a rectal bulge of more than 3 cm (from anal canal and the tip of the rectocele). Contrast-enhanced MR shows hyperintense T2 signal in peritoneal fat contents in peritoneoceles, the hyperintense fluid-filled small-bowel loops in enteroceles, and the hyperintense gel-filled rectum/sigmoid colon in rectoceles/sigmoidoceles. Intussusception of the rectum on MR is seen as rectum invaginating distally toward the anal canal (MR defecography is superior to dynamic supine MR for this indication).

Performing MRI for pelvic floor dysfunction when indicated for surgical planning and the assessment if the extent of disease may reduce the risk of surgical failure.
This information is extremely useful to urogynecologists and surgeons.

MRI of pelvic floor dysfunction: review. Law YM, Fielding JR. AJR Am J Roentgenol. 2008.