ANATOMY

Three compartments form the pelvic floor: the anterior (containing the bladder and urethra), the middle (vagina, cervix, and uterus); and the posterior (rectum and anus) (Fig.1).

They are supported by the endopelvic fascia, the pelvic diaphragm, and the urogenital diaphragm.

Fig. 1: Sagittal midline T2WI MR image representing the three pelvic compartments in different colors.

Endopelvic fascia

• It´s the most superior layer of the pelvic floor and covers the pelvic diaphragm; its integrity can be inferred by the efficiency of support it provides;
• In the anterior compartment, it’s known as pubocervical fascia and is formed by ligaments that support the urethra. A tear in this fascia can lead to urinary incontinence, cystocele or urethral hypermobility;
• In the middle compartment, it’s formed by the paracolpium and parametrium, which prevent genital organ prolapse;
• In the posterior compartment, it originates the perineal body, in the anovaginal septum, which prevents the expansion of the urogenital hiatus. It also continues as the rectovaginal fascia. A rupture in these structures can induce a rectocele or enterocele.

Pelvic diaphragm (Fig.2)

• It consists of four muscles that are tonically contracted at rest: ischiococcygeus, iliococcygeus, pubococcygeus, and puborectalis (the last three forming the levator ani muscle). The most important are the puborectalis and iliococcygeus muscles;
• The puborectalis forms a U-shaped sling around the rectum, defining the anorectal junction (ARJ) and has an essential role in elevating the bladder neck;
• The iliococcygeus muscle arises from the external anal sphincter, and posteriorly forms the levator plate. It prevents posterior prolapse.

Fig. 2: Pelvic Diaphragm of Female.
References: Netter FH. (2014) Atlas of human anatomy. Saunders, 6th edition

Urogenital diaphragm

• It’s restricted to the anterior and middle compartments and is triangular in shape;
• Encloses the urethral sphincter and the deep perineal space.

PROTOCOL

It's recommended the use of a 1.5T MRI (at least) and a multicoil array around the pelvis.
The patient should void 2h before the study.
There is no need for previous preparation (enema, IV contrast, fasting).
Supine position with the knees elevated is recommended.
The rectum should be distended (120-250cc) and vaginal filling (20cc) may be useful, both with ultrasound gel.
Static and MRI dynamic images (during straining, squeezing, and defecation) should be used. High-resolution T2WI in three planes are indicated for static images, whereas steady-state or balanced state free precession sequences are recommended for dynamic sequences.
Optional: axial and coronal dynamic sequences during straining.

INTERPRETING MRI FINDINGS

MRI reference points have been proposed to assess the presence and severity of pelvic organ prolapse.
The pubococcygeal line (PCL) is the most commonly used (Fig.3). It represents the level of the pelvic floor and connects the inferior border of the pubic symphysis to the last coccygeal joint.
The H and M lines are also measured to identify pelvic floor relaxation (Fig.3).
The H line is drawn from the pubic symphysis’s inferior border to the posterior ARJ (the point where the puborectalis muscle slings around the rectum) and represents the anteroposterior width of the levator hiatus.
The M line extends from the PCL to the posterior aspect of the H line and is indicative of the descent of the levator hiatus.
The H line should not exceed 5cm and the M line 2cm.
The anorectal angle (ARA) is drawn between the posterior border of the rectum’s distal part and the axis of the anal canal. It’s crucial to access the pelvic diaphragm tonus and the ability to contract and relax.
At rest, ARA usually measures 108º to 127° (variable values depending on the author). During squeezing, it decreases by approximately 15°-20°, and during straining and defecation, it increases the same amount (Fig.4).

Fig. 3: Midsagittal T2WI MR image of a female with normal anatomy during rest, showing the reference lines most frequently used for the evaluation of PFD: PCL (yellow line); H line (dotted red line); M line (dotted green line).

Fig. 4: T2WI MR images showing normal anorectal angle (ARA) at rest (a) – ARA 113º - while performing squeeze maneuver (b) – ARA 102º - and during defecation (c) – ARA 127º.

Pelvic Organ Prolapse

Anterior Compartment:

A cystocele occurs when there is abnormal descent of the bladder at rest or straining and results from tearing of levator ani muscle or pubocervical fascia (Fig. 5-9).
The average vertical distance of the bladder neck from the pubococcygeal line should be less than 1cm. If >1cm, we are facing a cystocele.
Depending on the distance of the bladder base from de PCL, cystoceles are graded as mild (1-3cm), moderate (3-6cm) or severe (>6cm).
On sagittal images, the normal urethra has a vertical orientation. Anterior angulation of the proximal urethra is indicative of urethral hypermobility (>30º from its resting axis) (Fig. 5,6,8,9). It must be referred to because adequate repair requires a pubocervical sling procedure.

Middle Compartment:

Uterine and vaginal vault prolapse occurs when there is a weakness of the compartment’s supporting structures, such as the pubocervical fascia, rectovaginal fascia, paracolpium, and parametrium.
The prolapse is measured from the PCL to the most anteroinferior portion of the cervix or to the posterosuperior vaginal apex (Fig. 5, 7-9).
After a hysterectomy, the vaginal apex should be at least 1cm above the PCL at strain.
It can also be classified in mild (1-3cm), moderate (3-6cm) or severe (>6cm).
On axial images, there is a loss of the habitual H shape of the vagina.

Posterior Compartment

An anterior rectocele is caused by the herniation of the anterior wall of the rectum into the vagina, resulting from damage to the rectovaginal fascia.
It occurs and is considered relevant when there is a rectal bulge of more than 2cm during straining and/or evacuation, measured between the anterior wall of the anal canal and the rectocele’s tip (Fig. 5-10).
Rectoceles become important when symptomatic and may occur without defecatory dysfunction.
It should be graded as small, medium, or large, when <2cm, 2-4cm, and >4cm respectively.
MRI is also important to evaluate the rectocele’s emptying dynamic.
Posterior and lateral rectoceles are less common.

Rectal invagination (prolapse of full-thickness anorectal wall) or prolapse of the rectal mucosa alone are causes of mechanical obstruction (Fig. 6,7). They should be differentiated and mentioned for appropriate surgical treatment.

In healthy individuals, the rectovaginal space is closely apposed. The widening of this space can induce herniation of peritoneal fat (peritoneocele) (Fig.7), small bowel (enterocele), or the sigmoid colon (sigmoidocele) below the proximal one-third of the vagina. Hysterectomy increases the risk of enterocele formation. They are known as perineal hernias when they don’t reduce spontaneously.

Fig. 5: T2WI MR image during defecation demonstrating a moderate cystocele (red dotted line), a mild uterine prolapse (green dotted line) and a medium anterior rectocele (white dotted arrow). There are also signs of urethral hypermobility (blue dotted line) and of pelvic floor relaxation with descent of the anorectal junction and consequent elongation of the M line (orange line).

Fig. 6: T2WI MR image during defecation showing a moderate cystocele (orange dotted line) a medium anterior rectocele (blue dotted arrow). There are also signs of urethral hypermobility (blue dotted line) and an intrarectal invagination (red arrow).

Fig. 7: T2WI MR image during defecation demonstrating a severe uterine prolapse (orange dotted line), a mild cystocele (green dotted line) and a small anterior rectocele (blue dotted arrow). There are also a peritoneocele (*) and prolapse of the posterior rectal mucosa (red arrow).

Fig. 8: T2WI MR images during defecation demonstrating a moderate uterine prolapse (red dotted line), a mild cystocele (blue dotted line) and a small anterior rectocele (orange dotted arrow). There are also a prominent puborectalis impression (red arrow) and signs of urethral hypermobility (green dotted line).

Descending perineal syndrome (DPS)

DPS is caused by a loss of muscular tone, resulting in abnormal descent of the entire pelvic floor at rest and/or during evacuation (Fig. 5,9).
Pudendal nerve’s injury and chronic straining at defecation are some causes of this entity.
MRI suspicious findings are: increased ARA at rest; decreased elevation of the pelvic floor during contraction; an elongated H line and a low ARJ level on dynamic images (M line >2,5cm); caudal angulation of the levator plate (>10° with respect to the PCL).
It’s frequently associated with pelvic prolapses.

Fig. 9: T2WI MR images at rest (a) and during defecation (B and C) demonstrating descent of the anorectal junction with elongation of the M and H lines (green and orange lines). There is also a caudal angulation of the levator plate (arrow in B). A moderate cystocele (red dotted line) and uterine prolapse (orange dotted line), a medium anterior rectocele (blue dotted arrow) and signs of urethral hypermobility (blue dotted line) are seen in C. These findings are suggestive of descending perineal syndrome with tricompartimental prolapse.

Spastic Pelvic Floor Syndrome

It’s also known as paradoxical contraction of the puborectalis muscle, dyssynergic defecation or anismus.
It’s characterized by involuntary contraction of the puborectalis muscle during defecation and may result in prolonged and incomplete evacuation.
A prominent puborectal impression (Fig. 8,10), lack of descent of the pelvic floor and absence of ARA opening during defecation are the MRI findings of this syndrome.
The puborectalis muscle may appear hypertrophied.

Fig. 10: T2WI MR image during defecation showing a large anterior rectocele (blue dotted arrow) bulging into the vagina and a prominent puborectalis impression (orange arrow).