Dynamic Pelvic Floor MRI Imaging Technique

The MR imaging evaluation is performed with the patient in the supine position, without contrast agents, and within fifteen minutes. If the examination is focused on the posterior compartment then rectal contrast can be considered. Despite the fact that pelvic floor dysfunction, i.e. prolapse or relaxation, are most easily identified when the patient is upright, imaging in the supine position has been shown to be perfectly satisfactory for evaluating symptomatic pelvic floor dysfunction. After the patient has voided, she is positioned on the MR imaging table with the knees bent. It is important to eliminate any tilt and strive to align the symphysis pubis and coccyx for optimal imaging. The legs are somewhat apart so as not to interfere with organ prolapse. A multicoil array, whether pelvis or torso, is wrapped low around the patient's pelvis. During positioning, the technologist explains the procedure and coaches the patient on how to maintain a maximum pelvic strain, i.e. Valsalva. Most women can maintain maximal pelvic pressure for less than 10 seconds. Scout images are obtained to identify a midline sagittal section that shows the pubic symphysis, urethra, vagina, rectum, and coccyx. Rapid half-fourier T2-weighted imaging sequences, such as single-shot fast spin echo (SSFSE) or half-acquisition single-shot turbo spin echo (HASTE), are used to obtain sagittal images while the patient is at rest and during pelvic strain, followed by axial images. Coronal images are optional. Real-time dynamic imaging can be obtained using trueFISP (true-fast imaging of steady-state precession) through one plane, for which the midline sagittal plane is most commonly used. Approximately 90 images are taken consecutively over 49 seconds of total acquisition time, during which the patient is asked to perform repetitive strain maneuver alternating between rest and Valsalva or a maneuver that provokes symptoms On sagittal MR images, the radiologist identifies and draws the following lines :

1. Pubococcygeal line (PCL) – Line that extends from the inferior border of the pubic symphysis to the last joint of the coccyx and represents the level of the pelvic floor.
   • The distance from the pubococcygeal line to the bladder neck, cervix, and anorectal junction should be measured on images obtained when the patients is at rest and at maximal pelvic strain.
   • In healthy women, there is minimal movement of the pelvic organs even with maximal strain.
   • In a symptomatic patient, organ descent of greater than 1 cm below the pubococcygeal line indicates pelvic floor laxity. Organ descent greater than 2 cm is often indicative of the need for surgical intervention.
2. H and M lines – These lines are helpful for quantifying the loss of pelvic floor support or pelvic relaxation. Both of these lines become elongated during the Valsalva maneuver in the patient with pelvic floor laxity.
   • The H line is the anteroposterior width of the levator hiatus and is drawn from the inferior aspect of the pubic symphysis to the posterior wall of the rectum at the level of the anorectal junction.
   • The M line is the vertical descent (height) of the levator hiatus and is drawn as a perpendicular line dropped from the pubococcygeal line to the most posterior aspect of the H line.
   • In asymptomatic population, during straining, the H line is < 6 cm long and does not descend (M line) more than 2 cm below the PCL. The upper urethra, urethrovesical junction, bladder, upper vagina, uterus, small bowel, sigmoid colon, mesenteric fat, and rectum are all above the hiatus (H line).
3. The angle of the levator plate with the pubococcygeal line is helpful in identifying small bowel prolapse.
   • In healthy women, the levator plate will parallel the pubococcygeal line at rest and during pelvic strain.
   • Increased caudal inclination is an indicator of loss of posterior muscle support.
   • Descent of the small bowel more than 2 cm between the vagina and rectum is indicative of enterocele. Anterior bulging of the rectum is evidence of an anterior rectocele.

On the axial and coronal images, the puborectal and iliococcygeal muscles should be examined for thinning and tears. The width of the levator hiatus has not proved to be significant in the identification of pelvic floor laxity; however, it rarely exceeds 4.5 cm in women with intact pelvic floors. Paravaginal fascial tears can be suggested from the posterior displacement of the vaginal fornix.

The radiology report should detail the location and number of compartments that demonstrate laxity and the measured descent of the bladder, vagina, and rectum. In some cases, the damage to the pelvic floor may be so severe that all three compartments are involved. This is called global pelvic floor weakness. The presence or absence of enterocele and rectocele should be noted. The location of muscle tears and inferred fascial tears should also be specified. Supporting measurements such as those of the H line, M line, and angle of the levator plate can be reported as desired by the radiologist and referring surgeon. These measurements may be useful for quantification of descent and for comparision with future images in patients who return for evaluation of recurrent symptoms. The following are MRI grading systems as proposed by Barbaric et al.:

MR Grading System for Organ Prolapse(“H Line” Used as Reference)
GRADE	ORGAN LOCATION
0 (No Prolapse)	Above the “H line”
1 (Mild, Small)	0-2 cm below the “H line”
2 (Moderate)	2-4 cm below the “H line”
3 (Severe, Large)	³ 4 cm below the “H line”
4 (Procidentia)	Only for cystourethrocele

Hiatal Enlargement (Relaxation)(“H line” Used as Reference)
GRADE	ENLARGEMENT (cm)
0 ( Normal )	< 6
1 (Mild)	6-8
2 (Moderate)	8-10
3 (Severe)	³ 10

Pelvic Floor Descent(“M Line” Used as Reference)
GRADE	DESCENT (cm)
0 ( Normal )	0-2
1 (Mild)	2-4
2 (Moderate)	4-6
3 (Severe)	³ 6