MRPT clearance is a measurement taken during MR proctography (also called MR defecography) that shows how completely your rectum empties when you attempt to have a bowel movement. It’s expressed as a percentage: healthy women in one study evacuated a median of 57% of rectal contents, and clearing less than 20% is considered abnormal. This number helps doctors determine whether difficulty with bowel movements is caused by a structural or functional problem in the pelvic floor.
How MRPT Clearance Is Measured
Before the scan, a radiologist or technician fills your rectum with ultrasound gel. This gel shows up clearly on MRI and acts as a stand-in for stool. You then lie on your back inside the MRI scanner with your knees elevated on a firm pillow, which makes straining and pushing easier.
The scanner takes images in real time while you rest, squeeze, and then attempt to push the gel out. The “clearance” is the proportion of gel you manage to evacuate compared to how much was placed inside. If the rectum started with a certain volume and you expelled most of it, your clearance percentage is high. If very little comes out despite your effort, the clearance is low, pointing toward a possible evacuation problem.
What Normal and Abnormal Clearance Look Like
Research on healthy volunteers found wide variation in how much gel people can push out during the test. The median was 57%, but the range stretched from about 18% to 82%. Because of that spread, a single cutoff is more useful than a target number. Evacuating less than 20% of the gel is generally considered abnormal and suggests something is interfering with rectal emptying.
Low clearance doesn’t automatically mean one specific diagnosis. It’s a flag that prompts the radiologist to look more closely at what’s happening structurally during the evacuation images.
What Low Clearance Can Reveal
When clearance is poor, the dynamic images from the scan often show one or more underlying causes. The most common findings include:
- Rectocele: A bulge in the rectal wall, usually toward the vagina in women. Gel can get trapped in this pouch instead of moving downward. Rectoceles are graded by size: small (under 2 cm), moderate (2 to 4 cm), and large (over 4 cm).
- Pelvic floor dyssynergia: Sometimes called spastic perineum syndrome. The puborectalis muscle, which wraps around the rectum like a sling, fails to relax during pushing. On imaging, it creates a persistent indentation on the back wall of the rectum and keeps the anorectal angle too tight for contents to pass.
- Intussusception: The rectal lining telescopes inward during straining, partially blocking the outlet.
- Enterocele or sigmoidocele: Loops of small bowel or sigmoid colon drop down into the space between the rectum and vagina, pressing on the rectum and obstructing evacuation.
These findings collectively fall under a condition called obstructive defecation syndrome (ODS). Clearance percentage is one of the first clues that ODS may be present, and the accompanying images explain why.
How MRPT Compares to X-Ray Defecography
The older version of this test uses X-ray fluoroscopy instead of MRI. A study comparing both methods in 32 patients found no significant difference in detecting rectoceles or pelvic floor dyssynergia. However, the two techniques have distinct strengths.
X-ray defecography was better at picking up rectal mucosal prolapse (found in 22 patients versus 12 on MRI), internal intussusception (18 versus 3), and a descending perineum (21 versus 8). MRI, on the other hand, showed more problems in the front and middle sections of the pelvic floor, such as bladder or uterine prolapse. It also provided far more anatomical detail of surrounding structures.
Because pelvic floor problems rarely affect only one compartment, the ability of MRI to evaluate the bladder, uterus, and bowel in a single exam is a major advantage. It also avoids radiation exposure, which matters when the patient population skews toward younger women.
What to Expect During the Test
No fasting is required before MR proctography. You won’t need to drink oral contrast or receive an IV injection. The main preparation is rectal filling with ultrasound gel, which the clinical team handles shortly before the scan. In some centers, a small amount of gel is also placed in the vagina to help distinguish structures on imaging, though this step is optional and may be skipped based on patient preference.
The scan itself uses a standard MRI machine (1.5 Tesla or stronger) with a coil positioned low over the pelvis. You’ll lie on your back with your knees propped up. The technologist will coach you through the phases: rest quietly, squeeze your pelvic floor muscles, then bear down as if having a bowel movement. The dynamic evacuation sequence captures several images per second so the radiologist can watch the entire process in motion. Most people find the pushing-while-lying-down position awkward, and it can reduce how much gel you’re able to expel compared to sitting upright. Radiologists account for this when interpreting your clearance number.
How Clearance Fits Into Your Results
Your MRPT report will typically include several measurements beyond clearance. The radiologist measures how far the pelvic organs descend below a reference line called the pubococcygeal line, which runs from the pubic bone to the tailbone. Any downward movement of the bladder, vaginal vault, or anorectal junction beyond this line gets measured in centimeters and graded by severity.
Clearance percentage ties all of these structural observations together into a functional picture. You might have a moderate rectocele on imaging, but if your clearance is 60%, the rectocele probably isn’t causing significant obstruction. Conversely, a small rectocele paired with 15% clearance and a tight anorectal angle suggests the real culprit is muscle coordination, not anatomy alone. This distinction matters because treatment for dyssynergia (typically pelvic floor physical therapy and biofeedback) is very different from treatment for a large, obstructing rectocele, which may require surgical repair.