Home › Topics › MRI or ultrasound
What the evidence says · 2026
When endometriosis reaches the rectum, the surgeon needs a good map before going in. Which scan gives that map? A 2026 study compares them.
By Dr. César Cantú · Laparoscopic Gynecologic Surgeon, Advanced Training in Minimally Invasive Surgery · 4-min read
When endometriosis grows until it reaches the rectum (the end of the bowel), the surgeon needs to know where it is, how deep it goes and how close to the anus before operating. Two scans are used for this: an MRI and an ultrasound done from inside the rectum.
The key idea: they don't compete, they complement each other. Each scan sees different things. Together they give the surgeon the most complete map to plan.
So if you're asked to have both before surgery, it isn't an extra expense: it's so the surgeon goes in knowing exactly what's there.
Deep endometriosis that reaches the rectum is among the most delicate to operate. Success begins before surgery: in knowing precisely where the disease is, how far it goes into the bowel wall, and how far it sits from the anus. That information decides what kind of operation is needed. A 2026 study put head to head the two imaging scans we use to build that map.
The study we reviewed
Published in the Journal of Minimally Invasive Gynecology · 2026
What it looked at: how well MRI and ultrasound got six details that matter for planning: the size of the lesion, where it is, how far from the anus, how deep it invades the wall, whether there's one lesion or several, and whether it touches other organs.
What it found: both scans did well at detecting the disease. But when comparing those six details, they rarely matched each other fully (only lesion size showed moderate agreement). It isn't that one fails: each one measures different things better.
In plain terms: there's no "winner." MRI and ultrasound complement each other, and using both gives the most complete map.
The MRI is like a wide, panoramic photo of the whole pelvis: it shows the overall picture, which organs are involved and how they relate to one another. The ultrasound from inside the rectum is like a close-up: it measures very finely how deep the lesion goes into the bowel wall and how far it sits from the anus.
Those two details — depth and distance to the anus — are exactly the ones that decide what kind of surgery is needed and how to plan it. That's why having both views, not just one, gives the surgeon the most complete information to choose the best strategy and avoid surprises in the operating room.
It connects with something we've said before: in deep disease, careful planning and precision are what protect a good outcome.
If you're going to have surgery for endometriosis that reaches the bowel and you're asked for both an MRI and an ultrasound, it isn't doubling up for no reason: each scan adds one part of the map. A useful question at your consultation is "which scans will you use to map my case before surgery?". Going in with a good map is what makes a difficult surgery come out well planned.
Do you have endometriosis that might involve the bowel and want to understand how your surgery is planned?
Schedule a consultationZhang W, Zhang X, Zhang H, et al. Complementary roles of rectal magnetic resonance imaging and endoscopic ultrasound in the preoperative assessment of rectal deep endometriosis: a retrospective cohort. J Minim Invasive Gynecol. 2026. doi:10.1016/j.jmig.2026.08.012
Educational content. It does not replace an individual medical evaluation. The indication for surgery and imaging is defined case by case in consultation.