A CT abd/pelvis without contrast is one of our go-to studies when evaluating for nephrolithiasis in a patient with acute flank pain. And it’s a really good one too–a recent meta-analysis of CT for suspected renal stone showed a sensitivity of 97% and a specificity of 95%. Pretty, pretty excellent.

But have you ever wonder why patients are positioned prone during CT abd/pelvis looking for a renal stone? Or perhaps you didn’t even realize that our patient’s often get flipped like a pancake on the scanner table. Read on…

According to our friendly neighborhood radiologists, a prone scan has been found to be superior to supine scans in distinguishing intramural UVJ stones from stones that have already passed into the bladder. In a study from 2017, authors found that 16% of identified renal stones on supine CT scans were in “equivocal” locations–meaning, they could not differential between a stone in the bladder vs. a stone in the UVJ. Alternatively, every one of the stones detected on the prone CT scans was determined to be in an “unequivocal” location, i.e. they were confident in the stone location.

So why do we care about this?

  1. Stone size and location is critically important to treatment. In a patient who’s stone has already passed, they likely just needs some reassurance and maybe guidance to strain their urine for said stone. A stone that is at the UVJ or even impacted will need pain control and perhaps even Flomax depending on it’s size.
  2. Avoid unnecessary procedures. An impacted stone may need to go to the OR with urology for cystoscopy, which is not without risks.
  3. We can prevent re-scanning these patients. Repeat imaging for exact location can delay diagnosis, prolong the patient’s length of stay, and expose them to additional radiation.

Tl;dr: For patients with acute flank pain who you suspect renal colic, ask the CT tech to place them prone on the scanner table. You’ll get a more accurate read on the stone’s location, which will help guide the appropriate treatment plan.

Special thanks to Dr. Colin Pesyna for the topic inspiration!

Source: Meissnitzer, M., et al (2017). Comparison of prone vs. supine unenhanced CT imaging in patients with clinically suspected ureterolithiasis. Abdominal Radiology (New York), 42(2), 569–576. http://doi.org/10.1007/s00261-016-0918-1