NEW ORLEANS, LA USA (UroToday.com) -
FBP versus AIDR: iPhone versus iPhone 6+
Since nephrolithiasis is a recurrent disease, most kidney stone formers will undergo numerous diagnostic studies, i.e., CT scans, over the course their lifetime. The majority of CT scanners in the US are equipped with filter back projection (or FBP), a simple image-processor inside the scanner that takes the images and converts them to something radiologists and urologists can read. Adaptive iterative dose reduction (or AIDR) is a newer image processor that requires much less CT radiation to acquire its images. It’s more expensive, has higher computational requirements, and is only available on CT scanners made after 2011. If FBP is an iPhone, think of AIDR as the iPhone 6+.
Since 2012, several small studies in highly select stone patients have shown that AIDR lowers the radiation dose that the CT scanner emits (known as CTDI) and is just as good as DLP at making a stone diagnosis. However, these studies were limited by small patient number, by lack of patient randomization, and by the use of CTDI radiation estimates. This group has developed formulas, based on cadavers, that can estimate true organ-specific radiation dose. They set out to use these formulas to determine whether or not AIDR can lower true organ doses compared to FBP.
The University of Florida houses 2 ADR- and 4 FBP-capable CT scanners. Over the course of one year, patients were randomly assigned to these based on scanner availability, creating a modified form of random assignment. They reviewed all CT scans done over one year’s time and identified 340 patients with stones in the urinary tract -- 90 patients had AIDR while 250 had FBP. Two blinded radiologists reviewed each scan to assess quality on a 5-point Likert scale, while a third was available as a tiebreaker for discrepancies. Absorbed skin and internal abdominal organ doses were calculated using their recently published validated equations. They found that both groups were similar in regards to gender, BMI (mean=30), stone burden detected, and image quality. Mean CTDI was 28.7mGy for FBP and 13.7mGy for AIDR. Using their validated formula, internal organ dose was reduced from mean of 24.0 mGy (FBP) to 9.3 mGy (AIDR). Using their cadaveric formulas, CTDI was an inconsistent measure of both skin and internal organ radiation dose and was not a reliable surrogate for patient radiation dose.
Overall, in this random cohort of 340 stone patients, AIDR reduced organ dose by almost 60% compared to FBP while maintaining excellent image quality for stone diagnosis. They suggest that all urologists ask their radiology colleagues what phone they are currently sporting. If it’s anything later than the first generation iPhone, ask them why they’ve updated their cell phones but not their hospital CT scanner! Then tell them to replace their old FBP CT technology with that of AIDR to better care for kidney stone patients.
Presented by Rishi Modh, MD at the American Urological Association (AUA) Annual Meeting - May 15 - 19, 2015 - New Orleans, LA USA
Gainesville, FL USA
Reported by Zhamshid Okhunov, MD (University of California-Irvine), medical writer for UroToday.com