[Intl_DxMedPhys] ACR CT test question

[David S. Winter]

All,

I apologize as this has no doubt been discussed in the past, but I have a two part inquiry related to ACR CT testing. I am happy to clarify directly with the ACR, but I figured I would touch base with this forum first. (Especially since only part 1 would be good to ask the ACR directly.)

Part 1:  The ACR’s scanning instructions webpage and the CT QC manual both give a weight range for the pediatric abdominal exam of 40-50lbs, but only the QC manual has a weight for the adult abdomen (i.e. 70 kg, ~154 lbs). The webpage merely says “average sized patient”. The exclusion of the 70 kg reference could be an accidental omission, but that requires the same omission to have been repeated in multiple different locations on their webpage.

“Average sized” could be interpreted to mean average across the entire US population, the region a CT scanner is in, or the specific patient population a specific CT scanner sees (which could be massively different if the scanner is the go-to scanner for bariatric patients.) The CDC says the current adult average weights are 199 lbs for men and 171.8 lbs for women. (Both significantly more than 154). In addition, the ACR’s mandatory dose calculation sheet calculates the adult abdomen SSDE for a 35 cm water equivalent diameter. While you can’t exactly convert abdominal water equivalent diameter to a patient weight, that value would likely correspond to a patient significantly heavier than 154 lbs.

Has anyone specifically seen anything that dictates which is definitively correct for determining what techniques to set, and by extension, determine whether or not the scanner is really passing the CNR test?

Part 2: In practice, I am curious how people are determining the mA that CT scanners are selecting for the “average”, particularly for brand new scanners without much/any historic patient data. I can imagine four different general approaches. 1) Analysis of historical data aggregated by a dose metric monitoring system like Radimetrics. 2) Manually looking for a few historical patients that look to be roughly “average size” and then averaging the mA from a representative slice from a few of those studies. 3) For new systems, using values from a similar system with similar settings. 4) Conducting scans of a standard (e.g. their CTDI phantom), seeing what the CT scanner picks for mA, and then applying a scaling factor to that mA to adjust for the size difference.

Sincerely,
Scott

D. SCOTT WINTER
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CHIEF DIAGNOSTIC MEDICAL PHYSICIST
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