Tube current modulation vs. size-specific dose estimates

Size-specific dose estimates (SSDEs) are credited with bringing new levels of accuracy to radiation dose estimates by allowing dose to be tailored to patient size in a straightforward way. But how well SSDE works with automated tube current modulation, another mainstay of CT imaging, is still up for debate.

In their study, Kyle McMillan and colleagues from the Mayo Clinic in Rochester, MN, wanted to know if SSDE remained a valid measurement in the face of automated tube current modulation -- a development that could affect CT dose index volume (CTDI_vol).

"In a previous paper, it was asserted that the concept of CTDI_vol breaks down mathematically when tube current modulation is used," wrote study co-author Cynthia McCollough, PhD, in an email to AuntMinnie.com. "Since SSDE relies on CTDI_vol and most modern body imaging is performed with the use of tube current modulation, this could affect the accuracy of SSDE calculations."

The study, performed with a 128-detector-row scanner, aimed to use previously validated Monte Carlo methods to determine the accuracy of SSDE, which represents the mean dose in the center of the scan region.

The results showed strong correlation between CTDI_vol-normalized effective dose and water-equivalent diameter, according to the researchers. A general relationship between CTDI_vol-to-effective-dose conversion coefficients and patient size is likely sufficient to estimate SSDE for exams acquired with both fixed and modulated tube currents.