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TH-AB-207A-08: Variation of Size-Specific Dose Estimates Across Patient Sizes Under the Conditions of Automatic Exposure Control.

PURPOSE
CT automatic exposure control (AEC) systems adjust tube current output in response to changes in patient attenuation. Some have interpreted this to mean that the use of AEC results in constant size-specific dose estimates (SSDE) across patient sizes. The purpose of this work was to derive the AEC parameters that yield constant SSDE across patient sizes and compare these conditions with clinical implementations of AEC.


METHODS
The relationship between tube current output and patient attenuation within AEC systems is governed by a modulation parameter (b-value), which controls the strength of the tube current adjustment. In order for SSDE to be constant over patient sizes, this b-value needs to yield tube current values, for a given patient size, that are inversely proportional to the AAPM Report 204 CTDIvol-to-SSDE conversion coefficients. A mathematical framework was developed to determine the b-value that produces constant SSDE across patient sizes. Using that b-value, along with b-values used in clinical practice (0.33, 0.5 and 1.0), theoretical estimates of SSDE as a function of water equivalent diameter (WED) were determined, and the range of SSDE values (max-min) was quantified over WED ranging from 10 to 50 cm.


RESULTS
A b-value of 0.185 was determined to yield constant SSDE across patient sizes. For this b-value, the range of SSDE was 0.11 mGy. Ranges of 10.27 mGy, 22.05 mGy and 97.00 mGy were determined for SSDE calculated using b-values of 0.33, 0.5 and 1.0, respectively.


CONCLUSION
Current clinical implementations of AEC do not yield constant SSDE across patient sizes. A b-value was calculated that would yield constant SSDE across patient sizes, but this value is much less than b-values used clinically. We conclude that constant SSDE should not be the goal of tube current adaption, as it would not yield clinically acceptable tube current adaption with patient size. CH McCollough is supported by industry funding from Siemens Healthcare.