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Linear and Polynomial Approximations to Correct the IEC CTDI Measurement for Wide Beam CT Scanner

V Weir1*, I Hamilton1, J Zhang2 , (1) Baylor University Medical Center, Plano, TX, (2) University of Kentucky, Lexington, KY


(Sunday, 7/14/2019) 3:00 PM - 3:30 PM

Room: Exhibit Hall | Forum 9

Purpose: The IEC recommends adjusting the traditional CTDI�₀₀ measurement by nominal beam width to estimate radiation dose of a wide-beam CT scanner. It is reported that this method may significantly underestimate radiation dose at wider beam width. We investigate a feasible approach to correct the IEC method for more accurate dose estimation.

Methods: Three methods were used to estimate radiation dose in a GE 256-slice CT scanner. The first used a 0.6cc Farmer chamber to measure dose length integral (DLI). The second method integrated dose profiles over 600mm with a pencil chamber. The third method followed IEC recommendations by adjusting traditional CTDI with beam width. We performed dose measurements using both PMMA body and head phantoms, combining with various available bowtie filters and at two kV settings. Correction factors were developed based on these measurements.

Results: CTDI measurements using the DLI method and the direct integration method align with each other with an average error of <7.66%. While the IEC method underestimates radiation dose relative to the DLI, with an error range from 11.19% to 21.88%, depending on the phantom and the filter. A correction factor of 15.4% is developed for body and head phantoms with the large body, head and pediatric head bowtie filter. A polynomial correction can better adjust the IEC measurements, with an average error of <6%. The a1 coefficient of the polynomial correction is approximately equal to the Aeq obtained from DLI measurements, with an average error <7%. a1 could be directly used to estimate DLI.

Conclusion: Linear or polynomial approximation can help align the IEC method with the direct integration of dose profiles or point detector approach for more accurate CT dose measurement. The polynomial correction may potentially bypass the need for an elongated phantom when Aeq is estimated by the a1 coefficient.


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