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The Relationship Between Beam Width,Scan Length, and Dosimeter Dimension in CT Equilibrium Dose Measurement

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


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: AAPM TG#111 proposed the equilibrium dose to address the concern of wide beam CT dose measurement. The just-published TG#200 detailed the implementation of this approach. This study is to elucidate, theoretically and experimentally, the relationship between x-ray beam width, scan length, and dosimeter dimension for accurate equilibrium dose measurement.

Methods: The equilibrium dose is an approach-to-equilibrium function which is typically presented as growth functions. Accordingly, two ordinary differential equations (ODEs) were derived to simulate two scenarios: varying x-ray beam width with a fixed active dosimeter size and a fixed beam width with varying dosimeter size (integration length). Both cases were validated in a 256-slice CT scanner with a 60-cm body phantom. The beam widths were set at 5, 40, 80, 120, 140, and 160mm, respectively. Eight sleeves were constructed and placed in different arrangements to create dosimeter sizes of 10, 20, 30, 40, 50, and 60 mm on a 100mm pencil chamber. The ratio of the beam width to the dosimeter size was used to predict the minimum beam width, while the minimum dosimeter length was the one that integrated 98% of exposure.

Results: ODE equations can accurately model the approach-to-equilibrium function. Linear regression analysis shows a good correlation between the measurement and the proposed equation (R2 of 0.96~0.99) at all beam widths except the smallest beam width of 5mm. The modeling results show that the minimum dosimeter length d for integrating 98% of the signal for a given beam width a is a=5.02a. Further analysis shows that the minimum beam width a, at which a dosimeter of length d is used as a point dosimeter, is a=5.56d.

Conclusion: Both ODEs yield equivalent solution and provide a better understanding of the relationship between beam width, scan length and dosimeter size for wide beam CT equilibrium dose measurements.

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