TU‐G‐103‐09: Measurement of Planar Average Equilibrium Dose of CT

Abstract

Purpose: The planar average equilibrium dose of CT is measured using the methodology suggested by AAPM report No. 111, and compared with the traditional weighted average estimate and the new proposed equal weighted average estimate. Methods: The study used a 320 slices Toshiba Aquilion ONE scanner, three CTDI body phantoms with total length of 46.5 cm (two of them are nested 3 piece CTDI phantoms with diameters of 10, 16, and 32 cm for each piece), a Radcal Accu‐Pro electrometer, and a Radcal 0.6 cc farmer chamber. In the helical mode (120 kVp, 200 mAs, nominal beam width 0.5×16 mm, pitch 0.938), we measured the cumulative dose DL(z=0) at the center of the phantoms and 12 clock position at radius of 4, 7, and 15 cm from the center, for scan length L=100, 200, 300, and 400 mm. The procedure was repeated for pitch setting of 0.641 and 1.438 and nominal beam width of 0.5×160 mm and 0.5×64 mm at L=400 mm. Results: The data obtained at four scan lengths were fit to estimate the equilibrium Dose (Deq) at radius of 0, 4, 7 and 15 cm from the phantom center. Three models (linear, quadratic, and exponential) were then used to fit DL(z=0) and Deq as a function of radius to phantom center for various scan length, pitch, and beam width combinations. We calculated the planar average cumulative (and equilibrium) dose using the above models and compared the results with the traditional weighted average estimate (Dcenter/3+2Dedge/3) and the new proposed equal weighted average estimate (Dcenter/2+Dedge/2). The planar average dose differs from the traditional weighted estimate within 2%, and equal weighted estimate within 10%. Conclusion: The traditional weighted average method provides a more accurate estimate to the CT planar average equilibrium dose than the proposed equal weighted average method.