SU‐E‐I‐49: Influence of the Phantom Length in Computed Tomography Dose Profile Dosimetry

Abstract

Purpose: We measured the computed tomography (CT) dose profile with CT dose phantom for several X‐ray beam widths on 64‐slice, and 320‐slice multi detector‐row CT (MDCT) using micro ionization chamber, and assessed the association of the CTDI100 value with CT dose phantom length. Methods: The CT dose profile (CTDP) was measured as Dm using radiation monitor (Model 9015, Radcal, Monrovia, CA) with micro ionization chamber (10X5‐0.18, Radcal, Monrovia, CA) which have an active length of 19 mm and a 0.18 cm3 active area on 64‐slice MDCT (Aquilion CX, Toshiba Medical Systems, Tochigi, Japan), and 320‐slice MDCT (Aquilion One, Toshiba Medical Systems, Tochigi, Japan). The MDCT systems were used with a tube voltage of 120 kV, tube current of 100 mA, rotation time of 1 second, and nominal X‐ray beam widths of 4, 12, 32, and 160 mm (4*1 mm, 4*3 mm, 64*0.5 mm, and 320*0.5 mm axial slice acquisitions, respectively). A micro ionization chamber was placed in the center of the single CT dose phantom, and we acquired several axial scans as 5 mm increments of phantom length. Then two, and three coupled dose phantom were placed together to simulate a 300‐mm‐long, and 450‐mm‐long phantom. We acquired several axial scans in similar method. Results: The observed percent increase of the Dm with extended length from 150 mm to 300 mm of the CT dose phantom was 120 % at range from −60 to 60 mm. However, it was greatly increased toward peripheral area of the DP. With extended length from 300 mm to 450 mm of CT dose phantom, the observed percent increase of the Dm was slightly increased. There was no difference in the form of the CTDP between 300‐mm‐long phantom and 450‐mm‐long phantom. Conclusion: In upward of the 300‐mm‐long CT dose phantom, there is no difference in the form of the CTDP.