Assessment of longitudinal beam property and contrast uniformity for 256‐ and 320‐row area detector computed tomography scanners in the 160‐mm nonhelical volume‐acquisition mode

BACKGROUND: Because the x‐ray property of patient longitudinal axis in area detector computed tomography (ADCT) depends on a heel effect, radiation dose and beam quality are not uniform along the long axis of the patient. OBJECTIVE: This study aimed to measure the longitudinal beam properties and contrast uniformity of ADCT scanners in the 160‐mm nonhelical volume‐acquisition (NVA) mode and provide useful datasets for the radiation dose reduction in ADCT examinations. MATERIALS AND METHODS: Two different types of ADCT scanners were used in this study. To assess the heel effect in 256‐ and 320‐row ADCT scanners, we measured dose profile, half‐value layer, and iodine contrast uniformity along longitudinal beam direction. RESULTS: The maximum effective energy difference within a 160‐mm x‐ray beam is approximately 4 keV. Maximum radiation dose on the anode side of the x‐ray tube showed approximately 40%–45% reduction compared with that on the isocenter position; the heel effect properties longitudinally differed throughout the x‐ray beam, and the decrease in the radiation dose in 256‐ and 320‐row ADCT scanners was observed on the patient table side and gantry side respectively. The CT numbers of iodinated solutions for 256‐row ADCT scanner were independent of the heel effect; nevertheless, the CT numbers of 320‐row ADCT scanner tended to increase on the patient table (cathode) side. CONCLUSION: This study reveals that the radiation dose on the anode side of the x‐ray tube shows approximately 40%–45% reduction compared with that on the isocenter position, and the heel effect properties for 256‐ and 320‐row ADCT scanners longitudinally differ throughout the x‐ray beam. The x‐ray tube for individual ADCT scanners is mounted in an opposite direction along the long axis of the patient.