Computed Tomography Dose Index Measurements in Wide-beam Computed Tomography System

Abstract

Computed tomography (CT) produces cross-sectional images for medical diagnosis; however, in wide cone-beam CT, conventional CT dose index (CTDI) measurements using a 100-mm ionization chamber (IC) underestimate dose for larger beam widths. This study evaluated and compared CTDI measurements in wide beam using different methods for wide-beam CT, following International Atomic Energy Agency Human Health Report No. 5. Measurements were performed on an Aquilion™ ONE CT scanner (160 mm beam width) using 100-and 300-mm ICs (Radcal 10 × 6–3CT and PTW TM30017). CTDI<inf>free-in-air</inf> and weighted CTDI (CTDI<inf>w</inf>) were obtained under brain and abdomen protocols, both in free air and phantom conditions, at the Advanced Diagnostic Imaging Center, Faculty of Medicine Ramathibodi Hospital, Thailand. For free-in-air measurements at 80 mm beam width, CTDI<inf>100</inf><inf>air</inf><inf>(no-step)</inf> was slightly smaller than CTDI<inf>300</inf><inf>air</inf> with percentage differences of −1.27% and −1.94%, while CTDI<inf>100air</inf><inf>(two-step)</inf> showed +6.79% and +6.48% differences for brain and abdomen protocols, respectively. At 160 mm beam width, CTDI<inf>100</inf><inf>air</inf><inf>(no-step)</inf> was significantly lower due to incomplete dose coverage, whereas two-and three-step methods yielded slightly higher values. For CTDI<inf>w</inf> at 80 mm, percentage differences were −17.06% and −15.05% (no-step), −3.29% and +1.88% (two-step), and −8.94% and −3.45% (calculated two-step) for brain and abdomen, respectively. At 160 mm, CTDI<inf>100w</inf><inf>(no-step)</inf> was 41.27% and 36.38% lower than CTDI<inf>300W</inf>, while CTDI<inf>100w</inf><inf>(three-step)</inf> exceeded CTDI<inf>300w</inf> due to scattering or overlap. Overall, CTDI measurements using a 100 mm IC underestimate dose for beam widths >NT +40 mm. The two-step technique is sufficient for wide-beam CT dosimetry measurements.