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

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_free-in-air and weighted CTDI (CTDI_w) 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_{100 air (no-step)} was slightly smaller than CTDI_{300 air} with percentage differences of -1.27% and -1.94%, while CTDI_{100air (two-step)} showed +6.79% and +6.48% differences for brain and abdomen protocols, respectively. At 160 mm beam width, CTDI_{100 air (no-step)} was significantly lower due to incomplete dose coverage, whereas two-and three-step methods yielded slightly higher values. For CTDI_w 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_{100w (no-step)} was 41.27% and 36.38% lower than CTDI_300W, while CTDI_{100w (three-step)} exceeded CTDI_300w 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.