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

Area of Science:

• Medical Physics
• Radiological Imaging
• Radiation Dosimetry

Background:

• Computed tomography (CT) is crucial for medical diagnosis, but conventional CT dose index (CTDI) measurements using a 100-mm ionization chamber (IC) underestimate radiation dose in wide cone-beam CT (CBCT) with larger beam widths.
• Accurate dose assessment is vital for patient safety and optimizing imaging protocols in CT.

Purpose of the Study:

• To evaluate and compare CTDI measurements in wide-beam CT using different methods, adhering to International Atomic Energy Agency Human Health Report No. 5.
• To determine the accuracy of various CTDI measurement techniques for wide beam widths on a specific CBCT scanner.

Main Methods:

• Measurements were conducted on an Aquilion™ ONE CT scanner with a 160 mm beam width using 100-mm and 300-mm ICs.
• CTDI_free-in-air and weighted CTDI (CTDI_w) were measured under brain and abdomen protocols in both free air and phantom conditions.
• Different measurement steps (no-step, two-step, three-step) were employed to assess their impact on CTDI values.

Main Results:

• At 80 mm beam width, CTDI_100air measurements showed minor differences compared to CTDI_300air, with the two-step method yielding slightly higher values.
• At 160 mm beam width, CTDI_100air (no-step) significantly underestimated dose, while multi-step methods provided closer estimates.
• CTDI_w measurements at 160 mm revealed substantial underestimation with the no-step method using a 100-mm IC, while the three-step method showed slight overestimation.

Conclusions:

• Standard CTDI measurements with a 100-mm IC significantly underestimate radiation dose for beam widths exceeding approximately 40 mm in wide-beam CT.
• The two-step measurement technique is demonstrated to be sufficient for accurate dosimetry in wide-beam CT applications.
• These findings highlight the need for appropriate dosimetry methods in wide-beam CT to ensure accurate dose reporting and patient safety.