Purpose: For a conventional CT with a 32 cm body phantom, the ratio between the peripheral and the center CTDI measurement (the P/C ratio) is close to 2. For the Airo mobile CT, the P/C ratio is approximately 4. This study is to investigate what causes the ratio difference and how it impacts radiation dose and image quality.
Methods: We measured the HVL at the iso-center to verify the x-ray beam quality. For the assessment of the beam-shaping filter, exposure was measured under service mode with a RaySafe R/F detector in 14 lateral positions starting from the iso-center. The measurements were made with 2cm intervals between them for 120kV, 192mAs using service mode. All measurements were normalized to the measurement at the iso-center. The center and peripheral CTDIs were measured with a 32-cm body phantom for 80kV, 100kV, and 120kV. The image uniformity between the center and edge was analyzed. For comparison the same experiment was repeated in a Siemens Confidence.
Results: The x-ray quality is comparable (6.85 vs. 7.84mmAl). The higher P/C ratio in the Airo is due to the beam-shaping filter where a flat filter is applied, while a bowtie filter is used for the Siemens Confidence. For the Airo, the P/C ratios are 4.83, 4.03, 3.64 for 80kV, 100kV, and 120kV, respectively. The P/C ratio for the Confidence is 2.11 at 120kV. The dose distribution across the body phantom is more homogeneous in the Confidence than the Airo. The use of a bowtie filter can homogenize the photon flux at the detectors and reduce radiation to the patient in those peripheral regions of the body.
Conclusion: The Airo with a flat filter delivers relatively more radiation to patient especially to the skin. It may be of a concern for a complicated orthopedic procedure.