Megavoltage computed tomography image quality assessment in a modern tomotherapy system: a frequency-domain analysis

This study aimed to comprehensively evaluate the image quality of iterative (IR) and filtered back-projection (FBP) reconstruction in a modern megavoltage computed tomography (MV-CT) system using frequency-domain methods in both the in-plane and longitudinal directions. MV-CT scans of a Catphan 504 phantom were obtained using various pitch factors (PFs). The in-plane image quality was assessed using a task-based transfer function (TTF) and a noise power spectrum (NPS). The longitudinal characteristics were evaluated using the slice sensitivity profile (SSP) and modulation transfer function (MTF). Finally, the helical artifacts and radiation doses were quantified. The in-plane TTF and NPS for IR were independent of the PF. Compared to FBP, IR provided significant noise reduction (e.g., 73.3% lower NPS at 0.2 mm⁻1) but showed slightly lower TTF for moderate-contrast objects at low frequencies. In contrast, the longitudinal resolution was directly dependent on the PF; specifically, with increasing PF, the effective slice thickness widened from 3.7 mm (Fine) to 6.0 mm (Coarse), and the MTF degraded. This was accompanied by more pronounced helical artifacts as the circularity of the object decreased from 0.95 (PF 1.0) to 0.69 (PF 3.0). The IR provided remarkable noise reduction and improved soft-tissue visualization. However, the disparate behavior of resolution between imaging directions is a key finding: while the in-plane quality is consistent across pitch factors, the longitudinal resolution degrades significantly with a higher pitch. This characteristic has direct implications for the choice of imaging protocol, particularly for high-precision treatments where geometric accuracy is paramount.