Room: Exhibit Hall | Forum 9
Purpose: CT protocols with ultra low radiation dose have become popular choices in many clinical applications, such as lung cancer screening and colonography. This study aims to demonstrate that low helical pitch may cause increased photon starvation artifacts in ultra-low-dose CT.
Methods: A cylindrical water phantom with a diameter of 30 cm was scanned on a 64-slice CT scanner (Sensation 64, Siemens) at multiple effective mAs levels (mAs/pitch = 200, 100, 50, 25, and 12). The corresponding CTDIvol were 15.2, 7.6, 3.8, 1.9, and 0.95 mGy, respectively. For each dose setting, the scan was repeated at three helical pitches: 0.45, 0.8, and 1.2. The tube current was automatically adjusted by the scanner so that the effective mAs, and thus CTDIvol, was kept the same for different pitches. Images were reconstructed with a medium-smooth kernel B30 and a slice thickness of 5 mm. Noise level in the central region of the phantom was measured. Images acquired at the same dose level but different helical pitches were visually inspected to assess the photon starvation artifacts.
Results: At the same radiation dose, image noise increased with the decreasing helical pitch. When the helical pitch decreased from 1.2 to 0.45, noise increased by 7%, 9%, and 13% for 200, 100, and 50 effective mAs, respectively. More importantly, photon starvation artifacts became more severe with the decreasing pitch at the same radiation dose. The fundamental reason for the increased photon starvation at a lower helical pitch was because of the reduced tube current per projection view, compared to that at a higher helical pitch, to maintain the same radiation output.
Conclusion: A low helical pitch may increase image noise and photon starvation artifacts compared to a higher pitch for the same dose level. This effect becomes more significant in ultra-low dose CT.
Funding Support, Disclosures, and Conflict of Interest: CHM receives research funding from Siemens Healthcare