On the Improvement of CBCT Image Quality for Soft Tissue-Based SRS Localization
Mao W, Snyder K, Gardner S, Wen N, Zhong H, Jackson P, Shah M, and Chetty I. On the improvement of CBCT image quality for soft tissue-based SRS localization. Med Phys 2017; 44(6):3203-3204.
Purpose: On-board cone-beam CT (CBCT) imaging modes have limited soft-tissue resolution. Soft-tissue matching is desired for stereotactic radiosurgery (SRS) patient setup. Here, we explore the optimal gantry rotation speed and other parameters to improve CBCT image quality to potentially improve SRS localization. Methods: A series of CBCT acquisition modes were generated on a Varian Edge linac. All scans were designed with full-fan bowtie filter at 100 kVp. Gantry rotation speed was varied from 1.5°/s to 6.0°/s, corresponding to a total projection number from 2000 to 500. Each series had fixed x-ray tube current: 15 mA, 30 mA, 45 mA, or 60 mA and full gantry rotation (360°) or half gantry rotation (200°). Exposure was < 600 mAs. A CATPHAN phantom was scanned using each mode. Image metrics including uniformity, constancy, spatial resolution, and noise level were analyzed using a commercially available software. Contrast to noise ratio (CNR) was analyzed at 1% contrast level at a diameter of 15 mm. Results: Phantom studies show that noise level is reduced and CNR is increased by slowing down the gantry rotation with fixed gantry rotation range and x-ray tube current while other image qualities are maintained. For half rotation scans with a tube current of 15 mA, when gantry rotation is slowed from 6°/s to 1.5°/s, noise is reduced from 36.6 HU to 18.5 HU and CNR is increased from 0.5 to 1.0. Combining three scanning parameters together, signal noise is proportional to the inverse square root of the total exposure, although a full rotation leads slightly less noise than a half rotation. Patient images show a slow-full-rotation CBCT imaging improves soft-tissue resolution. Conclusion: Soft-tissue resolution is enhanced by slowing down gantry rotation, expanding gantry rotation range, or increasing tube current, which will enhances soft tissue-based localization for SRS treatments.