Program Information
Suitability of KVCT Image Quality Phantoms for MVCT Image QA
J Shea*, E Chao , B Harstad , Accuray Incorporated, Madison, WI
Presentations
SU-K-601-7 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 601
Purpose: To investigate the energy dependence of image quality phantoms commonly used in kilo-voltage imaging and to recommend suitable usage for mega-voltage imaging.
Methods: Four commonly available CT image quality phantoms were tested: Gammex CT ACR 464 Phantom, The Phantom Laboratory CatPhan® 412, QRM ConeBeam Phantom, and CIRS AAPM CT Performance Phantom Model 610. The CT linearity and low-contrast sections of each phantom were investigated for energy dependence. Phantom specifications at kilovoltage energies were taken from vendor specifications, where available, and supplemented by images acquired on a fan-beam kVCT ring. MVCT images were acquired on a TomoTherapy® system. In the CIRS Model 610 Phantom, saline solutions were prepared to fill the cavities of its low-contrast module. CT number and image contrast were measured using mean pixel values of ROIs inside and adjacent to the contrast targets in slice-averaged images.
Results: The absolute CT numbers of the linearity materials vary over energy according to the proportion of photoelectric and Compton interactions. The measured MVCT numbers differ from the nominal and measured kVCT numbers, but are calculable from electron density when the linearity materials are sufficiently specified. The relative CT numbers of the low contrast objects are energy independent for all but the Model 610 phantom, showing agreement to better than ±2 HU. In the Model 610, the CT number of the acrylic background increases with energy while the CT number of the saline contrast solution decreases with energy, resulting in energy-dependent contrast.
Conclusion: Low-contrast resolution in MVCT is best assessed using the QRM ConeBeam phantom, which offers the widest range of energy-independent contrast levels. CT number accuracy in MVCT is best assessed by the CatPhan, which specifies the electron density of each of its linearity materials.
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