CBCT Image Quality Assessment Testing Clinically Relevant Orientation and Position

Objectives: Some physical measures of CBCT image quality have been shown to correlate well with diagnostic image quality. Traditionally, these objective measures have been assessed in the center of the volume in a standard orientation. The purpose of this study was to test whether measures of image quality vary as a function of test tool location, test tool orientation, and dose.
Methods: CBCT image quality was assessed with one standard and three modified phantoms (Quart GmbH, Zorneding, Germany). The test tool was located at the center of the phantom (standard), at the periphery (Mod1), angled with respect to the image acquisition plane (Mod2), or angled with respect to the image acquisition plane and at the periphery (Mod3). Each phantom was imaged with a Carestream CS 9300 CBCT scanner (Carestream, Rochester, NY), using regular dose (180µ voxel/90kVp/64mAs) and low dose (400µ voxel/85kVp/14.5mAs) with an 8x8cm field-of-view. Contrast-to-Noise-Ratio (CNR) and 10% modulation-transfer-function (MTF) were assessed in three repeated volumes. Data were analyzed using ANOVA and Tukey’s HSD.
Results: CNR differed by phantom (p<0.0001) and dose (p<0.0001). Mod3 displayed significantly greater CNR than other phantoms. Low dose provided higher CNR. MTF differed only by dose (p<0.0001).
Conclusions: CNR improved for a peripherally positioned tilted test tool (Mod3). Reduced kVp and larger voxels appear to counteract the effect of reduced mAs producing improved CNR at low dose. Thus, image quality parameters are different at the center of a CBCT volume when compared to the periphery, depend on the orientation of the object and vary as a function of kVp and voxel size.