Assessment of image quality of two cone-beam computed tomography of the Varian Linear accelerators: Comparison with spiral CT simulator

To assess and compare image quality characteristics of x-ray computed tomography (CT) and cone beam (CBCT) imaging systems of the Varian linear accelerator.

The CatPhan®504, was examined on the CT simulator (SOMATOM Definition AS, VA48A) and two CBCTs (TrueBeam™ and Clinac® iX linear accelerators) attached to Varian linear accelerator. Image quality parameters including pixel value stability, spatial linearity, pixel size verification, uniformity, noise, spatial resolution, low contrast resolution, and contrast-to-noise ratio (CNR) were assessed using different scanning protocols.

The mean pixel values of regions of interest were stable for CT, TB, and iX-CBCT imaging. Noise on CT was slightly lower and was seen to decrease with increasing mAs, while CNR increased with CT mAs and two CBCTs. For all schemes, the Modulation Transfer Function (MTF) of the reconstructed image was limited by the pixel size. Low contrast targets for TB-CBCT were visible, with up to 6 and 2 targets for 1% and 0.5% for contrast, respectively. However, up to 4 targets of 1% contrast on iX-CBCT images are visible for the low-contrast objectives. Also, up to 8, 4, and 1 targets of 1%, 0.5%, and 0.3% contrast were visible for the low-contrast targets on CT images.

CT and CBCT image quality parameters have been quantified and compared for clinical protocols in different mAs conditions. Selecting the right protocol will boost contrast, based on image quality criteria. The mAs can be decreased to minimize patient dosage.