Cone-beam computed tomography with a flat-panel imager: Magnitude and effects of x-ray scatter

Abstract

A system for cone-beam computed tomography (CBCT) based on a flat-panel imager (FPI) is used to examine the magnitude and effects of x-ray scatter in FPI-CBCT volume reconstructions. The system is being developed for application in image-guided therapies and has previously demonstrated spatial resolution and soft-tissue visibility comparable or superior to a conventional CT scanner under conditions of low x-ray scatter. For larger objects consistent with imaging of human anatomy (e.g., the pelvis) and for increased cone angle (i.e., larger volumetric reconstructions), however, the effects of x-ray scatter become significant. The magnitude of x-ray scatter with which the FPI-CBCT system must contend is quantified in terms of the scatter-to-primary energy fluence ratio (SPR) and scatter intensity profiles in the detector plane, each measured as a function of object size and cone angle. For large objects and cone angles (e.g., a pelvis imaged with a cone angle of 6 degrees), SPR in excess of 100% is observed. Associated with such levels of x-ray scatter are cup and streak artifacts as well as reduced accuracy in reconstruction values, quantified herein across a range of SPR consistent with the clinical setting. The effect of x-ray scatter on the contrast, noise, and contrast-to-noise ratio (CNR) in FPI-CBCT reconstructions was measured as a function of SPR and compared to predictions of a simple analytical model. The results quantify the degree to which elevated SPR degrades the CNR. For example, FPI-CBCT images of a breast-equivalent insert in water were degraded in CNR by nearly a factor of 2 for SPR ranging from approximately 2% to 120%. The analytical model for CNR provides a quantitative understanding of the relationship between CNR, dose, and spatial resolution and allows knowledgeable selection of the acquisition and reconstruction parameters that, for a given SPR, are required to restore the CNR to values achieved under conditions of low x-ray scatter. For example, for SPR = 100%, the CNR in FPI-CBCT images can be fully restored by: (1) increasing the dose by a factor of 4 (at full spatial resolution); (2) increasing dose and slice thickness by a factor of 2; or (3) increasing slice thickness by a factor of 4 (with no increase in dose). Other reconstruction parameters, such as transaxial resolution length and reconstruction filter, can be similarly adjusted to achieve CNR equal to that obtained in the scatter-free case.

Optimizing the target detectability of cone beam CT performed in image‐guided radiation therapy for patients of different body sizes

“…Scatter is a very important artifact causing factor in CBCT. The scatter‐to‐primary ratio (SPR) is about 0.01 for single‐ray CT and 0.05–0.15 for fan‐beam and spiral CT, and may be as large as 0.4–2.0 in CBCT 10, 11, 12, 13, 29, 30, 31. Typical scatter artifacts show as shading or streaks, which would result in reduced contrast resolution and increased noise in CBCT imaging.…”

“…During daily treatment, cone beam CT (CBCT) mounted on the gantry of linear accelerator is used to detect target position relative to the planned radiation beams to improve the accuracy of treatment delivery through geometric corrections. Target detectability of CBCT is a very important image quality metric to achieve a high level of patient positioning and treatment accuracy 10, 11, 12, 13. In spite of the increasing use of CBCT to verify and correct patient setup, the contrast resolution of CBCT in delineating soft tissue structures is lower than that of MDCT 14, 15.…”

Flat-detector computed tomography (FD-CT)

“…So-called data truncation artefacts result, which influence the CT-value accuracy and disturb the diagnostic quality of the images. Correction algorithms [31,32] allow to restore The increased collimation in the z-direction results in increased scatter fractions [33][34][35]. This will induce severe cupping artefacts, i.e.…”

CTmod—A toolkit for Monte Carlo simulation of projections including scatter in computed tomography

“…to make sure that codes based on the toolkit are applicable in practice, scatter-to-primary ratios of exposure in [40] were compared to data simulated using a CTmod based code. A notable discrepancy was found, see [41].…”