A method to correct the influence of carbon fiber couchtop and patient positioning device on image quality of cone beam CT

Assessment of scatter for the micro-CT subsystem of the trimodality FLEX Triumph preclinical scanner

PURPOSE

This work aims at assessing, through experimental measurements and Monte Carlo calculations, the scatter to primary ratio (SPR) for the micro-CT subsystem of the FLEX Triumph preclinical PET-CT scanner to improve its quantitative capabilities.

METHODS

Experimental measurements were carried out using the single blocker method, where five cylindrical blockers with diameters ranging between 3 and 11.65 mm were used to assess the SPR without the blocker through interpolation. Because of the vertical layout of the imaging device, the blocker was placed over rat-sized and mouse-sized phantoms and central and peripheral SPR values were obtained by rotating the source and detector. The influence of beam energy (30, 50, and 80 kVp), geometrical magnification (1.3 and 2.0) and phantom diameter (25 and 50 mm) and density (polyethylene and water) were investigated. Monte Carlo (MC) simulations using the MCNP4C code were also performed and compared to experimental results to validate their accuracy.

RESULTS

The highest difference was found in the extreme peripheral region of the small phantom, while the maximum difference at the center of the phantom is about 6%, indicating that MC simulations can reproduce well the experimental results, at least in the region inside the phantom. The maximal SPR (0.562) was obtained for the large phantom at 30 kVp and a magnification of 1.3. The full SPR profile was calculated using MC simulations and used to express its dependency on beam energy (quadratic), air gap (asymptotic), and phantom diameter (quadratic).

CONCLUSIONS

The obtained results are in good agreement with theoretical predictions. MC simulations were valuable for the evaluation of the influence of various acquisition parameters on the SPR estimates.

Correction for patient table-induced scattered radiation in cone-beam computed tomography (CBCT)

PURPOSE

In image-guided radiotherapy, an artifact typically seen in axial slices of x-ray cone-beam computed tomography (CBCT) reconstructions is a dark region or "black hole" situated below the scan isocenter. The authors trace the cause of the artifact to scattered radiation produced by radiotherapy patient tabletops and show it is linked to the use of the offset-detector acquisition mode to enlarge the imaging field-of-view. The authors present a hybrid scatter kernel superposition (SKS) algorithm to correct for scatter from both the object-of-interest and the tabletop.

METHODS

Monte Carlo simulations and phantom experiments were first performed to identify the source of the black hole artifact. For correction, a SKS algorithm was developed that uses separate kernels to estimate scatter from the patient tabletop and the object-of-interest. Each projection is divided into two regions, one defined by the shadow cast by the tabletop on the imager and one defined by the unshadowed region. The region not shadowed by the tabletop is processed using the recently developed fast adaptive scatter kernel superposition (fASKS) method which employs asymmetric kernels that best model scatter transport through bodylike objects. The shadowed region is convolved with a combination of slab-derived symmetric SKS kernels and asymmetric fASKS kernels. The composition of the hybrid kernels is projection-angle-dependent. To test the algorithm, pelvis phantom and in vivo data were acquired using a CBCT test stand, a Varian Acuity simulator, and a Varian On-Board Imager, all of which have similar geometries and components. Artifact intensities and Hounsfield unit (HU) accuracies in the reconstructions were assessed before and after the correction.

RESULTS

The hybrid kernel algorithm provided effective correction and produced substantially better scatter estimates than the symmetric SKS or asymmetric fASKS methods alone. HU nonuniformities in the reconstructed pelvis phantom were reduced from 220 to 50 HU (i.e., 22%-5%). In the in vivo scans, the black hole artifact was reduced by up to 147 HU, a 73% improvement, and anatomical details in the prostate and rectum areas were made considerably more visible.

CONCLUSIONS

Radiotherapy tabletops, which are generally flatter and larger than those used for diagnostic CT, can produce significant scatter-related artifacts. The proposed hybrid SKS algorithm accurately estimates scatter from both the object-of-interest and the patient tabletop, and resulting image uniformities and HU accuracies are greatly improved.