Cone-beam CT images of a patient with a pre-defined distribution of noise in the image and dose to the patient can be
accomplished through the development of advanced compensation schemes. Such compensation schemes involve
delivery of x-ray fluence patterns that vary in intensity both across a single projection image (u,v) and for different
projection view angles (θ) and provide the ability to perform intensity-modulated cone-beam CT. Implementation of an
intensity-modulated cone-beam CT system for task-specific imaging has potential for tremendous reductions in patient
dose and x-ray scatter reaching the detector. Pursuing this advanced imaging technique requires detailed characterization
of the cone-beam CT platform. Determination of appropriately modulated fluence patterns relies on knowledge of
numerous properties of the imaging system, including the constraints imposed by the modulator, the magnitude of x-ray
scatter under different patient sizes and modulator positions, and properties of the detector. With an estimate of the
patient anatomy and knowledge of the imaging system, an iterative process can be used to determine modulated fluence
patterns corresponding to an image prescribed for the specific task and patient. Delivery of such modulated fluence
patterns provide a CBCT image tailored to a specific patient and imaging task offering the optimum balance between
image quality and patient dose. Specifically, arbitrary regions of interest requiring high image signal-to-noise ratio can
be generated through knowledgeable spatio-angular intensity modulation, allowing image quality to degrade in other
regions in order to minimize x-ray scatter and imaging dose.
In this work Monte Carlo (MC) simulations are used to correct kilovoltage (kV) cone-beam computed tomographic (CBCT) projections for scatter radiation. All images were acquired using a kV CBCT bench-top system composed of an x-ray tube, a rotation stage and a flat-panel imager. The EGSnrc MC code was used to model the system. BEAMnrc was used to model the x-ray tube while a modified version of the DOSXYZnrc program was used to transport the particles through various phantoms and score phase space files with identified scattered and primary particles. An analytical program was used to read the phase space files and produce image files. The scatter correction was implemented by subtracting Monte Carlo predicted scatter distribution from measured projection images; these projection images were then reconstructed. Corrected reconstructions showed an important improvement in image quality. Several approaches to reduce the simulation time were tested. To reduce the number of simulated scatter projections, the effect of varying the projection angle on the scatter distribution was evaluated for different geometries. It was found that the scatter distribution does not vary significantly over a 30-degree interval for the geometries tested. It was also established that increasing the size of the voxels in the voxelized phantom does not affect the scatter distribution but reduces the simulation time. Different techniques to smooth the scatter distribution were also investigated.
Cone-beam computed tomography (CBCT) presents a highly promising and challenging advanced application of flat-panel detectors (FPDs). The great advantage of this adaptable technology is in the potential for sub-mm 3D spatial resolution in combination with soft-tissue detectability. While the former is achieved naturally by CBCT systems incorporating modern FPD designs (e.g., 200 - 400 um pixel pitch), the latter presents a significant challenge due to limitations in FPD dynamic range, large field of view, and elevated levels of x-ray scatter in typical CBCT configurations. We are investigating a two-pronged strategy to maximizing soft-tissue detectability in CBCT: 1) front-end solutions, including novel beam modulation designs (viz., spatially varying compensators) that alleviate detector dynamic range requirements, reduce x-ray scatter, and better distribute imaging dose in a manner suited to soft-tissue visualization throughout the field of view; and 2) back-end solutions, including implementation of an advanced FPD design (Varian PaxScan 4030CB) that features dual-gain and dynamic gain switching that effectively extends detector dynamic range to 18 bits. These strategies are explored quantitatively on CBCT imaging platforms developed in our laboratory, including a dedicated CBCT bench and a mobile isocentric C-arm (Siemens PowerMobil). Pre-clinical evaluation of improved soft-tissue visibility was carried out in phantom and patient imaging with the C-arm device. Incorporation of these strategies begin to reveal the full potential of CBCT for soft-tissue visualization, an essential step in realizing broad utility of this adaptable technology for diagnostic and image-guided procedures.