Image interpretation is crucial during ultrasound image acquisition. A skilled operator is typically needed to
verify if the correct anatomical structures are all visualized and with sufficient quality. The need for this
operator is one of the major reasons why presently ultrasound is not widely used in radiotherapy workflows.
To solve this issue, we introduce an algorithm that uses anatomical information derived from a CT scan to
automatically provide the operator with a patient-specific ultrasound probe setup. The first application we
investigated, for its relevance to radiotherapy, is 4D transperineal ultrasound image acquisition for prostate
As initial test, the algorithm was applied on a CIRS multi-modality pelvic phantom. Probe setups were
calculated in order to allow visualization of the prostate and adjacent edges of bladder and rectum, as
clinically required. Five of the proposed setups were reproduced using a precision robotic arm and ultrasound
volumes were acquired. A gel-filled probe cover was used to ensure proper acoustic coupling, while taking
into account possible tilted positions of the probe with respect to the flat phantom surface.
Visual inspection of the acquired volumes revealed that clinical requirements were fulfilled. Preliminary
quantitative evaluation was also performed. The mean absolute distance (MAD) was calculated between
actual anatomical structure positions and positions predicted by the CT-based algorithm. This resulted in a
MAD of (2.8±0.4) mm for prostate, (2.5±0.6) mm for bladder and (2.8±0.6) mm for rectum. These results
show that no significant systematic errors due to e.g. probe misplacement were introduced.
X-ray scatter has a significant impact on image quality in kV cone-beam CT (CBCT), its effects include: CT number inaccuracy, streak and cupping artifacts, and loss of contrast. Compensators provide a method for not only decreasing the magnitude of the scatter distribution, but also reducing the structure found in the scatter distribution. Recent Monte Carlo (MC) simulations examining X-ray scatter in CBCT projection images have shown that the scatter distribution in x-ray imaging contains structure largely induced by coherent scattering. In order to maximize the reduction of x-ray scatter induced artifacts a decrease in the magnitude and structure of the scatter distribution is sought through optimal compensator design. A flexible MC model that allows for separation of scattered and primary photons has been created to simulate the CBCT imaging process. The CBCT MC model is used to investigate the effectiveness of compensators in decreasing the magnitude and structure of the scatter distribution in CBCT projection images. The influence of the compensator designs on the scatter distribution are evaluated for different anatomy (abdomen, pelvis, and head and neck) and viewing angles using a voxelized anthropomorphic phantom. The effect of compensator material composition on the amount of contamination photons in an open field is also investigated.
Sonography has good topographic accuracy for superficial lymph node assessment in patients with head and neck
cancers. It is therefore an ideal non-invasive tool for precise inter-fraction volumetric analysis of enlarged cervical
nodes. In addition, when registered with computed tomography (CT) images, ultrasound information may improve target
volume delineation and facilitate image-guided adaptive radiation therapy. A feasibility study was developed to evaluate
the use of a prototype ultrasound system capable of three dimensional visualization and multi-modality image fusion for
cervical node geometry. A ceiling-mounted optical tracking camera recorded the position and orientation of a transducer
in order to synchronize the transducer's position with respect to the room's coordinate system. Tracking systems were
installed in both the CT-simulator and radiation therapy treatment rooms. Serial images were collected at the time of
treatment planning and at subsequent treatment fractions. Volume reconstruction was performed by generating surfaces
around contours. The quality of the spatial reconstruction and semi-automatic segmentation was highly dependent on the
system's ability to track the transducer throughout each scan procedure. The ultrasound information provided enhanced
soft tissue contrast and facilitated node delineation. Manual segmentation was the preferred method to contour structures
due to their sonographic topography.
In external-beam radiotherapy efforts are currently devoted to research on image-guided verification techniques. In brachytherapy the situation is far less advanced; usually, there is no treatment verification imaging. We are studying the possibility to use the photons emitted from a conventional 192Ir brachytherapy source for High Dose Rate (HDR) treatments, when inserted in a patient. We investigated whether the images can be used for dose delivery verification and to interrupt faulty dose deliveries. Phantoms were built to accommodate a remote controlled HDR 192Ir source. Images were collected with an x-ray intensifier, and predicted from calculations based on ray-tracing. For a bone/tissue/air/lung phantom with the source on top of the phantom measured contrasts were 8% (bone/tissue), 19% (tissue/lung) and 26% (lung/bone). When a thick Lucite slab was added on top of the contrast phantom, the contrasts decreased to 3, 7 and 10%, respectively, indicating that phantom scatter is an important issue. Differences between measured and simulated images and the influence of scatter were quantified. From this feasibility study it is concluded that imaging with 192Ir photons is possible but that work on scatter rejection through simulation and anti-scatter grids is needed.
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.