Purpose: To develop a fast‐converging SART‐type algorithm and show clinical feasibility in CBCT reconstructions by combining the algorithmic enhancements with a parallel computing hardware (GPU). Methods: SART reconstructs a volumetric image by iteratively conducting volume projection and correction backprojection. However, this reconstruction problem can also be cast as a least squares problem for minimizing the volume projection errors with respect to the scanner projection data. This way, SART can be viewed as equivalent to a gradient method for minimizing the quadratic objective function f(x), with a fixed step‐size. Novelty in this work is that we implemented a simple yet much faster algorithm by computing a unique step‐size at each iteration. We applied this variable step‐size (VS)‐SART algorithm to numerical and physical phantoms for reconstruction. Furthermore, we accelerated the reconstruction by implementing the algorithm on NVIDIA GTX 295 GPU card. CBCT projections of CatPhan phantom were acquired from the Varian TrueBeam system. Results: We first compared SART and VS‐SART using Shepp‐Logan numerical phantom with 180 parallel‐beam projections. As the iterations progress, f(x) is asymptotically minimized for both algorithms but VS‐SART is found to converge much faster. Therefore, for a fixed number of iterations, VS‐SART commands superior image quality. In addition, compared with the FDK algorithm with 364 projections, our VS‐SART algorithm produces visibly equivalent quality CBCT image for CatPhan phantom with only 120 projections, in 12 iterations completed in 33 seconds. This is a factor of three dose reduction while maintaining the reconstruction time acceptable. Conclusions: By approaching SART reconstruction problem from a gradient method perspective, we enhanced the reconstruction speed significantly (i.e., less number of iterations). In addition, with GPU acceleration, the overall reconstruction time is achieved within a clinically viable range. We anticipate that the VS‐SART algorithm can be applied to CT, PET, and SPECT also.
Purpose: The actual delivered dose to a moving tumor can deviate from prescribed dose not only during each fraction, but during each IMRT field. This deviation can be anticipated and incorporated into a treatment plan if the tumor specific motion probability density function (PDF) is accurately identified. In this study, a novel technique in determining the actual, tumor specific PDF of a moving gross tumor volume (GTV) is described and confirmed through phantom experiments using a quantitative approach of 4DCT imaging. Methods and Materials: We hypothesize that a PDF of the GTV can be obtained through weighted sums of phase‐sorted 4DCT images. Experimental validation is performed using a lung phantom attached to a programmable motion platform. The ground truth motion PDF of the targets was calculated through convolution of the motion pattern and the GTV dimension along the direction of motion. These PDF convolutions are compared to the normalized, reconstructed CT numbers obtained from averaging ten phase sorted 4DCT images after background subtraction. Results: The PDF reconstructed from averaged, weighted sets of 4DCT images was almost identical to the ground truth PDF in all comparisons made. Through sums of phase sorted 4DCT images, the resulting CT number represents a relative probability of finding some portion of the GTV in each geometric location (or voxel). In physical phantom reconstructions, the Pearson correlation coefficient between CT‐based PDF and the ground truth PDF was greater than .97 in all cases considered. An actual patient PDF was also evaluated in frontal and sagittal planes. Conclusions: Combining phase sorted CT images is an effective method in obtaining the actual tumor PDF as seen by an IMRT field. With the patient specific PDF, treatment planning to ensure proper target coverage can be achieved through intra‐field modulation, dose rate, and monitoring initial beam‐on phase.
We found that imaging dose did not have much impact on the visibility of the ITV volume, irrespective of the amplitude, I/E ratio, or period. Thus, it seems that low-dose FBCBCT may be just as suitable for clinical use while sparing a significant imaging dose to the patients.
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