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Noise analysis in intensity-weighted region-of-interest imaging for cone-beam CT

We have previously introduced an intensity-weighted region-of-interest (IWROI) technique for image-guided radiation therapy (IGRT), where the outer ROI receives lower exposure than the inner ROI by use of intensity weighting (IW-) filters during the scan. Image noise level, as a result, in the outer ROI will be higher than that in the inner ROI after reconstruction. CBCT images are to be registered to planning CT images and there exist a number of imageregistration algorithms, some of which are relatively more robust against image noise. The IW-filters should be designed so that the outer ROI images have noise levels slightly less than the tolerance of the registiration algorithm resulting in maximum reduction to patient exposure. Therefore, a proper study can guide the filter design based on noise characteristics of the image reconstruction. As a part of the study, we investigated the noise propagation from the projections to the reconstructed image in this work. Also, we obtained the experimental noise characteristics of projections for varying exposure, simulated a pelvis imaging using the NCAT phantom, and performed an image registration of the reconstructed image to the phantom image.