A method is presented for accurately simulating the effects of dose reduction in x-ray computed tomography (CT) by adding synthetic noise to raw projection data. A model for realistic noise in projection data was generated, incorporating the mechanisms of stochastic noise in energy-integrating x-ray detectors, electronic system noise, and bowtie beam filtering (used for patient dose reduction). Parameters for the model were extracted from phantom measurements on a variety of clinical CT scanners (helical single row, four-row, and 16-row). Dose reduction simulations were performed by adding synthetic noise based on the noise model to raw data acquired from clinical scanners. Qualitative and quantitative validation of the process was accomplished by comparing phantom scans acquired under high and low dose conditions with simulated imagery. The importance of including alternative noise mechanisms (bowtie filter and system noise) was demonstrated. Henceforth, scans of clinical patients were acquired using conventional protocols; through simulations, image sets were presented at a variety of lower dose procedures. The methodology promises to be a useful tool for radiologists to explore dose reduction protocols in an effort to produce diagnostic images with radiation dose “as low as reasonably achievable”.
ROI imaging techniques can improve image quality and reduce radiation dose to patient and staff when the optimal combination of filter material and image receptor is used. An investigation was conducted to evaluate the effect of thickness of a-Se (0.5 mm and 1.0 mm) on image contrast, contrast to noise ratio (CNR), and figure of merit (FOM) with or without ROI filters (various thickness' of Gd and Cu) and to compare the results with the corresponding values obtained using a 0.4 mm thick cesium iodide (CsI) image receptor. Simulated x-ray spectra and published attenuation coefficients were used to calculate the x-ray transmission through a broad range of thickness' of various contrast materials for beams of 50 to 100 kVp. The results indicate that a-Se provides substantially better contrast compared to CsI for barium and iodinated contrast media for all cases, especially when the combination of Gd filter and the thinner a-Se is used. Moreover, during ROI procedures, the thicker a-Se generally increases image contrast, CNR, and FOM compared to CsI. Although, the thinner a-Se provides the highest image contrast for all cases, its combination with Cu results in lower CNR and FOM at higher kVp's compared to CsI.