Energy-resolving x-ray detectors may enable producing iodine-specific images of the coronary arteries without the presence of motion artifacts. We refer to this approach as energy-resolved angiography (ERA), which uses basis material decomposition to produce iodine-specific images. We compared the theoretical iodine pixel signal-to- noise ratio (SNR) and the zero-frequency SNR of ERA with that of conventional digital subtraction angiography (DSA), the latter of which produces iodine-specific images by subtracting images acquired before and after iodine injection. For both ERA and DSA, we modeled iodine SNR with and without the response of realistic x-ray detectors. For ERA, we used a validated model of the energy response of a cadmium zinc telluride (CZT) spectroscopic x-ray detector to account for spectral degradation and spatio-energetic cross talk due to charge sharing. For DSA, we modeled the response of a cesium-iodine (CsI)-based detector and validated our model by comparison with published data. Incorporating a realistic energy response for spectroscopic x-ray detectors decreased the pixel SNR and zero-frequency SNR by greater than a factor of two. In the case of DSA, optical blur in the scintillator increased iodine SNR relative to ideal systems, a result attributable to reduced high-frequency noise in the presence of optical blur. Our results suggest that, for the same patient x-ray exposure, the pixel SNR and zero-frequency SNR of ERA will be ~1/6 and ~1/3 of that DSA, respectively.
We propose two-dimensional (2D) dual-energy (DE) x-ray imaging of lung structure and function for the assessment of COPD, and investigate the resulting image quality theoretically using the human observer detectability index (d') as a figure of merit. We modeled the ability of human observers to detect ventilation defects in xenon enhanced DE (XeDE) images and emphysema in unenhanced DE images. Our model of d' accounted for the extent of emphysematous destruction and functional impairment as a function of defect/lesion contrast, spatial resolution, x-ray scatter, quantum and background anatomical noise power spectrum (NPS), and the efficiency of human observers. The effect of x-ray spectrum and exposure allocation factor on d' was also explored. Our results suggest that, the detectability is maximized for exposure allocation factors that minimize quantum NPS. The optimal combination of tube voltage was found to be ~50/140 kV or 60/140 kV depending on the task and patient at an x-ray exposure equal to that of a standard chest x-ray. In 2D DE x-ray imaging of COPD, the detectability is primarily limited by low contrast, x-ray scatter, and anatomic noise, the latter two of which reduce the detectability of individual defects by 30% and ~>90%, respectively.
Halide lead perovskites have been proposed for direct-conversion x-ray imaging because of their high stopping power, high charge mobility and high bulk resistivity. We modeled the detective quantum efficiency (DQE) of methylammonium lead iodide (MAPbI3) and compared with that of amorphous selenium (a-Se) and columnar cesium iodide (CsI). For CsI, we calculated the DQE for RQA-5, RQA- 7 and RQA-9 x-ray spectra for 200 µm detectors elements; for a-Se we calculated the DQE for MMA 28 x-ray spectrum for 75 µm elements. Our DQE model included the quantum efficiency, x-ray fluorescence, fluorescence reabsorption, charge conversion, collection of secondary quanta (i.e. charges or optical photons), charge diffusion in MAPbI3, optical blur in CsI, noise aliasing, and electronic noise. The model DQE of CsI was compared with published data; the model photoelectric noise power spectrum of lead was compared with published Monte Carlo data; there was excellent agreement. For fluoroscopic applications, the theoretical DQE of MAPbI3 was approximately equal to that of CsI at exposures of 10µR per image, but was ~40% lower than CsI at an exposure of 0.1µR per image. This result is due to the relatively high levels of electronic noise present in prototype MAPbI3 systems. For chest radiography applications, the theoretical DQE of MAPbI3 was 25% greater than that of CsI at typical exposure levels (i.e. 0.04mR 3mR at the detector). For mammography, the theoretical DQE of MAPbI3 was ~5% greater than that of a-Se across all spatial frequencies and all exposures between 0.6mR 250mR. These results suggest that halide lead perovskites may provide superior dose efficiency than CsI-based systems in chest radiography applications, but may offer little to no improvement in mammographic or fluoroscopic applications.
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