Abstract
ObjectiveTo evaluate the dual-energy (DE) performance and spectral separation with respect to iodine imaging in a photon-counting CT (PCCT) and compare it to dual-source CT (DSCT) DE imaging.MethodsA semi-anthropomorphic phantom extendable with fat rings equipped with iodine vials is measured in an experimental PCCT. The system comprises a PC detector with two energy bins (20 keV, T) and (T, eU) with threshold T and tube voltage U. Measurements using the PCCT are performed at all available tube voltages (80 to 140 kV) and threshold settings (50–90 keV). Further measurements are performed using a conventional energy-integrating DSCT. Spectral separation is quantified as the relative contrast media ratio R between the energy bins and low/high images. Image noise and dose-normalized contrast-to-noise ratio (CNRD) are evaluated in resulting iodine images. All results are validated in a post-mortem angiography study.ResultsR of the PC detector varies between 1.2 and 2.6 and increases with higher thresholds and higher tube voltage. Reference R of the EI DSCT is found as 2.20 on average overall phantoms. Maximum CNRD in iodine images is found for T = 60/65/70/70 keV for 80/100/120/140 kV. The highest CNRD of the PCCT is obtained using 140 kV and is decreasing with decreasing tube voltage. All results could be confirmed in the post-mortem angiography study.ConclusionIntrinsically acquired DE data are able to provide iodine images similar to conventional DSCT. However, PCCT thresholds should be chosen with respect to tube voltage to maximize image quality in retrospectively derived image sets.Key Points• Photon-counting CT allows for the computation of iodine images with similar quality compared to conventional dual-source dual-energy CT.• Thresholds should be chosen as a function of the tube voltage to maximize iodine contrast-to-noise ratio in derived image sets.• Image quality of retrospectively computed image sets can be maximized using optimized threshold settings.
Highlights
Over the last decades, computed tomography (CT) has proven to be an invaluable tool in clinical diagnostics
Photon-counting CT allows for the computation of iodine images with similar quality compared to conventional dual-source dual-energy CT
Thresholds should be chosen as a function of the tube voltage to maximize iodine contrast-to-noise ratio in derived image sets
Summary
Over the last decades, computed tomography (CT) has proven to be an invaluable tool in clinical diagnostics. In the case of two energy bins, a single threshold is used and photons with energy below this threshold are sorted to the lower bin and photons with energy above this threshold are sorted to the upper bin This threshold can usually be placed almost arbitrarily with respect to the detected x-ray spectrum and influences the image quality of derived image sets, e.g., iodine maps or VMIs. Since the detectors always acquire at least two energy bins, spectral data are available even if no dedicated dual-energy acquisition was performed. An experimental CT system equipped with a prototype photon-counting detector is used in all studies presented here This detector has already proven to allow for dual- and multi-energy decomposition of conventional and potential novel contrast agents, to provide iodine quantification with high accuracy, and to allow for the computation of VMIs, among others, and several contrast-enhanced patient studies have been published [14,15,16,17,18,19,20]. Since the ionizing nature of x-rays prohibits multiple examinations of patients, the results will be verified in a post-mortem CT angiography study
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