Dual-energy CT is emerging as a dose-saving tool for coronary CT angiography that allows calcium-scoring without the
need for a separate unenhanced scan acquisition. Unfortunately the similar attenuation coefficient profiles of iodine and
calcium limits the accuracy of their decomposition in the material basis images. We evaluate a tungsten-based contrast
material with a more distinct attenuation profile from calcium, and compare its performance to a conventional iodinated
agent. We constructed a custom thorax phantom containing simulated sets of vessels 3, 6 and 9 mm in diameter. The
vessel sets were walled with concentric and eccentric calcifications (“plaque”) with concentrations of 0, 20, 30 and 40%
weight calcium hydroxyapatite (HAP). The phantom was filled sequentially with iodine and tungsten contrast material,
and scanned helically using a fast-kV-switching DECT scanner. At material decomposition, both iodine and tungsten
vessel lumens were separable from the HAP vessel walls, but separation was superior with tungsten which showed
minimal false positive signal in the HAP image. Assessing their relative performance using line profiles, the HAP signal
was greater in the tungsten separation in 6/9 of the vessel sets, and within 15% of the iodine separation for the remaining
3/9 sets. The robust phantom design enabled systematic evaluation of dual-energy material separation for calcium and a
candidate non-iodinated vascular contrast element. This approach can be used to screen further agents and also refine
dual energy CT material decomposition approaches.
Metal artifacts have been a problem associated with computed tomography (CT) since its introduction. Recent techniques to mitigate this problem have included utilization of high-energy (keV) virtual monochromatic spectral (VMS) images, produced via dual-energy CT (DECT). A problem with these high-keV images is that contrast enhancement provided by all commercially available contrast media is severely reduced. Contrast agents based on higher atomic number elements can maintain contrast at the higher energy levels where artifacts are reduced. This study evaluated three such candidate elements: bismuth, tantalum, and tungsten, as well as two conventional contrast elements: iodine and barium. A water-based phantom with vials containing these five elements in solution, as well as different artifact-producing metal structures, was scanned with a DECT scanner capable of rapid operating voltage switching. In the VMS datasets, substantial reductions in the contrast were observed for iodine and barium, which suffered from contrast reductions of 97% and 91%, respectively, at 140 versus 40 keV. In comparison under the same conditions, the candidate agents demonstrated contrast enhancement reductions of only 20%, 29%, and 32% for tungsten, tantalum, and bismuth, respectively. At 140 versus 40 keV, metal artifact severity was reduced by 57% to 85% depending on the phantom configuration.
Image artifacts generated by metal implants have been a problem associated with CT since its introduction. Recent techniques to mitigate this problem have included the utilization of certain Dual-Energy CT (DECT) features. DECT can produce virtual monochromatic spectral (VMS) images, simulating how the data would appear if scanned at a single x-ray energy (keV). High-keV VMS images can greatly reduce the severity of metal artifacts. A problem with these high-keV images is that contrast enhancement provided by all commercially-available contrast media is severely reduced. It is therefore impossible to generate VMS images with simultaneous high contrast and minimized metal artifact severity. Novel contrast agents based on higher atomic number elements can maintain contrast enhancement at the higher energy levels where artifacts are reduced. This study evaluated three such candidate elements: bismuth, tantalum, and tungsten, as well as two conventional contrast elements: iodine and barium. A water-based phantom with vials containing these five elements in solution, as well as different artifact-producing metal structures, was scanned with a DECT scanner capable of rapid operating voltage switching. In the VMS datasets, substantial reductions in the contrast were observed for iodine and barium, which suffered from contrast reduction of 97 and 91% respectively at 140 versus 40 keV. In comparison under the same conditions, the novel candidate agents demonstrated contrast enhancement reductions of only 20, 29 and 32% for tungsten, tantalum and bismuth respectively. At 140 versus 40 keV, metal artifact severity was reduced by 57-85% depending on the phantom configuration.
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