Tungsten-Based Contrast Agent for Photon-Counting Detector CT Angiography in Calcified Coronaries: Comparison to Iodine in a Cardiovascular Phantom

Abstract

Objectives: Calcified plaques induce blooming artifacts in coronary computed tomography angiography (CCTA) potentially leading to inaccurate stenosis evaluation. Tungsten represents a high atomic number, experimental contrast agent with different physical properties than iodine. We explored the potential of a tungsten-based contrast agent for photon-counting detector (PCD) CCTA in heavily calcified coronary vessels.

Materials and methods: A cardiovascular phantom exhibiting coronaries with calcified plaques was imaged on a first-generation dual-source PCD-CT. The coronaries with 3 different calcified plaques were filled with iodine and tungsten contrast media solutions equating to iodine and tungsten delivery rates (IDR and TDR) of 0.3, 0.5, 0.7, 1.0, 1.5, 2.0, 2.5, and 3.0 g/s, respectively. Electrocardiogram-triggered sequential acquisitions were performed in the spectral mode (QuantumPlus). Virtual monoenergetic images (VMIs) were reconstructed from 40 to 190 keV in 1 keV increments. Blooming artifacts and percentage error stenoses from calcified plaques were quantified, and attenuation characteristics of both contrast media were recorded.

Results: Blooming artifacts from calcified plaques were most pronounced at 40 keV (78%) and least pronounced at 190 keV (58%). Similarly, percentage error stenoses were highest at 40 keV (48%) and lowest at 190 keV (2%), respectively. Attenuation of iodine decreased monotonically in VMIs from low to high keV, with the strongest decrease from 40 keV to 100 keV (IDR of 2.5 g/s: 1279 HU at 40 keV, 187 HU at 100 kV, and 35 HU at 190 keV). The attenuation of tungsten, on the other hand, increased monotonically as a function of VMI energy, with the strongest increase between 40 and 100 keV (TDR of 2.5 g/s: 202 HU at 40 keV, 661 HU at 100 kV, and 717 HU at 190 keV). For each keV level, the relationship between attenuation and IDR/TDR could be described by linear regressions ( R2 ≥ 0.88, P < 0.001). Specifically, attenuation increased linearly when increasing the delivery rate irrespective of keV level or contrast medium. Iodine exhibited the highest relative increase in attenuation values at lower keV levels when increasing the IDR. Conversely, for tungsten, the greatest relative increase in attenuation values occurred at higher keV levels when increasing the TDR. When high keV imaging is desirable to reduce blooming artifacts from calcified plaques, IDR has to be increased at higher keV levels to maintain diagnostic vessel attenuation (ie, 300 HU), whereas for tungsten, TDR can be kept constant or can be even reduced at high keV energy levels.

Conclusions: Tungsten's attenuation characteristics in relation to VMI energy levels are reversed to those of iodine, with tungsten exhibiting high attenuation values at high keV levels and vice versa. Thus, tungsten shows promise for high keV imaging CCTA with PCD-CT as-in distinction to iodine-both high vessel attenuation and low blooming artifacts from calcified plaques can be achieved.

FIGURE 1

From left to right: image of full body phantom; thorax and abdomen of the phantom including the heart model; image of the coronary vessels laid on top of the heart model including yellow arrows pointing to the calcified lesions within the coronary model. Images courtesy of United Biologics Inc, Santa Ana, CA.

FIGURE 2

Overview of blooming artifacts (A) and percentage error stenoses (B) stratified by keV energy level.

FIGURE 3

Overview of attenuation characteristics of iodine and tungsten at various delivery rates.

FIGURE 4

Overview of attenuation characteristics of iodine and tungsten at a delivery rate of 2.5 g/s for a coronary vessel with a calcified plaque. VMIs are (40 to 190 keV) shown in 30 keV increments. Both contrast media and all keV levels are displayed with window settings centered at 500 HU and width 1700 HU. Note the increased attenuation of iodine at lower keV levels and the increased attenuation of tungsten at higher keV levels as shown schematically by means of overlaid attenuation curves. In addition, decreasing blooming artifacts can be seen when moving from lower to higher keV levels.

FIGURE 5

Overview of the relationship among attenuation, VMI energy level, and contrast medium delivery rate (IDR/TDR). Exemplarily, a range of VMI energy levels spanning from 40 to 190 keV are shown. Linear regression equations for each keV level and contrast medium allow for the computation of the required delivery rate to achieve a specific attenuation.

FIGURE 6

Relationship between VMI energy level and delivery rate for iodine and tungsten to achieve an attenuation of 300 HU in the vessel lumen.