Photon-counting detector coronary CT angiography: impact of virtual monoenergetic imaging and iterative reconstruction on image quality

Abstract

Objectives: To assess the impact of low kilo-electronvolt (keV) virtual monoenergetic image (VMI) energies and iterative reconstruction on image quality of clinical photon-counting detector coronary CT angiography (CCTA).

Methods: CCTA with PCD-CT (prospective ECG-triggering, 120 kVp, automatic tube current modulation) was performed in a high-end cardiovascular phantom with dynamic flow, pulsatile heart motion, and including different calcified plaques with various stenosis grades and in 10 consecutive patients. VMI at 40,50,60 and 70 keV were reconstructed without (QIR-off) and with all quantum iterative reconstruction (QIR) levels (QIR-1 to 4). In the phantom, noise power spectrum, vessel attenuation, contrast-to-noise-ratio (CNR), and vessel sharpness were measured. Two readers graded stenoses in the phantom and graded overall image quality, subjective noise, vessel sharpness, vascular contrast, and coronary artery plaque delineation on 5-point Likert scales in patients.

Results: In the phantom, noise texture was only slightly affected by keV and QIR while noise increased by 69% from 70 keV QIR-4 to 40 keV QIR-off. Reconstructions at 40 keV QIR-4 exhibited the highest CNR (46.1 ± 1.8), vessel sharpness (425 ± 42 ∆HU/mm), and vessel attenuation (1098 ± 14 HU). Stenosis measurements were not affected by keV or QIR level (p > 0.12) with an average error of 3%/6% for reader 1/reader 2, respectively. In patients, across all subjective categories and both readers, 40 keV QIR-3 and QIR-4 images received the best scores (p < 0.001).

Conclusion: Forty keV VMI with QIR-4 significantly improved image quality of CCTA with PCD-CT.

Advances in knowledge: PCD-CT at 40 keV and QIR-4 improves image quality of CCTA.

Figure 1.

Overview of the cardiovascular phantom. The phantom setup on the CT scanner table is shown on the left. A close-up picture of the heart is shown in the middle together with a volume rendered CT image in the lower left part. A representative CT image of the heart is shown on the top right, whereby calcified plaques in the coronary artery wall are marked with small orange arrows. Close-up short- and long-axis images of a coronary plaque are shown in the lower right part of the figure.

Figure 2.

Overview of the left coronary model. The model included three plaques composed of deposited calcium carbonate with occlusion percentages of 50% in diagonal branch of the left anterior descending artery (D/LAD), 70% in the main branch of the LAD, and 90% in the circumflex artery (CX) as defined by the vendor.

Figure 3.

Overview of objective image quality analysis in the phantom. (A) shows CNR and coronary attenuation (i.e., mean HU values from ROI measurements) at different keV and QIR levels. (B) indicates data from noise power spectrum analysis. CNR increases at lower keV levels and at higher QIR strength levels. Coronary attenuation increases at lower keV levels but remains virtually unaffected by the QIR strength level. NPS analysis shows comparable image texture among all reconstructions yet decreasing noise levels for higher keV levels and QIR strength levels (graph in the bottom right corner).

Figure 4.

Overview of the vessel sharpness analysis and stenosis grading in the phantom. (A) shows the line profile and corresponding vessel sharpness as defined by the maximum slopes of the line profile at the vessel edges. (B) shows the stenosis measurements by both readers for all plaques stratified for keV and QIR strength levels. The black-dotted line indicates the true percentage stenosis as provided by the phantom vendor. Vessel sharpness increased at lower keV levels but remained virtually unchanged across the various QIR strength levels. Stenosis measurements were not affected by keV or QIR strength level.

Figure 5.

Representative images of a 70-year-old female patient with an Agatston score of 475 and a calcified plaque in the left anterior descending (LAD) artery. 40 to 70 keV VMI reconstructions at QIR-4 are shown (upper row: standard field-of-view reconstruction, lower row: magnified view of the LAD). Note the high vascular contrast on 40 keV images along with the better delineation of the vessel lumen and plaque.

Figure 6.

Representative images of a 46-year-old male patient with an Agatston score of 61 and a mixed plaque in the proximal left anterior descending (LAD) artery. 40 keV VMI reconstructions at QIR-off to QIR-4 are shown (upper row: standard field-of-view reconstruction, lower row: magnified view of the proximal LAD). Note the lower noise levels at higher levels of QIR.