Ultra-High-Resolution Coronary CT Angiography With Photon-Counting Detector CT: Feasibility and Image Characterization

Abstract

Objectives: The aim of this study was to evaluate the feasibility and quality of ultra-high-resolution coronary computed tomography angiography (CCTA) with dual-source photon-counting detector CT (PCD-CT) in patients with a high coronary calcium load, including an analysis of the optimal reconstruction kernel and matrix size.

Materials and methods: In this institutional review board-approved study, 20 patients (6 women; mean age, 79 ± 10 years; mean body mass index, 25.6 ± 4.3 kg/m 2 ) undergoing PCD-CCTA in the ultra-high-resolution mode were included. Ultra-high-resolution CCTA was acquired in an electrocardiography-gated dual-source spiral mode at a tube voltage of 120 kV and collimation of 120 × 0.2 mm. The field of view (FOV) and matrix sizes were adjusted to the resolution properties of the individual reconstruction kernels using a FOV of 200 × 200 mm 2 or 150 × 150 mm 2 and a matrix size of 512 × 512 pixels or 1024 × 1024 pixels, respectively. Images were reconstructed using vascular kernels of 8 sharpness levels (Bv40, Bv44, Bv56, Bv60, Bv64, Bv72, Bv80, and Bv89), using quantum iterative reconstruction (QIR) at a strength level of 4, and a slice thickness of 0.2 mm. Images with the Bv40 kernel, QIR at a strength level of 4, and a slice thickness of 0.6 mm served as the reference. Image noise, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel sharpness, and blooming artifacts were quantified. For subjective image quality, 2 blinded readers evaluated image noise and delineation of coronary artery plaques and the adjacent vessel lumen using a 5-point discrete visual scale. A phantom scan served to characterize image noise texture by calculating the noise power spectrum for every reconstruction kernel.

Results: Maximum spatial frequency (f peak ) gradually shifted to higher values for reconstructions with the Bv40 to Bv64 kernel (0.15 to 0.56 mm -1 ), but not for reconstructions with the Bv72 to Bv89 kernel. Ultra-high-resolution CCTA was feasible in all patients (median calcium score, 479). In patients, reconstructions with the Bv40 kernel and a slice thickness of 0.6 mm showed largest blooming artifacts (55.2% ± 9.8%) and lowest vessel sharpness (477.1 ± 73.6 ΔHU/mm) while achieving highest SNR (27.4 ± 5.6) and CNR (32.9 ± 6.6) and lowest noise (17.1 ± 2.2 HU). Considering reconstructions with a slice thickness of 0.2 mm, image noise, SNR, CNR, vessel sharpness, and blooming artifacts significantly differed across kernels (all P 's < 0.001). With higher kernel sharpness, SNR and CNR continuously decreased, whereas image noise and vessel sharpness increased, with highest sharpness for the Bv89 kernel (2383.4 ± 787.1 ΔHU/mm). Blooming artifacts continuously decreased for reconstructions with the Bv40 (slice thickness, 0.2 mm; 52.8% ± 9.2%) to the Bv72 kernel (39.7% ± 9.1%). Subjective noise was perceived by both readers in agreement with the objective measurements. Considering delineation of coronary artery plaques and the adjacent vessel lumen, reconstructions with the Bv64 and Bv72 kernel (for both, median score of 5) were favored by the readers providing an excellent anatomic delineation of plaque characteristics and vessel lumen.

Conclusions: Ultra-high-resolution CCTA with PCD-CT is feasible and enables the visualization of calcified coronaries with an excellent image quality, high sharpness, and reduced blooming. Coronary plaque characterization and delineation of the adjacent vessel lumen are possible with an optimal quality using Bv64 kernel, a FOV of 200 × 200 mm 2 , and a matrix size of 512 × 512 pixels.

Trial registration: ClinicalTrials.gov NCT01368250.

FIGURE 1

Curves depict the normalized noise power spectrum (NPS) for the different kernels assessed in the phantom scan. Note the gradual shift toward higher peak spatial frequencies for reconstructions with the Bv40, Bv44, Bv56, Bv60, and Bv64 kernel. For reconstructions with the Bv72, Bv80, and Bv89 kernel, quantum iterative reconstruction (QIR) algorithm has a pronounced effect on noise reduction in the high frequency range seen, which manifests as a strong drop toward higher frequencies.

FIGURE 2

Boxplots showing measurements of the image noise, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel sharpness, and blooming artifacts of ultra-high-resolution CCTA (slice thickness, 0.2 mm) with PCD-CT in patients. Horizontal lines in the boxes correspond to the median. The top and bottom lines of the boxes correspond to the first and third quartiles, respectively. The whiskers show lowest and highest values within 1.5 interquartile range (IQR) of the lower and upper limits. Circles show outliers.

FIGURE 3

Cinematic rendering (A) and curved planar reformations (B) from ultra-high-resolution CCTA (slice thickness, 0.2 mm) with PCD-CT of the left anterior descending artery reconstructed with 8 different kernels in an 88-year-old male patient with severe aortic stenosis. Note the excellent visualization of small second-order coronary vessels across all reconstructions.

FIGURE 4

Axial images from ultra-high-resolution CCTA (slice thickness, 0.2 mm) with PCD-CT in an 88-year-old male patient with severe aortic stenosis. A mixed plaque at the distal left main to proximal left anterior descending artery is seen. Note the continuous improvement of the delineation of the partially calcified plaque at higher kernel strengths showing considerably superior anatomic detail of the small plaque.

FIGURE 5

Axial images from ultra-high-resolution CCTA (slice thickness, 0.2 mm) with PCD-CT of the left main and the proximal left anterior descending artery. Although borders of calcified plaques have a blurry appearance on reconstructions with low kernel strengths, reconstructions with moderate kernel strengths (eg, Bv64) enable an excellent delineation of calcified plaques and adjacent vessel lumen before increased image noise on reconstructions with high kernel strengths (eg, Bv89) hampers vessel lumen visualization.