Thin slice photon‐counting CT coronary angiography compared to conventional CT: Objective image quality and clinical radiation dose assessment
Authors: van der Bie Judith , Bos Daniel , Dijkshoorn Marcel L. , Booij Ronald , Budde Ricardo P. J. , van Straten Marcel 
Published: February 2024

Background

Photon‐counting CT (PCCT) is the next‐generation CT scanner that enables improved spatial resolution and spectral imaging. For full spectral processing, higher tube voltages compared to conventional CT are necessary to achieve the required spectral separation. This generated interest in the potential influence of thin slice high tube voltage PCCT on overall image quality and consequently on radiation dose.

Purpose

This study first evaluated tube voltages and radiation doses applied in patients who underwent coronary CT angiography with PCCT and energy‐integrating detector CT (EID‐CT). Next, image quality of PCCT and EID‐CT was objectively evaluated in a phantom study simulating different patient sizes at these tube voltages and radiation doses.

Methods

We conducted a retrospective analysis of clinical doses of patients scanned on a conventional and PCCT system. Average patient water equivalent diameters for different tube voltages were extracted from the dose reports for both EID‐CT and PCCT. A conical phantom made of polyethylene with multiple diameters (26/31/36 cm) representing different patient sizes and containing an iodine insert was scanned with a EID‐CT scanner using tube voltages and phantom diameters that match the patient scans and characteristics. Next, phantom scans were made with PCCT at a fixed tube voltage of 120 kV and with CTDIVOL values and phantom diameters identical to the EID‐CT scans. Clinical image reconstructions at 0.6 mm slice thickness for conventional CT were compared to PCCT images with 0.4 mm slice thickness. Image quality was quantified using the detectability index (d′), which estimated the visibility of a 3 mm diameter contrast‐enhanced coronary artery by considering noise, contrast, resolution, and human visual perception. Alongside d′, noise, contrast and resolution were also individually assessed. In addition, the influence of various kernels (Bv40/Bv44/Bv48/Bv56), quantum iterative reconstruction strengths (QIR, 3/4) and monoenergetic levels (40/45/50/55 keV) for PCCT on d′ was investigated.

Results

In this study, 143 patients were included: 47 were scanned on PCCT (120 kV) and the remaining on EID‐CT (74 small‐sized at 70 kV, 18 medium‐sized at 80 kV and four large‐sized at 90 kV). EID‐CT showed 7%–17% higher d′ than PCCT with Bv40 kernel and strength four for small/medium patients. Lower monoenergetic images (40 keV) helped mitigate the difference to 1%–6%. For large patients, PCCT's detectability was up to 31% higher than EID‐CT. PCCT has thinner slices but similar noise levels for similar reconstruction parameters. The noise increased with lower keV levels in PCCT (≈30% increase), but higher QIR strengths reduced noise. PCCT's iodine contrast was stable across patient sizes, while EID‐CT had 33% less contrast in large patients than in small‐sized patients.

Conclusion

At 120 kV, thin slice PCCT enables CCTA in phantom scans representing large patients without raising radiation dose or affecting vessel detectability. However, higher doses are needed for small and medium‐sized patients to obtain a similar image quality as in EID‐CT. The alternative of using lower mono‐energetic levels requires further evaluation in clinical practice.