Photon-counting computed tomography (PCCT) represents a significant advancement in diagnostic imaging, offering superior spectral imaging capabilities and reduced radiation exposure compared to conventional CT systems. By employing photon-counting detectors that measure the energy of individual X-ray photons, PCCT provides enhanced tissue differentiation and image clarity. However, for PCCT to be fully integrated into routine clinical practice, its long-term stability must be rigorously assessed. Consistent image quality and quantitative accuracy are essential, particularly for longitudinal studies where monitoring disease progression or treatment response over extended periods is required.
A two-year longitudinal study aimed to address this gap by evaluating the stability of a first-generation dual-source PCCT scanner. The research focused on the reproducibility of key imaging metrics, including Hounsfield Units (HU), image noise and iodine density measurements, under consistent experimental conditions. The results offer valuable insights into the reliability of PCCT for long-term diagnostic use and its potential applications in clinical care.
Methodology and Experimental Setup
The study was conducted over a two-year period from November 2021 to November 2023, involving weekly imaging sessions using a dual-source PCCT scanner (NAEOTOM Alpha, Siemens Healthineers). A Gammex multi-energy CT phantom with tissue-mimicking inserts, including various concentrations of iodine and calcium, was utilised to assess quantitative imaging stability. The phantom was chosen due to its ability to simulate different tissue compositions and contrast levels, providing a controlled environment for consistent measurements.
A uniform scanning protocol was maintained throughout the study, including the use of virtual monochromatic imaging (VMI) at four photon-energy levels: 40, 70, 100 and 190 keV. Imaging parameters such as tube voltage, scan modes and reconstruction filters were standardised to ensure comparability across all sessions. Both single-source and dual-source acquisition modes were evaluated, and each scan was repeated three times to minimise statistical variance.
To monitor system performance, the study documented all hardware and software modifications over the observation period. Significant software updates occurred at weeks 8, 35 and 69, while hardware adjustments were implemented at weeks 8, 19 and 80. These changes allowed the researchers to analyse how system improvements influenced imaging consistency.
Stability of Hounsfield Units and Image Noise
The results demonstrated that the PCCT system maintained stable Hounsfield Unit (HU) values over the two-year period, confirming its reliability for quantitative imaging. In single-source mode, the relative error for VMI 70 keV remained low at an average of 0.11%, while the dual-source mode exhibited a slightly higher variation of 0.30%. These minimal variations were largely associated with software updates aimed at enhancing cross-scatter correction, particularly during the early stages of the study.
Image noise, a critical factor in diagnostic clarity, also exhibited stability throughout the observation period. Noise levels for VMI 70 keV were recorded at 42 ± 1 HU in single-source mode and 35.5 ± 0.9 HU in dual-source mode. Although minor fluctuations were detected following software modifications, such as a 1 HU increase in dual-source mode, these changes were temporary and stabilised over time.
This consistent performance is significant for clinical practice, particularly in longitudinal studies where reproducible quantitative data is essential for tracking disease progression. The findings suggest that PCCT can provide reliable image quality over extended periods, supporting its suitability for oncology, cardiovascular monitoring and chronic disease management.
Iodine Density and Spectral Imaging Consistency
The quantification of iodine density, a crucial metric for contrast-enhanced imaging, demonstrated both stability and improvement over the two-year study period. Iodine density mapping allows for the differentiation of tissues based on contrast agent distribution, which is particularly valuable in vascular imaging and tumour assessments.
The dual-source mode initially exhibited a higher nominal error, with iodine density measurements showing an average error of 1.44 mg/mL. However, after software and hardware adjustments early in the study, the error decreased significantly to 0.03 mg/mL, indicating improved measurement accuracy. Similarly, the single-source mode maintained consistent results, with minimal variation in iodine density throughout the observation period.
These results underline the ability of PCCT to maintain accurate spectral imaging over time, ensuring consistent contrast-enhanced imaging quality. Such stability is crucial for clinical environments where quantitative precision can influence diagnostic decisions, particularly in oncology follow-ups and vascular health assessments.
The two-year longitudinal study of the first-generation dual-source PCCT scanner provides strong evidence of its long-term stability and reliability in diagnostic imaging. The consistent Hounsfield Unit values, minimal image noise fluctuations and stable iodine density measurements affirm PCCT’s suitability for clinical applications, particularly in longitudinal studies where consistent data is essential.
The findings support the broader adoption of PCCT in diagnostic radiology, especially for applications requiring high-resolution, quantitative accuracy over extended periods. Its ability to provide reliable spectral imaging while minimising radiation exposure represents a significant advancement in medical imaging technology. Future research could explore multi-centre trials and longer observation periods to further validate PCCT's stability across diverse clinical environments, reinforcing its role in modern diagnostic care.
Source: European Radiology
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