Photon count rates estimated from 1980 clinical CT scans

Area of Science:

• Medical Imaging Physics
• Radiological Technology

Background:

• Photon counting detectors (PCDs) are susceptible to performance degradation at elevated count rates, manifesting as increased noise, reduced quantification accuracy, and image artifacts.
• Count rate is influenced by patient attenuation, beam filtration, scanner geometry, and X-ray technique, necessitating characterization for clinical applications.

Purpose of the Study:

• To establish a comprehensive understanding of computed tomography (CT) count rates encountered in clinical settings across a wide range of indications and patient sizes.
• To provide data essential for guiding protocol and technology development in photon counting detector systems.

Main Methods:

• Retrospective analysis of 1980 patient CT exams across all body regions, sampling 36,951 axial slices.
• Voxel-based tissue classification (air, lung, fat, soft tissue, bone) and modeling of various bowtie filters, kVp spectra, and mA settings.
• Forward-projection to simulate detector-incident count rates using a GE Revolution Apex scanner geometry, reporting 99th percentile rates as a function of patient water equivalent diameter.

Main Results:

• Routine exams showed count rates around 3.5 x 10^8 counts/mm^2/s (torso, extremities, brain) and 6 x 10^8 counts/mm^2/s (neck).
• High-mA simulations (1000 mA) for pediatric, thoracic, and cardiac scans yielded rates >10 x 10^8 counts/mm^2/s (torso, extremities, brain) and close to 2 x 10^9 counts/mm^2/s (neck).
• Maximum count rate needs showed minimal variation with patient size; higher energies and less attenuating bowtie filters increased count rates.

Conclusions:

• This study presents the first characterization of CT count rates using a large, diverse patient cohort across all body regions.
• Findings indicate that clinical count rates can exceed 1 x 10^9 counts/mm^2/s, providing crucial data for detector development.
• The study highlights the need for detectors capable of handling these high rates but does not predict the impact on image quality at lower detector count rates.