Ultra-high Resolution Photon Counting Detector Computed Tomography Imaging for Quantitative Lung Assessment: An Anthropomorphic Phantom Study

Abstract

Background: Quantitative lung imaging is utilized to understand, characterize, and monitor lung disease and response to interventions. X-ray computed tomography has remained the modality of choice for clinical lung assessment, and photon counting detector-computed tomography (PCD-CT) is the latest advancement. PCD-CT provides increased spatial and contrast resolution, decreased image noise and artifacts (such as beam hardening) and, thus, a potential for enhanced image quality for equivalent or reduced radiation dose levels. However, evaluation of the ultra-high resolution capabilities of PCD-CT for quantitative lung imaging has not yet been systematically investigated.

Purpose: This study aims to evaluate 2 ultra-high resolution acquisition modes and 4 reconstruction kernels for optimal quantitative chest imaging at high radiation dose (9 mGy). We assess the stability of measurements across different scan modes and reconstruction kernels when the radiation dose level is reduced.

Methods: A customized anthropomorphic chest phantom, containing standardized insert materials, including air, water, various density foam inserts, and a modulation transfer function (MTF) cube, was repeatedly scanned with PCD-CT (NAEOTOM Alpha; Siemens Healthineers). Two ultra-high resolution acquisition modes, quantum plus (UHRQ+) and quantum with tin filtering (UHRQSn), and 4 reconstruction kernels (Br64, Bl60, Qr60, and Qr40, all with iterative reconstruction level 3) were examined with acquisitions at 3 radiation dose levels (9.1 mGy, 6.8 mGy, and 3.2 mGy). Quantitative density measures, airway measurements, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and MTF values were compared, along with the percentage change in measurement values from high to low radiation dose levels.

Results: At the highest radiation dose levels, UHRQ+ acquisition resulted in lower density values with higher SD compared with UHRQSn. UHRQ+ mode demonstrated higher CNR, SNR, and MTF values. Only UHRQ+ with Qr40 reconstruction provided accurate air measurements, both inside and outside the phantom, across all radiation dose levels. Quantitative density measurements remained highly stable (<2% change) as the radiation dose was reduced from 9.1 to 3.2 mGy. Airway wall thickness, diameter, and lumen area measurements were all larger with UHRQ+ acquisition compared with UHRQSn for the high radiation dose level. At low radiation dose levels, the UHRQ+ acquisition with Br64 reconstruction maintained the highest consistency in airway metrics compared with the values from the high dose acquisition, with <5% measurement percentage change.

Conclusion: The UHRQ+ mode is recommended for quantitative lung assessment, leveraging the PCD-CT voxel size potential (1024×1024 in plane matrix with 0.2 mm slice thickness). The choice of reconstruction kernel at ultra-high resolution should be task-specific, with Qr40 being optimal for density assessment due to its accuracy in air measurement across regions and Br64 for airway assessment. The high consistency of measurements across the radiation dose levels for these kernels (<5% measurement change from 9 mGy measurements) suggests that acquisition at 3 mGy is sufficient for quantitative analysis.

Keywords: airway lumen; airway wall thickness; chest; lung density measures; lung disease; protocol development; pulmonary; quantitative CT.