Size-specific weighted detectability index for computed tomography image characterization

Technological developments in computed tomography (CT) have increased the diversity of acquisition and reconstruction strategies available for clinical imaging. Task-based image quality metrics that account for patient-size variability are needed to characterize reconstructed image performance and support protocol optimization across clinically relevant imaging conditions.

To propose and evaluate a novel size-specific weighted detectability index (SSW-d'), a task-based image performance metric (d') that incorporates lesion contrast weighting and water-equivalent diameter variability for the characterization of CT images acquired at low dose.

First, a multi-sized image quality phantom was scanned on two CT systems: one photon-counting CT (PCCT, CT1) and one CT equipped with energy integrating detectors (EICT, CT2). Acquisitions were performed with a volume CT dose index (CTDI_vol) of 3.2 mGy at 120kV without a tin filter (Sn), at Sn100 and Sn140kV. Second, acquisitions at 3.2 mGy were performed on CT1 using three reconstruction algorithms (FBP, QIR-2, QIR-4). Subsequently, five reconstruction kernels (Br32, Br44, Br56, Br68, and Br76) were evaluated at 1.1 mGy. A higher CTDI_vol value of 6 mGy, combined with Br68 and QIR-4, was also tested. The detectability index values (d') were determined for two 10 mm diameter lesions according to two different levels of contrast, based on iodine and bone equivalent tissue. The SSW-d' was calculated for each CT system and for each individual task. Then, SSW-d' was expanded to include both tasks, applying contrast weights determined for each of them.

On CT1, the highest SSW-d' values were 19.3 ± 0.3 and 6.9 ± 0.0, at 120 kV for bone and iodine rods, respectively. On CT2, the highest SSW-d' results were 19.0 ± 0.3 and 4.9 ± 0.0 at Sn100 kV for bone and iodine rods, respectively. The lowest values were at Sn140kV for both rods and CT systems. When encompassing both tasks, the maximum SSW-d' values were 10.05 ± 0.04 and 8.25 ± 0.03, respectively obtained at 120 kV for CT1 and at Sn100 kV for CT2. The results showed that the lowest SSW-d' values were at Sn140 kV for both CT systems. Comparison of acquisition and reconstruction parameters showed that the highest SSW-d' value was obtained using the Br32 kernel (17.6 ± 0.7), approximately twofold higher than that obtained at 6 mGy (8.6 ± 0.3). The lowest SSW-d' value (1.9 ± 0.1) was obtained with FBP, while the highest performance was achieved with QIR-4 reconstruction.

A novel metric based on a size-specific weighted detectability index (SSW-d') was developed and evaluated for the characterization of reconstructed CT image performance across a range of patient-equivalent diameters and clinically relevant detection tasks. The proposed SSW-d' consolidates multiple detectability index measurements into a single descriptor that accounts for lesion contrast weighting and patient-size variability. The metric was sensitive to acquisition and reconstruction parameters and proved useful for identifying imaging conditions that maximize task-based performance in ultra-low-dose CT. These findings suggest that SSW-d' may serve as a practical tool for image-quality characterization and protocol optimization across clinically relevant imaging scenarios.