Influence of phantom design on evaluation metrics in photon counting spectral head CT: a simulation study

Accurate iodine quantification in contrast-enhanced head CT is crucial for precise diagnosis and treatment planning. Traditional CT methods, which use energy-integrating detectors and dual-exposure techniques for material discrimination, often increase patient radiation exposure and are susceptible to motion artifacts and spectral resolution loss. Photon counting detectors (PCDs), capable of acquiring multiple energy windows in a single exposure with superior energy resolution, offer a promising alternative. However, the adoption of these technological advancements requires corresponding developments in evaluation methodologies to ensure their safe and effective implementation. One critical area of concern is the accuracy of iodine quantification, which is commonly assessed using cylindrical phantoms that neither replicate the shape of the human head nor incorporate skull-mimicking materials. These phantoms are widely used not only for testing but also for calibration, which may contribute to an overestimation of system performance in clinical applications. We address the impact of phantom design on evaluation metrics in spectral head CT, comparing conventional cylindrical phantoms to anatomically realistic elliptical phantoms with skull simulants.

We conducted simulations using a photon-counting spectral CT system equipped with cadmium telluride (CdTe) detectors, utilizing the Photon Counting Toolkit and Tigre CT software for detector response and CT geometry simulations. We compared cylindrical phantoms (20 cm diameter) to elliptical phantoms in three different sizes, incorporating skull materials with major/minor diameters and skull thicknesses of 18/14/0.5, 20/16/0.6, and 23/18/0.7 cm. Iodine inserts at concentrations of 0, 2, 5, and $10\mathrm{mg}/\mathrm{mL}$ with diameters of 1, 0.5, and 0.3 cm were used. We evaluated the influence of bowtie filters, various tube currents, and operating voltages. Image reconstruction was performed after beam hardening correction using the signal-to-thickness calibration (STC) method with standard filtered back projection, followed by both image-based and projection-based material decomposition.

The results showed that image-based methods were more sensitive to phantom design, with cylindrical phantoms exhibiting enhanced performance compared with anatomically realistic designs across key metrics, including systematic error, root mean square error (RMSE), and precision. By contrast, the projection-based material decomposition method demonstrated greater consistency across different phantom designs and improved accuracy and precision. This highlights its potential for more reliable iodine quantification in complex geometries.

These findings underscore the critical importance of phantom design, especially the inclusion of skull-mimicking materials, in the assessment of quantitative results. Cylindrical phantoms, commonly used for calibration and testing, may overestimate performance in iodine quantification for head CT due to their simplified geometry. We emphasize the need for adopting anatomically realistic phantom designs, such as elliptical phantoms with skull simulants, to enable a more clinically relevant and accurate evaluation of spectral photon-counting head CT systems.