Quantifying photon counting detector (PCD) performance using PCD-CT images

Background: Photon counting detector CTs (PCD-CTs) have recently been introduced to clinical imaging. This development creates a new need for end-users to quantify and monitor the physical performance of PCDs. Traditionally, the characterization of PCD performance relied on detector counts, which are typically accessible to the manufacturer but are not usually available to clinical end-users. Purpose: The goal of this work was to develop a new method for quantifying PCD performance using reconstructed PCD-CT images, without requiring access to the PCD counts. Methods: The proposed method is based on a set of closed-form relationships that connect PCD-CT image noise, the PCD deadtime ((Formula presented.)), and the zero-frequency detective quantum efficiency ((Formula presented.)) of PCDs. At a low tube current (mA) level, the mean output counts of the PCD were estimated by fitting the measured PCD-CT noise power spectrum (NPS) to a parametric model. (Formula presented.) was then calculated by normalizing the estimated mean detector counts to the expected input x-ray photon number. To estimate (Formula presented.), the image variance of PCD-CT was measured at different mA levels. A novel quantitative relationship between PCD-CT image variance, (Formula presented.), and mA was employed to estimate (Formula presented.) through parametric fitting. The method was validated using both simulated and experimental PCD-CT data, covering a range of (Formula presented.), (Formula presented.), and system geometries. Results: For the simulated curved-detector PCD-CT, the estimation errors for (Formula presented.) and deadtime were −3.7% and 0.5%, respectively. For the simulated collinear-detector PCD-CT, the estimation errors for (Formula presented.) and deadtime were −3.3% and −1.0%, respectively. For the experimental collinear-detector PCD-CT, the estimation errors for (Formula presented.) and deadtime were −2.6% and 1.6%, respectively. Conclusions: By analyzing the variance and NPS of PCD-CT images, (Formula presented.) and deadtime of scanner's PCD can be accurately estimated, without access to raw detector counts or projection data.