An experimental method for direct measurement of CT detector presampling MTF from reconstructed images

Modulation transfer functions (MTFs) measured from reconstructed CT images conventionally represent the combined effects of detector blur, focal spot blur, gantry motion blur, and image reconstruction-induced blur. As a result, direct isolation and measurement of the detector MTF from reconstructed images has long been considered infeasible without explicit knowledge of the individual contributions to the overall CT image MTF.

To develop and validate a new experimental method for measuring the CT detector presampling MTF using reconstructed DICOM images.

The core idea of the proposed method is to utilize the longitudinal ( $z$ ) profile of an edge measured from in the reconstructed image to estimate the edge spread function (ESF), and to use a linear stage to translate the edge along the $z$ -axis to achieve oversampling of the ESF. A metallic straight edge was aligned parallel to the axial plane. When static-gantry reconstruction is available, the edge is placed near the surface of the parked detector to minimize focal spot blur; otherwise, it is placed at iso-center and the focal spot blur is measured and corrected. The edge is stepped along $z$ in sub-detector-pixel increments, and images from all positions are combined to reconstruct a densely sampled ESF and the detector presampling MTF. The method was tested on three CT systems: a Siemens NAEOTOM Alpha photon-counting detector (PCD) system, a Siemens SOMATOM Force energy-integrating detector (EID) system, and a GE Revolution HD EID system, with validation against manufacturer-reported detector presampling MTFs.

The measured detector MTFs showed good agreement with the manufacturer-reported reference values. Both the standard-resolution and ultra-high-resolution (UHR) modes of the PCD exhibited substantially higher spatial resolution than the EID. Specifically, the ${\mathrm{MTF}}_{10}$ values for the UHR-mode PCD, standard-mode PCD, Siemens EID, and GE EID were 29.1, 14.1, 9.6, and 9.5 lp/cm, respectively. Importantly, detector MTFs measured using the proposed method were consistent across images acquired with different focal spot sizes and reconstructed using different kernels, demonstrating insensitivity to these confounding system factors.

The proposed method enables accurate measurement of CT detector presampling MTFs directly from reconstructed images by mitigating confounding tomographic reconstruction blur effects, without requiring access to detector counts or projection data.