A method to measure slice sensitivity profiles of CT images under low-contrast and high-noise conditions

Noise reduction features of iterative reconstruction (IR) methods in computed tomography might accompany the sacrifice of the longitudinal resolution, or slice sensitivity profile (SSP), at low contrast-to-noise ratio (CNR) conditions. To assess the benefit of IR methods correctly, the difference of SSP between IR methods and filtered-backprojection (FBP) must be taken into account. Therefore, SSP measurement under low-CNR conditions is necessary. Although edge methods are predominantly used, their performance under low-CNR conditions appears to be not fully established. We developed a method that is compatible with extremely low-CNR conditions. Thin plastic disk-shaped sheets embedded in acrylic resin were used as low-contrast test objects. The lowest peak contrast used was approximately 17 [HU]. We assessed the performance of our method by using FBP images. We identified a source of measurement instability aside from noise: the measured thin-slice SSP is dependent on the orbital phase of helical scan, presumably because of cone-beam artifacts. This impediment to high accuracy is manageable using phase-controlled scans. We confirmed that table position repeatability is much better than the value of the specifications, and therefore the ensemble-averaged images of multiple scans can be used for SSP measurement. Accurate measurement of SSP under extremely low-CNR conditions is possible, even when the test object is visually indiscernible from the noisy background. Low-contrast SSP behavior is elucidated for IR methods (AIDR-3D, FIRST, and AiSR-V) by using this measurement method.