A data-efficient method for local noise power spectrum (NPS) estimation in FDK reconstructed 3D cone-beam CT.

PURPOSE: For CT systems in which noise is nonstationary, a local NPS is often needed to characterize its noise property. We have previously developed a data-efficient radial NPS method to estimate the two-dimensional (2D) local NPS for FBP reconstructed fan-beam CT utilizing the polar separability of CT NPS[1]. In this work we extend this method to estimate three-dimensional (3D) local NPS for FDK-reconstructed cone-beam CT (CBCT) volumes.

METHODS: Starting from the 2D polar separability, we analyze the CBCT geometry and FDK image reconstruction process to derive the 3D expression of the polar separability for CBCT local NPS. With the polar separability, the 3D local NPS of CBCT can be decomposed into a 2D radial NPS shape function and a 1D angular amplitude function with certain geometrical transforms. The 2D radial NPS shape function is a global function characterizing the noise correlation structure, while the 1D angular amplitude function is a local function reflecting the varying local noise amplitudes. The 3D radial local NPS method is constructed from the polar separability. We evaluate the accuracy of the 3D radial local NPS method using simulated and real CBCT data by comparing the radial local NPS estimates to a reference local NPS in terms of normalized mean squared error (NMSE) and a task-based performance metric (lesion detectability).

RESULTS: In both simulated and physical CBCT examples, a very small NMSE (<5%) was achieved by the radial local NPS method from as few as two scans, while for the traditional local NPS method about twenty scans were needed to reach this accuracy. The results also showed that the detectability based system performances computed using the local NPS estimated with the NPS method developed in this work from two scans closely reflected the actual system performance.

CONCLUSIONS: The polar separability greatly reduces the data dimensionality of the 3D CBCT local NPS. The radial local NPS method developed based on this property is shown to be capable of estimating the 3D local NPS from only two CBCT scans with acceptable accuracy. The minimum data requirement indicates the potential utility of local NPS in CBCT applications even for clinical situations. This article is protected by copyright. All rights reserved.