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Technical Note: Measuring contrast‐ and noise‐dependent spatial resolution of an iterative reconstruction method in CT using ensemble averaging
Author(s) -
Yu Lifeng,
Vrieze Thomas J.,
Leng Shuai,
Fletcher Joel G.,
McCollough Cynthia H.
Publication year - 2015
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4916802
Subject(s) - imaging phantom , image resolution , iterative reconstruction , optical transfer function , nuclear medicine , kernel (algebra) , noise (video) , contrast (vision) , tomography , physics , mathematics , optics , artificial intelligence , computer science , medicine , image (mathematics) , combinatorics
Purpose: The spatial resolution of iterative reconstruction (IR) in computed tomography (CT) is contrast‐ and noise‐dependent because of the nonlinear regularization. Due to the severe noise contamination, it is challenging to perform precise spatial‐resolution measurements at very low‐contrast levels. The purpose of this study was to measure the spatial resolution of a commercially available IR method using ensemble‐averaged images acquired from repeated scans. Methods: A low‐contrast phantom containing three rods (7, 14, and 21 HU below background) was scanned on a 128‐slice CT scanner at three dose levels (CTDI vol = 16, 8, and 4 mGy). Images were reconstructed using two filtered‐backprojection (FBP) kernels (B40 and B20) and a commercial IR method (sinogram affirmed iterative reconstruction, SAFIRE, Siemens Healthcare) with two strength settings (I40‐3 and I40‐5). The same scan was repeated 100 times at each dose level. The modulation transfer function (MTF) was calculated based on the edge profile measured on the ensemble‐averaged images. Results: The spatial resolution of the two FBP kernels, B40 and B20, remained relatively constant across contrast and dose levels. However, the spatial resolution of the two IR kernels degraded relative to FBP as contrast or dose level decreased. For a given dose level at 16 mGy, the MTF 50% value normalized to the B40 kernel decreased from 98.4% at 21 HU to 88.5% at 7 HU for I40‐3 and from 97.6% to 82.1% for I40‐5. At 21 HU, the relative MTF 50% value decreased from 98.4% at 16 mGy to 90.7% at 4 mGy for I40‐3 and from 97.6% to 85.6% for I40‐5. Conclusions: A simple technique using ensemble averaging from repeated CT scans can be used to measure the spatial resolution of IR techniques in CT at very low contrast levels. The evaluated IR method degraded the spatial resolution at low contrast and high noise levels.