Highlights
- •A method to measure low-contrast high-noise SSP of nonlinear CT images was developed.
- •Low-contrast plastic sheet is used as a weak impulsive test object.
- •Accurate SSP is obtainable even when a test object is indistinct against noise.
- •Each different peculiar behavior of low-CNR SSP is disclosed for three IR methods.
Abstract
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.
Keywords
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Article Info
Publication History
Published online: March 30, 2019
Accepted:
March 11,
2019
Received in revised form:
March 3,
2019
Received:
May 27,
2018
Identification
Copyright
© 2019 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.
