In previous studies of the noise power spectrum (NPS) of multi-detector CT (MDCT) systems, the image object
was usually placed at the iso-center of the CT system; therefore, the bowtie filter had negligible impact on the
shape of the two-dimensional (2D) NPS of MDCT. This work characterized the NPS of off-centered objects
when a bowtie filter is present. It was found that the interplay between the bowtie filter and object position has
significant impact on the rotational symmetry of the 2D NPS. Depending on the size of the bowtie filter, the
degree of object off-centering, and the location of the region of interest (ROI) used for the NPS measurements,
the symmetry of the 2D NPS can be classified as circular, dumbbell, and a peculiar cloverleaf symmetry. An
anisotropic NPS corresponds to structured noise texture, which may directly influence the detection performance
of certain low contrast detection tasks.
State of the art automatic exposure control modulates the tube current across view angle and Z based on patient anatomy for use in axial full scan reconstructions. Cardiac CT, however, uses a fundamentally different image reconstruction that applies a temporal weighting to reduce motion artifacts. This paper describes a phase based mA modulation that goes beyond axial and ECG modulation; it uses knowledge of the temporal view weighting applied within the reconstruction algorithm to improve dose efficiency in cardiac CT scanning. Using physical phantoms and synthetic noise emulation, we measure how knowledge of sinogram temporal weighting and the prescribed cardiac phase can be used to improve dose efficiency. First, we validated that a synthetic CT noise emulation method produced realistic image noise. Next, we used the CT noise emulation method to simulate mA modulation on scans of a physical anthropomorphic phantom where a motion profile corresponding to a heart rate of 60 beats per minute was used. The CT noise emulation method matched noise to lower dose scans across the image within 1.5% relative error. Using this noise emulation method to simulate modulating the mA while keeping the total dose constant, the image variance was reduced by an average of 11.9% on a scan with 50 msec padding, demonstrating improved dose efficiency. Radiation dose reduction in cardiac CT can be achieved while maintaining the same level of image noise through phase based dose modulation that incorporates knowledge of the cardiac reconstruction algorithm.
Today lowering patient radiation dose while maintaining image quality in Computed Tomography has become a very
active research field. Various iterative reconstruction algorithms have been designed to improve/maintain image quality
for low dose patient scans. Typically radiation dose variation will result in detectability variation for low contrast
objects. This paper assesses the low contrast detectability performance of the images acquired at different dose levels
and obtained using different image generation algorithms via two-alterative forced choice human observer method.
Filtered backprojection and iterative reconstruction algorithms were used in the study. Results showed that for the
objects and scan protocol used, the iterative algorithm employed in this study has similar low contrast detectability
performance compared to filtered backprojection algorithm at a 4 times lower dose level. It also demonstrated that well
controlled human observer study is feasible to assess the image quality of a CT system.
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