The purposes of this study are to analyze signal-to-noise ratio (SNR) changes for in-plane (axial plane) position and in-plane
direction in X-ray computed tomography (CT) system and to verify those visual effects by using simulated small low-contrast disc objects. Three-models of multi detector-row CT were employed. Modulation transfer function (MTF) was obtained using a thin metal wire. Noise power spectrum (NPSs) was obtained using a cylindrical water phantom. The measurement positions were set to center and off-centered positions of 64mm, 128mm and 192mm. One-dimensional MTFs and NPSs for the x- and y-direction were calculated by means of a numerical slit scanning method. SNRs were then calculated from MTFs and NPSs. The simulated low-contrast disc objects with diameter of 2 to 10mm and contrast to background of 3.0%, 4.5% and 6.0% were superimposed on the water phantom images. Respective simulated objects in the images are then visually evaluated in degree of their recognition, and then the validity of the resultant SNRs are examined. Resultant in-plane SNRs differed between the center and peripheries and indicated a trend that the SNR values increase in accordance with distance from the center. The increasing degree differed between x- and y-direction, and also changed by the CT systems. These results suggested that the peripheries region has higher low-contrast detectability than the center. The properties derived in this study indicated that the depiction abilities at various in-plane positions are not uniform in clinical CT images, and detectability of the low contrast lesion may be influenced.
Generally, the modulation transfer function (MTF) of a computed tomography (CT) scanner is calculated based on the
CT value. However, it is impossible to measure the MTF directly because the CT value is defined as a nonlinear function
of the X-ray intensity. Due to this characteristic, the MTF varies with the subject's contrast. Therefore, we measured the
MTFs of a CT scanner using high- and low-contrast wire phantoms. We selected thick copper wire in water as the
high-contrast subject and thin copper wire in water as the low-contrast subject. The MTF measured with the
low-contrast subject was decreased relative to that measured with the high-contrast subject because the CT value was
nonlinear. Thus, to evaluate the spatial resolution in a low-contrast subject such as the human body, we should measure
the MTF with a low-contrast wire phantom. In addition, by using low-contrast subjects, we can approximately determine
the CT value using a linear function.
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