An experimental study on the shift-variant MTF of CT systems using a simple cylindrical phantom

Soohwa Kam; Hanbean Youn; Ho Kyung Kim; Hosang Jeon

doi:10.1117/12.2007862

6 March 2013 An experimental study on the shift-variant MTF of CT systems using a simple cylindrical phantom

Soohwa Kam, Hanbean Youn, Ho Kyung Kim, Hosang Jeon

Proceedings Volume 8668, Medical Imaging 2013: Physics of Medical Imaging; 86684Q (2013) https://doi.org/10.1117/12.2007862
Event: SPIE Medical Imaging, 2013, Lake Buena Vista (Orlando Area), Florida, United States

Abstract

The modulation transfer function (MTF) is a typical parameter to measure the spatial resolution, which is an essential factor for evaluating the performance of computed tomography (CT) systems. It is known that the CT system does not follow the shift-invariant manner because of the cone-beam geometry and the transformation from the cylindrical coordinates to the axial coordinates when the image reconstruction is employed. Several studies reported that if the position of impulse receded from the center of a region of interest (ROI), the MTF degraded continuously. In this study, the trend of shift-variant characteristics of CT systems was measured and analyzed using a novel multi-cylindrical phantom. This study used to determine a point spread function (PSF) and MTF of a CT system using a simple cylindrical phantom. First of all, the optimal diameter of cylinder phantoms was experimentally determined as 70 mm to obtain reliable PSFs. Two kinds of field of views (FOVs), 40 cm and 60 cm, were used to vary reconstructed pixel sizes. The shift-variant MTF curves were acquired at five off-center positions per FOV. For the effective analysis of MTF shiftvariance, the integrated MTF values were calculated and used. In the result, the MTF slightly decreased as diameter increased from CT center in the central region within the distance of 10 cm. Moreover, a considerable MTF decrease suddenly occurred around the distance of 15 cm in the actual FOVs. The decreasing trend of the off-center spatial resolution of CT cannot be neglected in recent radiologic and radio-therapeutic fields requiring high degree of image precision, especially in sub-mm images. It is recommended that the ROI is laid on the CT center as close as possible. A novel cylindrical phantom was finally suggested to effectively measure PSFs with optimal diameters for clinical FOVs in this study. This phantom is cheap and convenient to use because it was only made of acryl with simple geometry. It is expected that the spatial resolution of CT can be easily monitored using our methodology in clinical CT sites.

Citation Download Citation

Soohwa Kam, Hanbean Youn, Ho Kyung Kim, and Hosang Jeon "An experimental study on the shift-variant MTF of CT systems using a simple cylindrical phantom", Proc. SPIE 8668, Medical Imaging 2013: Physics of Medical Imaging, 86684Q (6 March 2013); https://doi.org/10.1117/12.2007862

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