Evaluation of a quantitative method for measuring the spatial resolution of a gamma camera using a bar phantom

The method developed by Hander et al. (1997) [1] enables a quantitative determination of spatial resolution by analysing planar images of a lead bar phantom. The method is based on the determination of the modulation transfer function (MTF) and the full width at half maximum (FWHM) solely on the basis of the mean value and standard deviation within a region of interest (ROI). To assess the applicability of this method for constancy checks with extrinsic measurements, the influence of the phantom-to-collimator distance, the total counts and the matrix size was evaluated. In addition, the constancy of the method for repeated measurements under repositioning was analysed. The phantom-to-collimator distance significantly affects the determined resolution, with the FWHM increasing by approximately 0.4 mm per centimetre of distance. The matrix size also influences the determined values, leading to up to 20% lower MTF values for the larger bars (4 and 5 mm) when decreasing the matrix from a 1024 × 1024 to a 256 × 256 matrix. The total number of counts does not affect the MTF or FWHM values, provided that at least 3·10⁶ counts are acquired. However, this applies only to the larger bars (4 and 5 mm); if reliable values are required for all four quadrants, the total number of counts should be increased further. Under constant measurement conditions, variations below 2.4% were observed for the bars of 4 and 5 mm width. For the smaller bars of 2 and 3 mm width the variations were below 9%.