A novel method to evaluate gamma camera rotational uniformity and sensitivity variation.

An alternative to the conventional method of performing the AAPM Report 52 rotational uniformity and sensitivity test has been developed. In contrast to the conventional method in which a Co-57 sheet source is fastened to the collimator, this new point-source method acquires the images intrinsically using a Tc-99m point source placed near the isocenter of gantry rotation. As with the conventional method, the point-source method acquires 5 x 10(6) count flood images at four distinct gantry positions to calculate the maximum sensitivity variation (MSV)--a quantitative metric of rotational uniformity and sensitivity variation. The point-source method incorporates corrections for the decay of Tc-99m between acquisitions, the curvature in the image intensity due to variation in photon flux across the detector from a near-field source, and the source-to-detector distance variations between views. The raw point-source images were fitted with an analytic function in order to compute curvature- and distance-corrected images prior to analysis. Five independent MSV measurements were performed using both conventional and point-source methods on a single detector of a dual-headed SPECT system to estimate the precision of each method. The precision of the point-source method was further investigated by performing ten independent measurements of MSV on six different detectors. Correlation between the MSV calculated by the two methods was investigated by performing the test on nine different detectors using both methods. Different levels of sensitivity variations were also simulated on four detectors to generate 40 additional paired points for correlation analysis. The effect of the total image counts on the MSV estimated with the new method was evaluated by acquiring image sequences with 5 x 10(6), 10 x 10(6), and 20 x 10(6) count images. The MSV calculated using the conventional and point-source methods exhibited a high degree of correlation and consistency with equivalence. The precision of the point-source method (0.145%) is lower than the conventional method (0.04%) but sufficient to test MSV. No statistically significant dependence of MSV with the point-source method on the total image counts over a range of (5-20) x 10(6) counts was observed. Curvature correction of the images prior to the generation of difference images renders images more conducive to qualitative inspection for structured, nonrandom patterns. The advantages of the new methodology are that multiple detectors of a gamma camera can be evaluated simultaneously which substantially reduces the time required for MSV testing and the reduced risk of accidental damage to the collimators and patient proximity detection system from having to mount a sheet source on each of the detectors.