Using the NEMA NU 4 PET image quality phantom in multipinhole small-animal SPECT

Several commercial small-animal SPECT scanners using multipinhole collimation are presently available. However, generally accepted standards to characterize the performance of these scanners do not exist. Whereas for small-animal PET, the National Electrical Manufacturers Association (NEMA) NU 4 standards have been defined in 2008, such standards are still lacking for small-animal SPECT. In this study, the image quality parameters associated with the NEMA NU 4 image quality phantom were determined for a small-animal multipinhole SPECT scanner. Methods: Multiple whole-body scans of the NEMA NU 4 image quality phantom of 1-h duration were performed in a U-SPECT-II scanner using 99mTc with activities ranging between 8.4 and 78.2 MBq. The collimator contained 75 pinholes of 1.0-mm diameter and had a bore diameter of 98 mm. Image quality parameters were determined as a function of average phantom activity, number of iterations, postreconstruction spatial filter, and scatter correction. In addition, a mouse was injected with 99mTc-hydroxymethylene diphosphonate and was euthanized 6.5 h after injection. Multiple whole-body scans of this mouse of 1-h duration were acquired for activities ranging between 3.29 and 52.7 MBq. Results: An increase in the number of iterations was accompanied by an increase in the recovery coefficients for the small rods (RCrod), an increase in the noise in the uniform phantom region, and a decrease in spillover ratios for the cold-air– and water-filled scatter compartments (SORair and SORwat). Application of spatial filtering reduced image noise but lowered RCrod. Filtering did not influence SORair and SORwat. Scatter correction reduced SORair and SORwat. The effect of total phantom activity was primarily seen in a reduction of image noise with increasing activity. RCrod, SORair, and SORwat were more or less constant as a function of phantom activity. The relation between acquisition and reconstruction settings and image quality was confirmed in the 99mTc-hydroxymethylene diphosphonate mouse scans. Conclusion: Although developed for small-animal PET, the NEMA NU 4 image quality phantom was found to be useful for small-animal SPECT as well, allowing for objective determination of image quality parameters and showing the tradeoffs between several of these parameters on variation of acquisition and reconstruction settings.