A whole body mouse sized μSPECT image quality phantom

In this study a novel whole body mouse sized μSPECT image quality (IQ) phantom is introduced which can be used to assess hot contrast on a warm background. The proposed phantom contains five glass capillaries with sub-mm inner diameters (2.0, 1.5, 1.0, 0.8 and 0.6 mm) and a wall thickness ranging from only 0.2 mm for the smallest to 0.4 mm for the largest capillaries. Up until now the IQ phantoms used in μSPECT system performance evaluations only measured hot rods on a cold background. We scanned our μSPECT phantom on a standard U-SPECT-II (MILabs) mounted with respectively the 0.35 and 0.6 mm multipinhole collimators. The 2 mm capillary was filled with air to create a cold region in a warm background. The four smallest capillaries were filled with a 20 MBq/ml 99mTc solution and the background was changed to concentrations of 0, 5 and 10 MBq/ml to result in three different capillary to background ratios (2:1 and 4:1). The total activity in the phantom amounted 97 MBq and was scanned for 20 min and additionaly for 40 min for the 0.35 mm collimator. While visually contrasts were similar in the absence of a background for both collimators, significantly different results were obtained in the presence of a warm background. For the 0.35 mm collimator image noise was too high to recover the contrast in the 4:1 and the 2:1 ratios while the largest capillaries could be recovered for both contrast ratios when using the 0.6 mm collimator. These striking differences between the no background and warm background advocate the need for the phantom here described. The need for a warm background will be even more obvious when evaluating μSPECT systems that employ multiplexing.