NEMA NU4-2008 Performance Evaluation for the MicroPET FOCUS 120 and Iodine-124

This study aimed to evaluate the performance of the microPET FOCUS 120 in terms of counting rate capability and image quality when using 124I. The noise equivalent count rate (NECR) index was defined as: NECR = R^2T/RP + RGP (T = true, P = prompt, GP = gamma prompt). The maximum NECR (48 kcps) value was obtained for the 250-590 keV energy window (EW) with 6 ns time window (TW). The two other EW (350-590 and 350-650 keV) gave comparable maximum NECR of 43 kcps for the 6 ns TW. The 2 ns TW reduced the NECR by 45 to 50 %. Scatter and gamma plus scatter fractions were almost not TW dependent. The 250-590 keV window showed the largest scatter fraction with 23%. For the NEMA NU4-2008 image quality phantom, coefficients of variation in the uniform part increased from about 8 to 12% when decreasing TW from 6 to 2 ns. The lowest spill-over-ratios (SOR) for the non emitting regions were obtained for the 350-590 and 350-650 keV EW. Recovery coefficients (RC) of the hot rods were the highest for the 350-590 keV EW and 2 ns TW. Scatter correction led to a decrease in RC and occasionally (with FBP or 3DRP) to a negative value for the SOR.
The combination of the 350-590 keV energy window with 6 ns time window appeared to be a good compromise between counting rate capability and image quality for the FOCUS 120, especially when MAP reconstruction was used. The scatter correction algorithm provided by the constructor should be used with caution.