Quantitative Measurement of In Vivo Tracer Concentration in Rats with Multiplexed Multi-Pinhole SPECT

The goal of this study is to evaluate the quantitative accuracy of measuring in vivo tracer concentrations using multiplexed multi-pinhole microSPECT with CT-based attenuation correction (AC) and simple methods for scatter compensation. Phantom and in vivo rat cardiac images were acquired and reconstructed with no photon AC, with AC only, with attenuation and dual-energy window scatter correction, and using a reduced attenuation coefficient to compensate for scatter. Absolute calibration was also acquired using small sources to minimize self-attenuation and scatter. The phantom tracer concentrations measured with SPECT were compared to dose calibrator measurements. The rats were sacrificed and the cardiac activity measured in vivo was compared to well counter measurements of cardiac activity. With no correction, the tracer concentrations measured in phantoms was as much as 30% below the true value for the largest phantom (52 mm diameter). AC improved the accuracy of quantification, but overestimated activity concentrations by up to 5% for the larger phantoms. With DEW scatter correction, activity concentration measured with SPECT agrees with the dose calibrator measurements to within -2 ±2% and with a reduced attenuation coefficient, the agreement was better than -1 ±1%. In rats injected with ^99mTc-tetrofosmin, SPECT measurements of total cardiac activity were 24±2% below well counter measurements with no corrections, 5±3% above well counter measurements with only attenuation correction, 3±3% below with DEW scatter correction, and 1±3% below with reduced attenuation correction.