Improved system calibration for ¹³¹I SPECT image quantification

Objective. SPECT system calibration plays an essential role in the accuracy of image quantification. In this work, an improved SPECT calibration method is proposed, considering the partial-volume effect (PVE) and the source calibration position. Methods. In the method presented, the calibration factor (S₀) is determined by an exponential fit of the calibration curve, which uses different threshold levels for determination of Volume of Interest. The calibration factor was calculated for images with High Count Density (HCD) and Low Count Density (LCD), and varying the source position within the phantom. To validate the calibration method, the calibration factors were used for absolute quantification of the total reference activities. Results. As expected, the sensitivity increases exponentially with volume, reaching a saturation level for volumes larger than the spatial resolution of the system, at which point the PVE becomes negligible. We obtained 13% accuracy in the image quantification using a centralized calibration source, which concurs with previous work. However, the calibration factors obtained with HCD images present the highest relative deviation (−18%) between the central and the most peripheral position, whereas for LCD images the deviation was only 3.7% for the same positions. Overall, deviations were below 9% for the calculated activities for each source position and using the calibration factor for the corresponding position. Conclusions. The proposed method has proven to be easier and faster to implement than other suggested procedures. SPECT quantification can be improved if the target position is considered in the calibration system procedure.