Evaluation 99mTc and 123I quantification using SPECT/CT

English: A review of Single Photon Emission Tomography (SPECT) quantification shows that different protocols and phantoms are available to evaluate SPECT quantification accuracy. This study was necessitated by the lack of standardized protocols and the widespread use of a variety of non-standard phantoms. The aim of this work was to evaluate the influence of the geometry of a radionuclide distribution on SPECT quantification accuracy for 99mTc and 123I isotopes in an abdominal phantom. In order to achieve the aim, the following steps were taken: The preparatory phase of the study was to design and construct an abdominal phantom, verify the accuracy of the attenuation coefficients obtained with the Computed Tomography (CT) scanner, determine the accuracy of the source calibrator used in this study and then obtain a calibration factor in order to convert image counts to activity. During the quantification phase SPECT data were acquired, the influence of not applying scatter correction explicitly was evaluated and the final quantification was performed using the proposed standard clinical reconstruction protocols. The influence of different tumour sizes and locations in the abdominal phantom relative to a high uptake organ on the quantification accuracy was evaluated. Finally, parameters in the Ordered Subset Expectation Maximization (OSEM) reconstruction protocol were altered in order to investigate the influence of number of subsets and iterations on the quantified data. The non-standard Density Phantom with five different compounds was used for the verification of the 99mTc and 123I attenuation coefficients. The percentage difference between the measured and theoretical attenuation coefficients values were < 3%, except for Polystyrene (85% and 65% respectively). The SPECT calibration factor was determined for both 99mTc (11.0 ± 1.3 cpm/kBq) and 123I (10.8 ± 0.3 cpm/kBq) using the Cylindrical Phantom. The in-house built Abdominal Phantom was used to evaluate the tumour activity quantification accuracy. The quantification accuracy of 99mTc and 123I was found to change significantly (p < 0.05) as a function of tumour size after corrections for “spill out” counts due to the partial volume effect, scatter and attenuation were applied. On the other hand, there was no significant difference in the quantification accuracy (p > 0.05) for each tumour at different tumour-liver distances when appropriate scatter and attenuation corrections were applied. The influence of OSEM parameters showed no dependence on the tumour-liver distance and no significant difference (p > 0.05) between quantification with background activity as compared to no background activity. In conclusion, the study showed that the quantification accuracy for 99mTc and 123I was comparable to other published studies. It was found that the tumour quantification accuracy is not influenced by proximity of high uptake organs when appropriate correction factors were applied. Tumour size influenced the accuracy of SPECT quantification for both radionuclides. The results of this study also showed that at least 128 Maximum Likelihood Expectation Maximization (MLEM) equivalent iterations were needed during iterative reconstruction to achieve convergence and consistent SPECT quantification accuracy. Finally, it is recommended that the evaluated quantification protocol may be used in our nuclear medicine clinic for 99mTc and 123I quantification.