ACCURACY IN THE QUANTIFICATION OF 131I ACTIVITY FROM SPECT IMAGES: A SIMIND MONTE CARLO STUDY

Abstract

The possibility of quantifying the activity of a radiopharmaceutical from SPECT images depends, among others, on the technical specifications of the imaging equipment, the isotope/collimator combination, and the presence of physical
phenomena experienced by the emitted radiation that degrades the image. These factors can be investigated and optimized using Monte Carlo-based simulations. The aim of this work is to evaluate the accuracy on the quantification of ¹³¹I activity from SPECT images obtained with a General Electric SPECT equipment, model Discovery NM 630, under different operational conditions. To this end, a gamma camera was configured in SIMIND MC as a computational model of the clinical equipment and lesions containing ¹³¹I in different locations of an anthropomorphic phantom was
configured as a model of the patient. Simulated SPECT images were obtained using general purpose medium- and highenergy collimators and different thicknesses of scintillation crystal, and ¹³¹I activity was quantified from the images, with and without attenuation correction. The quantified ¹³¹I activity from simulated SPECT images without attenuation correction is always smaller than the activity configured in the simulation, yielding errors of between 39 and 64%. The error in the quantification of ¹³¹I activity markedly decreases when the image is corrected for attenuation, yielding error values less than 0.4% in the thyroid remnants. The work verified that it is possible to quantify 131I activity from SPECT images if an attenuation correction method is available.