A COUNT-BASED RADIONUCLIDE METHOD FOR VOLUME QUANTITATION USING CONJUGATE IMAGING AND AN EXTERNAL REFERENCE SOURCE - THEORETICAL CONSIDERATION AND PHANTOM STUDY

A count-based method was developed for ventricular volume determinatio n during multigated equilibrium cardiac blood-pool imaging (MUGA). Met hods: Two sets of conjugate images of the volume were obtained, one wi th an external reference overlying the volume and the other without. T he reference has known activity and dimension. The ratio of the geomet ric mean count rates over a pixel obtained from these two conjugate im ages, Ratio(geo), and the ratio of the specific activity in the refere nce to that in the volume, Ratio(geo) were used to calculate the lengt h of the volume over that pixel perpendicular to the camera face, H, a s a function of the length of the reference source, R, and the attenua tion coefficient, u(b), of the volume: ubH/2)-e(-ubH/2)=Ratio(ext)/Rat io(geo)(1-e-(ubR)). Based on H, the volume is then calculated. This me thod corrects for attenuation directly and determines the volume expli citly. To validate this method, phantom studies are carried out with k nown volumes of Tc-99m containing saline solutions in situations of va riable amount of attenuating medium and background activity. Results: In all cases, the calculated volume agrees closely with the actual vol ume. Conclusion: This is an accurate method of volume quantitation tha t is well suited for determining ventricular volume during MUGA studie s.