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A count-based radionuclide method for volume quantitation using conjugate imaging and an external reference source:
1Department of Nuclear Medicine, San Francisco General Hospital, CA 94110.
Unlabelled:
A count-based method was developed for ventricular volume determination during multigated equilibrium cardiac blood-pool imaging (MUGA).
Methods:
Two sets of conjugate images of the volume were obtained, one with an external reference overlying the volume and the other without. The reference has known activity and dimension. The ratio of the geometric mean count rates over a pixel obtained from these two conjugate images, Ratio(geo), and the ratio of the specific activity in the reference to that in the volume, Ratio(ext), were used to calculate the length of the volume over that pixel perpendicular to the camera face, H, as a function of the length of the reference source, R, and the attenuation coefficient, ub, of the volume: [formula: see text] Based on H, the volume is then calculated. This method corrects for attenuation directly and determines the volume explicitly. To validate this method, phantom studies are carried out with known volumes of 99mTc containing saline solutions in situations of variable amount of attenuating medium and background activity.
Results:
In all cases, the calculated volume agrees closely with the actual volume.
Conclusion:
This is an accurate method of volume quantitation that is well suited for determining ventricular volume during MUGA studies.