Planar gamma camera quantitation of 123I, 99mTc or 111In in the liver and spleen of an abdominal phantom.

Three planar, gamma camera methods for quantitating radiopharmaceuticals, such as radiolabeled antibodies, were investigated. Iodine-123 (123I), technetium-99m (99mTc) or indium-111 (111In) in the liver and spleen of an abdominal phantom were assessed in this study. In the first approach, the number of counts detected in a "single image" of the liver or spleen was used to measure radionuclide content using an attenuation correction factor (ACF) calculated from data obtained without radionuclide in the background volume of the phantom. In the other two methods, radionuclide content was derived from either the geometric mean (GM) of counts in conjugate, opposed images ("global conjugate") or in individual, opposed pixels of the conjugate, opposed images ("pixel conjugate") of the liver and spleen. Both of the conjugate image methods were corrected for attenuation with a first order ACF derived from a transmission image. The influence of background radionuclide on the accuracy of quantitation was studied by filling the background volume of the phantom with water containing 7 or 14 percent of the concentration of the radioactive water placed in the liver and spleen. The best estimates of radionuclide content were obtained by quantitation from the GM of counts in conjugate images of the liver and spleen. Radionuclide content of the liver and spleen could be determined from a single image if correction for attenuation was available. In all instances, measurements were less accurate for the spleen and for either organ when 111In was used. These results further validate and extend observations reported by others and provide a basis for radiation dosimetry for these and similar radionuclides and organs in patients.