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

Three planar, gamma camera methods for quantitating radiopharmaceuticals, s uch as radiolabeled antibodies, were investigated iodine-123 (I-123), techn etium-99m (Tc-99m) or indium-111 (In-111) in the fiver and spleen of an abd ominal phantom were assessed in this study. In the first approach, the numb er of counts detected in a "single image" of the liver or spleen was used t o measure radionuclide content using an attenuation correction factor (ACF) calculated from data obtained without radionuclide in the background volum e of the phantom. In the other two methods, radionuclide content was derive d from either the geometric mean (GM) of counts in conjugate, opposed image s ("global conjugate") or in individual opposed pixels of the conjugate, op posed images ("pixel conjugate") of the liver and spleen. Both of the conju gate image methods were corrected for attenuation with a first order ACF de rived from a transmission image. The influence of background radionuclide o n the accuracy of quantitation was studied by filling the background volume of the phantom with wafer containing 7 or 14 percent of the concentration of the radioactive water placed in the fiver and spleen. The best estimates of radionuclide content were obtained by quantitation from the GM of count s 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 accurat e for the spleen and for either organ when In-111 was used These results fu rther validate and extend observations reported by others and provide a bas is for radiation dosimetry for these and similar radionuclides and organs i n patients.