A Comparison of Results of Brain Scanning Using Ga68-EDTA and the Positron Scintillation Camera, with Hg203-Neohydrin and the Conventional Focused Collimator Scanner
The results of isotope brain scanning utilizing two different isotopes and the same scanning system have been reported in the literature for such agents as positronemitting AS74vs. CU64 (1), AS74vs. Ga68 (2), and gamma-emitting RI13lSA vs. Hg203_ Neohydrin (3,4). This study, however, is unique in that a comparison has been made between conventional photoscanning with a gamma-emitter and scintiphotography with the scintillation camera, employing a positron-emitter. Scanning Technic Hg203_Neohydrin: Patients were given an intramuscular injection of nonradioactive Mercuhydrin the day before scanning. Radio-Neohydrin Hg203 was injected intravenously in a dose of 8.0 microcuries per kilogram of body weight. Four hours later a brain scan was performed with at least 2 projections obtained in each case. Photoscans were made with the Picker Magnascanner, utilizing the 19-hole collimator. The range differential was usually set at 30 per cent, with factors of speed, background cut-off, voltage, and light intensity adjusted for the individual study. The average time to complete a scan in two projections was one hour. Ga68-EDTA: Details of the method and of the positron scintillation camera are available in the literature (5, 6) so only a brief summary is included here. Ga68 is a positron-emitter with a physical half-life of sixty-eight minutes, and an effective half-life of about forty minutes. As the ethylenediamine tetraacetic acid (EDTA) chelate, the isotope remains primarily in the extracellular space, and is rejected by the normal brain. Tumors are visualized because of the breakdown of the bloodbrain barrier, an abnormal vascular supply, or a combination of both factors. An intravenous dose of 700 to 750 microcuries of Ga68-EDTA gives a wholebody dose of less than 30 millirads, and a renal dose of less than 150 millirads. Initial scintiphotographs are taken five minutes after injection. The time of each exposure varies from four to ten minutes and depends on the decay of the short-lived isotope. Since the scintillation camera sees its entire field of view at all times, problems introduced by the decreasing count rate of Ga68 are met simply by exposing until 40,000 dots are recorded. This value has been empirically determined to produce satisfactory brain scintiphotos. Frontal and lateral exposures are always obtained, and because of the speed with which each picture is made, both right and left lateral, face-up and face-down exposures are frequently taken. In the immediate postinjection period before equilibrium with the extracellular space has occurred, a relative predominance of Ga68 is found in the plasma. I t is sometimes possible to detect optimum visualization at this time, as compared to later exposures. This suggests a highly vascular tumor such as a meningioma or glioblastoma.