THOUGH many radioactive isotopes have been tried for brain scanning, the most widely used have been arsenic 74 and radioactive iodine 1~1 tagged onto serum albumin (RISA). With these agents and a variety of scanning methods satisfactory detection has been reported in 80 per cent or more of patients who subsequently proved to have intracranial tumours. 2,4,~,9 But each of these radioisotopes, despite its usefulness, has certain disadvantages. Arsenic 74, although applied successfully to brain scanning over the past 10 years, remains costly and is difficult to obtain for routine use. A high dosage is required to exploit the low percentage of emission of positrons and optimal localization is dependent upon an elaborate scanning system with matching of scans of gamma radiation and positrons. RISA is readily available, le,ss costly and more familiar from its common use in a variety of clinical applications. But it has two properties that also limit its value. First, in addition to the main emission of gamma rays at 860 Key (87 per cent), iodine in gives higher-energy gamma rays at 640 Key (9 per cent) and 7~0 Key (3 per cent). This makes shielding of scattered radiation a difficult problem. Moreover, this high-energy gamma radiation is scattered and reduced in energy as it passes through the tissues of the head, so that some of it contributes to the main level of energy detected at 360 Key. This results in less efficient resolution and identification of the gamma radiation derived primarily from the site of the lesion.
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Feindel et al. (1964) studied this question.