Technetium-99m in the diagnosis of left-to-right shunts.

Radio-isotope techniques for the detection of intracardiac shunts have often employed extemal counting over the praecordium (Prinzmetal, Corday, Spritzler, and Flieg, 1949; Huff, Parrish, and Crockett, 1957; Greenspan, Lester, Marvin, and Amplatz, 1959; Turner, Salazar, and Gorlin, 1960; Shapiro and Sharpe, 1960). However, Folse and Braunwald (1962) counted over the upper lung field and claimed to have resolved by this means those effects from blood flow through the various chambers of the heart on curves from praecordial counting. Shunts were detected by a lowered clearance rate of isotope from the lung field due to the continuous reintroduction of blood into the right heart and lesser circulation through the septal defect. Limitations in the accuracy of external counting techniques are imposed largely by the amount of isotope which can be administered with safety to the patient, and by the degree of definition of the anatomic field viewed by the detector head. The usual detection system employed in external counting incorporates a y-ray sensitive crystal usually made of thallium-activated sodium iodide, which is covered for most of its surface by a lead shield so that radioactivity from sites other than the desired one is excluded. The field viewed can be further limited by fitting a lead tube or collimator to the detector head; this excludes the reception of virtually all radioactivity other than that in the body tissues directly beneath the tube (Fig. 1). With these limitations, the use of 1311 for shunt diagnosis is somewhat inadequate, since with the usual dose of 20 to 50 tAc, and a collimation similar to that employed in the present detector head, the counting rates achieved are too low to afford precise information. LEAD SHIELD