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The development of cardiovascular nuclear medicine imaging at the Texas Heart Institute.

ince its inception in 1969, the Department of Nuclear Medicine at St. Luke's Episcopal Hospital has maintained a strong emphasis on cardiovascular nuclear medicine (CVNM) for the patients served by the Texas Heart Institute. While its foremost goal is to provide state-of-the-art clinical imaging services, the department continues to be a national leader in the evaluation of gamma-ray imaging systems, radiopharmaceuticals, and specialized computer software. 1 Although radionuclide imaging was not new in 1969, its early use was restricted by equipment limitations and the by lack of appropriate radiopharmaceuticals. Most of the initial emphasis in the field had been upon malignant diseases, with imaging limited to rectilinear scanning of static targets. Two major advances in the mid-1960s, however-the introduction of the Anger scintillation camera and the availability of the artificially produced radioisotope, technetium-99m (Tc99m)-made dynamic studies of the cardiovascular system and other organs possible, thereby changing the entire complexion of nuclear medicine. Nothing better exemplifies the development of CVNM than the history of myocardial perfusion scanning at the Texas Heart Institute. The current radiopharmaceutical of choice for evaluating myocardial perfusion is thallium-201 (thallous) chloride (Tl-201), which was introduced at the Texas Heart Institute in 1974 while still an Investigational New Drug (IND). Tl-201 is a potassium analog that is deposited intracellularly in myocytes and other cells in proportion to perfusion and cellular metabolism. Initially, Tl-201 imaging was accomplished by obtaining planar images of the chest from 3 different views with the patient at rest. The test was soon modified, however, to incorporate images obtained immediately after the patient had exercised on a treadmill or bicycle. These simple film images were interpreted visually by comparing the relative intensities of tracer activity in different regions of the myocardium. Though the format of the test remained essentially unchanged for the next 10 years, other aspects of the procedure changed dramatically. A number of instrument adaptations have been made to improve image quality. The most notable by-product has been single photon emission computed tomography (SPECT), which is similar to x-ray transmission computed tomography (CT-the "CAT scan"). In the mid 1970s, SPECT was evaluated at St. Luke's with a prototype instrument:1 principles of that instrument form the basis of stateof-the-art thallium imaging today. Along with the improvement of thallium imaging equipment, advances in computer applications have generated increasingly sophisticated techniques of semi-quantitative analysis and display, culminating in today's polar coordinate mapping ("bullseye") technique. The "bullseye" is so named because it comprises multiple tomographic slices along the short axis of the left ventricle, arranged in a series of concentric rings. This configuration condenses digital information from multiple slices into a more readable pattern and allows comparison of a particular patient's thallium distribution with a "normal pattern" based on a composite image from patients without cardiac disease (Fig. 1). Further assessment and display of the data allow the patient's tracer uptake to be analyzed in a qualitative and quantitative manner,