Radionuclide determination of myocardial blood flow

The diffusible-indicator method of determining tissue blood flow was devised by Kety and his associates, ^1-3who observed the washout of an intraarterially injected tracer from the tissue of interest, and found that it was proportional to tissue blood flow. Kety's original tracer was nitrous oxide, but he soon adapted his method to the use of ²⁴Na. Other investigators developed Kety's method further, substituting ⁸⁵Kr, and ultimately, ¹³³Xe as diffusible indicators. It has been found that if ¹³³Xe is injected directly into a coronary artery and its washout from myocardium is observed with a scintillation probe over the precordium, the resulting determination, mean myocardial blood flow, is of limited application. Since coronary artery disease is a regional process, the most useful determination is one that yields regional myocardial blood flow. This may be determined by one of the original Kety methods,³ i.e., observing the washout of tracer injected directly into the myocardium, but since it requires thoracotomy this method is not widely applicable. Several groups have assembled instrument systems based on the use of scintillation camera-computer combinations with which they can enter the image of the passage of a bolus of intracornoary arterially injected tracer, and by means of image data quantification derive regional myocardial blood flow values by Kety's method. The authors have studied more than 130 dogs before and after experimental coronary embolization and have described a complete method of deriving regional myocardial blood flows with an Anger camera-small computer system. Analysis of flow curves thus generated has suggested the existence of more than one compartment within myocardial blood flow. These compartments might be related to primary/collateral flow or to the volume of perfused tissue incorporated in the region of interest. Cannon et al.^25-27 have employed a multicrystal camera of the autofluoroscope type and an IBM 360/91 computer, in which the camera functions as 294 isolated detectors for the purpose of identifying as many regions of myocardial blood flow. Cannon et al. have studied both normal human subjects and patients with radiographically demonstrable coronary artery disease and have found regional flow to be a valid method both for identifying the myocardial flow inhomogeneities expected with coronary artery disease and for evaluating the results of reparative surgery.