Frederick J. Bonte

Myocardial infarct imaging with Technetium 99m phosphates

Technetium-99m-phosphate imaging is particularly valuable in detecting (1) small transmural infarcts (3 g and larger in size); (2) new acute transmural infarcts in or near regions of old infarction; (3) acute subendocardial infarcts (larger than 3 g in size); (4) acute infarction in patients with left bundle branch block; and (5) perioperative myocardial infarction. Localization of inferior and posterior myocardial infarction is improved with imaging. Sizing of acute anterior and lateral infarcts has been accurately done in dogs and should prove helpful in patients. Extensive evaluation in both experimental animals and in patients has shown 99mTc-phosphate myocardial imaging to be a useful clinical tool, and it may be one of the most sensitive noninvasive ways presently available to identify acute myocardial necrosis. It is important to understand that 99mTc-phosphate imaging has a different pathophysiology basis from EKGs or serum enzymes. These tests do not compete but instead should complement one another.

Pathophysiologic considerations and clinicopathological correlates of technetium-99m stannous pyrophosphate myocardial scintigraphy

99mTc-PYP myocardial scintigrams represent a means to detect and localize acute myocardial necrosis. These scintigrams are expected to be abnormal with acute myocardial infarcts of at least 3 grams in weight if serial imaging is utilized and proper attention to technique is provided. Any etiology of myocardial necrosis may produce abnormal 99mTc-PYP scintigrams if the damage is relatively localized and includes at least 3 grams of tissue. It is possible to accurately size acute anterior and anterolateral transmural myocardial infarcts using area or 2 dimensional measurements. Further development in imaging cameras and computer techniques allowing three dimensional reconstruction of myocardial infarcts with this and similar imaging techniques may allow relatively precise quantitation of other types of myocardial infarcts. The doughnut and persistently abnormal 99mTc-PYP scintigrams appear to have anatomic and prognostic significance at least in subsets of patients studied, but larger numbers of individuals need to be evaluated before final conclusions regarding their ultimate prognostic significance can be reached.

Radionuclide evaluation of cardiac trauma

Radionuclides were first used in the evaluation of myocardial trauma as a noninvasive means to detect hemopericardium. At present an important use is in the diagnosis of myocardial contusion, which can be difficult to recognize clinically, and often has nonspecific EKG and enzyme alterations. Technetium-99m pyrophosphate scintigraphy has been shown to be of significant value in confirming this diagnosis. Myocardial scintigrams are helpful in determining the degree of damage produced by penetrating wounds of the heart and can also detect electrical injury to the heart from accidental shock or cardioversion. In addition, multiple gated blood pool scans can determine the hemodynamic significance of mycardial trauma and evaluate for late sequelae such as aneurysm formation.

Multiple Parameter Estimation from Tomographic Inert Gas Clearance Curves: A Modification on the Double Integral Method

This paper describes a method for measuring relative perfusion (uncoupled from partition coefficient), relative “pseudo” partition coefficient, and input function delay using data from a dynamic single photon emission tomograph (DSPECT). The method produces regional transverse section images of these three parameters. While the examples presented here will use data from the TOMOMATIC 64*, the method is valid for application to data from inert gas clearance studies where (a) the input function is known, and (b) the observed tissue can be assumed to be dominated by a single compartment washout.

Technetium-99m-Pyrophosphate Myocardial Imaging in Acute Myocardial Infarction

The recognition of acute myocardial infarcts is not always easily accomplished. Infarct recognition is especially difficult using electrocardiography in individuals who had previous myocardial infarcts, those with left bundle branch block, those who have been cardioverted, and those with acute non-transmural (subendocardial) myocardial infarcts. Even the most sophisticated enzymatic techniques presently available have certain limitations in identifying the presence of absence of acute myocardial infarcts in patients including: (1) there is a temporal dependency in the ability of various enzyme markers to detect acute myocardial infarcts, and (2) certain clinical settings preclude using traditional enzyme techniques (including creatine kinase — MB isoenzyme) for infarct recognition and to be emphasized in this regard is the perioperative and postoperative setting after coronary artery revascularization. Therefore, it is important to have additional relatively noninvasive means that allow infarct detection, localization and provide some estimate of the size of the lesion.

Mechanisms of Technetium-99m-Pyrophosphate Accumulation in Damaged Myocardium

The purpose of this chapter is to summarize the available information concerning the pathophysiological basis for the use of the “hot spot” myocardial imaging technique, 99mTc stannous pyrophosphate in the detection of irreversibly damaged myocardial tissue, including acute myocardial infarcts. 99mTc-pyrophosphate is classified as a “hot spot” imaging technique since it concentrates in acutely infarcted myocardium. Table 3-1 identifies other “hot spot” imaging techniques that also allow the recognition of acute myocardial necrosis. This chapter will concentrate on the use of 99mTc-pyrophosphate as an “infarct avid” or “hot spot” agent to detect acute myocardial infarcts.

Technetium-99m-Pyrophosphate Myocardial Imaging in Patients with Atypical Chest Pain

Technetium-99m-pyrophosphate myocardial scintigrams may be utilized to help exclude the presence of acute myocardial infarcts in patients with atypical chest pain that are admitted to the coronary care unit. Our previous clinicopathologic correlates have suggested that 99mTc-pyrophosphate myocardial scintigrams are capable of identifying acute myocardial necrosis with 89% sensitivity and high specificity; this scintigraphic approach has an even higher sensitivity (one approaching 100%) in the identification of acute myocardial necrosis amounting to 3 g or more in weight when serial myocardial imaging is utilized and imaging is performed within the proper time frame (Table 13-1) [1–3]. When two 99mTc-pyrophosphate myocardial scintigrams are obtained in the first 24 hr to 5 days after acute myocardial infarction, one may be confident that serial negative 99mTc-pyrophosphate myocardial scintigrams exclude acute myocardial necrosis amounting to 3 g or more tissue with better than 95% sensitivity (Table 13-1)[1,2]. This, of course, requires optimal imaging technique and imaging within the appropriate time periods after the onset of symptoms (chapters 3 and 7) and some experience so that one may properly interpret the 99mTc-pyrophosphate myocardial scintigrams.

Technetium-99m-Pyrophosphate Myocardial Imaging in Unstable Angina

Technetium-99m-pyrophosphate myocardial scintigrams may be abnormal in patients with unstable angina pectoris even if serum enzymes are normal and the electrocardiogram is either normal or nonspecifically abnormal and demonstrates ST-T wave changes [1–3]. Abnormal 99mTc-pyrophosphate scintigrams in these patients often demonstrate faint (“2+”) and poorly localized increased 99mTc-pyrophosphate uptake (Figures 11-1, 11-2 and 11-3).

Technetium-99m-Pyrophosphate Myocardial Imaging

Technetium-99m is the most widely used radioisotope in nuclear medicine presently. It has nearly ideal decay characteristics, including a half-life of 6 hr, monoenergetic and highly abundant gamma rays (140 keV, 90%) and absence of beta or alpha particle emission [1]. It is also easy to produce and readily available in generator form. It has the ability to complex with many organic and inorganic compounds [1]. Technetium-99m is the daughter product of 99Mo. Molybdenum-99 has a half-life of 67 hr and undergoes both gamma and beta decay, the beta-decay leading to its daughter product 99mTc. Tech-netium-99m can be separated easily from its parent product by simple solvent extraction with methylethylketone under basic conditions or by column chro-matography on an alumina column. Simple elution with isotonic saline brings ionic pertechnetate (99mTcO4) through the column, leaving 99mMo behind, which, by its subsequent decay, generates more 99mTc that can be eluted later. Technetium-99m stannous pyrophosphate was chosen as a potential imaging agent to identify acute myocardial infarcts with the rationale being that pyrophosphate might complex with calcium which is deposited in irreversibly damaged myocardial cells following acute myocardial infarction [2, 3].

Comparative Value and Limitations of Thallium-201 and Technetium-99m-Pyrophosphate Myocardial Imaging in Acute Myocardial Infarction

Comparing the results of 201T1 and 99mTc-pyrophosphate myocardial imaging in patients with acute myocardial infarction, it is evident that both imaging techniques provide different information and each has its own advantages and disadvantages. For the practical application in the coronary care unit, it is essential to realize that each method yields its best results at different time intervals after onset of myocardial infarction (Figure 8-1).