Radioisotopic studies of the regional myocardial circulation

Application-specific nuclear medicalin vivoimaging devices

Nuclear medical imaging devices, such as those enabling photon emission imaging (gamma camera, single photon emission computed tomography, or positron emission imaging), that are typically used in today's clinics are optimized for assessing large portions of the human body, and are classified as whole-body imaging systems. These systems have known limitations for organ imaging, therefore application-specific devices have been designed, constructed and evaluated. These devices, given their compact nature and superior technical characteristics, such as their higher detection sensitivity and spatial resolution for organ imaging compared to whole-body imaging systems, have shown promise for niche applications. Several of these devices have further been integrated with complementary anatomical imaging devices. The objectives of this review article are to (1) provide an overview of such application-specific nuclear imaging devices that were developed over the past two decades (in the twenty-first century), with emphasis on brain, cardiac, breast, and prostate imaging; and (2) discuss the rationale, advantages and challenges associated with the translation of these devices for routine clinical imaging. Finally, a perspective on the future prospects for application-specific devices is provided, which is that sustained effort is required both to overcome design limitations which impact their utility (where these exist) and to collect the data required to define their clinical value.

A beat-to-beat calculator for the diastolic pressure time index and the tension time index

We have developed a beat-to-beat calculator which can calculate in real-time the ratio of the diastolic pressure time index (DPTI), and the tension time index (TTI) as an index of the myocardial oxygen supply/demand balance. Physicians set up presumed value for the left ventricular endodiastolic pressure, a search area for the dicrotic notch, a threshold for the onset of the up-slope and the corresponding value of the calibration signal on the digital switches of the calculator. Next, the arterial pressure analog signal is input into the calculator. The calculator searches automatically for both the onset of the up-slope and the dicrotic notch. The arterial pressure curve is displayed beat-to-beat with the recognized onset and the dicrotic notch on the CRT to be confirmed by physicians. When physicians do not agree with the automatic recognition they can fit the automatic recognition to the observation. If the recognition of the onset is inadequate, the threshold can be re-adjusted to trigger the onset. If recognition of the dicrotic notch is inadequate, the physician can adjust the search-area. Therefore, physicians who operate the calculator can rely on the calculated DPTI/TTI. This calculator can continuously monitor the myocardial oxygen supply/demand balance in patients with acute myocardial infarction or just after open-heart surgery.

Measurement of regional myocardial perfusion using collimated miniature radiation detectors and the method of 133-xenon washout

A new approach to the measurement of local myocardial perfusion based on the application of highly-collimated miniature cadmium telluride radiation detectors to measure washout of 133-Xenon from well-defined tissue volumes is presented. Single-hole collimators with length/diameter ratios of 1 (L = 4 mm, D = 4 mm) and 4 (L = 12 mm, D = 3 mm) were employed as prototype designs. The probe field of view was characterized theoretically using a spherical model of the myocardium in conjunction with experimental point source response measurements for each collimator. Method evaluation using two medium resolution collimators (L/D = 1) was effected by performing left main and circumflex coronary artery perfusion studies at controlled but variable flows in the dog heart. An excellent correlation (r greater than 0.99) between actual and estimated perfusion determined from 65 washout curves over the flow range 0.38 to 3.18 ml/min/gm was demonstrated. The ability to resolve regional flow differences was verified by measuring tracer washout from circumflex and left anterior descending regions of the myocardium, using two high resolution collimators (L/D = 4), in a preparation where the left main coronary artery was cannulated and a snare was placed around the circumflex coronary artery to selectively reduce flow to that region.

Quantitative aspects of myocardial perfusion imaging

Myocardial perfusion measurements have traditionally been performed in a quantitative fashion using application of the Sapirstein, Fick, Kety-Schmidt, or compartmental analysis principles. Although global myocardial blood flow measurements have not proven clinically useful, regional determinations have substantially advanced our understanding of and ability to detect myocardial ischemia. With the introduction of thallium-201, such studies have become widely available, although these have generally undergone qualitative evaluation. Using computer-digitized data, several methods for the quantification of myocardial perfusion images have been introduced. These include orthogonal and polar coordinate systems and anatomically oriented region of interest segmentation. Statistical ranges of normal and time-activity analyses have been applied to these data, resulting in objective and reproducible means of data evaluation.

Rationale and radiopharmaceuticals for myocardial imaging

Static radionuclide imaging procedures are now available for evaluating regional myocardial perfusion and for detecting acute myocardial infarction. Thallium-201, a radiopharmaceutical that possesses many of the characteristics of potassium analogues, at present is receiving the greatest attention as a regional blood flow indicator. Ischemic lesions appear as areas of decreased tracer uptake. Unfortunately, this agent is expensive, is in limited supply, and has a photopeak that is low for optimum imaging. Positive infarct images can be obtained with various Technetium-99m chelates. Pyrophosphate appears to be the best of the technetium compounds studied to date, although the mechanism of uptake of the chelates has not yet been fully elucidated. Therefore, quantitative measurements of infract size are not justified. As perfusion imaging and infarct imaging provide useful, complementary data, a dual tracer approach to evaluating patients with suspected coronary artery disease and/or myocardial infarction, is probably justifiable.

Studies of regional myocardial blood flow: results in patients with left anterior descending coronary artery disease

Measurements of regional myocardial blood flow have been performed rapidly and safely in man at the time of coronary arteriography. Xenon-133 was injected into the coronary artery and estimates of myocardial capillary perfusion were made by computer analysis of the multiple precordial tracer washout curves recorded with a scintillation camera. Blood flow rates/100 g/min in different areas of the heart were calculated by the Schmidt-Kety formula, using an assumed partition coefficient, and were related to the coronary vascular lesions by landmarks provided by the arteriograms. The flow significance of lesions was assessed by making measurements at rest and during interventions that increase coronary blood flow. Perfusion was also estimated in areas supplied by collateral vessels, and regions of scar were detected from the initial distribution of peak tracer activity. In patients with less than 50% obstructions apparent on coronary arteriograms and in those with greater than 50% isolated left anterior descending lesions average mean left ventricular (LV) myocardial perfusion values were not reduced below values found in patients with normal coronary arteries and normal cardiac function. However, in patients with significant two-vessel disease (LAD + right, LAD + circ) mean LV perfusion was significantly reduced. Average regional myocardial perfusion distal to significant lesions was not selectively reduced below the remainder of the ventricle unless the lesions were 100% obstructions. However, both total and distal regional myocardial perfusion increased less in 12 patients with greater than 50% lesions than in 12 others with normal arteriograms or less than 50% lesions. Subnormal flow responses in all of the former group were associated with angina pectoris compared with no angina in the latter. The studies indicate the potential utility of the measurements of regional myocardial blood flow in the evaluation of patients with ischemic heart disease who are potential candidates for myocardial revascularization procedures.

Quantitative evaluation of coronary perfusion in man

The usefulness and limitations of currently available techniques for quantitating coronary flow in ischemic heart disease are summarized. There are appreciable difficulties in assessing coronary flow solely from an arteriographic evaluation of the epicardial arteries. There is a considerable reserve mechanism for vasodilation at the arteriolar level, and a proximal occlusive lesion produces a reduction in flow only after this distal reserve has been exhausted. In addition, small increments in the severity of established lesions sometimes cause profound reductions in flow. The development of clinically useful flow measurements has been impeded by methodological problems related to nonuniformity of flow within the left ventricle in coronary artery disease. Validation of specific techniques for abnormal situations is difficult but possible and should probably be a prerequisite to the clinical application of any technique. When a methodologically appropriate technique is employed, average left ventricular flow per unit weight is found to be reduced systematically at rest in patients with double- and triple-vessel disease. This reduction is a group difference, however, and is not always evident in individual patients. Accordingly, more recent measurements have concentrated on the assessment of regional perfusion, and two general approaches, selective venous sampling and selective precordial sampling are illustrated. While only preliminary measurements of regional flow are available, it is clear that these measurements offer a more sensitive tool for detecting abnormalities of flow in individual patients and thereby for contributing to the management of specific clinical problems. Measurements of regional flow need to be performed during stress as well as at rest. For the future, there is also need for techniques which can assess transmural variations of flow in man and relate measurements of regional flow to regional oxygen demand. Because of the complexity of current techniques which are methodologically adequate, measurements of coronary flow will, for the immediate future, probably remain confined to clinical centers which have a special interest in them. The effort in these centers will hopefully include significant emphasis on the refinement of existing techniques so that they are more widely applicable.