Positron emission tomography of the heart

The diagnostic value of QRS changes for prediction of coronary artery disease during exercise testing in women: false-positive rates

OBJECTIVES

The false positive rate of electrocardiographic exercise testing (ET) for coronary artery disease (CAD) in women ranges from 38 to 67%, using the ST-T changes (ST-T) criteria. The aim of this study was to compare the diagnostic accuracy of QRS duration change criteria with ST-T change criteria during ET.

METHODS

We studied 234 women (mean age 58+/-17 years, range 27-83 years), of whom 160 were pre-menopausal (PrMW; mean age 41+/-9, range 27-56) and 74 were post-menopausal (PoMW; mean age 65+/-7, range 57-83). All participants underwent ET to rule out CAD, followed by thallium stress testing (TL). QRS duration, measured with a computerized optical scanner and ST-T changes at peak ET were compared with TL. An ischemic QRS response was defined as an exercise-induced prolongation of QRS duration >3 ms.

RESULTS

The sensitivities of QRS duration changes for the entire study group, the PrMW group and the PoMW group in comparison with TL, were 93, 88 and 92%, respectively, while the corresponding rates of specificity were 91, 85 and 91%, respectively. The sensitivities of ET ST-T changes for the entire study group, for the PrMW group and for the PoMW group were 48, 46 and 54%, respectively, while the corresponding rates of specificity were 62, 75 and 79%, respectively. The false-positive rate was 20% for ischemic ST-T and 4% for ischemic QRS duration for the entire study population.

CONCLUSIONS

Computer-measured QRS duration changes during ET are more sensitive and specific than ST-T changes for the detection of ischemia in women.

Significance of reduced uptake of iodinated fatty acid analogue for the evaluation of patients with acute chest pain

OBJECTIVES

To assess whether 15-(p-[iodine-123] iodophenyl)-3-(R,S) methylpentadecanoic acid (BMIPP) imaging can identify previous ischemic areas, BMIPP SPECT was performed in patients with acute chest pain to compare its findings with those of technetium-99m-tetrofosmin (tetrofosmin) SPECT and coronary angiography.

BACKGROUND

Basic studies indicate that BMIPP can identify previous ischemia as reduced tracer uptake.

METHODS

This study prospectively enrolled 111 consecutive patients with acute chest pain without myocardial infarction. Tetrofosmin SPECT was performed at rest within 24 h after the last episode of chest pain. Coronary angiography and BMIPP SPECT were also performed on the following day.

RESULTS

Sixty-four of the 87 patients with coronary stenosis or spasm showed BMIPP abnormalities corresponding to the areas of coronary abnormalities (sensitivity 74%), whereas only 33 of them showed perfusion abnormalities (sensitivity 38%) (p < 0.001). Of 24 patients [corrected] without coronary stenosis or spasm, 22 showed normal BMIPP SPECT (specificity = 92%) [corrected] and 23 showed normal tetrofosmin SPECT (sensitivity = 96%) [corrected]. Coronary stenosis was more often seen in the group with abnormal tetrofosmin/abnormal BMIPP (82%) and with normal tetrofosmin/abnormal BMIPP (69%) than in the group with normal tetrofosmin/normal BMIPP (36%) (p < 0.05). Coronary spasm was observed more often in the group with abnormal tetrofosmin/abnormal BMIPP (83%) and with normal tetrofosmin/abnormal BMIPP (90%) than in the group with normal tetrofosmin/normal BMIPP (27%) (p < 0.05). The extent and severity scores of tetrofosmin and BMIPP in the patients with organic stenosis were significantly higher than those of patients with no organic stenosis or spasm (p < 0.0001).

CONCLUSIONS

These data indicate that BMIPP SPECT may specifically identify previous ischemic lesions due to coronary stenosis or spasm in patients with acute chest pain.

Manufacture of strontium-82/rubidium-82 generators and quality control of rubidium-82 chloride for myocardial perfusion imaging in patients using positron emission tomography

We describe a protocol to manufacture 82Sr/82Rb generators and 82RbCl for myocardial imaging with PET. The generators are manufactured in 3 stages: (1) preparation of a tin oxide column, (2) leak test of the generator column and (3) loading of the generator with 82Sr. The generators produced sterile and non-pyrogenic 82RbCl for i.v. injection. No significant 82Sr/85Sr breakthroughs were observed after elution with 20 1 of saline. The automated system delivered human doses of 82RbCl accurately.

Prognostic value of iodine-123 labelled BMIPP fatty acid analogue imaging in patients with myocardial infarction

This study was undertaken to evaluate the prognostic value of iodine-123 labelled 15-iodophenyl-3-R,S-methyl pentadecanoic acid (BMIPP) imaging in patients with myocardial infarction. BMIPP is an iodinated methyl branched fatty acid analogue which is trapped in the myocardium with little washout, thereby reflecting fatty acid utilization in the myocardium. We previously reported that in patients with myocardial infarction, regions are often observed where reduced BMIPP uptake is seen relative to thallium-201 perfusion at rest. However, the clinical significance of such discordant BMIPP uptake remains unknown. Fifty consecutive patients with chronic myocardial infarction referred for stress thallium scan and coronary arteriography underwent BMIPP imaging at rest. Each patient was in a stable condition at the time of the radionuclide study. Follow-up was performed at a mean interval of 23 months to investigate the prognostic implications of the radionuclide studies. Nine patients had cardiac events during the follow-up period. Univariate analysis showed that the number of discordant BMIPP versus 201TL uptake segments was the best predictor of future cardiac events (P=0.0245), followed by the presence of discordant BMIPP uptake (P=0.0388) and the number of 201TL redistribution segments (P=0.0444). When all the clinical and radionuclide variables were analysed by Cox regression analysis, the presence of discordant BMIPP uptake was the best, and an independent, predictor of future cardiac events (chi 2=8.5) followed by the number of coronary stenoses on angiography (chi 2=3.9). These preliminary data suggest that decreased BMIPP uptake relative to 201TL is a valuable predictor of future cardiac events in patients with myocardial infarction. Areas with such discordant BMIPP uptake may contain jeopardized myocardium where fatty acid utilization has been severely suppressed relative to myocardial perfusion.

Synthesis of fatty acids specifically labelled with 11C in various positions, including 2H substitution, for in vivo studies of myocardium using PET

Fatty acids were labelled with 11C in several positions by reacting [11C]carbon dioxide with the appropriate Grignard reagent or by reacting a alpha, omega-bis-(bromo magnesium) alkane with a 11C-labelled alkyl iodide followed by a reaction with carbon dioxide. The methyl and methylene 11C-labelled fatty acids were obtained in 12-36% (decay corrected) radiochemical yield within 45-65 min, and with radiochemical purities higher than 96%. Perdeuterated alpha, omega-dibromo hexane, decane and tetradecane were synthesized from dimethylacetylene dicarboxylate by means of a Raney-nickel reduction in D2O, Kolbe electrolysis and LAD reduction. The use of multiple isotopic labelling by the combination of position specific 11C labelling and 2H substitution, has the potential to highlight different aspects of a complex biochemical system by PET. This principle is illustrated by results of the kinetics of different types of 11C label of dodecanoic acid and the corresponding moieties of acetate. The combination of tracers allows the kinetics of beta-oxidation of middle length carbon chain fatty acids and citric acid cycle metabolism to be separately assured, whilst deuteration of the tracers opens the possibility of highlighting the kinetics of the proton extraction processes reflecting rate limiting steps.

Contrast induced myocardial signal reduction: effect of lanthanide chelates on ultra high speed MR images

The myocardial MR signal reduction associated with an intravenous bolus of Gd-DTPA and Dy-DTPA was studied in a canine model. Imaging was performed with a high speed echo-planar type imaging system (Instascan, Advanced NMR Systems, Inc.). Gated spin-echo images were obtained with TE of 30 ms, which permits image acquisition in approximately 40 ms. The gated TR was dependent on the heart rate, with an average TR of 2.4 s. After 0.1 mmol/kg of contrast was injected, 70 images were acquired, which showed in an 80-image data set a reduction in myocardial signal with a gradual return to normal. After dipyridamole infusion, the signal loss was significantly more pronounced, and earlier than in the control data set. There was no significant difference between Gd-DTPA and Dy-DTPA in these imaging studies despite the theoretical prediction of better Dy signal reduction, possibly due to physiological variability during the course of a study or between studies. The cause of enhanced contrast effect after dipyridamole infusion is discussed, as is the basis for dipyridamole enhancement, and the possible role of contrast enhanced MR imaging in the detection of cardiac disease.

Detection and assessment by positron emission tomography of a genetically determined defect in myocardial fatty acid utilization (long-chain acyl-CoA dehydrogenase deficiency)

Genetic defects in fatty acid oxidation are important, inherited causes of cardiomyopathy, skeletal myopathies, and childhood sudden death. The clinical manifestations and their severity vary widely among affected subjects and different age groups. Although measurement of serum and urinary fatty acid intermediary metabolites and enzymatic assays establish the diagnosis of a defect in fatty acid oxidation, they do not predict the specific clinical manifestations nor their severity in a given subject. To determine whether impaired myocardial fatty acid utilization, indicative of cardiac phenotypic expression of a specific genetic abnormality in fatty acid oxidation, can be detected, cardiac positron emission tomography with the metabolic tracers carbon-11-labeled palmitate and acetate was performed in 6 patients with long-chain acyl-CoA dehydrogenase (ACD) deficiency and in 9 control subjects. The myocardial extraction of both tracers was similar in patients and controls. The rate of clearance of palmitate from myocardium was significantly prolonged in patients compared with that in control subjects (0.022 +/- 0.012 vs 0.061 +/- 0.033 min-1; p < 0.025), indicative of a decreased rate of oxidation of long-chain fatty acids. Furthermore, the extent of diminution of clearance of palmitate, quantified in terms of the rate of clearance for palmitate divided by that for acetate (to correct for individual differences in overall mitochondrial oxidative metabolic flux), correlated with the clinical severity of the long-chain ACD deficiency. Accordingly, noninvasive evaluation with positron emission tomography may not only facilitate diagnosis, but also enable assessment of the pathogenetic impact and effects of therapeutic interventions in the hearts of subjects with specific, inherited defects in fatty acid oxidative metabolism.

Use of carbon-11 acetate and dynamic positron emission tomography to assess regional myocardial oxygen consumption in patients with acute myocardial infarction receiving thrombolysis or coronary angioplasty

Carbon-11 (C-11) acetate has been introduced for the noninvasive measurements of myocardial oxygen consumption. This study was designed to assess regional C-11 acetate clearance in patients with acute myocardial infarction. Thirty-one patients were studied within 8 days of acute myocardial infarction. C-11 acetate washout-rate constants were significantly lower in the infarct territory than in the remote myocardium (p < 0.008). The scintigraphic measurements correlated with heart rate-blood pressure product in the remote as well as infarct areas (0.52 and 0.48, respectively). There was no significant correlation to left ventricular ejection fraction. C-11 washout rates were significantly affected by beta-receptor therapy as assessed by multiple regression analysis. Thus, C-11 acetate kinetics allow noninvasive characterization of regional myocardial oxygen demand, which may be useful in assessing the extent of myocardial injury and myocardial oxygen demand of remote myocardium.

Assessment of myocardial fatty acid metabolism with positron emission tomography at rest and during dobutamine infusion in patients with coronary artery disease

The myocardial clearance rate of C-11 palmitate as an index of fatty acid oxidation was assessed by means of positron emission tomography (PET) at rest and during dobutamine infusion in seven normal subjects and 10 patients with coronary artery disease. In the normal subjects the clearance half time was homogeneous in the left ventricle at rest and uniformly shortened during dobutamine infusion. In the myocardium at risk, clearance half time tends to be longer in the segments with an abnormal Q wave on ECG, exhibiting regional wall motion abnormality, and supplied by severely stenosed coronary arteries, particularly during dobutamine infusion. These data indicate that fatty acid oxidation may be decreased in infarcted myocardium and associated with regional asynergy. Such an abnormality was most striking in those with severe coronary stenosis during dobutamine infusion. We conclude that PET with the use of C-11 palmitate at rest and during dobutamine is a useful means of identifying impaired fatty acid oxidation and decreased metabolic reserve in patients with coronary artery disease.

A feasibility study on quantitating myocardial perfusion with Albunex, an ultrasonic contrast agent

Quantitating regional myocardial perfusion has been the much sought-after but still elusive goal of many intensive investigations over the years. Videodensitometry of the variation of myocardial echogenicity in two-dimensional (2-D) echocardiograms as a function of time in conjunction with the injection of a bolus of an ultrasound contrast agent has been used clinically as a tool for a direct assessment of regional myocardial perfusion, despite that the precise relationship between tissue echogenicity observed on an image and the echoes detected by the ultrasonic probe is unknown. A study was undertaken to determine whether ultrasonic backscatter calculated from unprocessed radio frequency (RF) echoes returned from myocardium could be used to quantitate regional myocardium perfusion. A real-time ultrasonic scanner has been modified and interfaced to a microcomputer to acquire RF data at a rate up to 10 frames per second. Preliminary experimental data were obtained from four open-chest dogs following intracoronary injection of a bolus of Albunex and two dogs following intravenous injection with this modified scanner. On one hand, these results indicate that the integrated backscatter measured from the region of myocardium perfused by the coronary artery where Albunex is injected and selected for monitoring initially increases, reaches a peak, and then decreases as the contrast agent is washed out and that the magnitude of the peak is approximately linearly proportional to the volume concentration of Albunex microspheres injected, clearly demonstrating the feasibility of this approach for quantitating region myocardial perfusion. On the other hand, intravenous injections did not result in any appreciable change in myocardial backscatter in the left ventricle although a response could be observed in the left ventricular blood pool.

Myocardial oxidative metabolism in normal subjects in fasting, glucose loading and dobutamine infusion states

Experimental studies indicated the clearance rate constant of 11C-acetate as an index of regional myocardial oxygen consumption. To assess the response of the clearance rate from the left ventricular (LV) myocardium to the change in plasma substrate levels and to the increase in the cardiac work load in normal subjects, a total of 18 dynamic positron emission tomographic studies were performed at rest in the fasting state (control) (n = 7), after oral glucose administration (n = 4), and during dobutamine infusion (n = 7) in 7 normal volunteers. The clearance rate constant (Kmono) was similar in the control (0.065 +/- 0.017 min-1) and glucose loading states (0.059 +/- 0.008 min-1), whereas a significant increase in Kmono was observed during dobutamine infusion (0.106 +/- 0.018 min-1) (p < 0.01) in relation to the increase in the pressure-rate product with a correlation coefficient of 0.873 (p < 0.01). When the LV myocardium was divided into 6 segments, there were no significant differences among the segments in Kmono values in any condition. These normal responses should be valuable for assessing oxidative metabolic reserve and regional changes in oxidative metabolism in patients with coronary artery disease.

Cyclotrons and positron emission tomography radiopharmaceuticals for clinical imaging

Positron emission tomography (PET) requires positron-emitting radionuclides that emit 511-keV photons detectable by PET imagers. Positron-emitting radionuclides are commonly produced in charged particle accelerators, eg, linear accelerators or cyclotrons. The most widely available radiopharmaceuticals for PET imaging are carbon-11-, nitrogen-13-, and oxygen-15-labeled compounds, many of which, either in their normal state or incorporated in other compounds, serve as physiological tracers. Other useful PET radiopharmaceuticals include fluorine-18-, bromine-75-, gallium-68 (68Ga)-, rubidium-82 (82Rb)-, and copper-62 (62Cu)-labeled compounds. Many positron emitters have short half-lives and thus require on-site cyclotrons for application, and others (68Ga, 82Rb, and 62Cu) are available from radionuclides generators using relatively long-lived parent radionuclides. This review is divided into two sections: cyclotrons and PET radiopharmaceuticals for clinical imaging. In the cyclotron section, the principle of operation of the cyclotron, types of cyclotrons, medical cyclotrons, and production of radionuclides are discussed. In the section on PET radiopharmaceuticals, the synthesis and clinical use of PET radiopharmaceuticals are described.

Hibernation and myocardial ischemia: clinical detection by positron emission tomography

Regions of myocardium supplied by severely diseased epicardial arteries may develop chronic ischemia at rest and exhibit reduced contractility, contributing to a reduction in global left ventricular function. However, after revascularization, contractility in these regions may return to normal. These regions of asynergy are described as "hibernating myocardium." Such myocardium in which normal contractility may be restored often coexists with areas of infarcted, or scar, tissue, leading to the definition of hypoperfused hibernating myocardium as viable myocardium. It is important to identify viable myocardium, as revascularization of these areas should lead to the greatest improvement in left ventricular function and, thus, improvement in survival. Positron emission tomography is the best noninvasive method for quantifying regional myocardial blood flow and metabolism. Using 18F-fluorodeoxyglucose, which measures myocardial glucose utilization, it is possible to identify myocardial tissue that is hypoperfused at rest with preserved or increased glucose uptake. This mismatch of blood flow to metabolism has a high predictive accuracy in the recovery of contractile function. In order to reduce the need for metabolic imaging in documenting myocardial viability, a regional index of perfusable tissue derived from imaging with 15O water has been recently developed that also allows the quantification of tissue viability.

Implementation and evaluation of a two-compartment model for quantification of myocardial perfusion with rubidium-82 and positron emission tomography

Positron emission tomography offers the ability to noninvasively assess regional myocardial perfusion in absolute terms (i.e., milliliters per gram per minute). Accurate estimates have been difficult to achieve with generator-produced 82Rb because of the complex behavior of this tracer in the myocardium. The aim of the present study was to determine whether regional myocardial blood flow could be assessed quantitatively with 82Rb and positron emission tomography by using a two-compartment kinetic model. Regional perfusion in milliliters per gram per minute was estimated from dynamic tomographic scans after intravenous administration of 82Rb in 18 studies in 13 intact dogs studied without intervention, after 2 and 24 hours of induced ischemia, during reperfusion after transient occlusion, or at rest and after pharmacological hyperemia after induced coronary artery stenosis. Regional flow was estimated along with the forward and backward rates of transport (k1 and k2 [minutes-1]) after the relative volume of distribution of the first compartment was fixed to 0.53 ml/ml and the tomographic parameters, the recovery and spillover fractions, were fixed to averaged values obtained in previous studies. In 36 comparisons, estimates of regional flow with 82Rb correlated well with flow measured with concomitantly administered radiolabeled microspheres (r = 0.91, p less than 0.05) over the flow range from 0.14 to 4.25 ml/g/min. A putative index of viability, k2, increased significantly in regions with severe ischemia. The results suggest that quantification of regional myocardial perfusion is possible in centers using 82Rb for estimates of myocardial perfusion when a physiologically appropriate, two-compartment model is used.

Radiopharmaceucticals for cardiovascular imaging

Radionuclide cardiac imaging is a noninvasive technique routinely used to detect coronary artery disease (CAD). This imaging modality includes techniques such as planar, single photon emission computed tomography (SPECT), positron emission tomography (PET) and radionuclide ventriculography--each technique having unique features of its own. Each technique employs various radiopharmaceuticals suitable for assessing different physiological and functional parameters that may become abnormal in the presence of CAD. Various cardiac imaging techniques include myocardial perfusion or blood flow, myocardial metabolism and cardiac function and wall motion. While radionuclide ventriculography gives the global functional status of the heart, SPECT and PET techniques provide information as to the regional blood flow and metabolic status of the myocardium. The following is a review of radiopharmaceuticals that are utilized clinically and in research in different types of nuclear cardiac imaging. Radiopharmaceuticals have been grouped according to the technique employed in which they are used. Various characteristics, merits and disadvantages of each radiopharmaceutical are discussed in detail.

Assessment of coronary vasomotor tone in humans

The assessment of endothelium-mediated modulation of coronary vasomotor tone in the intact human circulation under physiologic conditions requires very precise determination of both epicardial artery diameters, reflecting effects within the conduit vessels, as well as coronary blood flow, reflecting effects within the resistance vasculature during cardiac catheterization. In the present report, the accuracy and limitations of quantitative approaches to assess arterial dimensions from coronary angiograms are discussed. Using state-of-the-art image-processing techniques and x-ray imaging, epicardial artery diameter changes within the range of 8-10% can be reliably detected by quantitative coronary angiography. In addition, advances in interventional techniques do provide a means to selectively assess intracoronary blood-flow velocities using intracoronary Doppler catheters. Combining epicardial artery diameter measurements and intracoronary blood-flow velocity parameters allows for a reasonably accurate instantaneous estimate of coronary arterial blood flow. The advantages and limitations of the intracoronary Doppler technique compared to other techniques are discussed.

Noninvasive quantification of regional myocardial perfusion with rubidium-82 and positron emission tomography. Exploration of a mathematical model

Positron emission tomography (PET) centers without cyclotrons use generator-produced rubidium-82 (82Rb) for assessment of myocardial perfusion. The aim of the present study was to determine whether myocardial blood flow could be assessed quantitatively with 82Rb and PET. Because the myocardial extraction fraction of 82Rb varies inversely and nonlinearly with flow and cannot be measured conveniently with PET, we used an experimentally derived mathematical function defining the relation between single-pass extraction fraction of 82Rb and flow to obviate the necessity of measuring the extraction fraction directly. Myocardial blood flow in absolute terms (ml/g/min) was estimated from dynamic PET scans after intravenous administration of 82Rb in intact dogs and compared with flows measured with radiolabeled microspheres. In 36 comparisons in 13 dogs studied at rest, or after coronary occlusion, reperfusion, or after coronary hyperemia induced with intravenous dipyridamole, over the flow range from 0.2 to 2.0 ml/g/min, estimates of perfusion with rubidium correlated well with flows measured concomitantly with microspheres, although there was a slight underestimation of flow with rubidium (flow by 82Rb = 0.92 x flow by microspheres-0.021, r = 0.83). In general, estimates of flow in ischemic regions were less reliable than estimates for regions with normal flow. Thus, although the relation between myocardial extraction and retention of 82Rb and flow can vary under a variety of physiological and pathophysiological conditions, this study demonstrates the ability to obtain quantitative estimates of myocardial blood flow with 82Rb and PET under carefully defined conditions without measuring the extraction fraction directly.

Imaging of myocardial metabolism by positron emission tomography

Tracer techniques have provided new insight in cardiology by allowing noninvasive studies of myocardial perfusion, function, metabolism, and, more recently, ligand-receptor interaction. Positron emission tomography allows accurate quantification and the use of natural substrates labelled with 11C, 13N, or 15O. Myocardial metabolism is complex and utilizes a number of substrates, primarily fatty acids. Fatty acids utilization can be studied with 11C palmitate, while 11C acetate more selectively traces TCA cycle activity and reflects myocardial oxygen utilization. Glucose uptake can be traced using 18F deoxyglucose, a glucose analog that is a substrate for hexokinase but is not further metabolized. Flow and oxidative glucose metabolism are usually coupled, and thereby the uptake of FDG and perfusion tracers are usually similar. In myocardial ischemia, however, glucose utilization can persist due to anaerobic glycolysis, and its uptake is frequently enhanced. Clinical applications of the use of metabolic studies in patients with ischemic heart disease are presented.

Augmented myocardial perfusion reserve after coronary angioplasty quantified by positron emission tomography with H2(15)O

Effects of coronary angioplasty on myocardial flow reserve have been difficult to characterize noninvasively because conventional imaging techniques cannot quantitate blood flow in absolute terms. The effects of coronary angioplasty on myocardial perfusion and perfusion reserve were delineated with positron emission tomography and oxygen-15-labeled water (H2(15)O) in 13 patients before and after single vessel angioplasty. In 11 patients, angioplasty was successful (minimal cross-sectional area increased from 0.60 +/- 0.59 to 3.45 +/- 1.09 mm2, p less than 0.001). In these patients, regional H2(15)O radioactivity (the ratio of nutritional perfusion in regions distal to the stenosis compared with regions supplied by angiographically normal arteries) at rest before angioplasty was 55 +/- 22% of peak myocardial radioactivity and did not increase significantly afterward (70 +/- 16%, p = NS). However, after administration of intravenous dipyridamole, hyperemic perfusion in regions distal to a stenosis averaged only 39 +/- 18% of peak myocardial counts before angioplasty, but increased to 66 +/- 22% after angioplasty (p less than 0.02). Perfusion reserve in the two patients in whom angioplasty was angiographically unsuccessful showed no change. Quantitative estimates of perfusion in absolute rather than relative terms were obtained with positron emission tomographic data from seven of the patients with successful angioplasty. At rest, perfusion in regions distal to a stenosis was not different from the values in regions supplied by normal coronary arteries (1.54 +/- 0.54 compared with 1.46 +/- 0.38 ml/g per min, p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrasonic tissue characterization with a real time integrated backscatter imaging system in normal and aging human hearts

Experimental studies have shown that variation in the magnitude of integrated ultrasonic backscatter during the cardiac cycle represents acoustic properties of myocardium that are affected by pathologic processes; however, there are few clinical studies using integrated backscatter. Forty subjects without cardiovascular disease (aged 22 to 71 years, mean 41) were studied with use of a new M-mode format integrated backscatter imaging system to characterize the range of cyclic variation of integrated backscatter in normal subjects. Cyclic variation in integrated backscatter was noted in both the septum and the posterior wall in all subjects. The magnitude of the cyclic variation of integrated backscatter and the interval from the onset of the QRS wave of the electrocardiogram to the minimal integrated backscatter value were measured using an area of interest of variable size for integrated backscatter sampling and a software resident in the ultrasound scanner. The magnitude of cyclic variation was larger for the posterior wall than for the septum (6.3 +/- 0.8 versus 4.9 +/- 1.3 dB, p less than 0.01). The interval to the minimal integrated backscatter value was 328 +/- 58 ms for the septum and 348 +/- 42 ms for the posterior wall (p = NS). There was a weak correlation between the magnitude of cyclic variation of integrated backscatter and subject age for the posterior wall (r = -0.47, p less than 0.01), but this was not significant for the septum (r = -0.21) (partially because of inability to exclude specular septal echoes) and septal endocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial ischemia and clinical applications of positron emission tomography

Positron emission tomography (PET) uniquely permits the noninvasive study and quantification of regional rates of blood flow and metabolism in human myocardium. In myocardial ischemia, PET identifies the presence and extent of viability in dysfunctional myocardium and distinguishes it from irreversible ischemic injury and scar tissue formation. Because metabolic markers have proved more accurate and reliable for identifying viability than conventional diagnostic approaches, PET has become increasingly useful for characterizing the severity and extent of an ischemic injury and for assigning patients with acute or chronic coronary artery disease to the most efficacious treatment.

Noninvasive quantitation of myocardial blood flow in human subjects with oxygen-15-labeled water and positron emission tomography

Noninvasive measurement of myocardial blood flow in absolute terms (i.e., milliliters per gram per min) has been difficult to accomplish despite the intrinsically quantitative power of positron emission tomography because of the nonphysiologic nature of tracers that have been employed conventionally as well as the limited spatial resolution of currently available instruments. It was previously demonstrated that myocardial blood flow in animals can be quantitated accurately with the diffusible tracer oxygen-15-labeled water (H2(15)O) when the arterial input function and myocardial radiotracer concentration were measured directly. To extend the approach for completely noninvasive measurement of blood flow, a parameter estimation procedure was developed whereby effects of limited tomographic spatial resolution and cardiac motion were compensated for within the operational flow model. In validation studies in 18 dogs, myocardial blood flow measured with positron emission tomography after intravenously administered H2(15)O correlated closely with flow measured with concomitantly administered radiolabeled microspheres over the range of 0.29 to 5.04 ml/g per min (r = 0.95). Although regional ischemia was clearly identifiable tomographically, absolute flow could not be determined accurately in ischemic regions in four dogs because of poor count statistics related to wall thinning. Subsequently, myocardial blood flow was measured in 11 normal human subjects. Flow was homogeneous throughout the myocardium, averaged 0.90 +/- 0.22 ml/g per min at rest and increased to 3.55 +/- 1.15 ml/g per min after intravenous administration of dipyridamole. Therefore, positron emission tomography with H2 15O and the approach developed permits noninvasive measurement of myocardial blood flow in absolute terms in humans and should facilitate objective assessment of interventions designed to enhance nutritive perfusion.

The high count rate performance of a two-dimensionally position-sensitive detector for positron emission tomography

In positron tomographs using a small number of position-sensitive detectors, each detector must operate at high singles event rates, especially during dynamic studies. To enable the PENN-PET tomography to perform studies involving high data rates, the high count rate behaviour of the position-sensitive scintillation detector used in the tomograph was investigated at singles rates in excess of 2 million counts per second (MCPS). Detector dead-time, minimised through the use of pulse clipping (clipping time, 120 ns), is a maximum of 20% at the highest data rates. At 2 MCPS and 240 ns pulse integration time, the full width at half maximum of the point spread function (PSF) worsens by approximately 20% over its low count rate value of 5.2 mm. Furthermore, at high count rates, pulse pile-up produces long tails in the PSF along the detector's long axis. These tails were reduced or eliminated through the use of a shortened pulse integration time (160 ns instead of 240 ns), an upper level energy discriminator and a local centroid event positioning algorithm. Detector performance was characterised for different combinations of these event processing techniques, and the mechanisms by which pulse pile-up distorts the high count rate PSF were investigated using computer simulations. With the incorporation of the high count rate event processing techniques, the detector's count rate capability enables the PENN-PET tomograph to handle most current imaging protocols.

Noninvasive assessment of canine myocardial oxidative metabolism with carbon-11 acetate and positron emission tomography

Noninvasive quantification of regional myocardial metabolism would be highly desirable to evaluate pathogenetic mechanisms of heart disease and their response to therapy. It was previously demonstrated that the metabolism of radiolabeled acetate, a readily utilized myocardial substrate predominantly metabolized to carbon dioxide (CO2) by way of the tricarboxylic acid cycle, provides a good index of oxidative metabolism in isolated perfused rabbit hearts because of tight coupling between the tricarboxylic acid cycle and oxidative phosphorylation. In the present study, in a prelude to human studies, the relation between myocardial clearance of carbon-11 (11C)-labeled acetate and myocardial oxygen consumption was characterized in eight intact dogs using positron emission tomography. Anesthetized dogs were studied during baseline conditions and again during either high or low work states induced pharmacologically. High myocardial extraction and rapid blood clearance of tracer yielded myocardial images of excellent quality. The turnover (clearance) of 11C radioactivity from the myocardium was biexponential with the mean half-time of the dominant rapid phase averaging 5.4 +/- 2.2, 2.8 +/- 1.3 and 11.1 +/- 1.3 min in control, high and low work load studies, respectively. No significant difference was found between the rate of clearance of 11C radioactivity from the myocardium measured noninvasively with positron emission tomography and the myocardial efflux of 11CO2 measured directly from the coronary sinus. The rate of clearance of the 11C radioactivity from the heart correlated closely with myocardial oxygen consumption (r = 0.90, p less than 0.001) as well as with the rate-pressure product (r = 0.95, p less than 0.001). Hence, the rate of oxidation of 11C-acetate can be determined noninvasively with positron emission tomography, providing a quantitative index of oxidative metabolism under diverse conditions.

Insights into coronary artery disease gained from metabolic imaging

Positron emission tomography offers the possibility of evaluating and quantifying regional myocardial blood flow and metabolism. Used in patients with coronary artery disease, positron emission tomography has demonstrated sustained metabolic activity in regions with reduced blood flow and impaired contractile function, and it thereby enables differentiation between viable myocardium and myocardium that has succumbed to necrosis and scar formation. Viable myocardial regions identified by metabolic rather than functional or blood-flow criteria are frequently observed in patients after an acute coronary event and in patients with stable coronary artery disease. Positron emission tomography reflects either acute myocardial ischemia, "hibernation," as well as "myocardial stunning." Findings from metabolic imaging have proved useful in characterizing more accurately coronary artery disease and its functional consequences. These findings have been found equally useful for clinical management.

Delineation of impaired regional myocardial perfusion by positron emission tomography with H2(15)O

Positron emission tomography with 15O-labeled water (H2(15)O) can be used to delineate abnormal regional myocardial blood flow in experimental animals. To determine the feasibility of this method in humans, we studied 33 subjects (9 normal volunteers and 24 patients with angiographically documented coronary artery disease) at rest and after myocardial hyperemia induced with intravenous infusion of dipyridamole. At rest, the myocardial region demonstrating the lowest relative H2(15)O activity exhibited 71 +/- 8% of activity in the region with peak activity in control subjects and 62 +/- 17% in patients (p = NS). After the dipyridamole infusion, differences between the two groups were accentuated. In control subjects, activity in the region with lowest relative radioactivity averaged 77 +/- 5% of that in the region with peak activity. In patients, it averaged 55 +/- 22% of activity in the region with peak activity (p less than 0.01). Results in patients with ischemia with or without a history of remote myocardial infarction were not significantly different. In 22 of the 24 patients, the region with lowest relative perfusion corresponded anatomically to the region of myocardium distal to a stenosis. Thus, delineation of regional myocardial perfusion in patients with coronary artery disease is possible with positron emission tomography and H2(15)O. Further studies will be necessary to prospectively determine sensitivity and specificity.

Positron emission tomography. Diagnostic and therapeutic implications in human myocardial ischemia

Positron emission tomography and tracers of blood flow and of metabolism offer a most unique capability: The noninvasive study of regional myocardial metabolism and its derangements as a result of regional or global myocardial disease. Research with PET not only has confirmed the existence of metabolic fluxes and reactions as established previously through highly invasive or even destructive investigational techniques but has provided new insights into pathophysiologic processes, especially in ischemic and post-ischemic myocardium. From these investigations in both animal experiments and in humans, observations have emerged which indicate a place for PET in clinical cardiology. PET is likely to contribute to detection of disease, to characterizing its extent and severity as well as to decide upon the most appropriate therapeutic strategy and assessing its results. It is recognized that many of these observations with clinical implications await confirmation through larger clinical trials, follow-up studies as well as independent confirmation. Besides exploring ischemic heart disease, PET is equally suitable for examining substrate fluxes and interactions in other disorders as for example in intrinsic myocardial disease like primary and secondary cardiomyopathies. While derangements of metabolism in these disorders may be an expression of the consequences of the disease process or its underlying mechanisms itself, findings on PET will allow formulation of new hypotheses on disease mechanisms that conversely can then be tested. In addition to F-18 2-deoxyglucose and C-11 palmitate, the number of tracers for substrate metabolism is likely to increase. An example is C-11 acetate currently intensely investigated as a tool for measuring overall myocardial oxidative metabolism. Others as for example C-11 labeled short chain fatty acids are on the horizon. The study of cardiac receptors is similarly possible. Thus, a set of tools will soon be available for dissection of entire metabolic pathways and for determination of rate limiting steps in health and disease and to more clearly define specific defects in biochemical reaction steps that critically contribute to or even ae the specific cause of disease.

Delineation of myocardial oxygen utilization with carbon-11-labeled acetate

Although positron-emission tomography (PET) with labeled fatty acid delineates infarct size and permits qualitative assessment of fatty acid utilization, quantification of oxidative metabolism is limited by complex alterations in the pattern of utilization of fatty acid during ischemia and reperfusion. Because metabolism of acetate by myocardium is less complex than that of glucose or palmitate, we characterized kinetics of utilization of radiolabeled acetate in 37 isolated rabbit hearts perfused with modified Krebs-Henseleit buffer and performed a pilot tomographic study in man. Results of initial experiments with carbon-14-labeled acetate (14C-acetate) indicated that the steady-state extraction fraction of acetate averaged 61.5 +/- 4.0% in control hearts (n = 4), 93.6 +/- 0.9% in hearts rendered ischemic (n = 4), and 54.8 +/- 4.0% in hearts reperfused after 60 min of ischemia (n = 3). Oxidation of 14C-acetate, assessed from the rate of efflux of 14CO2 in the venous effluent, correlated closely with the rate of oxygen consumption under diverse metabolic conditions (r = .97, p less than .001). In addition, no significant differences were observed between rates of efflux of total 14C in all chemical species (reflecting total clearance of tracer from myocardium) and efflux of 14CO2. Clearance of 11C-acetate, measured externally with gamma probes in normal and ischemic myocardium, correlated closely with clearance of 14C-acetate measured directly in the effluent (r = .99, p less than .001) and with overall myocardial oxygen consumption (r = .95, p less than .001). Accumulation and clearance of 11C-acetate from human myocardium with PET demonstrated kinetics comparable to those seen with radiolabeled acetate in vitro. Thus externally detectable clearance of 11C-acetate provides a quantitative index of myocardial oxidative metabolism despite variation in the patterns of intermediary metabolism that confounds interpretation of results with conventionally used tracers such as glucose and fatty acid.

Methods of measurement of myocardial blood flow in patients: a critical review

During the past decade, major progress has been made in the evolution of technology directed toward the accurate measurement of regional myocardial perfusion in patients. The deficiencies of some of the older methods (thermodilution and gas clearance) are better appreciated and improved approaches (Doppler catheters, positron-emission tomography, and digital subtraction angiography) have been developed. The new approaches should play a major role in research and for most applications the older methods will gradually be replaced. Efforts to bring these new methods to community hospitals and practicing cardiologists should be stimulated. Doppler catheters, positron-emission tomography, and digital-subtraction angiography are commercially available and Doppler catheters and digital-subtraction angiography could be easily incorporated into routine cardiac catheterization procedures. The Doppler catheter is the most inexpensive and probably the simplest to apply. In our opinion, routine measurements of coronary flow reserve will significantly improve the care of patients with coronary obstructive disease and other diseases that impair myocardial perfusion. If coronary reserve measurements are used frequently, patient selection for coronary angioplasty and bypass surgery will no longer depend entirely on visual assessment of percent diameter stenosis, a very poor criterion in many situations. Also, patients with chest pain syndromes, normal coronary vessels, and impaired coronary reserve will be identified and perhaps some effective treatment for this condition will be devised.(ABSTRACT TRUNCATED AT 250 WORDS)

Current status and prospects of new radionuclides and radiopharmaceuticals for cardiovascular nuclear medicine

The rapid emergence of new imaging modalities like positron emission tomography (PET) and single photon emission computerized tomography (SPECT) and their advance into the clinical arena offered new opportunities for, but also stimulated research and development of new radiopharmaceuticals suitable for cardiac imaging. While tracers of myocardial blood flow remained in the center of interest, other trends heralded possibilities of studying more comprehensively cardiac physiology and pathophysiology as, for example, metabolism, the severity of tissue injury, neural activity and membrane function. N-13 ammonia and rubidium-82 became the primary tracers for evaluating and possibly quantifying regional myocardial blood flow with PET, while cationic Tc-99m isonitrile complexes have now reached a stage where high contrast images of the human heart are obtained on planar scintigraphy and SPECT. These radiopharmaceuticals hold considerable promise for routine clinical use. Tracers of metabolism, especially those labeled with positron emitting isotopes as for example, C-11 palmitate, F-18 2-deoxyglucose, are approaching the phase of clinical use and provide information on regional myocardial substrate metabolism and oxidative processes. Less successful and more limited were developments of single photon emitting tracers of metabolism which remained largely confined to radioiodinated fatty acid analogs. Exploration and characterization of the metabolic fate of the radiolabel in tissue and its relation to the externally observed signal have been truly impressive. Tested in humans primarily in western European countries, these tracers promise to yield metabolic information on a more limited scope. Most widely applied are iodohepta- and hexadecanoic acid and, more recently, the aromatic fatty acid analog, paraiodophenylpentadecanoic acid. Labeled monoclonal antibodies rapidly advanced to the point of clinical use. Accurate identification and sizing of acute myocardial infarction is now possible with Tc-99m or indium-111 labeled specific antimyosin antibody fragments. This success stimulated new research activities for use of labeled antibody techniques in other areas as for example, scintigraphic evaluation of formation and presence of vascular thrombi. While promising, these efforts have however remained in an early stage of development. The same holds true for single photon and positron emitting tracers that are suitable for assessing sympathetic neuron densities in myocardium as well as imaging of both cholinergic and adrenergic receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Coronary thrombolysis with tissue-type plasminogen activator (t-PA): emerging strategies

Fundamental observations and the conceptual framework underlying coronary thrombolysis have a history dating back to 1789. Recent enthusiasm for it is predicated on the recently established safety of cardiac catheterization in critically ill patients, the high incidence of coronary thrombosis underlying acute transmural myocardial infarction and demonstrable benefit conferred to the heart and the patient when thrombolysis is initiated early after the onset of ischemia. Clot-selective activators of the fibrinolytic system offer promise for safe induction of coronary thrombolysis without marked predisposition to bleeding. One such activator, tissue-type plasminogen activator (t-PA), has been synthesized by recombinant deoxyribonucleic acid (DNA) technology, amenable to large scale production of pharmaceutical agents and hence widespread availability. Initial clinical trials conducted with t-PA have demonstrated opening rates of completely occluded, infarct-related coronary arteries of approximately 75% without marked depletion of fibrinogen. The focus of research in progress includes: noninvasive delineation of recanalization and estimation of the extent of myocardium salvaged by initial recanalization, development of alternative routes of administration of thrombolytic agents potentially exploitable by paramedical personnel and, perhaps, high risk patients themselves, and definitive elucidation of the extent to which benefits conferred by thrombolysis can be enhanced with adjunctive pharmacologic interventions as well as early angioplasty or surgery.