Abnormal myocardial fatty acid metabolism in dilated cardiomyopathy detected by iodine-123 phenylpentadecanoic acid and tomographic imaging

Clinical use of nuclear cardiology in the assessment of heart failure

A nuclear cardiology test is the most commonly performed non-invasive cardiac imaging test in patients with heart failure, and it plays a pivotal role in their assessment and management. Quantitative gated single positron emission computed tomography (QGS) is used to assess quantitatively cardiac volume, left ventricular ejection fraction (LVEF), stroke volume, and cardiac diastolic function. Resting and stress myocardial perfusion imaging, with exercise or pharmacologic stress, plays a fundamental role in distinguishing ischemic from non-ischemic etiology of heart failure, and in demonstrating myocardial viability. Diastolic heart failure also termed as heart failure with a preserved LVEF is readily identified by nuclear cardiology techniques and can accurately be estimated by peak filling rate (PFR) and time to PFR. Movement of the left ventricle can also be readily assessed by QGS, with newer techniques such as three-dimensional, wall thickening evaluation aiding its assessment. Myocardial perfusion imaging is also commonly used to identify candidates for implantable cardiac defibrillator and cardiac resynchronization therapies. Neurotransmitter imaging using (123)I-metaiodobenzylguanidine offers prognostic information in patients with heart failure. Metabolism and function in the heart are closely related, and energy substrate metabolism is a potential target of medical therapies to improve cardiac function in patients with heart failure. Cardiac metabolic imaging using (123)I-15-(p-iodophenyl)3-R, S-methylpentadecacoic acid is a commonly used tracer in clinical studies to diagnose metabolic heart failure. Nuclear cardiology tests, including neurotransmitter imaging and metabolic imaging, are now easily preformed with new tracers to refine heart failure diagnosis. Nuclear cardiology studies contribute significantly to guiding management decisions for identifying cardiac risk in patients with heart failure.

Clinical features of myocardial triglyceride in different types of cardiomyopathy assessed by proton magnetic resonance spectroscopy: comparison with myocardial creatine

BACKGROUND

Myocardial lipid overstorage may produce cardiomyopathy, leading to dysfunction, but advanced heart failure may cause lipolysis via sympathetic nerve activation. In the failing heart, the creatine kinase system may also be impaired. The aims of this study were to assess myocardial triglyceride (TG) and creatine (CR) in different types of cardiomyopathy and to investigate whether they are related to the severity of cardiac dysfunction.

METHODS AND RESULTS

In patients with hypertrophic cardiomyopathy (HCM, n = 8), dilated cardiomyopathy (DCM, n = 12) or ischemic cardiomyopathy (ICM, n = 10), and normal subjects (NML, n = 22), myocardial TG and CR were evaluated using proton magnetic resonance spectroscopy. To assess cardiac sympathetic nerve activity, myocardial MIBG (a radioactive guanethidine analog) uptake was measured in DCM. Myocardial TG was significantly lower in hypertrophic cardiomyopathy (HCM) (1.92 ± 0.99 μmol/g), but higher in ICM (7.59 ± 4.36 μmol/g) than in NML hearts (4.05 ± 1.94 μmol/g). There was no significant difference in TG between DCM (4.84 ± 6.45 μmol/g) and NML. Myocardial CR in HCM (20.4 ± 8.4 μmol/g), DCM (14.8 ± 4.8 μmol/g), and ICM (19.4 ± 6.3 μmol/g) was significantly lower than that in NML hearts (27.1 ± 4.3 μmol/g). Overall, myocardial CR correlated positively with the severity of heart failure estimated by ejection fraction or myocardial BMIPP (a radioactive fatty acid analog) uptake, but TG did not. In DCM, myocardial TG correlated with body mass index, but not with MIBG uptake.

CONCLUSIONS

Myocardial TG may be related to the specific cause of disease rather than the severity of cardiac dysfunction. In contrast, myocardial CR reflects the severity of heart failure despite different pathoetiologic mechanisms of dysfunction. In DCM, myocardial TG may be affected by an overweight state rather than cardiac sympathetic nerve dysfunction. Thus, myocardial CR has a closer relationship to heart failure severity than does myocardial TG.

Combined thallium-201 and dynamic iodine-123 iodophenylpentadecanoic acid single-photon emission computed tomography in patients after acute myocardial infarction with effective reperfusion

BACKGROUND

Considerable derangements of energy metabolism are to be expected during ischemia and reperfusion. In ischemic myocardium, the oxidative degradation of carbohydrates is shifted toward the anaerobic production of lactate and the oxidation of fatty acids is suppressed.

HYPOTHESIS

The aim of this study was to examine the uptake and metabolism of iodine-123 (123I) iodophenylpentadecanoic acid (IPPA) in stunned myocardium.

METHODS

In 15 patients, SPECT with 201Tl and 123I IPPA as well as echocardiography with low-dose dobutamine stimulation were performed 12 +/- 5 days after myocardial infarction with reperfusion. Follow-up echocardiography was carried out 24 +/- 8 days later for documentation of functional improvement. Uptake of 201Tl and 123I IPPA were obtained in five left ventricular segments, and dynamic SPECT imaging was used for calculation of the fast and the slow components of the biexponential myocardial 123I IPPA clearance.

RESULTS

Wall motion improved in 14 of 26 dysfunctional segments (54%). Stunned segments were characterized by a reduced 123I IPPA extraction, a shorter half-life of the fast, and a longer half-life of the slow clearance component. All parameters of the combined 201Tl/123I IPPA study predicted functional recovery with similar accuracies (area under the receiver operator characteristic curves between 0.68 and 0.76; p = NS). Analysis of 201Tl uptake alone could not predict functional recovery in this study.

CONCLUSIONS

Stunned myocardium is characterized by a disturbance of fatty acid metabolism. For prediction of functional improvement, 123I IPPA imaging added significant diagnostic information.

Clinical Value of Iodine-123 Beta-Methyliodophenyl Pentadecanoic Acid (BMIPP) Myocardial Single Photon Emission Computed Tomography for Predicting Cardiac Death Among Patients With Chronic Heart Failure

In the present study, the effectiveness of 123I-beta-methyliodophenyl pentadecanoic acid (BMIPP) single photon emission computed tomography (SPECT) for predicting cardiac death of patients with chronic heart failure was evaluated. Abnormalities of fatty acid metabolism are found in patients with chronic heart failure and BMIPP was developed as a tracer for scintigraphic assessment of myocardial fatty acid utilization. The study group comprised 74 patients with chronic heart failure with a left ventricular ejection fraction (LVEF) <45% on left ventriculography or radionuclide angiocardiography. They underwent both 201Tl SPECT and BMIPP SPECT. The uptake of tracer was scored semiquantitatively from 0 (normal) to 4 (defect) in 20 segments and a total defect score (TDS) for all 20 segments was calculated. On planar images the mediastinum to heart count ratio (H/M) was calculated for the BMIPP and Tl studies, and the H/M(BMIPP):H/M(Tl) (H/M(BMIPP) divided by H/M(Tl)) was also calculated. The mean follow-up period was 660 days and there were 17 cases of cardiac death. Multivariate analysis identified H/M(BMIPP):H/M(Tl) (p<0.05) and LVEF (p<0.05) as independent predictors of cardiac death. The receiver-operating characteristic curve of H/M(BMIPP):H/M (Tl) was situated to the left relative to LVEF. Analysis of the myocardial metabolism by BMIPP SPECT can predict the high-risk patients with chronic heart failure.

[123I]-phenylpentadecanoic acid uptake in patients with dilated cardiomyopathy

BACKGROUND

The rate constant for global fatty acid influx (k(1)) was studied in 12 male patients with dilated cardiomyopathy (DCM).

METHOD

10 normal subjects served as controls. 201-Thallium (201TI) and [123I]-phenyl-pentadecanoic acid (IPPA) were administered during bicycle exercise under fasting conditions.

RESULTS

All patients showed non-homogeneous tracer uptake defects for 201TI and IPPA. k(1) was significantly higher in DCM patients than controls. k(1) showed significant inverse correlation between cardiac index, left-ventricular ejection fraction, left-ventricular enddiastolic pressure and echocardiographic left-ventricular ejection fraction.

CONCLUSION

We presume that an increased regional rate constant of IPPA influx into the myocardial tissue in patients with DCM reflects a compensatory mechanism of altered myocardium.

Fatty acids for myocardial imaging

Radioiodinated free fatty acids are tracers that can be used to assess both myocardial perfusion and metabolism. There have been several fatty acids and structurally modified fatty acids studied since Evans' initial report of radiolabeled I-123 oleic acid in 1965. The radiolabeling of a phenyl group added to the long chain fatty acids in the omega-terminal position opposite the carboxyl terminal group prevents nonspecific deiodination and the rapid release of free iodine as the tracer undergoes beta-oxidation. The additional inclusion of a methyl or dimethyl group to the chain slows oxidation resulting in prolonged myocardial retention. The longer retention of the radiolabel permits longer image acquisitions more compatible with single photon emission computed tomography (SPECT) imaging, especially with single-detector imaging systems. Several protocols have been implemented using these compounds, particularly 15-(para-iodophenyl)-3-R,S-methyl pentadecanoic BMIPP, to detect abnormal fatty acid metabolism in ischemic heart disease as well as in nonischemic and hypertrophic cardiomyopathies. Successful management of patients with ischemic cardiomyopathies depends on the accurate identification of hibernating myocardium. The studies covered in this review suggest that both IPPA and BMIPP, especially when combined with markers of myocardial perfusion, may be excellent tracers of viable and potentially functional myocardium. Future studies with larger numbers of patients are needed to confirm the results of these studies and to compare their efficacy with that of other available imaging modalities. Cost and distribution issues will have to be resolved for these metabolic tracers to compete in the commercial marketplace. Otherwise they will likely be available only on a limited basis for research use. As progress is made with these issues and with the development of newer imaging systems, the use of radioiodinated and fluorinated fatty acids is likely to be increasingly attractive.

Assessment of myocardial fatty acid metabolic abnormalities in patients with idiopathic dilated cardiomyopathy using 123I BMIPP SPECT: correlation with clinicopathological findings and clinical course

OBJECTIVE

To determine the clinical and prognostic value of identifying metabolic abnormalities of myocardial fatty acid metabolism in idiopathic dilated cardiomyopathy using iodine-123 beta-methyl-iodophenyl pentadecanoic acid (123I BMIPP).

SETTING

Cardiac care division in national hospital.

PATIENTS

32 consecutive patients with idiopathic dilated cardiomyopathy in whom both 123I BMIPP and thallium-201 myocardial single photon emission computed tomography were performed.

METHODS

The uptake of each tracer was scored visually from 0 (normal) to 3 (defect) in 17 segments (eight basal, eight midventricular, and one apical). A total score for all 17 segments was compared with clinicopathological variables. Prognostic value of mismatches between the two tracers were also evaluated.

RESULTS

The 123I BMIPP total score was correlated with pulmonary capillary wedge pressure (r = 0.68, p < 0.001), left ventricular end diastolic pressure (r = 0.65, p < 0.001), percentage fractional shortening at six months' follow up (r = -0.58, p = 0. 001), myocyte diameter (r = 0.66, p < 0.001), and percentage area of interstitial fibrosis (r = 0.69, p < 0.001) measured by morphometry in the biopsy specimens. During a mean (SD) follow up of 20 (11) months, deterioration of the New York Heart Association functional class was observed in 11 of the 32 patients; four of these died. Segments with a greater decrease in 123I BMIPP than thallium-201 uptake (type B mismatching) were often observed in patients with deterioration (88/187, 29% v 58/357, 16%; p < 0.001).

CONCLUSIONS

The extent of the abnormality of myocardial fatty acid metabolism in idiopathic dilated cardiomyopathy reflects the severity of haemodynamic deterioration and histopathological changes. Type B mismatching is one of the important prognostic indicators in idiopathic dilated cardiomyopathy.

Predicting the effects on patients with dilated cardiomyopathy of beta-blocker therapy, by using iodine-123 15-(p-iodophenyl)-3- R,S-methylpentadecanoic acid (BMIPP) myocardial scintigraphy

We examined whether the iodine-123 15-(p-iodophenyl)-3- R,S-methylpentadecanoic acid (BMIPP) myocardial scintigraphy was useful for predicting the treatment response to beta-blocker in patients with dilated cardiomyopathy (DCM). Sixteen patients with DCM were studied. BMIPP single photon emission computed tomography (SPECT) was performed before beta-blocker therapy. The count ratio of the heart (H) to the upper mediastinum (M) (H/M ratio) was calculated. Several measurements including the BMIPP H/M ratio before the administration of metoprorol were retrospectively compared among the 10 "good responders" (showing improvement by at least one NYHA class or an increase in the ejection fraction of > or = 0.10, 6 months after the start of the drug therapy) and the 6 "poor responders." The bull's eye map of BMIPP was divided into 17 areas. Each segmental score was analyzed quantitatively by means of a two-point scoring system (good uptake > or = 67%, poor uptake < 67%). The total score was regarded as the uptake score. The H/M ratio was significantly higher in the good responders than in the poor responders (2.41 +/- 0.24 vs. 1.86 +/- 0.17 p < 0.01). There were no significant differences between the two groups in any other variable data at entry. The uptake score was also a good index for predicting the therapeutic effect. When a relative uptake of 67% or higher was scored as 1, uptake scores of 9 to 17 corresponded to good responses (sensitivity = 100%, specificity = 100%, accuracy = 100%, positive and negative predictive value = 100%). Although the number of patients studied is small, our results suggests that BMIPP myocardial scintigraphy can predict the response to a beta-blocker in patients with DCM.

Myocardial defect detected by 123I-BMIPP scintigraphy and left ventricular dysfunction in patients with idiopathic dilated cardiomyopathy

The present study examined the role of myocardial fatty acid in patients with idiopathic cardiomyopathy (DCM) by means of 123I-beta-methyl-p-iodophenyl pentadecanoic acid (123I-BMIPP) scintigraphy. Thirteen patients underwent 123I-BMIPP imaging, 201Tl imaging and echocardiography. All patients showed defective myocardial uptake of 123I-BMIPP and 201Tl. The left ventricular end-diastolic dimension (64.1 +/- 7.3 mm vs. 55.6 +/- 1.5 mm, p < 0.05) and end-systolic dimension (52.4 +/- 8.0 mm vs. 40.6 +/- 2.1 mm, p < 0.01) were significantly large in the defect group (123I-BMIPP defect score (DS) > 8) than the small defect group (DS < 7). The % fractional shortening (%FS) was also significantly smaller (18.6 +/- 3.8% vs. 27.0 +/- 3.3%, p < 0.01) in the large defect group. The 123I-BMIPP DS correlated statistically with %FS (r = 0.75, p < 0.01), while the 201Tl DS did not (r = 0.41, ns). We conclude that the patients with DCM revealed a 123I-BMIPP uptake defect and the defect reflected the degree of left ventricular dysfunction.

Prognosis of hypertrophic cardiomyopathy: assessment by 123I-BMIPP (beta-methyl-p-(123I)iodophenyl pentadecanoic acid) myocardial single photon emission computed tomography

123I-BMIPP (beta-methyl-iodophenyl pentadecanoic acid) has shown unique properties for potential use in assessing myocardial metabolism. Previous basic and clinical studies demonstrated that the disturbances of myocardial metabolism precede the occurrence of myocardial perfusion abnormalities by using 201Tl in hypertrophic myocardium. The present study was therefore undertaken to determine whether or not 123I-BMIPP myocardial SPECT is useful in predicting the prognosis of hypertrophic cardiomyopathy (HCM) in 65 patients in 6 facilities. There were 33 patients with non-obstructive HCM, 12 with obstructive HCM, 12 with apical HCM and 8 with dilated-phase HCM. Fasted patients at rest received an intravenous injection of 111 MBq of 123I-BMIPP. Twenty to thirty minutes later, myocardial SPECT was carried out. The BMIPP severity score (BMIPP SS) was evaluated semiquantitatively by using representative short axial SPECT images. We followed up the incidence of cardiac events for a mean period of 3.0 +/- 0.6 years. Cardiac events occurred in 13 patients. Of these, 11 developed heart failure and 6 died (4 from heart failure and 2 from sudden death). The BMIPP SS in the dilated-phase HCM was significantly lower than that for the nonsurvivors. The BMIPP SS was particularly high in patients with fatal heart failure. Furthermore, there was a close negative correlation between the BMIPP SS and percent fractional shortening measured by echocardiography (r = -0.49). Finally, the mortality over the three years increased according to the extent of the BMIPP SS. In conclusion, these results indicate that BMIPP SS is useful in evaluating the severity of HCM. We conclude that 123I-BMIPP is a valuable metabolic tracer in predicting the outcome of HCM.

Iodine-123-labelled fatty acids for myocardial single-photon emission tomography: current status and future perspectives

Renewed interest in the clinical use of iodine-123-labelled fatty acids is currently primarily focused on the use of iodine-123-labelled 15-(p-iodophenyl)pentadecanoic acid (IPPA) and "modified" fatty acid analogues such as 15-(p-iodophenyl)-3-R,S-methylpentadecanoic acid (BMIPP) which show delayed myocardial clearance, thus permitting single-photon emission tomographic imaging. Interest in the use of BMIPP and similar agents results from the differences which have often been observed in various types of heart disease between regional myocardial uptake patterns of [123I]BMIPP and flow tracer distribution. Although the physiological basis is not completely understood, differences between regional fatty acid and flow tracer distribution may reflect alterations in important parameters of metabolism which can be useful for patient management or therapy planning. These tracers may also represent unique metabolic probes for correlation of energy substrate metabolism with regional myocardial viability. The two agents currently most widely used clinically are 123I-labelled IPPA and BMIPP. While [123I]IPPA is commercially available as a radiopharmaceutical in Europe (Cygne) and Canada (Nordion), multicenter trials are in progress in the United States as a prelude to approval for broad use. [123I]BMIPP was recently introduced as Cardiodine for commercial distribution in Japan (Nihon Medi-Physics, Inc.). [123I]BMIPP is also being used in clinical studies on an institutional approval basis at several institutions in Europe and the United States. In this review, the development of a variety of radioiodinated fatty acids is discussed. The results of clinical trials with [123I]IPPA and [123I]BMIPP are discussed in detail, as are the future prospects for fatty acid imaging.

Detection of alterations in left ventricular fatty acid metabolism in patients with acute myocardial infarction by 15-(p-123I-phenyl)-pentadecanoic acid and tomographic imaging

15-(p-iodine 123-phenyl)-pentadecanoic acid (IPPA) is a synthetic radiolabeled fatty acid with kinetics similar to palmitate. Fourteen patients who had had an acute myocardial infarction 7 +/- 6 days earlier and 9 normal volunteers were studied after being injected with IPPA. The volunteers were remarkable for homogeneous uptake and metabolism of IPPA; 13 of 14 infarct patients showed areas of decreased uptake > 2 SDs below the mean of the volunteers. Metabolism was homogeneous in the volunteers (14.2% +/- 5.8%) and was significantly higher than in regions identified as infarcted (3.9% +/- 12.1%, p < 0.001). Noninfarcted regions in the patients demonstrated significantly increased rates of IPPA metabolism compared to rates in volunteers (23.0% +/- 9.6% p < 0.001). We conclude that patients with recent myocardial infarction have abnormalities of fatty acid metabolism such as decreased uptake and clearance of fatty acid in regions of infarction and normal uptake but relatively increased fatty acid clearance in unaffected regions of the myocardium.

Serial evaluation of dilated cardiomyopathy with exercise thallium-201 tomography: correlation with the evolution of left ventricular parameters

The purpose of this prospective study was to correlate (1) the initial findings of exercise thallium-201 tomography with the evolution of left ventricular parameters at long term follow-up in patients with dilated cardiomyopathy and (2) the changes of exercise thallium-201 tomography repeated 1 year later. We studied 19 men with dilated cardiomyopathy and normal coronary angiogram. Two patients died and three patients had heart transplantation during follow-up. The other 14 patients were assessed at baseline and 1-year follow-up. Thallium-201 tomograms were divided into 20 segments for each patient. Two groups were defined according to the evolution of left ventricular ejection fraction: group 1 (n = 7) had unchanged or decreased ejection fraction at follow-up (24 +/- 11% at baseline versus 22 +/- 11% at follow-up, ns) and group 2 (n = 7) had improved ejection fraction at follow-up (25 +/- 9% at baseline versus 49 +/- 8% at follow-up, P < 0.03). The number of total abnormal segments at stress were not statistically different at baseline between groups 1 and 2, and in group 1 between baseline and follow-up. Group 2 at follow-up had a reduced number of total abnormal segments (P < 0.03). The percentage of reversibility was similar in both groups at baseline and follow-up. On exercise thallium-201 tomography, neither the presence nor the reversibility of stress myocardial perfusion abnormalities can predict improvement of left ventricular ejection fraction in dilated cardiomyopathy. However, regression of dilated cardiomyopathy is accompanied by a reduction of stress myocardial perfusion abnormalities.

Complementarity of magnetic resonance spectroscopy, positron emission tomography and single photon emission tomography for the in vivo investigation of human cardiac metabolism and neurotransmission

The three techniques allowing the noninvasive study of cardiac metabolism, namely magnetic resonance spectroscopy (MRS), positron emission tomography (PET) and single photon emission computed tomography (SPET), all use external detection with stable or radioactive isotopes. These techniques yield different information. PET is quantitative and very sensitive, and therefore only tracer amounts of molecules need to be injected. It allows neurotransmitters and receptors to be studied and a global view of metabolism (oxygen consumption, glucose and fatty acid utilization) to be obtained. SPET also has good sensitivity, but uses gamma-emitting isotopes of heteroatoms. Their longer half-lives allow follow-up for hours or days. MRS is based on stable elements with high (hydrogen 1, phosphorus 31, fluorine 19...) or low (carbon 13, Deuterium) natural abundance. It has very low sensitivity and only millimolar concentrations of substrates can be detected, but various parts of metabolism can be studied. The in vivo measurement of myocardial concentration of substances has many problems that are common to all three techniques (measurement of the volume, measurement of the quantity of each molecule, resolution, partial volume effect, improvement of the signal-to-noise ratio, movement of the organ). The complementarity of the techniques is illustrated by their applications to the study of cardiac metabolism. For instance, the energy metabolism can be studied by 31P-MRS, which detects the high-energy compounds ATP and phosphocreatine, and 13C-MRS yields information on the tricarboxylic acid cycle activity. PET and SPET allow the utilization of fatty acids, the normal fuels of the heart, to be studied. During ischaemia, PET with 18F-fluorodeoxyglucose (18FDG) can determine the glucose consumption and 1H-MRS shows the increase in lactic acid, reflecting anaerobic glycolysis. Comparison of the use of acetate labelled with 11C for PET or 13C for MRS shows the potentials and limitations of each technique. Myocardial perfusion can be evaluated directly with various PET tracers or indirectly with thallium 201 or various technetium-99m-labelled tracers by SPET. No MRS marker of perfusion is so far clinically available. Mainly SPET and PET are used clinically for the investigation of ischaemic heart disease as well as cardiomyopathies, but some initial results using 31P-MRS are being obtained.

Recent developments in 99mTc and 123I-radiopharmaceuticals for SPECT imaging

Availability of 123I of high radionuclidic purity has encouraged the development of 123I-based radiopharmaceuticals for the assessment of myocardial fatty acid metabolism, myocardial neuronal activity, and for receptor and antibody imaging. Advances in the chemistry of technetium have resulted in the development of novel agents for myocardial and cerebral perfusion and renal function studies. Monoclonal antibodies labeled with 99mTc show promise for imaging neoplastic lesions, myocardial infarcts, and thrombus localization. Recent developments in 123I and 99mTc agents for myocardial and brain imaging studies are discussed.

Radioiodinated tracers for myocardial imaging

Recent advances in the efficient production of high purity radioiodine (123I) and new efficient radiolabeling techniques have allowed the development of new classes of cardiovascular radiopharmaceuticals. These include 123I-labeled fatty acids to assess myocardial metabolism, 123I-metaiodobenzylguanidine (MIBG) for myocardial neuronal activity, labeled monoclonal antibodies for myocardial necrosis, and labeled lipoproteins for receptor concentration. 123I-labeled fatty acids and MIBG are under clinical investigation with encouraging results. 123I- and 111In-labeled fragments of monoclonal antibodies to myosin have been used for imaging myocardial necrosis in humans. The development of radiotracers for imaging of cholinergic and adrenergic receptors is still in the experimental stage. Recent advances in imaging instrumentation and radiopharmaceuticals have resulted in cardiac imaging applications beyond blood pool ventriculography, perfusion, and infarct-avid imaging. Developments of radioiodine (123I)-labeled agents promise to play an important role in the assessment of myocardial metabolism, neuronal activity, and receptor concentration. The chemistry of iodine is well defined compared with that of 99mTc; therefore, iodine isotopes are well suited for labeling biologically important molecules. Among the iodine isotopes, 123I has nearly ideal nuclear properties for nuclear medical applications with a 13.3-hour half-life (T1/2) and 159 keV gamma emission (83%). Despite the nearly ideal chemical and nuclear properties of 123I, the widespread application of 123I-based radiopharmaceuticals in clinical practice has been limited by high production costs (123I is produced in a cyclotron), relatively limited availability, and the presence of undesirable radionuclidic impurities (124I, T1/2 = 4.2 days; 125I, T1/2 = 60 days; 126I, T1/2 = 13.1 days). The relatively long T1/2 and beta particle emission can substantially increase the higher radiation burden to the patient. High energy gamma rays (greater than 600 KeV) from these impurities can degrade images obtained using low energy collimators. Recent developments in production techniques have greatly reduced the levels of the undesirable radionuclides in 123I. Ready availability of pure 123I at modest cost, in concentrations suitable for the radio-labeling of a variety of useful biomolecules, should enhance the clinical applications of 123I-labeled compounds. Molecules labeled with 123I that are useful in cardiac imaging studies are fatty acid analogs, monoclonal antibodies, receptor binding agents, and norepinephrine analogs. This article will discuss developments in radioiodine (123I)-labeled radiotracers for myocardial imaging.

Radiopharmaceuticals for studying cardiac metabolism

(1) Metabolism is the link between myocardial blood flow and physiological performance of the heart. (2) Metabolic myocardial radiopharmaceuticals have the potential to identify metabolic alterations unique to a given intrinsic cardiac disease (e.g. cardiomyopathies), to assess acute metabolic changes or in delineating a specific chronic metabolic defect (e.g. coronary artery disease). (3) Two approaches can be employed to evaluate in vivo myocardial utilization of subtracts: (a) use of radiolabeled "physiologic" substrates e.g. positron emitting 11C-palmitic acid was successfully employed for assessing the in vivo metabolic sequelae of myocardial ischemia, infarction and cardiomyopathies, and (b) use of modified tracers which enter known metabolic pathways. However, because of their unique structure, metabolism of the tracer stops at a certain state thus leaving the radiolabel trapped in the cell, e.g. [18F]FDG for measuring glucose metabolic rate in the human brain and myocardium. (4) Among the radiopharmaceuticals for planar and single photon tomography, the para and the ortho isomers of 123I-phenyl iodoheptadecanoic acids and their beta-methyl derivatives are the most promising tracers for myocardial metabolic studies. (5) Ortho-(123I-phenyl)-pentadecanoic acid (o-IPPA) human myocardial uptake was rapidly and markedly elevated in well perfused segments; myocardial turnover was strikingly prolonged, suggesting some "trapping" phenomenon, resulting in excellent scintigrams. This is in contrast to the relatively shorter clearance of the para isomer from the myocardium. (6) 11C-Palmitic acid and [18F]FDG are the most widely used for PET scanning for following myocardial metabolism. The most important clinical application of these agents is predicting viability of ischemic myocardium. (7) A significant proportion of fixed perfusion defects seen on thallium studies can be demonstrated to be viable myocardium on PET scans using metabolic agents. If the markers of perfusion alone are relied on to assess tissue viability, the extent of salvageable myocardium may be underestimated. The demonstration of myocardial viability is crucial in the decision of the optimal treatment of the disease.

Abstention from alcohol in dilated cardiomyopathy: complete regression of the clinical disease but persistence of myocardial perfusion defects on exercise thallium-201 tomography

This case report describes a 43-year-old man with dilated cardiomyopathy reversed by abstention from alcohol over 1 year but with persistence of previous myocardial perfusion defects on exercise thallium-201 tomography. This suggests that despite the near normalization of left ventricular function, a permanent myocardial disease seems to persist.