Usefulness of 123I-metaiodobenzylguanidine myocardial scintigraphy for predicting the effectiveness of ?-blockers in patients with dilated cardiomyopathy from the standpoint of long-term prognosis

[11C]meta-hydroxyephedrine PET evaluation in experimental pulmonary arterial hypertension: Effects of carvedilol of right ventricular sympathetic function

BACKGROUND

Little is known about the sequelae of chronic sympathetic nervous system (SNS) activation in patients with pulmonary arterial hypertension (PAH) and right heart failure (RHF). We aimed to, (1) validate the use of [11C]-meta-hydroxyephedrine (HED) for assessing right ventricular (RV) SNS integrity, and (2) determine the effects of β-receptor blockade on ventricular function and myocardial SNS activity in a PAH rat model.

METHODS

PAH was induced in male Sprague-Dawley rats (N = 36) using the Sugen+chronic hypoxia model. At week 5 post-injection, PAH rats were randomized to carvedilol (15 mg·kg-1·day-1 oral; N = 16) or vehicle (N = 16) for 4 weeks. Myocardial SNS function was assessed with HED positron emission tomography(PET).

RESULTS

With increasing PAH disease severity, immunohistochemistry confirmed selective sympathetic denervation within the RV and sparing of parasympathetic nerves. These findings were confirmed on PET with a significant negative relationship between HED volume of distribution(DV) and right ventricular systolic pressure (RVSP) in the RV (r = -0.90, p = 0.0003). Carvedilol did not reduce hemodynamic severity compared to vehicle. RV ejection fraction (EF) was lower in both PAH groups compared to control (p < 0.05), and was not further reduced by carvedilol. Carvedilol improved SNS function in the LV with significant increases in the HED DV, and decreased tracer washout in the LV (p < 0.05) but not RV.

CONCLUSIONS

PAH disease severity correlated with a reduction in HED DV in the RV. This was associated with selective sympathetic denervation. Late carvedilol treatment did not lead to recovery of RV function. These results support the role of HED imaging in assessing SNS innervation in a failing right ventricle.

Cardiac Imaging With 123I-meta-iodobenzylguanidine and Analogous PET Tracers: Current Status and Future Perspectives

Autonomic innervation plays an important role in proper functioning of the cardiovascular system. Altered cardiac sympathetic function is present in a variety of diseases, and can be assessed with radionuclide imaging using sympathetic neurotransmitter analogues. The most studied adrenergic radiotracer is cardiac ¹²³I-meta-iodobenzylguanidine (¹²³I-mIBG). Cardiac ¹²³I-mIBG uptake can be evaluated using both planar and tomographic imaging, thereby providing insight into global and regional sympathetic innervation. Standardly assessed imaging parameters are the heart-to-mediastinum ratio and washout rate, customarily derived from planar images. Focal tracer deficits on tomographic imaging also show prognostic utility, with some data suggesting that the best approach to tomographic image interpretation may differ from conventional methods. Cardiac ¹²³I-mIBG image findings strongly correlate with the severity and prognosis of many cardiovascular diseases, especially heart failure and ventricular arrhythmias. Cardiac ¹²³I-mIBG imaging in heart failure is FDA approved for prognostic purposes. With the robustly demonstrated ability to predict occurrence of potentially fatal arrhythmias, cardiac ¹²³I-mIBG imaging shows promise for better selecting patients who will benefit from an implantable cardioverter defibrillator, but clinical use has been hampered by lack of the randomized trial needed for incorporation into societal guidelines. In patients with ischemic heart disease, cardiac ¹²³I-mIBG imaging aids in assessing the extent of damage and in identifying arrhythmogenic regions. There have also been studies using cardiac ¹²³I-mIBG for other conditions, including patients following heart transplantation, diabetic related cardiac abnormalities and chemotherapy induced cardiotoxicity. Positron emission tomographic adrenergic radiotracers, that improve image quality, have been investigated, especially ¹¹C-meta-hydroxyephedrine, and most recently ¹⁸F-fluorbenguan. Cadmium-zinc-telluride cameras also improve image quality. With better spatial resolution and quantification, PET tracers and advanced camera technologies promise to expand the clinical utility of cardiac sympathetic imaging.

Molecular imaging in heart failure patients

This review focuses on molecular imaging using various radioligands for the tissue characterization of patients with heart failure. ¹²³I-labeled metaiodobenzylguanidine (MIBG), as a marker of adrenergic neuron function, plays an important role in risk stratification in heart failure and may be useful for predicting fatal arrhythmias that may require implantable cardioverter-defibrillator treatment. MIBG has also been used for monitoring treatment effects under various medications. Various positron emission tomography (PET) radioligands have been introduced for the quantitative assessment of presynaptic and postsynaptic neuronal function in vivo. ¹¹C-hydroxyephedrine, like MIBG, has potential for assessing the severity of heart failure. Our PET study using the β-receptor antagonist ¹¹C-CGP 12177 in patients with heart failure showed a reduction of β-receptor density, indicating downregulation, in most of the patients. More studies are needed to confirm the clinical utility of these molecular imaging modalities for the management of heart failure patients.

Cardiac autonomic imaging with SPECT tracers

Radionuclide cardiac imaging has potential to assess underlying molecular, electrophysiologic, and pathophysiologic processes of cardiac disease. An area of current interest is cardiac autonomic innervation imaging with a radiotracer such as (123)I-meta-iodobenzylguanidine ((123)I-mIBG), a norepinephrine analogue. Cardiac (123)I-mIBG uptake can be assessed by planar and SPECT techniques, involving determination of global uptake by a heart-to-mediastinal ratio, tracer washout between early and delayed images, and focal defects on tomographic images. Cardiac (123)I-mIBG findings have consistently been shown to correlate strongly with heart failure severity, pre-disposition to cardiac arrhythmias, and poor prognosis independent of conventional clinical, laboratory, and image parameters. (123)I-mIBG imaging promises to help monitor a patient's clinical course and response to therapy, showing potential to help select patients for an ICD and other advanced therapies better than current methods. Autonomic imaging also appears to help diagnose ischemic heart disease and identify higher risk, as well as risk-stratify patients with diabetes. Although more investigations in larger populations are needed to strengthen prior findings and influence modifications of clinical guidelines, cardiac (123)I-mIBG imaging shows promise as an emerging technique for recognizing and following potentially life-threatening conditions, as well as improving our understanding of the pathophysiology of various diseases.

Cardiac applications of 123I-mIBG imaging

Cardiac autonomic innervation plays a key role in maintaining hemodynamic and electrophysiologic harmony. Cardiac sympathetic function is adversely altered in many disease states, such as congestive heart failure, myocardial ischemia, and diabetes. (123)I-mIBG, a sympathetic neurotransmitter radionuclide analog, aids in the detection of sympathetic innervation abnormalities and can be imaged with planar and single-photon emission computed tomographic techniques. Cardiac (123)I-mIBG uptake can be assessed by the heart mediastinal ratio (H/M), tracer washout rate, and focal uptake defects. These parameters have been widely studied and shown to correlate strongly and independently with congestive heart failure progression, cardiac arrhythmias, cardiac death, and all-cause mortality. There is accumulating evidence that (123)I-mIBG imaging can help to monitor a patient's clinical course and response to therapy. The ability to predict potentially lethal ventricular arrhythmias promises to help more accurately select patients for implantable cardioverter defibrillators, limiting unnecessary devices and identifying additional patients at risk who do not meet current guidelines. (123)I-mIBG shows potential to help determine whether greater risk and usually more expensive ventricular assist device therapies or cardiac transplantation might be needed. Although more investigation in larger populations is needed to strengthen previous findings, cardiac (123)I-mIBG imaging shows promise as a new technique for recognizing and following potentially life-threatening cardiac conditions.

Quantitative I-123 mIBG SPECT in differentiating abnormal and normal mIBG myocardial uptake

BACKGROUND

The purpose of this study was to evaluate global quantitation of cardiac uptake on I-123 mIBG SPECT.

METHODS

The study included a pilot group of 67 subjects and a validation group of 1,051 subjects. SPECT images were reconstructed by filtered backprojection, ordered subsets expectation maximization, and deconvolution of septal penetration, respectively. SPECT heart-to-mediastinum ratio (H/M) was calculated by comparing the mean counts between heart and mediastinum volumes of interest drawn on transaxial images. Receiver operating characteristic (ROC) analysis was used to assess the capability of each SPECT method to differentiate the heart disease subjects from controls in comparison with that of the planar H/M.

RESULTS

In the validation group, the areas under the ROC curves were not significantly different between the SPECT and planar H/M. Order subsets expectation maximization had significantly larger area under the ROC curve than the other two SPECT methods.

CONCLUSION

H/M obtained from I-123 mIBG SPECT was equivalent to the planar H/M for differentiating between subjects with normal and abnormal mIBG uptake. Global quantification of cardiac I-123 mIBG SPECT may represent a viable alternative to the planar H/M.

Nuclear cardiology and heart failure

The prevalence of heart failure in the adult population is increasing. It varies between 1% and 2%, although it mainly affects elderly people (6-10% of people over the age of 65 years will develop heart failure). The syndrome of heart failure arises as a consequence of an abnormality in cardiac structure, function, rhythm, or conduction. Coronary artery disease is the leading cause of heart failure and it accounts for this disorder in 60-70% of all patients affected. Nuclear techniques provide unique information on left ventricular function and perfusion by gated-single photon emission tomography (SPECT). Myocardial viability can be assessed by both SPECT and PET imaging. Finally, autonomic dysfunction has been shown to increase the risk of death in patients with heart disease and this may be applicable to all patients with cardiac disease regardless of aetiology. MIBG scanning has a very promising prognostic value in patients with heart failure.

Long-term prognostic stratification by a combination of (123)I-metaiodobenzylguanidine scintigraphy and ejection fraction in dilated cardiomyopathy

OBJECTIVE

(123)I-metaiodobenzylguanidine (MIBG) scintigraphy is a useful tool for predicting the prognosis in patients with congestive heart failure; however, little is known regarding long-term prognostic evaluations. The aim of this study was to evaluate long-term prognosis in a roughly 10-year period, in dilated cardiomyopathy (DCM) by MIBG imaging, compared to other conventional functional parameters.

METHODS

Eighty-six DCM patients (50 ± 14 years of age, 57 males) underwent MIBG imaging, at 15 min and 4 h after tracer injection, from which the delayed heart to mediastinum ratio (H/M) and washout rate (WR) were obtained. The left ventricular ejection fraction (EF) and end-diastolic diameter (LVDd) were also measured by echocardiogram. All patients were followed up for 8-14 years, and the death event was investigated.

RESULTS

Kaplan-Meier curves revealed a poor prognosis only in the group above the third quartile of WR (=50%) (10-year prognosis, 35%); however, there were no statistically significant differences in prognosis among the other 3 groups (10-year prognosis, 75-84%). A Cox hazard univariate analysis selected WR (p = 0.0004), H/M (p < 0.0001), EF (p = 0.0024), and LVDd (p = 0.0189) as significant prognostic indicators. Multivariate analysis revealed the H/M (p = 0.0023) and EF (p = 0.024) to be an independent prognostic predictor. The 10-year prognosis of patients with both WR < 50% and EF > 30%; WR < 50% and EF < 30%; and both WR > 50% and EF < 30% were 89, 71, and 33%, respectively. These three groups were well stratified, significantly (log-rank test: χ (2) = 30.0, p < 0.0001). However, even patients with WR ≥ 50% had few death events after 3 years following MIBG imaging.

CONCLUSIONS

The MIBG parameter, delayed H/M or WR combined with the EF is a useful tool for the prediction of a long-term prognosis in DCM, which is superior to MIBG parameters alone. However, patients with WR > 50% but no event in a 3-year follow-up period should undergo an additional MIBG imaging for prognostic prediction.

Scintigraphic techniques for early detection of cancer treatment-induced cardiotoxicity

New antitumor agents have resulted in significant survival benefits for cancer patients. However, several agents may have serious cardiovascular side effects. Left ventricular ejection fraction measurement by (99m)Tc multigated radionuclide angiography is regarded as the gold standard to measure cardiotoxicity in adult patients. It identifies left ventricular dysfunction with high reproducibility and low interobserver variability. A decrease in left ventricular ejection fraction, however, is a relatively late manifestation of myocardial damage. Nuclear cardiologic techniques that visualize pathophysiologic processes at the tissue level could detect myocardial injury at an earlier stage. These techniques may give the opportunity for timely intervention to prevent further damage and could provide insights into the mechanisms and pathophysiology of cardiotoxicity caused by anticancer agents. This review provides an overview of past, current, and promising newly developed radiopharmaceuticals and describes the role and recent advances of scintigraphic techniques to measure cardiotoxicity. Both first-order functional imaging techniques (visualizing mechanical [pump] function), such as (99m)Tc multigated radionuclide angiography and (99m)Tc gated blood-pool SPECT, and third-order functional imaging techniques (visualizing pathophysiologic and neurophysiologic processes at the tissue level) are discussed. Third-order functional imaging techniques comprise (123)I-metaiodobenzylguanidine scintigraphy, which images the efferent sympathetic nervous innervations; sympathetic neuronal PET, with its wide range of tracers; (111)In-antimyosin, which is a specific marker for myocardial cell injury and necrosis; (99m)Tc-annexin V scintigraphy, which visualizes apoptosis and cell death; fatty-acid-use scintigraphy, which visualizes the storage of free fatty acids in the lipid pool of the cytosol (which can be impaired by cardiotoxic agents); and (111)In-trastuzumab imaging, to study trastuzumab targeting to the myocardium. To define the prognostic importance and clinical value of each of these functional imaging techniques, prospective clinical trials are warranted.

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.

Intracellular and extracellular targets of molecular imaging in the myocardium

Utilization of molecular imaging has significantly advanced the field of cardiovascular medicine. In addition to the targets currently in use, novel targets are being developed, including those involved in the processes of myocardial metabolism, myocardial injury, cardiac neurotransmission, and interstitial dysregulation. Further development of these imaging targets may lead to improved characterization of disease processes and guide provision of individualized therapies.

Prognostic value of myocardial 123I-metaiodobenzylguanidine (MIBG) parameters in patients with heart failure: a systematic review

AIMS

To derive a more precise estimate of the prognostic significance of myocardial 123I-metaiodobenzylguanidine (MIBG) parameters [early heart mediastinal ratio (H/M), late H/M, and myocardial washout] in heart failure (HF).

METHODS AND RESULTS

Eighteen studies with a total of 1755 patients, stratifying survival, and cardiac events in patients with HF by MIBG, were eligible for analysis. The pooled hazard ratio (HR) estimates for cardiac death and cardiac events associated with washout showed no significant heterogeneity and were 1.72 [95%CI (confidence interval), 1.72-2.52; P = 0.006] and 1.08 (95%CI: 1.03-1.12; P < 0.001), respectively. The pooled HR estimates for cardiac death and cardiac events associated with early H/M and late H/M showed significant heterogeneity (I2 > or = 75%). Limiting the pooling to the qualitative best three studies rendered I2 insignificant (I2 = 0) and resulted in a pooled HR of late H/M for cardiac death of 1.82 (95%CI: 0.80-4.12; P = 0.15) and for cardiac events of 1.98 (95%CI: 1.57-2.50; P < 0.001).

CONCLUSION

Our results indicate that patients with HF and decreased late H/M or increased myocardial MIBG washout have a worse prognosis compared with those with normal semi-quantitative myocardial MIBG parameters.

Neuronal imaging using SPECT

(123)I-metaiodobenzylguanidine (MIBG) myocardial scintigraphy is one of only a few methods available for objective evaluation of cardiac sympathetic function at the clinical level. Disorders of cardiac sympathetic function play an important role in various heart diseases, and MIBG provides an abundance of useful information for the evaluation of severity, prognosis and therapeutic effects; this is particularly useful in cases of heart failure, ischaemic heart disease and arrhythmic disease. On the other hand, the quantitative indices for MIBG differ between institutions, and evidence has not been sufficiently well established for MIBG scintigraphy when compared with myocardial perfusion imaging in ischaemic heart diseases. In consideration of these difficulties, this review provides fundamental information regarding MIBG, its usefulness for various diseases and future difficulties.

Neuronal imaging using SPECT

BACKGROUND

123I-metaiodobenzylguanidine (MIBG) myocardial scintigraphy is one of only a few methods available for objective evaluation of cardiac sympathetic function at a clinical level. Disorders in cardiac sympathetic function play an important role in various heart diseases, and MIBG provides an abundance of useful information for evaluation of disease severity, prognosis, and therapeutic effects; this information is of particular value in patients with heart failure, ischemic heart diseases, or arrhythmic disorders. On the other hand, the quantitative indices for MIBG differ between institutions, and evidence has not been sufficiently well established for MIBG, compared with myocardial perfusion imaging, in ischemic heart diseases.

REVIEW

In view of these difficulties, this review provides fundamental information regarding MIBG, its usefulness for various diseases and future difficulties.

Targeting neuronal dysfunction and receptor imaging

The sympathetic nervous system has great influence on cardiovascular physiology, and the importance of cardiac innervation abnormalities in the physiopathology of various cardiac diseases has been emphasized. Cardiac neurotransmission imaging with single-photon emission computed tomography (SPECT) allows in vivo assessment of the myocardial nervous system. At present, the most commonly used SPECT tracer to assess cardiac neurotransmission is metaiodobenzylguanidine labelled with iodine-123 ((123)I-MIBG). In patients with heart transplantation, ischemic heart disease, dysautonomias and drug-induced cardiotoxicity, assessment of neuronal function can help characterise the disease and improve the prognostic stratification. Cardiac (123)I-MIBG scintigraphy allows autonomic neuropathy to be detected in the early stages of diabetes mellitus. In patients with heart failure, the assessment of cardiac sympathetic activity has important prognostic implications. Future directions in cardiac sympathetic neurotransmission include the development of new tracers, targeting of second-messenger molecules and early assessment of cardiac neurotransmission in genetically predisposed subjects for prevention of heart failure.

Usefulness of meta-[123I]iodobenzylguanidine myocardial scintigraphy for predicting cardiac events in patients with dilated cardiomyopathy who receive long-term beta blocker treatment

OBJECTIVE

Large-scale clinical trials have demonstrated that beta blocker therapy improves the prognosis of dilated cardiomyopathy (DCM), but cardiac events sometimes occur even in patients showing favourable response to the therapy. In this study, the usefulness of meta-iodobenzyguanidine (MIBG) in predicting cardiac events was investigated in DCM patients successfully receiving long-term treatment with beta blockers.

METHODS

The subjects were 53 patients with DCM (including 10 women; mean age, 56.5+/-10.9 years) who could continue beta blocker therapy for more than 6 months. MIBG was performed within 1 year of commencing the therapy. The extent score, severity score and washout rate were obtained from single photon emission computed tomography images. At the same time, left ventricular ejection fraction (LVEF) and left ventricular end diastolic diameters were measured by echocardiography. The endpoints were cardiac events (cardiac death and admission to hospital due to heart failure or arrhythmia), and patients were observed for an average of 1314+/-986 days (150-4100 days).

RESULTS

Cardiac events occurred in nine patients during the observation period. The multivariate statistical analysis demonstrated that the delayed extent score was the strongest significant predictive factor, (hazard ratio 1.036, P<0.01). while LVEF was not a predictive factor. Both the improvement of LVEF and MIBG were significant predictive factors. The improvement of washout rate was the strongest.

CONCLUSION

Parameters of MIBG but not of LVEF were useful in predicting cardiac events in DCM patients whose condition had been successfully stabilized by the introduction of beta blockers.