Assessment of cardiac sympathetic neuronal function using PET imaging

Noradrenergic cardiac denervation is associated with gait velocity in Parkinson disease: a dual ligand PET study

PURPOSE

Preliminary data suggest that gait abnormalities in Parkinson disease (PD) may be associated with sympathetic cardiac denervation. No kinematic gait studies were performed to confirm this observation. We aimed to correlate spatiotemporal kinematic gait parameters with cardiac sympathetic denervation as determined by cardiac [11C]HED PET in PD.

METHODS

Retrospective database analysis of 27 PD patients with cardiac sympathetic denervation. All patients underwent spatiotemporal kinematic gait assessment (medication 'off' state), cardiac [11C]HED and dopaminergic brain [11C]DTBZ PET scans. We employed a hierarchical regression approach to examine associations between the extent of cardiac denervation, dopaminergic nigrostriatal neurodegeneration, and three gait parameters - velocity, step length and cadence.

RESULTS

More extensive cardiac denervation was associated with slower velocity (estimate: -1.034, 95% CI [-1.65, -0.42], p = 0.002), shorter step length (estimate: -0.818, 95% CI [-1.43, -0.21], p = 0.011) and lower cadence (estimate: -0.752, 95% CI [-1.28, -0.23], p = 0.007) explaining alone 30% (Adjusted-R²: 0.297), 20% (Adjusted-R²: 0.202) and 23% (Adjusted-R²: 0.227) of the variability, respecivetly. These associations remained independent of striatal dopaminergic impairment and confounding factors such as age, Hoehn and Yahr (HY) stages, peripheral neuropathy, cognition, and autonomic symptoms. In contrast, striatal dopaminergic denervation was significantly associated with step length (estimate: 0.883, 95% CI [0.29, 1.48], p = 0.005), explaining about 24% of the variability but was dependent of HY stage.

CONCLUSIONS

More severe cardiac noradrenergic denervation was associated with lower gait velocity, independent of striatal dopaminergic denervation and HY stage, impacting both step length and cadence. These results suggest independent contributions of the peripheral autonomic system degeneration on gait dynsfunction in PD.

Cardiac Sympathetic Positron Emission Tomography Imaging with Meta-[18F]Fluorobenzylguanidine is Sensitive to Uptake-1 in Rats

Dysfunction of the cardiac sympathetic nervous system contributes to the development of cardiovascular diseases including ischemia, heart failure, and arrhythmias. Molecular imaging probes such as meta-[¹²³I]iodobenzylguanidine have demonstrated the utility of assessing neuronal integrity by targeting norepinephrine transporter (NET, uptake-1). However, current radiotracers can report only on innervation due to suboptimal kinetics and lack sensitivity to NET in rodents, precluding mechanistic studies in these species. The objective of this work was to characterize myocardial sympathetic neuronal uptake mechanisms and kinetics of the positron emission tomography (PET) radiotracer meta-[¹⁸F]fluorobenzylguanidine ([¹⁸F]mFBG) in rats. Automated synthesis using spirocyclic iodonium(III) ylide radiofluorination produces [¹⁸F]mFBG in 24 ± 1% isolated radiochemical yield and 30-95 GBq/μmol molar activity. PET imaging in healthy rats delineated the left ventricle, with monoexponential washout kinetics (k_mono = 0.027 ± 0.0026 min^-1, A_mono = 3.08 ± 0.33 SUV). Ex vivo biodistribution studies revealed tracer retention in the myocardium, while pharmacological treatment with selective NET inhibitor desipramine, nonselective neuronal and extraneuronal uptake-2 inhibitor phenoxybenzamine, and neuronal ablation with neurotoxin 6-hydroxydopamine reduced myocardial retention by 33, 76, and 36%, respectively. Clearance of [¹⁸F]mFBG from the myocardium was unaffected by treatment with uptake-1 and uptake-2 inhibitors following peak myocardial activity. These results suggest that myocardial distribution of [¹⁸F]mFBG in rats is dependent on both NET and extraneuronal transporters and that limited reuptake to the myocardium occurs. [¹⁸F]mFBG may therefore prove useful for imaging intraneuronal dysfunction in small animals.

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.

Electrophysiological characteristics of the rat azygos vein under electrical pacing and adrenergic stimulation

Rodent thoracic veins are characterized by an extended myocardial coating. In the present study, the electrical activity in the cardiac tissue of the rat azygos vein (AZV) was investigated for the first time. The atrial-like action potentials (AP) and atrial-like conduction of the excitation were observed in the rat AZV under continuous electrical pacing. Termination of electrical pacing resulted in spontaneous positive shift of resting membrane potential (RMP) in AZV. Boradrenaline induced biphasic effects on RMP in all quiescent AZV preparations but only in 25% preparations-bursts of spontaneous AP, which were suppressed by both α- and β-adrenoreceptor antagonists. Phenylephrine induced additional depolarization of RMP in quiescent AZV preparations, while isoproterenol caused hyperpolarization. In conclusion, bioelectrical properties of the rat AZV resemble those of atrial myocardium under continuous electrical pacing; however, depolarized RMP and NA-induced spontaneous AP characterize AZV as a tissue prone to rare automaticity.

Optimizing Parkinson's disease diagnosis: the role of a dual nuclear imaging algorithm

The diagnosis of Parkinson's disease (PD) currently relies almost exclusively on the clinical judgment of an experienced neurologist, ideally a specialist in movement disorders. However, such clinical diagnosis is often incorrect in a large percentage of patients, particularly in the early stages of the disease. A commercially available, objective and quantitative marker of nigrostriatal neurodegeneration was recently provided by 123-iodine ¹²³I-ioflupane SPECT imaging, which is however unable to differentiate PD from a variety of other parkinsonian syndromes associated with striatal dopamine deficiency. There is evidence to support an algorithm utilizing a dual neuroimaging strategy combining ¹²³I-ioflupane SPECT and the noradrenergic receptor ligand ¹²³I-metaiodobenzylguanidine (MIBG), which assesses the post-ganglion peripheral autonomic nervous system. Evolving concepts regarding the synucleinopathy affecting the central and peripheral autonomic nervous systems as part of a multisystem disease are reviewed to sustain such strategy. Data are presented to show how MIBG deficits are a common feature of multisystem Lewy body disease and can be used as a unique feature to distinguish PD from atypical parkinsonisms. We propose that the combination of cardiac (MIBG) and cerebral ¹²³I-ioflupane SPECT could satisfy one of the most significant unmet needs of current PD diagnosis and management, namely the early and accurate diagnosis of patients with typical Lewy body PD. Exemplary case scenarios will be described, highlighting how dual neuroimaging strategy can maximize diagnostic accuracy for patient care, clinical trials, pre-symptomatic PD screening, and special cases provided by specific genetic mutations associated with PD.

Current Clinical Applications and Next Steps for Cardiac Innervation Imaging

PURPOSE OF REVIEW

Autonomic innervation is crucial for regulating cardiac function. Sympathetic innervation imaging with ¹²³I-mIBG and analogous PET tracers assesses disease in ways that differ from customary methods. This review describes practical use in various clinical scenarios, discusses recent guidelines, presents new data confirming risk stratification power, describes an ongoing prospective study, and looks forward to wider use in patient management.

RECENT FINDINGS

ASNC ¹²³I-mIBG guidelines are available, expanding on European guidelines. ADMIRE-HF patient follow-up increased to 2 years in ADMIRE HFX, demonstrating independent mortality risk reclassification. ADMIRE-HF findings were substantiated in a Japanese consortium study and in the PAREPET ¹¹C-HED PET study. Exciting potential uses of adrenergic imaging are management of LVADs and VT ablation. CZT cameras provide advantages, but derived parameters differ from Anger camera values. Independent risk stratification utility of adrenergic imaging with ¹²³I-mIBG and PET tracers is continuously being confirmed. An ongoing prospective randomized study promises to establish patient management utility. There is potential for wider use and improved images with newer cameras and PET.

2-Year Natural Decline of Cardiac Sympathetic Innervation in Idiopathic Parkinson Disease Studied with 11C-Hydroxyephedrine PET

The objective of this study was to detect regional patterns of cardiac sympathetic denervation in idiopathic Parkinson disease (IPD) using ¹¹C-hydroxyephedrine (¹¹C-HED) PET and determine the denervation rate over 2 y.

METHODS

We obtained 62 cardiac ¹¹C-HED PET scans in 39 patients (30 men and 9 women; mean age ± SD, 61.9 ± 5.9 y), including 23 patients with follow-up scans at 2 y. We derived ¹¹C-HED retention indices (RIs; mL of blood/min/mL of tissue) reflecting nerve density and integrity for 480 left ventricular (LV) sectors. We compared IPD patients with 33 healthy controls using z score analysis; RI values ≤ 2.5 SDs were considered abnormal. We expressed global and regional LV denervation as the percentage extent of z score severity and severity-extent product (SEP) on 9-segment bullseye maps and decline in cardiac sympathetic innervation as the 2-y difference in SEP (diff-SEP).

RESULTS

Baseline ¹¹C-HED PET in the 39 IPD patients revealed an RI mean of 0.052 ± 0.022 mL of blood/min/mL of tissue. In comparison with data from normal controls, 12 patients had normal ¹¹C-HED PET, 5 showed mild denervation (percentage extent < 30%), and 22 had moderate to severe denervation (percentage extent > 30%, z score ≤ 2.5 SD). In the 23 paired PET scans, worsening cardiac denervation (global diff-SEP > 9) occurred in 14 of 23 (60.9%) patients over 2 y, including percentage LV abnormality (59% increasing to 66%), z-severity (-2.4 down to -2.5), and SEP (-195 to -227) (P = 0.0062). We found a mean annual decline of 4.6% ± 5.6 (maximum, 13%) in ¹¹C-HED retention from a baseline global RI mean of 0.0481 ± 0.0218 to 0.0432 ± 0.0220 (P = 0.0009). At baseline, 5 patients with normal uptake had no interval change; 3 with mild denervation developed interval decline in lateral and inferior segments (diff-SEP -82 to -99) compared with anterior and septal segments (-65 to -79), whereas the reverse pattern occurred in 15 patients with severe baseline denervation.

CONCLUSION

Progressive decline in cardiac sympathetic neural integrity in IPD patients occurs at a modest rate over 2 y on ¹¹C-HED scans with marked heterogeneity and a regional pattern of involvement and decline.

Inter-relation between autonomic and HPA axis activity in children and adolescents

Stress research in youth typically considers either the autonomic nervous system or HPA axis. However, these systems are highly coordinated and physically interconnected. We examined whether the inter-relation between cardio-autonomic and HPA axis measures was better associated with perceived stress than their singular associations. Children and adolescents (N=201) collected saliva samples to measure cortisol (AUCAG, AUCI, maximum), wore an electrocardiogram monitor for 24h to derive heart rate variability (HRV; LF, HF, LF/HF ratio), and completed the Perceived Stress Scale. The interaction between sympathovagal modulation (LF, LF/HF ratio) and cortisol awakening response (AUCAG, AUCI, maximum) explained significantly greater variance in perceived stress than either stress system alone. Higher sympathovagal modulation combined with higher cortisol awakening response was associated with greater perceived stress. Findings suggest that the inter-relation between cardio-autonomic and HPA axis activity may advance our understanding of how stress impacts health.

N-(11)C-Methyl-Dopamine PET Imaging of Sympathetic Nerve Injury in a Swine Model of Acute Myocardial Ischemia: A Comparison with (13)N-Ammonia PET

Objective. Using a swine model of acute myocardial ischemia, we sought to validate N-¹¹C-methyl-dopamine (¹¹C-MDA) as an agent capable of imaging cardiac sympathetic nerve injury. Methods. Acute myocardial ischemia was surgically generated in Chinese minipigs. ECG and serum enzyme levels were used to detect the presence of myocardial ischemia. Paired ¹¹C-MDA PET and ¹³N-ammonia PET scans were performed at baseline, 1 day, and 1, 3, and 6 months after surgery to relate cardiac sympathetic nerve injury to blood perfusion. Results. Seven survived the surgical procedure. The ECG-ST segment was depressed, and levels of the serum enzymes increased. Cardiac uptake of tracer was quantified as the defect volume. Both before and immediately after surgery, the images obtained with ¹¹C-MDA and ¹³N-ammonia were similar. At 1 to 6 months after surgery, however, ¹¹C-MDA postsurgical left ventricular myocardial defect volume was significantly greater compared to ¹³N-ammonia. Conclusions. In the Chinese minipig model of acute myocardial ischemia, the extent of the myocardial defect as visualized by ¹¹C-MDA is much greater than would be suggested by blood perfusion images, and the recovery from myocardial sympathetic nerve injury is much slower than the restoration of blood perfusion. ¹¹C-MDA PET may provide additional biological information during recovery from ischemic heart disease.

Mechanistic Insights into Sympathetic Neuronal Regeneration: Multitracer Molecular Imaging of Catecholamine Handling After Cardiac Transplantation

BACKGROUND

Post-transplant reinnervation is a unique model to study sympathetic neuronal regeneration in vivo. The differential role of subcellular mechanisms of catecholamine handling in nerve terminals has not been investigated.

METHODS AND RESULTS

Three different carbon-11-labeled catecholamines were used for positron emission tomography of transport (C-11 m-hydroxyephedrine, HED), vesicular storage (C-11 epinephrine, EPI), and metabolic degradation (C-11 phenylephrine). A 2-day protocol was used, including quantification of myocardial blood flow by N-13 ammonia. Resting myocardial blood flow and EPI, HED and phenylephrine retention were homogeneous in healthy volunteers (n=7). Washout was only observed for phenylephrine (T(1/2) 49±6 min). In nonrejecting, otherwise healthy heart transplant recipients (>1 year after surgery, n=10), resting myocardial blood flow was also homogenous. Regional catecholamine uptake of varying degrees was observed in the anterior left ventricular wall and septum. Overall, 24±19% of left ventricle showed HED uptake levels comparable with healthy volunteers, whereas it was only 8±7% for EPI (P=0.004 versus HED). Phenylephrine washout was not different from healthy volunteers in the area with restored EPI and HED retention (T(1/2) 41±7 min; P>0.05), but was significantly enhanced in the EPI/HED mismatch area (T(1/2) 36±8 min; P=0.008), consistent with inefficient vesicular storage and enhanced metabolic degradation.

CONCLUSIONS

Regeneration of subcellular components of sympathetic nerve terminal function does not occur simultaneously. In the reinnervating transplanted heart, a region with normal catecholamine transport and vesicular storage is surrounded by a borderzone, where transport is already restored but vesicular storage remains inefficient, suggesting that vesicular storage is a more delicate mechanism. This observation may have implications for other pathologies involving cardiac autonomic innervation.

11C-meta-hydroxyephedrine: a promising PET radiopharmaceutical for imaging the sympathetic nervous system

Dysfunction of the sympathetic nervous system underlies many cardiac diseases and can be assessed by molecular imaging using SPECT tracers as I-metaiodobenzylguanidine (I-MIBG). The norepinephrine analog C-meta-hydroxyephedrine (HED) has been used with PET to map the regional distribution of cardiac sympathetic neurons. Hydroxyephedrine is rapidly transported into sympathetic neurons by the norepinephrine transporter and stored in vesicles. This review describes the mechanism of action, radiosynthesis, and application of HED in the assessment of the cardiac sympathetic nervous system in heart failure, myocardial infarction, and arrhythmias. Noncardiac applications of HED in the clinical setting of sympathetic nervous system tumors and other emerging research applications are described.

Introduction to cardiac neuronal imaging: a clinical perspective

Procedures for noninvasive and minimally invasive imaging of cardiac neurons and neuronal function using radiolabeled compounds were developed in the second half of the 20th century. The foundation for these procedures was several centuries of research that identified the structural components of the autonomic nervous system and explored the means by which neurotransmitters such as acetylcholine and norepinephrine contributed to neuronal control of target organ effector cells. This article provides a brief clinical overview of modern approaches to the assessment of cardiac neurons as an introduction to the in-depth articles on the current status of cardiac neuronal imaging presented in this supplement.

Clinical Applications of Myocardial Innervation Imaging

Cardiac autonomic innervation plays an important role in regulating function. Adrenergic innervation imaging is possible with the norepinephrine analogue radiotracer iodine 123 meta-iodobenzylguanidine ((123)I-mIBG) and positron emitting tracers such carbon-11 hydroxyephedrine. (123)I-mIBG uptake is assessed globally via the heart to mediastinum ratio on planar images and regionally with tomographic imaging and has utility in various cardiac diseases. There is promise for guiding expensive invasive therapies such as implantable defibrillators, ventricular assist devices, and transplant. There are reports of utility in primary arrhythmic conditions, ischemic heart disease, and diabetes and after cardiac damaging chemotherapy.

Molecular imaging to predict ventricular arrhythmia in heart failure

Ventricular tachycardia (VT) is a major cause of sudden cardiac death (SCD) in patients with heart failure (HF). Left ventricular ejection fraction (LVEF) and heart failure class according to the New York Heart association (NYHA) are in most common use to identify patients that may benefit from implantable cardioverter defibrillator (ICD) therapy. But during 3 years of follow up only 35% of patients receive appropriate ICD action. Therefore, there is a continued need for refinement of selection criteria for ICD implantation. In this regard, molecular imaging of the autonomic nervous system, which plays a central role in HF progression and cardiac electro-mechanical regulation, can make a substantial contribution. This article reviews the currently available literature concerning the value of molecular neuronal cardiac imaging for prediction of ventricular arrhythmias in HF patients.

Sudden Cardiac Death: A Review Focused on Cardiovascular Imaging

Sudden cardiac death (SCD) is defined as natural death due to cardiac causes, heralded by abrupt loss of consciousness within 1 h of the onset of acute symptoms; pre-existing heart disease may have been known to be present but the time and mode of death are unexpected. Prediction and prevention of SCD is an area of active investigation, but considerable challenges persist that limit the efficacy and cost-effectiveness of available methodologies. It was well-recognized that optimization of SCD risk stratification would require integration of multi-disciplinary efforts at the bench and bedside, with studies in the general population. This integration has yet to be effectively accomplished. There is also increasing awareness that more investigation needs to be directed toward the identification of early predictors of SCD. Significant advancements have recently occurred for risk prediction in the inherited channelopathies and other inherited conditions that predispose to SCD, but there is much to be accomplished in this regard for the more common complex phenotypes, such as SCD among patients with coronary artery disease. A multimodality imaging approach is actually the most important tool to provide comprehensive information on different pathophysiological mechanisms related to SCD.

6-[18F]fluoro-L-DOPA uptake in the rat pancreas is dependent on the tracer metabolism

PURPOSE

6-[(18)F]fluoro-L-3,4-dihydroxyphenyl alanine ([(18)F]FDOPA) positron emission tomography (PET) is a diagnostic tool which can detect malignancies of the pancreas. We aimed to study whether the manipulation of the [(18)F]FDOPA metabolic pathway would change the (18)F-behavior to provide a biochemical foundation for PET imaging of rat pancreas with [(18)F]FDOPA.

PROCEDURES

Inhibitors of aromatic amino acid decarboxylase, catechol-O-methyltransferase, monoamine oxidases A and B, or their combinations on [(18)F]FDOPA uptake, metabolism, and the regional distribution in the rat pancreas was evaluated using in vivo PET/computed tomography imaging, chromatographic metabolite analyses, and autoradiography.

RESULTS

Enzyme inhibition generally increased the uptake of [(18)F]FDOPA derived (18)F-radioactivity in rat pancreas. Dependent on which enzymatic pathway is blocked (or a combination of pathways), different radiolabeled metabolites in pancreas are responsible for this increase in uptake.

CONCLUSIONS

Altering the metabolism of [(18)F]FDOPA by using various enzymatic inhibitors increased the radioactivity uptake and changed the radiometabolic profile in the pancreas allowing better discrimination between pancreas and surrounding tissues of rat. However, these manipulations did not separate islets from the exocrine pancreas. Elucidating the metabolic behavior of [(18)F]FDOPA provides a biochemical foundation of PET imaging of the rat pancreas.

Regional myocardial sympathetic denervation predicts the risk of sudden cardiac arrest in ischemic cardiomyopathy

OBJECTIVES

The PAREPET (Prediction of ARrhythmic Events with Positron Emission Tomography) study sought to test the hypothesis that quantifying inhomogeneity in myocardial sympathetic innervation could identify patients at highest risk for sudden cardiac arrest (SCA).

BACKGROUND

Left ventricular ejection fraction (LVEF) is the only parameter identifying patients at risk of SCA who benefit from an implantable cardiac defibrillator (ICD).

METHODS

We prospectively enrolled 204 subjects with ischemic cardiomyopathy (LVEF ≤35%) eligible for primary prevention ICDs. Positron emission tomography (PET) was used to quantify myocardial sympathetic denervation ((11)C-meta-hydroxyephedrine [(11)C-HED]), perfusion ((13)N-ammonia) and viability (insulin-stimulated (18)F-2-deoxyglucose). The primary endpoint was SCA defined as arrhythmic death or ICD discharge for ventricular fibrillation or ventricular tachycardia >240 beats/min.

RESULTS

After 4.1 years follow-up, cause-specific SCA was 16.2%. Infarct volume (22 ± 7% vs. 19 ± 9% of left ventricle [LV]) and LVEF (24 ± 8% vs. 28 ± 9%) were not predictors of SCA. In contrast, patients developing SCA had greater amounts of sympathetic denervation (33 ± 10% vs. 26 ± 11% of LV; p = 0.001) reflecting viable, denervated myocardium. The lower tertiles of sympathetic denervation had SCA rates of 1.2%/year and 2.2%/year, whereas the highest tertile had a rate of 6.7%/year. Multivariate predictors of SCA were PET sympathetic denervation, left ventricular end-diastolic volume index, creatinine, and no angiotensin inhibition. With optimized cut-points, the absence of all 4 risk factors identified low risk (44% of cohort; SCA <1%/year); whereas ≥2 factors identified high risk (20% of cohort; SCA ∼12%/year).

CONCLUSIONS

In ischemic cardiomyopathy, sympathetic denervation assessed using (11)C-HED PET predicts cause-specific mortality from SCA independently of LVEF and infarct volume. This may provide an improved approach for the identification of patients most likely to benefit from an ICD. (Prediction of ARrhythmic Events With Positron Emission Tomography [PAREPET]; NCT01400334).

Traditional and novel methods to assess and prevent chemotherapy-related cardiac dysfunction noninvasively

The field of cardio-oncology is challenged to address an ever greater spectrum of cardiotoxicity associated with combination chemotherapy, greater dose intensity, extremes of age, and enhanced patient survival which exposes more protracted risk of developing congestive heart failure (CHF). Recent reports of chemotherapy-induced hypertension as a common adverse effect of angiogenesis inhibitors and immunosuppressants clarify the need for routine blood pressure (BP) monitoring and guideline-based management of hypertension as an integral strategy to preserve LV function. Serial monitoring of radionuclide left ventricular ejection fraction (LVEF) in adults and echocardiography in children continues to provide outcome based, cost-effective prevention of CHF in high risk patients receiving chemotherapy. To optimize treatment and monitoring strategies to eliminate late-onset LV dysfunction and CHF, traditional and novel candidate methods for assessment of chemotherapy-induced LV dysfunction are reviewed. These include serial assessment of LV volume indices by gated SPECT ERNA and gated SPECT MPI, 3D echocardiography and contrast 2D echocardiography; longitudinal strain imaging, diastolic functional parameters, (123)I-MIBG, (111)In-Antimyosin antibody imaging, and (99m)Tc-Annexin V apoptosis imaging, biomarkers including troponins and BNP; genetic markers, and both functional and tissue characterization techniques with T1 weighted and T2 weighted images with cardiac magnetic resonance imaging (CMR). In our quest to optimize strategies for long-term cancer survival and prevention of CHF for patients receiving chemotherapy, rigorous modality and guideline-specific clinical outcome trials are required. A new multi-modality monitoring approach is proposed, which integrates evidence-based strengths of CMR, echocardiography, ERNA, biomarkers, and BP management for surveillance and validation of cardiotoxicity and prevention of clinical heart failure in patients receiving a broad spectrum of cancer therapies.

Assessment of cardiac autonomic neuronal function using PET imaging

The autonomic nervous system is the primary extrinsic control of cardiac performance, and altered autonomic activity has been recognized as an important factor in the progression of various cardiac pathologies. Molecular imaging techniques have been developed for global and regional interrogation of pre- and postsynaptic targets of the cardiac autonomic nervous system. Building on established work with the guanethidine analogue ¹²³I-metaiodobenzylguanidine (MIBG) for single-photon emission tomography (SPECT), development of radiotracers and protocols for positron emission tomography (PET) investigation of autonomic signaling has expanded. PET is limited in availability and requires specialized centers for radiosynthesis and interpretation, but the higher resolution allows for improved regional analysis and kinetic modeling provides more true quantification than is possible with SPECT. A wider array of radiolabeled catecholamines, analogues of catecholamines, and receptor ligands have been characterized and evaluated. Sympathetic neuronal PET tracers have shown promise in the identification of several cardiac pathologies. In particular, recent studies have elucidated a mechanistic role for heterogeneous sympathetic innervation in the development of lethal ventricular arrhythmias. Evaluation of cardiomyocyte adrenergic receptor expression and the parasympathetic nervous system has been slower to develop, with clinical studies beginning to emerge. This review summarizes the clinical and the experimental PET tracers currently available for autonomic imaging and discusses their application in health and cardiovascular disease, with particular emphasis on the major findings of the last decade.

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.

Assessment of global cardiac I-123 MIBG uptake and washout using volumetric quantification of SPECT acquisitions

BACKGROUND

Assessment of cardiac innervation using single-photon emission computer tomography (SPECT) is less established than planar imaging, but may be more suitable for quantification. Therefore, a volumetric quantification of I-123 MIBG SPECT acquisitions was performed. Reproducibility, the effects of extra cardiac I-123 MIBG uptake and the relation with conventional planar indices were evaluated.

METHODS

54 patients referred for planar and SPECT I-123 MIBG acquisitions were included. Ellipsoidal or box-shaped volumes of interest were placed on the left ventricle, cardiac lumen, mediastinum, lung and liver. SPECT segmentation was performed twice in all patients. Indices were determined based on the heart-to-mediastinum (HM), myocardial wall-to-mediastinum and myocardial wall-to-lumen regions. HM ratios and washout rates were also determined based on anterior planar images.

RESULTS

Cardiac count densities were highly reproducible (CV 1.5-5.4, ICC 0.96-0.99) and inter-rater variability was low (CV 1.8-6.8, ICC 0.94-0.99). Mediastinal uptake was an important explanatory variable of uptake in the entire heart (early R(2) = 0.36; delayed R(2) =0.43) and myocardial wall (early R(2) = 0.28; delayed R(2) = 0.37). Lung washout was an explanatory variable of organ washout of the heart (heart R(2) = 0.38; myocardial wall R(2) = 0.33). In general, SPECT indices showed moderate-to-good correlations with the planar uptake (PCC 0.497-0.851).

CONCLUSION

By applying a volumetric segmentation method we were able to segment the heart in all patients. SPECT I-123 MIBG quantification was found to be highly reproducible and had a moderate to good correlation with the planar indices.

Pattern of cardiac sympathetic denervation in idiopathic Parkinson disease studied with 11C hydroxyephedrine PET

PURPOSE

To determine whether cardiac sympathetic denervation in idiopathic Parkinson disease (IPD) affects the left ventricle in a distinct regional pattern versus a more global pattern with use of carbon 11 (11C) meta-hydroxyephedrine (HED) positron emission tomography (PET).

MATERIALS AND METHODS

This prospective study was approved by the institutional review board and was compliant with HIPAA. Informed consent was obtained from all subjects. Cardiac PET was performed with 11C HED in 27 patients with IPD (20 men and seven women aged 50-74 years; mean age, 62 years±6 [standard deviation]). 11C HED retention indexes (RIs), which reflect nerve density and integrity, were determined. RIs for 33 healthy control subjects (15 men and 18 women aged 20-78 years; mean age, 47 years±17) were used as a control database. Patients with IPD were compared with control subjects by using z score analysis. Global and segmental measurements of sympathetic denervation were expressed as percentage extent, z score severity, and severity-extent product (SEP). Group comparisons were performed with the Student t test.

RESULTS

The mean 11C HED RI was 0.086 mL of blood per minute per milliliter tissue±0.015 for control subjects and 0.043 mL of blood per minute per milliliter tissue±0.016 for patients with IPD (P<0001). When compared with normative data from the control database, profound cardiac denervation (global extent>50%) was seen in most patients (19 of 27 patients, 70%). Four patients had normal 11C HED studies and four had mild denervation (global extent<25%). The mean global denervation extent was 62%±38, the mean severity z score was -2.7±1.2, and the mean SEP was -202±131 (range, -358 to 0). Segmental analysis revealed relative sparing of anterior and proximal septal segments (mean extent, 48%-51%; mean severity z score, -2.47 to -2.0; mean SEP, -167 to -139), with lateral and proximal inferior segments more severely affected (mean extent, 68%-73%; mean severity z score, -2.8 to -2.62; mean SEP, -271 to -230). Patients with normal findings or preserved denervation did not significantly differ in mean age (t=1.09) or disease duration (t=0.44) compared to patients with severe sympathetic denervation.

CONCLUSION

Cardiac sympathetic denervation in IPD is extensive, with a segmental pattern that involves the proximal lateral left ventricular wall most severely, with relative sparing of the anterior and proximal septal walls.

(123)I-Labelled metaiodobenzylguanidine for the evaluation of cardiac sympathetic denervation in early stage amyloidosis

Purpose

Cardiac amyloidosis is a rare disorder, but it may lead to potentially life-threatening restrictive cardiomyopathy. Cardiac manifestations frequently occur in primary amyloidosis (AL) and familial amyloidosis (ATTR), but are uncommon in secondary amyloidosis (AA). Echocardiography is the method of choice for assessing cardiac amyloidosis. Amyloid deposits impair the function of sympathetic nerve endings. Disturbance of myocardial sympathetic innervations may play an important role in the remodelling process. ¹²³I-MIBG can detect these innervation changes.

Methods

Patients with biopsy-proven amyloidosis underwent general work-up, echocardiography and ¹²³I-MIBG scintigraphy. Left ventricular internal dimensions and wall thickness were measured, and highly refractile cardiac echoes (sparkling) were analysed. Early (15 min) and late (4 h) heart-to-mediastinum ratio (HMR) and wash-out rate were determined after administration of MIBG.

Results

Included in the study were 61 patients (30 women and 31 men; mean age 62 years; 39 AL, 11 AA, 11 ATTR). Echocardiographic parameters were not significantly different between the groups. Sparkling was present in 72 % of ATTR patients, in 54 % of AL patients and in 45 % of AA patients. Mean late HMR in all patients was 2.3 ± 0.75, and the mean wash-out rate was 8.6 ± 14 % (the latter not significantly different between the patient groups). Late HMR was significantly lower in patients with echocardiographic signs of amyloidosis than in patients without (2.0 ± 0.70 versus 2.8 ± 0.58, p < 0.001). Wash-out rates were significantly higher in these patients (−3.3 ± 9.9 % vs. 17 ± 10 %, p < 0.001). In ATTR patients without echocardiographic signs of amyloidosis, HMR was lower than in patients with the other types (2.0 ± 0.59 vs. 2.9 ± 0.50, p = 0.007).

Conclusion

MIBG HMR is lower and wash-out rate is higher in patients with echocardiographic signs of amyloidosis. Also, ¹²³I-MIBG scintigraphy can detect cardiac denervation in ATTR patients before signs of amyloidosis are evident on echocardiography.

Radionuclide imaging in stage B heart failure

This article discusses currently available radionuclide techniques in the diagnostic and prognostic evaluation of patients with chronic heart failure, with a focus on stage B/asymptomatic left ventricular dysfunction. Radionuclide imaging is promising for such patients because it can simultaneously determine left ventricular function, evaluate for the presence of obstructive coronary disease, determine the extent of viable myocardium, and evaluate dyssynchronous left ventricular contraction. Radionuclide imaging can thus provide important noninvasive insights into the pathophysiology, prognosis, and management of patients with asymptomatic left ventricular dysfunction as well as more advanced heat failure.

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.

Use of cardiac radionuclide imaging to identify patients at risk for arrhythmic sudden cardiac death

Sudden cardiac death (SCD) accounts for about ½ of all cardiovascular deaths, in most cases the result of a lethal ventricular arrhythmia. Patients considered at risk are often treated with an implantable cardiac defibrillator (ICD), but current criteria for device use, based largely on left ventricular ejection fraction (LVEF), leads to many patients receiving ICDs that they do not use, and many others not receiving ICDs but who suffer SCD. Thus, better methods of identifying patients at risk for SCD are needed, and radionuclide imaging offers much potential. Recent work has focused on imaging of cardiac autonomic innervation. (123)I-mIBG, a norepinephrine analog, is the tracer most studied, and a variety of positron emission tomographic tracers are also under investigation. Radionuclide autonomic imaging may identify at-risk patients with ischemic coronary artery disease, particularly following myocardial infarction and in the setting of hibernating myocardium. Most studies have been done in the setting of congestive heart failure (CHF), with a recent large multicenter study of patients with advanced disease, typically at high risk of SCD, showing that (123)I-mIBG can identify a low risk subgroup with an extremely low incidence of lethal ventricular arrhythmias and cardiac death, therefore, perhaps not requiring an ICD. Cardiac neuronal imaging has been shown to be better predictive of lethal arrhythmias/cardiac death than LVEF and New York Heart Association class, as well as various ECG parameters. Autonomic imaging will likely play an important role in the advancement of cardiac molecular imaging.

Synthesis of oncological [11C]radiopharmaceuticals for clinical PET

Positron emission tomography (PET) is a nuclear medicine modality which provides quantitative images of biological processes in vivo at the molecular level. Several PET radiopharmaceuticals labeled with short-lived isotopes such as (18)F and (11)C were developed in order to trace specific cellular and molecular pathways with the aim of enhancing clinical applications. Among these [(11)C]radiopharmaceuticals are N-[(11)C]methyl-choline ([(11)C]choline), l-(S-methyl-[(11)C])methionine ([(11)C]methionine) and 1-[(11)C]acetate ([(11)C]acetate), which have gained an important role in oncology where the application of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) is suboptimal. Nevertheless, the production of these radiopharmaceuticals did not reach the same level of standardization as for [(18)F]FDG synthesis. This review describes the most recent developments in the synthesis of the above-mentioned [(11)C]radiopharmaceuticals aiming to increase the availability and hence the use of [(11)C]choline, [(11)C]methionine and [(11)C]acetate in clinical practice.

Regulation of catecholamine-synthesising enzymes and beta-adrenoceptors gene expression in ventricles of stressed rats

Stress exposure activates the sympathoneural system, resulting in catecholamine release. Chronic stress is associated with development of numerous disorders, including cardiovascular diseases. Here we investigated the expression of mRNAs for catecholamine biosynthetic enzymes tyrosine-hydroxylase, dopamine-beta-hydroxylase and phenylethanolamine N-methyl-transferase, and for beta(1)- and beta(2)-adrenoceptors in the right and left ventricles of rats exposed to chronic unpredictable mild stress. The tyrosine-hydroxylase and dopamine-beta-hydroxylase mRNA levels were not affected by stress, whereas the phenylethanolamine N-methyltransferase mRNA levels significantly increased in both right and left ventricles. No changes in beta(1)-adrenoceptor mRNA levels in either right or left ventricles were observed. At the same time, stress produced a significant increase of beta(2)-adrenoceptor mRNA levels in left ventricles. These results suggest that elevated expression of phenylethanolamine N-methyltransferase in both ventricules and beta(2)-adrenoceptor genes in left ventricles could provide a molecular mechanism that leads to altered physiological response, which is important for the organism coping with stress.

Multimodality and nanoparticles in medical imaging

A number of medical imaging techniques are used heavily in the provision of spatially resolved information on disease and physiological status and accordingly play a critical role in clinical diagnostics and subsequent treatment. Though, for most imaging modes, contrast is potentially enhanced through the use of contrast agents or improved hardware or imaging protocols, no single methodology provides, in isolation, a detailed mapping of anatomy, disease markers or physiological status. In recent years, the concept of complementing the strengths of one imaging modality with those of another has come to the fore and been further bolstered by the development of fused instruments such as PET/CT and PET/MRI stations. Coupled with the continual development in imaging hardware has been a surge in reports of contrast agents bearing multiple functionality, potentially providing not only a powerful and highly sensitised means of co-localising physiological/disease status and anatomy, but also the tracking and delineation of multiple markers and indeed subsequent or simultaneous highly localized therapy ("theragnostics").

Evaluation of LMI1195, a novel 18F-labeled cardiac neuronal PET imaging agent, in cells and animal models

BACKGROUND

Heart failure has been associated with impaired cardiac sympathetic neuronal function. Cardiac imaging with radiolabeled agents that are substrates for the neuronal norepinephrine transporter (NET) has demonstrated the potential to identify individuals at risk of cardiac events. N-[3-Bromo-4-(3-[18F]fluoro-propoxy)-benzyl]-guanidine (LMI1195) is a newly developed 18F-labeled NET substrate designed to allow cardiac neuronal imaging with the high sensitivity, resolution, and quantification afforded by positron emission tomography (PET).

METHODS AND RESULTS

LMI1195 was evaluated in comparison with norepinephrine (NE) in vitro and 123I-meta-iodobenzylguanidine (MIBG) in vivo. The affinity (Ki) of LMI1195 for NET was 5.16 ± 2.83 μmol/L, similar to that of NE (3.36 ± 2.77 μmol/L) in a cell membrane-binding assay. Similarly, LMI1195 uptake kinetics examined in a human neuroblastoma cell line had Km and Vmax values of 1.44 ± 0.76 μmol/L and 6.05 ± 3.09 pmol/million cells per minute, comparable to NE (2.01 ± 0.85 μmol/L and 6.23 ± 1.52 pmol/million cells per minute). In rats, LMI1195 heart uptake at 15 and 60 minutes after intravenous administration was 2.36 ± 0.38% and 2.16 ± 0.38% injected dose per gram of tissue (%ID/g), similar to 123I-MIBG (2.14 ± 0.30 and 2.19 ± 0.27%ID/g). However, the heart to liver and lung uptake ratios were significantly higher for LMI1195 than for 123I-MIBG. In rabbits, desipramine (1 mg/kg), a selective NET inhibitor, blocked LMI1195 heart uptake by 82%, which was more effective than 123I-MIBG (53%), at 1 hour after dosing. Sympathetic denervation with 6-hydroxydopamine, a neurotoxin, resulted in a marked (79%) decrease in LMI1195 heart uptake. Cardiac PET imaging with LMI1195 in rats, rabbits, and nonhuman primates revealed clear myocardium with low radioactivity levels in the blood, lung, and liver. Imaging in rabbits pretreated with desipramine showed reduced heart radioactivity levels in a dose-dependent manner. Additionally, imaging in sympathetically denervated rabbits resulted in low cardiac image intensity with LMI1195 but normal perfusion images with flurpiridaz F 18, a PET myocardial perfusion imaging agent. In nonhuman primates pretreated with desipramine (0.5 mg/kg), imaging with LMI1195 showed a 66% decrease in myocardial uptake. In a rat model of heart failure, the LMI1195 cardiac uptake decreased as heart failure progressed.

CONCLUSIONS

LMI1195 is a novel (18)F imaging agent retained in the heart through the NET and allowing evaluation of the cardiac sympathetic neuronal function by PET imaging.

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.