Cardiac PET: metabolic and functional imaging of the myocardium

Total-Body PET/CT Applications in Cardiovascular Diseases: A Perspective Document of the SNMMI Cardiovascular Council

Digital PET/CT systems with a long axial field of view have become available and are emerging as the current state of the art. These new camera systems provide wider anatomic coverage, leading to major increases in system sensitivity. Preliminary results have demonstrated improvements in image quality and quantification, as well as substantial advantages in tracer kinetic modeling from dynamic imaging. These systems also potentially allow for low-dose examinations and major reductions in acquisition time. Thereby, they hold great promise to improve PET-based interrogation of cardiac physiology and biology. Additionally, the whole-body coverage enables simultaneous assessment of multiple organs and the large vascular structures of the body, opening new opportunities for imaging systemic mechanisms, disorders, or treatments and their interactions with the cardiovascular system as a whole. The aim of this perspective document is to debate the potential applications, challenges, opportunities, and remaining challenges of applying PET/CT with a long axial field of view to the field of cardiovascular disease.

Metabolic activity of the left and right atria are differentially altered in patients with atrial fibrillation and LV dysfunction

BACKGROUND

Alterations in atrial metabolism may play a role in the perpetuation of atrial fibrillation (AF). This study sought to compare 18F-fluorodeoxyglucose (FDG) uptake on PET, in patients with LV dysfunction versus those without AF.

METHODS

Seventy-two patients who underwent myocardial viability assessment were evaluated. AF patients (36) had persistent or permanent AF based on history and ECG. Patients without AF (36) were matched to AF patients based on sex, diabetes, age, and LVEF. Maximum and mean FDG Standard Uptake Values (SUV) in the left atrial (LA) wall and right atrial (RA) wall were measured. Tissue-to-blood ratios (TBR) were calculated as atrial wall to blood-pool activity. Atrial volumes were measured by echocardiography.

RESULTS

Maximum and mean FDG SUV and TBRs were significantly increased in the RA (but not the LA) of patients with AF compared to those without (P < 0.01). When accounting for changes in atrial volume, the presence of AF remained a significant predictor of higher RAMAX, but not RAMEAN FDG uptake.

CONCLUSION

In patients with LV dysfunction from ischemic cardiomyopathy, LA and RA glucose metabolism are differentially altered in those with persistent atrial fibrillation. Further investigations should elucidate the temporal relationship between AF and glucose metabolic changes, as a potential target for therapy.

PET Tracers for Imaging Cardiac Function in Cardio-oncology

PURPOSE OF REVIEW

Successful treatment of cancer can be hampered by the attendant risk of cardiotoxicity, manifesting as cardiomyopathy, left ventricle systolic dysfunction and, in some cases, heart failure. This risk can be mitigated if the injury to the heart is detected before the onset to irreversible cardiac impairment. The gold standard for cardiac imaging in cardio-oncology is echocardiography. Despite improvements in the application of this modality, it is not typically sensitive to sub-clinical or early-stage dysfunction. We identify in this review some emerging tracers for detecting incipient cardiotoxicity by positron emission tomography (PET).

RECENT FINDINGS

Vectors labeled with positron-emitting radionuclides (e.g., carbon-11, fluorine-18, gallium-68) are now available to study cardiac function, metabolism, and tissue repair in preclinical models. Many of these probes are highly sensitive to early damage, thereby potentially addressing the limitations of current imaging approaches, and show promise in preliminary clinical evaluations. The overlapping pathophysiology between cardiotoxicity and heart failure significantly expands the number of imaging tools available to cardio-oncology. This is highlighted by the emergence of radiolabeled probes targeting fibroblast activation protein (FAP) for sensitive detection of dysregulated healing process that underpins adverse cardiac remodeling. The growth of PET scanner technology also creates an opportunity for a renaissance in metabolic imaging in cardio-oncology research.

Positron Emission Tomography Techniques to Measure Active Inflammation, Fibrosis and Angiogenesis: Potential for Non-invasive Imaging of Hypertensive Heart Failure

Heart failure, which is responsible for a high number of deaths worldwide, can develop due to chronic hypertension. Heart failure can involve and progress through several different pathways, including: fibrosis, inflammation, and angiogenesis. Early and specific detection of changes in the myocardium during the transition to heart failure can be made via the use of molecular imaging techniques, including positron emission tomography (PET). Traditional cardiovascular PET techniques, such as myocardial perfusion imaging and sympathetic innervation imaging, have been established at the clinical level but are often lacking in pathway and target specificity that is important for assessment of heart failure. Therefore, there is a need to identify new PET imaging markers of inflammation, fibrosis and angiogenesis that could aid diagnosis, staging and treatment of hypertensive heart failure. This review will provide an overview of key mechanisms underlying hypertensive heart failure and will present the latest developments in PET probes for detection of cardiovascular inflammation, fibrosis and angiogenesis. Currently, selective PET probes for detection of angiogenesis remain elusive but promising PET probes for specific targeting of inflammation and fibrosis are rapidly progressing into clinical use.

Cardiac 18F-FDG PET/CT procedure for the diagnosis of prosthetic endocarditis and intracardiac devices

Infective endocarditis (IE) is a serious condition with a poor prognosis, its mortality unchanged significantly despite diagnostic and therapeutic advances in the last 30years. The diagnostic ability of the modified Duke criteria in prosthetic endocarditis and/or devices does not exceed 50%, so new tools are necessary for the diagnosis of this entity in this context. The ¹⁸F-FDG PET/CTA combines a highly sensitive technique to detect inflammatory-infectious activity with a technique with high anatomical resolution to assess the structural lesions associated with endocarditis. With a diagnostic sensitivity between 91-97%, this hybrid technique has become a useful diagnostic tool for patients with prosthetic valves or devices and suspicion of IE, becoming a major criterion in the diagnostic algorithm of current guidelines. This excellent diagnostic ability depends directly on the quality of the obtained exploration and the knowledge at the time of interpreting the images. The aim of this review is to describe and standardize the methodology of cardiac ¹⁸F-FDG PET/CTA in the diagnosis of endocarditis in prosthetic valves and intracardiac devices, with special emphasis on the particularities of the patient's preparation, the PET and CT acquisition procedures, and the subsequent imaging postprocessing and interpretation.

Cardiovascular PET/MR imaging: Quo Vadis?

With the recent advent of PET/MRI scanners, the combination of molecular imaging with a variety of known and novel PET radiotracers, the high spatial resolution of MRI, and its potential for multi-parametric imaging are anticipated to increase the diagnostic accuracy in cardiovascular disease detection, while providing novel mechanistic insights into the initiation and progression of the disease state. For the time being, cardiac PET/MRI emerges as potential clinical tool in the identification and characterization of infiltrative cardiac diseases, such as sarcoidosis, acute or chronic myocarditis, and cardiac tumors, respectively. The application of PET/MRI in conjunction with various radiotracer probes in the identification of the vulnerable atherosclerotic plaque also holds much promise but needs further translation and validation in clinical investigations. The combination of molecular imaging and creation of multi-parametric imaging maps with PET/MRI, however, are likely to set new horizons to develop predictive parameters for myocardial recovery and treatment response in ischemic and non-ischemic cardiomyopathy patients. Molecular imaging and multi-parametric imaging in cardiovascular disease with PET/MRI at current stage are at its infancy but bear a bright future.

Novel Noninvasive Nuclear Medicine Imaging Techniques for Cardiac Inflammation

PURPOSE OF REVIEW

Inflammation is a key player in a wide range of cardiovascular and myocardial diseases. Given the numerous implications of inflammatory processes in disease initiation and progression, functional imaging modalities including positron emission tomography (PET) represent valuable diagnostic, prognostic, and monitoring tools in patient management. Since increased glucose metabolism is a hallmark of inflammation, PET using the radiolabeled glucose analog [18F]-2-deoxy-2-fluoro-d-glucose (FDG) is the mainstay diagnostic test for nuclear imaging of (cardiac) inflammation. Recently, new approaches using more specific tracers to overcome the limited specificity of FDG have emerged.

RECENT FINDINGS

PET imaging has proven its value in a number of inflammatory conditions of the heart including myocarditis, endocarditis, sarcoidosis, or reactive changes after myocardial infarction. In infection-related endocarditis, FDG-PET and white blood cell scintigraphy have been implemented in current guidelines. FDG-PET is considered as nuclear medical gold standard in myocarditis, pericarditis, or sarcoidosis. Novel strategies, including targeting of somatostatin receptors or C-X-C motif chemokine receptor CXCR4, have shown promising results in first studies.

SUMMARY

Nuclear medicine techniques offer valuable information in the assessment of myocardial inflammation. Given the possibility to directly visualize inflammatory activity, they represent useful tools for diagnosis, risk stratification, and therapy monitoring.

Quantitative iodine-123-metaiodobenzylguanidine (MIBG) SPECT imaging in heart failure with left ventricular systolic dysfunction: Development and validation of automated procedures in conjunction with technetium-99m tetrofosmin myocardial perfusion SPECT

BACKGROUND

The purpose of this study was to develop and validate new approaches to quantitative MIBG myocardial SPECT imaging in heart failure (HF) subjects.

METHODS AND RESULTS

Quantitative MIBG myocardial SPECT analysis methods, alone and in conjunction with 99mTc-tetrofosmin perfusion SPECT, were adapted from previously validated techniques for the analysis of SPECT and PET perfusion imaging. To account for underestimation of MIBG defect severity in subjects with global reduction in uptake, a mixed reference database based on planar heart/mediastinum (H/M) ratio categories was used. Extent and severity of voxel-based defects and number of myocardial segments with significant dysinnervation (derived score ≥2) were determined. MIBG/99mTc-tetrofosmin mismatch was quantified using regions with preserved innervation as the reference for scaling 99mTc-tetrofosmin voxel maps. Quantification techniques were tested on studies of 619 ischemic (I) and 319 non-ischemic (NI) HF subjects. Using all analytical techniques, IHF subjects had significantly greater and more severe MIBG SPECT abnormalities compared with NIHF subjects. Innervation/perfusion mismatches were also larger in IHF subjects. Findings were consistent between voxel- and myocardial-segment-based quantitation methods.

CONCLUSIONS

Multiple objective methods for quantitation of MIBG SPECT imaging studies provided internally consistent results for distinguishing the different patterns of uptake between IHF and NIHF subjects.

[Multimodal imaging of ischemic heart diseases: A 2015 update]

Current realities and future possibilities of imaging in the ischemic heart diseases are very broad and constantly evolving, with the improvement of existing technologies and the introduction of new features such as dual-energy CT, strain ultrasound, multimodality fusion or perfusion MRI. Regular collaboration between prescribing clinicians, cardiologists, radiologists and nuclear radiologists is therefore essential to tailor the examination to the specific clinical question. The indications for each modality will therefore depend on its diagnostic performance, cost, acquisition and post-processing times and eventual radiation exposure. This review will detail principles and applications of current cardiac imaging examinations: echocardiography, nuclear medicine, MRI, CT and coronary angiography, emphasizing their current strengths and weaknesses in the ischemic heart diseases management.

Novel plasma and imaging biomarkers in heart failure with preserved ejection fraction

Existing diagnostic guidelines for heart failure with preserved ejection fraction (HFPEF) primarily comprise natriuretic peptides and echocardiographic assessment, highlighting the role of diastolic dysfunction. However, recent discoveries of novel plasma markers implicated in pathophysiology of heart failure and technological advances in imaging provide additional biomarkers which are potentially applicable to HFPEF. The evidence base for plasma extra-cellular matrix (ECM) peptides, galectin-3, ST2, GDF-15 and pentraxin-3 is reviewed. Furthermore, the capabilities of novel imaging techniques to assess existing parameters (e.g. left ventricular ejection fraction, systolic & diastolic function, chamber size) and additional derangements of the ECM, myocardial mechanics and ischaemia evaluation are addressed.

Coronary microvascular dysfunction: sex-specific risk, diagnosis, and therapy

Cardiovascular disease is the leading cause of death worldwide. In the presence of signs and symptoms of myocardial ischaemia, women are more likely than men to have no obstructive coronary artery disease (CAD). Women have a greater burden of symptoms than men, and are often falsely reassured despite the presence of ischaemic heart disease because of a lack of obstructive CAD. Coronary microvascular dysfunction should be considered as an aetiology for ischaemic heart disease with signs and symptoms of myocardial ischaemia, but no obstructive CAD. Coronary microvascular dysfunction is defined as impaired coronary flow reserve owing to functional and/or structural abnormalities of the microcirculation, and is associated with an adverse cardiovascular prognosis. Therapeutic lifestyle changes as well as antiatherosclerotic and antianginal medications might be beneficial, but clinical outcome trials are needed to guide treatment. In this Review, we discuss the prevalence, presentation, diagnosis, and treatment of coronary microvascular dysfunction, with a particular emphasis on ischaemic heart disease in women.

Current and future trends in multimodality imaging of coronary artery disease

Nowadays, there is a wide array of imaging studies available for the evaluation of coronary artery disease, each with its particular indications and strengths. Cardiac single photon emission tomography is mostly used to evaluate myocardial perfusion, having experienced recent marked improvements in image acquisition. Cardiac PET has its main utility in perfusion imaging, atherosclerosis and endothelial function evaluation, and viability assessment. Cardiovascular computed tomography has long been used as a reference test for non-invasive evaluation of coronary lesions and anatomic characterization. Cardiovascular magnetic resonance is currently the reference standard for non-invasive ventricular function evaluation and myocardial scarring delineation. These specific strengths have been enhanced with the advent of hybrid equipment, offering a true integration of different imaging modalities into a single, simultaneous and comprehensive study.

Multimodality Imaging in Ischemic Cardiomyopathy

Cardiac multimodality (hybrid) imaging can be obtained from a variety of techniques, such as nuclear medicine with single photon emission computed tomography (SPECT) and positron emission tomography (PET), or radiology with multislice computed tomography (CT), magnetic resonance (MR) and echography. They are typically combined in a side-by-side or fusion mode in order to provide functional and morphological data to better characterise coronary artery disease, with more proven efficacy than when used separately. The gained information is then used to guide revascularisation procedures. We present an up-to-date comprehensive overview of multimodality imaging already in clinical use, as well as a combination of techniques with promising or developing applications.