Test-retest repeatability of myocardial oxidative metabolism and efficiency using standalone dynamic 11C-acetate PET and multimodality approaches in healthy controls

Molecular Imaging of Heart Failure: An Update and Future Trends

Molecular imaging can detect and quantify pathophysiological processes underlying heart failure, complementing evaluation of cardiac structure and function with other imaging modalities. Targeted tracers have enabled assessment of various cellular and subcellular mechanisms of heart failure aiming for improved phenotyping, risk stratification, and personalized therapy. This review outlines the current status of molecular imaging in heart failure, accompanied with discussion on novel developments. The focus is on radionuclide methods with data from clinical studies. Imaging of myocardial metabolism can identify left ventricle dysfunction caused by myocardial ischemia that may be reversible after revascularization in the presence of viable myocardium. In vivo imaging of active inflammation and amyloid deposition have an established role in the detection of cardiac sarcoidosis and transthyretin amyloidosis. Innervation imaging has well documented prognostic value in predicting heart failure progression and arrhythmias. Tracers specific for inflammation, angiogenesis and myocardial fibrotic activity are in earlier stages of development, but have demonstrated potential value in early characterization of the response to myocardial injury and prediction of cardiac function over time. Early detection of disease activity is a key for transition from medical treatment of clinically overt heart failure towards a personalized approach aimed at supporting repair and preventing progressive cardiac dysfunction.

Phenotyping heart failure by nuclear imaging of myocardial perfusion, metabolism, and molecular targets

Nuclear imaging techniques can detect and quantify pathophysiological processes underlying heart failure, complementing evaluation of cardiac structure and function with other imaging modalities. Combined imaging of myocardial perfusion and metabolism can identify left ventricle dysfunction caused by myocardial ischaemia that may be reversible after revascularization in the presence of viable myocardium. High sensitivity of nuclear imaging to detect targeted tracers has enabled assessment of various cellular and subcellular mechanisms of heart failure. Nuclear imaging of active inflammation and amyloid deposition is incorporated into clinical management algorithms of cardiac sarcoidosis and amyloidosis. Innervation imaging has well-documented prognostic value with respect to heart failure progression and arrhythmias. Emerging tracers specific for inflammation and myocardial fibrotic activity are in earlier stages of development but have demonstrated potential value in early characterization of the response to myocardial injury and prediction of adverse left ventricular remodelling. Early detection of disease activity is a key for transition from broad medical treatment of clinically overt heart failure towards a personalized approach aimed at supporting repair and preventing progressive failure. This review outlines the current status of nuclear imaging in phenotyping heart failure and combines it with discussion on novel developments.

Detection of apoptosis by [18F]ML-10 after cardiac ischemia-reperfusion injury in mice

OBJECTIVE

Myocardial infarction leads to ischemic heart disease and cell death, which is still a major obstacle in western society. In vivo imaging of apoptosis, a defined cascade of cell death, could identify myocardial tissue at risk.

METHODS

Using 2-(5-[18F]fluoropentyl)-2-methyl-malonic acid ([18F]ML-10) in autoradiography and positron emission tomography (PET) visualized apoptosis in a mouse model of transient ligation of the left anterior descending (LAD) artery. 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET imaging indicated the defect area. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) histology stain indicated cardiac apoptosis.

RESULTS

[18F]ML-10 uptake was evident in the ischemic area after transient LAD ligation in ex vivo autoradiography and in vivo PET imaging. Detection of [18F]ML-10 is in line with the defect visualized by [18F]FDG and the histological approach of TUNEL staining.

CONCLUSION

The tracer [18F]ML-10 is suitable for detecting apoptosis after transient LAD ligation in mice.

Detection of cardiac apoptosis by [18F]ML-10 in a mouse model of permanent LAD ligation

PURPOSE

The loss of viable cardiac cells and cell death by myocardial infarction (MI) is still a significant obstacle in preventing deteriorating heart failure. Imaging of apoptosis, a defined cascade to cell death, could identify areas at risk.

PROCEDURES

Using 2-(5-[18F]fluoropentyl)-2-methyl-malonic acid ([18F]ML-10) in autoradiography and positron emission tomography (PET) visualized apoptosis in murine hearts after permanent ligation of the left anterior descending artery (LAD) inducing myocardial infarction (MI). 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET imaging localized the infarct area after MI. Histology by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining validated apoptosis in the heart.

RESULTS

Accumulation of [18F]ML-10 was evident in the infarct area after permanent ligation of the LAD in autoradiography and PET imaging. Detection of apoptosis by [18F]ML-10 is in line with the defect visualized by [18F]FDG and the histological approach.

CONCLUSION

[18F]ML-10 could be a suitable tracer for apoptosis imaging in a mouse model of permanent LAD ligation.

Cardiac 18F-FDG Positron Emission Tomography: An Accurate Tool to Monitor In vivo Metabolic and Functional Alterations in Murine Myocardial Infarction

Cardiac monitoring after murine myocardial infarction, using serial non-invasive cardiac 18F-FDG positron emissions tomography (PET) represents a suitable and accurate tool for in vivo studies. Cardiac PET imaging enables tracking metabolic alterations, heart function parameters and provides correlations of the infarct size to histology. ECG-gated 18F-FDG PET scans using a dedicated small-animal PET scanner were performed in mice at baseline, 3, 14, and 30 days after myocardial infarct (MI) by permanent ligation of the left anterior descending (LAD) artery. The percentage of the injected dose per gram (%ID/g) in the heart, left ventricular metabolic volume (LVMV), myocardial defect, and left ventricular function parameters: end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and the ejection fraction (EF%) were estimated. PET assessment of the defect positively correlates with post-infarct histology at 3 and 30 days. Infarcted murine hearts show an immediate decrease in LVMV and an increase in %ID/g early after infarction, diminishing in the remodeling process. This study of serial cardiac PET scans provides insight for murine myocardial infarction models by novel infarct surrogate parameters. It depicts that serial PET imaging is a valid, accurate, and multimodal non-invasive assessment.

SGLT2 Inhibition Does Not Affect Myocardial Fatty Acid Oxidation or Uptake, but Reduces Myocardial Glucose Uptake and Blood Flow in Individuals With Type 2 Diabetes: A Randomized Double-Blind, Placebo-Controlled Crossover Trial

Sodium-glucose cotransporter 2 (SGLT2) inhibition reduces cardiovascular morbidity and mortality in individuals with type 2 diabetes. Beneficial effects have been attributed to increased ketogenesis, reduced cardiac fatty acid oxidation, and diminished cardiac oxygen consumption. We therefore studied whether SGLT2 inhibition altered cardiac oxidative substrate consumption, efficiency, and perfusion. Thirteen individuals with type 2 diabetes were studied after 4 weeks' treatment with empagliflozin and placebo in a randomized, double-blind, placebo-controlled crossover study. Myocardial palmitate and glucose uptake were measured with ¹¹C-palmitate and ¹⁸F-fluorodeoxyglucose positron emission tomography (PET)/computed tomography (CT). Oxygen consumption and myocardial external efficiency (MEE) were measured with ¹¹C-acetate PET/CT. Resting and adenosine stress myocardial blood flow (MBF) and myocardial flow reserve (MFR) were measured using ¹⁵O-H₂O PET/CT. Empagliflozin did not affect myocardial free fatty acids (FFAs) uptake but reduced myocardial glucose uptake by 57% (P < 0.001). Empagliflozin did not change myocardial oxygen consumption or MEE. Empagliflozin reduced resting MBF by 13% (P < 0.01), but did not significantly affect stress MBF or MFR. In conclusion, SGLT2 inhibition did not affect myocardial FFA uptake, but channeled myocardial substrate utilization from glucose toward other sources and reduced resting MBF. However, the observed metabolic and hemodynamic changes were modest and most likely contribute only partially to the cardioprotective effect of SGLT2 inhibition.

First-Pass Techniques Applied to Standard Dynamic Cardiac PET: A reappraisal of Old Invasive Techniques to Assess Cardiac Function

Cardiac PET is increasingly performed with dynamic imaging to measure tracer pharmacokinetics in the myocardium. If the early time frames of the PET protocol are sufficiently short and the total amount of injected radioactivity is correctly measured the indicator dilution principle can be applied to PET with most tracers in clinical use, similar to invasive and other noninvasive techniques. The first-pass of the tracer through the heart and lungs can be used to quantify some highly important aspects of cardiovascular function, such as forward cardiac output, transit times, and partial volumes in the central compartments. Additionally, ECG-gated first pass images provide direct access to cardiac volumes and ejection fractions, even for tracers with poor trapping in the myocardial wall, for instance 15O-water. This review summarizes the basic approaches of the indicator dilution principle in clinical use with invasive techniques, and how these techniques can be integrated into a cardiac PET scan.

Cardiovascular Effects of Treatment With the Ketone Body 3-Hydroxybutyrate in Chronic Heart Failure Patients

Supplemental Digital Content is available in the text.

Background:

Myocardial utilization of 3-hydroxybutyrate (3-OHB) is increased in patients with heart failure and reduced ejection fraction (HFrEF). However, the cardiovascular effects of increased circulating plasma-3-OHB levels in these patients are unknown. Consequently, the authors’ aim was to modulate circulating 3-OHB levels in HFrEF patients and evaluate: (1) changes in cardiac output (CO); (2) a potential dose-response relationship between 3-OHB levels and CO; (3) the impact on myocardial external energy efficiency (MEE) and oxygen consumption (MVO₂); and (4) whether the cardiovascular response differed between HFrEF patients and age-matched volunteers.

Methods:

Study 1: 16 chronic HFrEF patients (left ventricular ejection fraction: 37±3%) were randomized in a crossover design to 3-hour of 3-OHB or placebo infusion. Patients were monitored invasively with a Swan-Ganz catheter and with echocardiography. Study 2: In a dose-response study, 8 HFrEF patients were examined at increasing 3-OHB infusion rates. Study 3 to 4: 10 HFrEF patients and 10 age-matched volunteers were randomized in a crossover design to 3-hour 3-OHB or placebo infusion. MEE and MVO₂ were evaluated using 11C-acetate positron emission tomography.

Results:

3-OHB infusion increased circulating levels of plasma 3-OHB from 0.4±0.3 to 3.3±0.4 mM (P<0.001). CO rose by 2.0±0.2 L/min (P<0.001) because of an increase in stroke volume of 20±2 mL (P<0.001) and heart rate of 7±2 beats per minute (bpm) (P<0.001). Left ventricular ejection fraction increased 8±1% (P<0.001) numerically. There was a dose-response relationship with a significant CO increase of 0.3 L/min already at plasma-3-OHB levels of 0.7 mM (P<0.001). 3-OHB increased MVO₂ without altering MEE. The response to 3-OHB infusion in terms of MEE and CO did not differ between HFrEF patents and age-matched volunteers.

Conclusions:

3-OHB has beneficial hemodynamic effects in HFrEF patients without impairing MEE. These beneficial effects are detectable in the physiological concentration range of circulating 3-OHB levels. The hemodynamic effects of 3-OHB were observed in both HFrEF patients and age-matched volunteers. 3-OHB may potentially constitute a novel treatment principle in HFrEF patients.

Myocardial Efficiency: A Fundamental Physiological Concept on the Verge of Clinical Impact

Myocardial external efficiency is the relation of mechanical energy generated by the left (or right) ventricle to the consumed chemical energy from aerobic metabolism. Efficiency can be calculated invasively, and, more importantly, noninvasively by using positron emission tomography, providing a single parameter by which to judge the adequacy of myocardial metabolism to generated mechanical output. This parameter has been found to be impaired in heart failure of myocardial or valvular etiology, and it changes in a characteristic manner with medical or interventional cardiac therapy. The authors discuss the concept, strengths, and limitations, known applications, and future perspectives of the use of myocardial efficiency.

Automatic calculation of myocardial external efficiency using a single 11C-acetate PET scan

BACKGROUND

Myocardial external efficiency (MEE) is defined as the ratio of kinetic energy associated with cardiac work [forward cardiac output (FCO)*mean systemic pressure] and the chemical energy from oxygen consumed (MVO2) by the left ventricular mass (LVM). We developed a fully automated method for estimating MEE based on a single 11C-acetate PET scan without ECG-gating.

METHODS AND RESULTS

Ten healthy controls, 34 patients with aortic valve stenosis (AVS), and 20 patients with mitral valve regurgitation (MVR) were recruited in a dual-center study. MVO2 was calculated using washout of 11C -acetate activity. FCO and LVM were calculated automatically using dynamic PET and parametric image formation. FCO and LVM were also obtained using cardiac magnetic resonance (CMR) in all subjects. The correlation between MEEPET-CMR and MEEPET was high (r = 0.85, P < 0.001) without significant bias. MEEPET was 23.6 ± 4.2% for controls and was lowered in AVS (17.2 ± 4.3%, P < 0.001) and in MVR (18.0 ± 5.2%, P = 0.004). MEEPET was strongly associated with both NYHA class (P < 0.001) and the magnitude of valvular dysfunction (mean aortic gradient: P < 0.001, regurgitant fraction: P = 0.009).

CONCLUSION

A single 11C-acetate PET yields accurate and automated MEE results on different scanners. MEE might provide an unbiased measurement of the phenotypic response to valvular disease.