Radionuclide Imaging of Myocardial Metabolism

Introduction

Cardiac health is dependent upon the hearts' ability to utilize different substrates to support overall oxidative metabolism in order to generate ATP. Indeed a loss in plasticity in substrate preference leading to an over dependence on the metabolism of an individual category of substrates is characteristic of a variety cardiac diseases such as diabetic heart disease where fatty acid metabolism predominates and dilated cardiomyopathy where glucose is the primary metabolic substrate. Because of the pleiotropic actions of myocardial substrate metabolism, the detrimental effects on cardiac health of a loss in plasticity in substrate preference can extend well beyond impairment in energy production. Examples include perturbations in various cell signaling pathways, alterations in cell growth and decreased cell survival. Despite the rapid growth in our understanding of the relationship between altered myocardial metabolism and cardiac disease many questions remain. For example, it is still unclear what are the key determinants of changes in myocardial substrate use and when these changes do occur to what extent are they adaptive or have the propensity to become maladaptive? Are these metabolic patterns of prognostic significance? Transgenic models targeting key aspects of myocardial substrate use are providing mechanistic insights into myocardial metabolic-functional relationships in various cardiac diseases. However, the relevance of the observed phenotypes to the corresponding human condition is frequently unclear. Finally, applied genomics have identified numerous gene variants intimately involved in the regulation of myocardial substrate use. Yet, identifying the clinically relevant genetic variants remains elusive. As a consequence, there is an ever growing demand for accurate non-invasive imaging approaches of myocardial substrate metabolism that provide linkage between the bench and the bedside leading to improved patient management paradigms. In this regard, radionuclide approaches has led the way. The most notable example that has achieved clinical applicability is the the detection of ischemic but viable myocardium with PET and ¹⁸F-fluorodexoglucose (FDG) for the management of patients with ischemic cardiomyopathy. In the subsequent discussion advances in metabolic imaging using radionuclide approaches and their potential future applications in the study of cardiovascular disease are discussed.

Methods to Image Myocardial Metabolism

Three methods are currently available to image myocardial metabolism noninvasively, magnetic resonance spectroscopy (MRS), single photon emission computed tomography (SPECT) and positron emission tomography (PET). A summary of each technique is listed below:

MRS

The advantages of MRS are the ability to measure multiple metabolic pathways simultaneously, the relative ease in performing serial measurements, and the lack of ionizing radiation. It can be combined with MRI permitting near simultaneous measurements of myocardial perfusion and mechanical function. A number of biologically important nuclei can be measured with MRS with phosphorous (³¹P) and hydrogen (¹H), being the most common. The fundamental principle of MRS is that the chemical environment of nuclei induces local magnetic fields that shift their resonance frequency. Different metabolites exhibit a unique frequency shift resulting in a signal consisting of one or more discrete resonance frequencies. The Fourier transform of the acquired signal produces a spectrum with peaks at distinct frequencies. The MRS spectrum displays the signal intensity as a function of frequency measured in parts per million (ppm) relative to the frequency of a reference compound. The signal intensity at a given frequency is proportional to the amount of the respective metabolite and can be used to determine the absolute concentration of the metabolite using appropriate calibrating reference signal.^{1, 2}

The disadvantages of MRS include low signal-to-noise, concomitant limited spatial resolution, intravoxel signal contamination and long acquisition times. Compared with nuclear imaging methods, MRS has a much lower sensitivity (detecting millimolar as opposed to nanomolar concentrations). As a consequence the initial success of imaging of cardiac metabolism using C-13 labeled agents in intact animals has not been translated to the study of humans.³ Cardiac applications for MRS become more limited as one moves from rodent to man as opposed to nuclear methods where the reverse occurs. This appears to be a function of both the higher field strength in the small bore systems and the use of radiofrequency coils that are in closer proximity to the entire heart used in small animal imaging. In contrast to rodent hearts where measurements of the entire left ventricular myocardium are obtained, measurements in human myocardium are typically limited to the anterior myocardium. Currently, only ³¹P and ¹H have been widely used for in vivo clinical cardiac examinations focusing on myocardial energetics (³¹P) and lipid accumulation (¹H).^{1, 3, 4}

Single Photon Emission Computed Tomography (SPECT)

The advantages to SPECT for cardiac metabolic include; the inherent high sensitivity of the radionuclide method to measure metabolic processes, wide availability of the technology, with ECG-gating the ability to measure myocardial function simultaneously, and the long-physical half-life of SPECT radiotracers. The latter is an important attribute for it allows delivery of radiotracer to multiple sites which facilitates the performance of multi-center studies designed to answer clinically relevant questions about the potential utility of measurements of myocardial substrate metabolism in various cardiac diseases. Small animal SPECT and SPECT/CT systems are rapidly advancing, allowing the performance of myocardial metabolic studies in rodent models of cardiac disease. The major disadvantage of SPECT is the inability to quantify cellular metabolic processes primarily because of the technical limitations of SPECT (relatively poor temporal and spatial resolution and inaccurate correction for photon attenuation) and the complexity of the metabolism of the SPECT radiotracers relative to a metabolic process of interest.

Metabolic processes that can be measured by SPECT include (Table 1):

Table 1. Currently used Radiotracers of Metabolism.

Radionuclide	Half-life	Compounda	Present use
SPECT
123I	13.3 hr	IPPA	Fatty acid metabolism
		BMIPP	Fatty acid metabolism
PET
15O	2.04 min	15O2	Oxygen consumption
11C	20.4 min	Acetate	Oxygen consumption
		Palmitate	Fatty acid metabolism
		Glucose	Glucose metabolism
		Lactate	Lactate metabolism
18F	110 min	Deoxyglucose	Glucose metabolism
		FTHA	Fatty acid metabolism
		FTP	Fatty acid metabolism
		FCPHA	Fatty acid metabolism

^aSee text for names of compounds

• Glucose Metabolism: No specific SPECT radiotracers are currently available to measure myocardial glucose metabolism. However, when combined with the appropriate detection scheme or collimator design, myocardial glucose metabolism can be measured with SPECT and FDG ⁵.

• Fatty Acid Metabolism: One of the earliest and most promising SPECT radiotracers of fatty acid metabolism was 15-(p-iodophenyl)-pentadecanoic acid (IPPA).^6-8 This radiotacer demonstrated rapid accumulation in the heart and exhibited clearance kinetics that followed a biexponential function characteristic for ¹¹C-palmitate. Moreover, the clearance rates correlated directly with β-oxidation. In humans with coronary atherosclerosis, radtiotracer uptake and washout was reduced in regions subtended by occlude arteries consistent with ischemia ⁹. However, the poor temporal resolution of SPECT systems did not take advantage of the rapid turnover of IPPA. As a consequence quantification of myocardial fatty acid metabolism was not possible and image quality was reduced. This led to the development of branched-chain analogs of IPPA, such as ¹²³I-beta-methyl-P-iodophenylpentadecanoic acid (BMIPP) (Table 1).^8-11 Alkyl branching inhibits β-oxidation shunting radiolabel to the triglyceride pool thereby increasing radiotracer retention and improving image quality.

PET

One major advantage of PET is a detection scheme that permits quantification of radioactivity within the field of view. A second advantage is the use of radiopharmaceuticals labeled with the positron-emitting radionuclides. These radionuclides of the biologically ubiquitous elements oxygen (¹⁵O), carbon (¹¹C), and nitrogen (¹³N), as well as fluorine (¹⁸F) substituting for hydrogen, can be incorporated into a wide variety of substrates or substrate analogues that participate in diverse biochemical pathways without altering the biochemical properties of the substrate of interest (Table 1). By combining the knowledge of the metabolic pathways of interest with kinetic models that faithfully describe the fate of the tracer in tissue, an accurate interpretation of the tracer kinetics as they relate to the metabolic process of interest can be achieved. The major disadvantages of PET are its complexity in both radiotracer design and image quantification schemes and expense. Metabolic processes that are typically measured with PET are:

• Myocardial Oxygen Consumption (MVO₂):

  • ¹⁵O-oxygen: Because oxygen is the final electron acceptor in all pathways of aerobic myocardial metabolism, PET with ¹⁵O-oxygen has also been used to measure MVO₂. The approach provides a measure of myocardial oxygen extraction which when combined with measurements of myocardial blood flow and arterial oxygen content measures MVO₂ directly. Due to its short physical half-life, ¹⁵O-oxygen is readily applicable in studies requiring repetitive assessments, such as those with an acute pharmacologic intervention. Its major disadvantages are the need for a multiple-tracer study (to account for myocardial blood flow and blood volume) and fairly complex compartmental modeling to obtain the measurements. ^12-14

  • ¹¹C-acetate: Currently, PET with ¹¹C-acetate is the preferred method of measuring MVO₂ noninvasively. Acetate is a two-carbon chain free fatty acid whose primary metabolic fate is rapidly conversion to acetyl–CoA and metabolism through the tricarboxylic acid cycle. Because of the close coupling between the tricarboxylic acid cycle and oxidative phosphorylation, myocardial turnover of ¹¹C-acetate reflects overall oxidative metabolism or MVO₂. Either exponential curve fitting or compartmental modeling is used to calculate MVO₂. Modeling is typically preferable when cardiac output is low because marked dispersion of the input function and spill-over of activity from the lungs to the myocardium which decreases the accuracy of the curve-fitting method.^15-19 However, it more complex that exponential curve-fitting and requires correction of blood radioactivity for ¹¹CO₂.

• Carbohydrate metabolism:

  • FDG: Most studies of myocardial glucose metabolism with PET have used FDG. This radiotracer competes with glucose for facilitated transport into the sarcolemma then for hexokinase-mediated phosphorylation. In general, FDG-6-phosphate is trapped in the cytosol and the myocardial uptake of FDG is thought to reflect overall anaerobic and aerobic myocardial glycolytic flux.^20-23 Overall image quality of FDG uptake is excellent but must be referenced to standardization of the substrate environment (e.g., glucose loading versus fasting). Myocardial glucose uptake can be assessed in either relative or in absolute terms (i.e., in nmol•g^-1•min^-1). For quantification, a mathematical correction for the kinetic differences between FDG and glucose called the lumped constant” must be used calculate rates of glucose. This value may vary depending upon the prevailing plasma substrate and hormonal conditions decreasing the accuracy of the measurement of myocardial glucose uptake.^{22, 24-26} Other disadvantages of FDG include the limited metabolic fate of FDG in tissue precluding determination of the metabolic fate (i.e., glycogen formation versus glycolysis) of the extracted tracer and glucose, and limitations on the performance of serial measurements of myocardial glucose uptake because of the relatively long physical half-life of fluorine-18.

  • Carbon-11 glucose: More recently, quantification of myocardial glucose metabolism has been performed with PET using glucose radiolabeled in the 1-carbon position with ¹¹C (¹¹C-glucose). An advantage of this approach is the lack of need for a lumped constant correction because ¹¹C-glucose is chemically identical to unlabeled glucose and thus has the same metabolic fate as glucose. Other advantages include more accurate measurements of myocardial glucose uptake compared with FDG and the ability estimate the metabolic fate of extracted glucose (Figures 1 and 2).^27-29 Disadvantages of this method include, a fairly complex synthesis of the tracer, the short physical half-life of ¹¹C requires an on-site cyclotron, compartmental modeling that is more demanding with ¹¹C-glucose than it is with FDG, and the need to correct the arterial input function for the production of ¹¹CO₂ and ¹¹C-lactate.

  • ¹¹C-lactate Lactate metabolism in the heart is a key source of energy production, particularly during periods of increased cardiac work. Its impairment typifies various cardiac diseases such as diabetic heart disease.³⁰ However, to date the ability to measure myocardial lactate has been limited by the lack the availability of an appropriate radiotracer and analysis scheme. Recently, a multi-compartmental model was developed for the assessment of myocardial lactate metabolism using PET and L-3-¹¹C-lactate. PET derived extraction of lactate correlated well with lactate oxidation measured by arterial and coronary sinus sampling over a wide range of conditions, (Figure 3).³¹ When combined with either FDG or ¹¹C-glucose it permits a more comprehensive measurement of myocardial carbohydrate metabolism.

• Fatty acid metabolism:

  • ¹¹C-palmitate: The major advantage of ¹¹C-palmitate is that its myocardial kinetics are identical to labeled palmitate. Exponential curve fitting typically identifies two clearance rates, an early rapid washout that reflects β-oxidation and a late slower washout reflecting incorporation into fatty acid intermediates and triglycerides. However, this method only provides semi-quantitative measurements of fatty acid metabolism and its accuracy is reduced under many pathologic conditions where radiotracer back-diffusion is present. Consequently, mathematical modeling techniques are typically used to circumvent these problems permitting the assessment of various aspects of myocardial fatty acid metabolism uptake such as uptake, oxidation and storage.^32-34 However, this method does suffer from several disadvantages including, reduced image quality and specificity, a more complex analysis, and the need for an on-site cyclotron and radiopharmaceutical production capability.

  • Fatty acid analogues: Most of the PET tracers in this category have been designed to reflect myocardial β-oxidation. 14-(R,S)-¹⁸F-fluoro-6-thiaheptadecanoic acid (FTHA) was one of the first radiotracers developed using this approach. Myocardial uptake and retention tracked accordingly with changes in substrate delivery, blood flow and workload in animal models.^{35, 36} The effects of various diseases such as coronary artery disease and dilated cardiomyopathy on myocardial fatty acid metabolism have been evaluated with PET using this radiotracer.^{37, 38} However, uptake and retention of FTHA has been shown to be insensitive to the inhibition of β-oxidation by hypoxia reducing enthusiasm for this radiotracer to measure myocardial fatty acid metabolism. ³⁹ To circumvent this problem 16-¹⁸F-fluoro-4-thia-palmitate (FTP) has been developed. This radiotracer retains the metabolic trapping function that is proportional to fatty acid oxidation under normal oxygenation and hypoxic conditions.^{39, 40} Similar to FDG, quantification of myocardial fatty acid metabolism with FTP requires the use of a lumped constant to correct for kinetic differences between the radiotracer and unlabeled palmitate. Furthermore the extent to which myocardial fatty acid uptake can be separated from oxidation based on the myocardial kinetics of FTP is unknown. Recently, a new F-18–labeled fatty acid radiotracer, trans-9(RS)-¹⁸F-fluoro-3,4(RS,RS) methyleneheptadecanoic acid (FCPHA), has been developed.⁴¹ This radiotracer is also trapped after undergoing several steps of β-oxidation. Preliminary results show that uptake of FCPHA into rat myocardium is approximately 1.5% injected dose per gram tissue at 5 minutes and is retained in tissue. However, the impact of alterations in plasma substrates, work load and blood flow on myocardial kinetics is unknown.

Figure 1.

Correlation in dog hearts between Fick-derived (x-axis) measurements of the rate of myocardial glucose utilization (rMGU) and PET-derived (y-axis) rMGU using (A)1-¹¹C-glucose, (B) FDG before correcting PET values for the lumped constant (LC), (C) FDG after correcting PET values by the LC, and (D) FDG after correcting PET values by a variable LC (LC_v) that accounts for varying substrate, hormonal and work environments. Correlation with Fick-derived values was significantly close when 1-¹¹C-glucose (panel A), as opposed to ¹⁸F-FDG was used, regardless of whether an LC was used or the type of LC used (panels B-D). Reproduced with permission reference 27.

FIGURE 2.

Correlation between PET and arterial and coronary sinus (ART/CS) measurement of fractional glycolysis (A) and glucose oxidation (B). Reproduced with permission reference 29.

FIGURE 3.

Representative PET time–activity curves of l-3-¹¹C-lactate obtained from intralipid (IL), insulin clamp (CLAMP), lactate infusion (LACTATE), or lactate and phenylepherine (LAC/PHEN) studies and corresponding myocardial images obtained 5–10 min after tracer injection and depicting primarily early tracer uptake. Images are displayed on horizontal long axis. Blood ¹¹C = ¹¹C time–activity curves obtained from region of interest (ROI) placed on left atrium; blood ¹¹C-lactate = blood ¹¹C time–activity curves after removing ¹¹CO₂, ¹¹C-neutral, and ¹¹C-basic metabolites; myocardial ¹¹C = ¹¹C time–activity curves obtained from ROI placed on lateral wall. A = apical wall; S = septal wall; L = lateral wall; LV = left ventricle. Reproduced with permission reference 31.

Overview of Myocardial Metabolism - Plasticity in Substrate Use

The heart is an omnivore capable of switching between one substrate to another for energy production depending upon numerous factors such as the plasma substrate environment, neurohumoral milieu and level of cardiac work (Figure 4).⁴² Control of substrate switching can either represent an acute or chronic adaptation in response to either short or prolonged alterations in the physiological environment. The inter-regulation of short-term modulation is based on an effective interplay between various substrates with the metabolism of one substrate automatically inhibiting the pathway of another substrate via rapid enzymatic changes. Such is the case of the Randle Cycle where fatty acid oxidation inhibits glucose oxidation, glycolysis and ultimately glucose uptake via the coordinated inhibition of pyruvate dehydrogenase, phosphofructokinase and hexokinase.⁴³

Figure 4.

Summary of myocardial substrate metabolism demonstrating the need for flexibility in myocardial substrate use tomaintain myocardial health. DCM, Dilated cardiomyopathy; IR, insulin resistance; DM, diabetes mellitus. Reproduced with permission Journal of Nuclear Cardiology 2005;12:345

In contrast, chronic metabolic adaptations reflect alterations in gene expression regulating various metabolic pathways. These changes can occur at the transcriptional and/or post-translational level through the coordinated up-regulation of enzymes and proteins in key metabolic pathways. One example is the hypoxia inducible transcription factor alpha (HIFα) which has several target genes that increase O₂ delivery or survival during hypoxia such as genes involved in the up-regulation of glucose metabolism.⁴⁴ Another example is the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARα), which is a key regulator of myocardial fatty acid uptake, oxidation and storage.⁴⁵ In diabetes mellitus, PPARα activity is increased leading to an up-regulation in genes controlling fatty acid uptake and oxidation.⁴⁶ In contrast, pressure-overload hypertrophy PPARα activity is reduced leading to a down-regulation of genes controlling fat metabolism and in turn leading to an up-regulation of glucose use.⁴⁷ Numerous detrimental effects ensue that extend beyond alterations in energy production and may include increases in oxygen free radical production, impaired energetics, increases in apoptosis, and the induction of left ventricular dysfunction.^{46, 48, 49} Discussed in the sections that follow is how metabolic imaging has helped characterize this loss of metabolic flexibility due to these chronic adaptations in various disease processes.

Aging and Gender

Marked changes in myocardial substrate metabolism occur with age. The fetal heart is typified by a preference for glucose primarily because of the relative hypoxia that occurs in utero. With birth and progression to adulthood fatty acids become the preferred energy source. In experimental models of aging the contribution of fatty acid oxidation to overall myocardial substrate metabolism declines with advancing age.^{50, 51} It appears the cause for the decrease in fatty acid oxidation is multi-factorial ranging from changes in mitochondrial lipid content, oxygen free radical injury, key enzyme systems, and a age-related decline in myocardial PPARα activity.^52-54 Healthy older humans demonstrate a similar metabolic shift.⁵⁵ However, despite the preference for glucose use, older individuals demonstrated a blunted increase in glucose utilization in response to dobutamine when compared to younger individuals. ⁵⁶ Potential clinical ramifications of this blunted metabolic response include stress-related energy deprivation state in the aging heart or possibly increase the susceptibility of the heart to injury during periods of ischemia. However, these age related changes in substrate metabolism are not irreversible as the impairment in metabolic reserve can be ameliorated by endurance exercise training (Figure 5).⁵⁷ Of interest, it appears the myocardial metabolic response to dobutamine following endurance exercise training is gender specific with men demonstrating an increase in myocardial glucose metabolism and a decline in fatty acid use whereas women exhibited increase in both glucose and fatty acid metabolism.

Figure 5.

Effect of endurance exercise training (EET) on myocardial glucose metabolism at rest and during intravenous dobutamine infusion both pre- and post-EET. MGUp, fractional myocardial glucose uptake; MGU, myocardial glucose utilization. Values are means ± SD. *P = 0.03 vs. Pre; †P = 0.04 vs. Pre. Reproduced with permission reference 5.

It is now apparent that gender is also a determinant of the pattern in the myocardial substrate metabolism. For example in female rats myocardial glucose metabolism was lower and fatty acid metabolism was higher compared with male rats.^{58, 59} These observations were confirmed in young healthy volunteers using PET with ¹¹C-glucose and ¹¹C-palmitate.⁶⁰ Women exhibited lower levels of glucose metabolism compared with men. Although, no differences in myocardial fatty acid metabolism were noted, women also exhibited higher MVO₂ compared with men as measured by PET with ¹¹C-acetate. It appears these gender differences in substrate metabolism become more pronounced as one transitions to more pathologic conditions. For example in addition to the changes in glucose metabolism and MVO₂, obese women exhibited higher fatty acid uptake and oxidation compared with obese men.⁶¹ The differences in myocardial metabolism could not be explained by differences in myocardial blood flow, insulin sensitivity, or hemodynamics. However, it should be noted that fatty acid release into the plasma is higher in women compared with men primarily due to a higher fat mass.⁶² Thus, differences in myocardial fatty acid delivery may be contributing to these metabolic differences. Sex hormone stimulation may also play a role since results of animal studies have shown that estrogen decreases glucose oxidation, gluconeogenesis, and glycogenolysis, and increases fatty acid oxidation in liver and skeletal muscle. ^{63 64 65 66} Estrogen also upregulates endogenous nitric oxide synthase which reduces GLUT-4 translocation to the cell surface, thereby inhibiting myocardial glucose uptake. ^{67 68 69} Indeed, in a small retrospective study PET-derived measurements of myocardial fatty acid metabolism were higher in post-menopausal women were higher compared with either post-menopausal women not receiving hormonal replacement therapy or age-matched men. ⁷⁰ Although, requiring further study, these gender and age differences in metabolism may provide a partial explanation for the gender- and age-related outcome differences for various cardiovascular diseases where altered myocardial metabolism plays a role. Moreover, these observations highlight the need to account for age and gender differences when performing measurements of myocardial substrate metabolism.

Diabetes Mellitus

Cardiovascular disease is the leading cause of morbidity and mortality in patients with diabetes mellitus.⁷¹ Although numerous mechanisms such as increased prevalence of hyperlipidemia and hypertension, impaired fibrinolysis, abnormal myocardial endothelial function, and reduced sympathetic neuronal function are likely responsible, there is extensive evidence to suggest that abnormalities in myocardial substrate metabolism also contribute to the increased cardiac risk. ^{72, 73} As mentioned previously the overdependence on fatty acid metabolism and a decrease in glucose use typifies the metabolic phenotype in diabetes. This increase in fatty acid metabolism reflects in large part the effects of peripheral insulin resistance which increase plasma fatty acid levels resulting in increased fatty acid delivery to the myocardium. This results increased myocardial fatty acid uptake which activates key transcriptional pathways such as the PPARα/PGC-1 signaling network resulting in further myocardial fatty acid uptake and oxidation.^74-76. Both insulin-mediated glucose transport and glucose transporter expression decline in diabetes mellitus. However, rates of myocardial glucose uptake are frequently normal due to the presence of hyperglycemia. The increase in myocardial fatty acid metabolism results in increased citrate levels which inhibit phosphofructokinase. Glucose oxidation is inhibited at the level of pyruvate dehydrogenase complex due to increased mitochondrial acetyl-CoA levels and the phosphorylation of pyruvate dehydrogenase kinase 4 by PPARα activation. Consequently, the maintenance of myocardial glucose uptake but a decrease in its downstream metabolism results in an accumulation of glucose metabolites. Potential detrimental effects associated with this shift in metabolism include: impaired mechanical function due to the inability to increase glucose metabolism in response to increase myocardial work, depletion of tricarboxylic acid cycle intermediates due to reduced anapleurosis, electrical instability and apoptosis, a greater sensitivity to myocardial ischemia and myocardial lipid and glucose metabolite accumulation leading to increased apoptosis.

Small animal radionuclide imaging has helped clarify the mechanisms responsible for the metabolic alterations that occur in diabetes mellitus. For example, mice with cardiac-restricted overexpression of PPARα (a metabolic phenotype that is similar to diabetic hearts).⁷⁷ demonstrate an increase in fatty acid uptake and an abnormal suppression of glucose uptake using small animal PET with ¹¹C-palmitate and FDG. Conversely, small animal PET measurements demonstrate an increase glucose uptake and reduced fatty acid uptake in mice with cardiac-restricted overexpression of PPARβ/σ.⁷⁸ Together these observations demonstrate that PPARα and PPARβ/σ drive different metabolic regulatory programs in the heart and that imaging can help characterize genetic manipulations in mouse heart. However, the small size of the mouse heart permits only semi-quantitative measurements of tracer uptake highlighting the imaging challenges that still must be overcome before quantification of metabolic processes are possible in this species. In contrast, quantitative measures of myocardial substrate metabolism are now possible with small animal PET in rat heart. Rates of myocardial glucose uptake correlate directly and closely and GLUT 4 gene expression in the Zucker-Diabetic-Fat (ZDF) rat, a model of type-2 diabetes mellitus (Figure 6).⁷⁹ Moreover, rates of myocardial glucose uptake and fatty acid uptake and oxidation measured with PET in the same disease model demonstrated the importance of increased fatty acid delivery to defining the metabolic phenotype in diabetes.⁸⁰

Figure 6.

Regression model M3 was used to characterize dependence of MGUp on gene expression of GLUT-4 with R² = 0.81, which is significant at P = 0.0003. Reproduced with permission reference 105.

Numerous studies have utilized PET with FDG to characterize myocardial glucose metabolism in patients with diabetes mellitus. These studies have been primarily limited to PET using FDG.^81-83 In general, rates myocardial glucose uptake are reduced in patients with either type 1 or type 2 diabetes mellitus compared with non-diabetics except in under conditions of marked hyperglycemia or supraphysiological levels in plasma insulin (such as occurs with a hyperinsulinemic-euglycemic clamp. Increased myocardial fatty acid uptake measured by arterial-coronary sinus balance studies has been reported in humans with type-1 diabetes mellitus without coronary artery disease.⁸⁴ Although, the impact of plasma levels of free fatty acids on the level of myocardial fatty acid uptake was not determined, a negative correlation between myocardial glucose uptake and plasma fatty acid levels was observed. Recently studies using PET and ¹¹C-plamitate and ¹¹C-glucose in patients with type-1 diabetes mellitus have helped verify these findings noninvasively. Patients with diabetes exhibited higher levels of fatty acid uptake and oxidation compared with non-diabetics primarily due to increased plasma fatty acid levels. In contrast, glucose uptake was reduced in these patients primarily due to decreased glucose transport mechanisms.³³. Consistent to what has been observed in experimental models of diabetes mellitus, the metabolic fate of extracted glucose is impaired in diabetes with reduced rates of glycolysis and glucose oxidation which become more pronounced with increases in cardiac work induced by dobutamine (Figure 7).⁸⁵ However, despite the overdependence on fatty acid metabolism the diabetic heart still is responsive to changes in plasma insulin and fatty acid levels but at a cost. For example, higher insulin levels are needed to achieve the same level of glucose uptake and glucose oxidation compared with non-diabetics consistent with myocardial insulin resistance. In response to higher plasma fatty acid levels myocardial fatty acid uptake is increased however limitations on the downstream oxidation rate results in a greater esterification rate.^{85, 86}

Figure 7.

Measurements of overall myocardial glucose uptake (MGU), glycolysis, glucose oxidation and glycogen synthesis for normal volunteers (NV), type-1 diabetic patients studied under baseline metabolic conditions (DM1) and diabetic patients studied during hyperinsulinemic-euglycemic clamp (DM1-C) both at rest (open bars) and during dobutamine (closed bars). Reproduced with permission reference 79.

Therapeutic strategies are being designed to decrease fatty acid delivery to the in order to reduce the over dependence of the myocardium on fatty acid metabolism and perhaps improve energetics and function of the left ventricle. For example, the use of the insulin sensitizing agent troglitazone in ZDF rats reduces plasma fatty acid levels, decreases myocardial lipid accumulation, reduces apoptosis and improves left ventricular function.⁴⁹ In PET with FDG studies in patients with type-2 diabetes mellitus, before and 26 weeks after treatment with rosiglitazone, demonstrated nearly a 40% increase in insulin-stimulated myocardial glucose uptake, implying reduced fatty acid uptake, which was attributed in large part to suppression in plasma fatty acid levels.⁸⁷ Similar metabolic changes were not seen with the biguanide, metformin whose mechanism of action is designed to reduce hepatic glucose production. Of note, the improvement in insulin-stimulated glucose metabolism could not be predicted by changes in the plasma glucose or HgBA1C levels. These data suggest that metabolic imaging may provide unique insights into therapeutic myocardial metabolic modulation not attainable with more readily available clinical parameters are not predictive of a therapeutic response.

Obesity and Insulin Resistance

Given that obesity is a risk factor for type-2 diabetes mellitus it is not surprising that a significant increase in body mass index (BMI) induces marked increases in myocardial fatty acid metabolism. In either dietary-induced or transgenic models of obesity myocardial fatty acid uptake and oxidation are significantly increased.^{49, 88} This increase at least initially, reflects the increase in fatty acid delivery to the heart due to increased lipolysis from both visceral/abdominal and subcutaneous fat stores secondary to insulin resistance. As in diabetes mellitus the increase in cardiac fatty acid delivery initiates a cascade of events that increase myocardial fatty acid metabolism. Ultimately, fatty acid uptake may exceed oxidation leading to extracted fatty acids entering non-oxidative pathways resulting in lipid formation which can compromise cell survival and cardiac function.⁴⁹

Using PET and ¹¹C-acetate and ¹¹C-palmitate it is now apparent that similar abnormalities in myocardial fatty acid metabolism occur in humans with obesity. For example, in otherwise healthy young obese women, myocardial fatty acid metabolism increases with an increase in BMI with the overdependence in fatty acid metabolism becoming more pronounced with worsening insulin resistance.⁸⁹ There is little change in myocardial glucose metabolism. The increase in myocardial fatty acid use is paralleled by an increase in MVO₂ and a decrease in energy transduction. These findings suggest that metabolic changes in obesity may play a role in the pathogenesis of cardiac dysfunction which is more pronounced in obese women.⁹⁰ Of note, it appears there the myocardial metabolic response to obesity differs between women and men.⁶¹ In contrast to obese women, obese men exhibit a greater impairment in myocardial glucose metabolism per level plasma insulin, suggesting greater myocardial insulin resistance (Figure 8). In addition, obesity had less effect on myocardial fatty acid metabolism in men. In contrast, MVO₂ is higher in the obese women compared with obese men. Thus it appears there appears to be a complex interplay between gender and obesity in influencing myocardial substrate metabolism.

Figure 8.

Shown are the levels for fractional myocardial glucose uptake and glucose utilization/plasma insulin, respectively, for the 4 groups. Obesity's and gender's significant interaction in the prediction of glucose uptake (P < .05) is shown graphically because both the magnitude and direction of the effect of obesity on myocardial glucose uptake differ in men and women. Obese men have lower uptake than nonobese men; obese women, however, have greater uptake than nonobese women. Obesity and gender also interacted in determining glucose utilization/plasma insulin (P < .01), as shown by the magnitude of the difference between the obese and nonobese men and that between the obese and nonobese women. Reproduced with permission reference 61.

Ischemia

With the induction of myocardial ischemia, fatty acid oxidation ceases and glucose becomes the primary substrate for both increased anaerobic glycolysis and for continued, albeit, diminished oxidative metabolism.⁹¹ This metabolic switch is prerequisite for continued a energy production and cell survival. When the ischemic insult is reversed, oxygen availability increases and oxidative metabolism resumes. It appears that these abnormalities in myocardial substrate metabolism may persist well after the resolution of ischemia, so called “ischemic memory”.⁹² Demonstration of either accelerated myocardial glucose metabolism or reduced fatty acid metabolism using FDG and BMIPP, respectively, has been used to document this phenomena. For example, increased myocardial FDG uptake has been shown in patients with unstable angina during pain-free episodes.¹² Moreover, in patients with stable angina increased FDG uptake was demonstrated following exercise-induced ischemia-in the absence of either perfusion deficits or ECG abnormalities⁹³. Similar observations have been made with SPECT using BMIPP. It is apparent that in patients with acute chest pain that abnormalities in myocardial BMIPP uptake may persist 24-36 hrs following the resolution of symptoms ^{94, 95} Moreover, this “metabolic fingerprint” appears superior to perfusion imaging for either identifying coronary artery disease as the cause of the chest pain or assigning prognosis.⁹⁴ The persistence in the metabolic defect increases the flexibility of radiotracer administration for it allows for delivery of a unit dose after the patient has already been evaluated. This is in contrast to the use of perfusion radiotracers which frequently must be available on-site due to the narrow time window from the resolution of symptoms and normalization of the flow deficit. Based on these observations, BMIPP is currently undergoing Phase 3 evaluation for acute chest pain imaging. Recent studies also suggest that patterns of reduced fatty acid metabolism may be a marker of cardiac risk in certain high risk patient populations. For example in patients with end-stage renal disease, cardiac death was significantly associated with highly abnormal BMIPP uptake.⁹⁶ Metabolic imaging with either FDG or BMIPP has also been used for direct ischemia detection during stress testing.^{5, 97, 98} The thought process being that abnormalities in vasodilator reserve with perfusion tracers will underestimate ischemia if oxygen demand and supply remain balanced. Results of initial studies where FDG were injected during exercise appears to support this contention with a greater detection rate for moderately severe coronary artery stenoses compared with perfusion imaging.⁵ Moreover, it appears defects in either glucose or fatty acid metabolism with exercise will persist for ∼24hrs (Figure 9).^{97, 98} Taken in sum, these observations suggest metabolic imaging may provide unique role in the management of patients with ischemic heart disease. However despite these promising results, numerous questions still remain as to the optimal imaging protocols, the impact of alterations in the plasma substrate environment on diagnostic accuracy, the usefulness in diabetic patients, whether added diagnostic and prognostic information is provided over perfusion imaging, and whether this information alters clinical management.

Figure 9.

Figure 9A: Left, Thallium-201 stress and reinjection (Reinj) images after treadmill exercise in the short-axis (SA) and vertical long-axis (VLA) SPECT tomograms (left). Thallium images demonstrate a severe reversible inferior defect (arrows), consistent with exercise stress-induced ischemia. Right, A similar defect is seen on the early BMIPP images in the same tomographic cuts (arrows), with BMIPP injected 22 hours after the stress-induced ischemia. The defect on the delayed BMIPP image is less prominent than on the early image. These image data suggest that BMIPP detects prolonged postischemic suppression of fatty acid metabolism for up to 22 hours after stress-induced ischemia. Reproduced with permission reference 95.

Figure 9B: Exercise and rest ^99mTc-sestamibi perfusion and ¹⁸F-FDG images of 71-y-old man (patient 7) with exertional angina, in short-axis and vertical and horizontal long-axis slices. Reversible perfusion abnormality involving anterior and septal walls (white arrows) is shown. Intense ¹⁸F-FDG uptake in anterior and septal walls on exercise images is demonstrated. ¹⁸F-FDG uptake persists on rest images in anterior and septal walls, and lateral wall also showed intense ¹⁸F-FDG uptake (yellow arrows). This patient had 90% stenosis of left anterior descending and 80% stenosis of circumflex and right coronary artery. Reproduced with permission reference 96.

Hypertension/Left Ventricular Hypertrophy

There are numerous lines of evidence to support the notion of a relationship between between abnormalities in myocardial substrate metabolism and left ventricular hypertrophy. In experimental models of pressure-overload left ventricular hypertrophy myocardial fatty acid oxidation is decreased and there is an increase in glucose use.^{99, 100} Moreover, interventions that inhibit mitochondrial fatty acid β-oxidation result in cardiac hypertrophy.¹⁰⁰ In humans, variants in genes regulating key aspects of myocardial fatty acid metabolism ranging from PPARα to various key β-oxidative enzymes are associated with left ventricular hypertrophy.^{101, 102}

In a rat model of hypertrophy PET with FDG demonstrated myocardial glucose uptake tracked directly with increasing hypertrophy confirming this shift in substrate preference in-vivo.¹⁰³. Similar results have been found in man. PET with ¹¹C-palmitate in humans has shown the reduction in myocardial fatty acid oxidation is an independent predictor of left ventricular mass in hypertension.¹⁰⁴ Combining measurements of left ventricular myocardial external work (either by echocardiography or MRI) with measurements of MVO₂ performed by PET with ¹¹C-acetate or ¹⁵O-oxygen, it is possible to estimate cardiac efficiency. Using this approach in patients with hypertension-induced left ventricular hypertrophy has shown that the decline in myocardial fatty acid metabolism is associated with a decline in efficiency, a condition that may increase the potential for the development of heart failure. ^{13, 105} Another potential application of PET is the phenotyping of patients with genetic variants related to myocyte growth. For example in patients with hypertrophic cardiomyopathy attributable to a known specific variant in the α-tropomyosin gene, it was observed that increased myocardial perfusion, fatty acid metabolism, and efficiency characterize patients with mild hypertrophy whereas these metabolic alterations decrease as hypertrophy becomes more advanced.¹⁰⁶. Although, requiring further study in larger patient populations, this study suggests that metabolic imaging may identify relevant gene variants without waiting for more end-stage manifestations such as left ventricular remodeling and dysfunction to occur.

Nonischemic Dilated Cardiomyopathy

Perturbations in myocardial substrate metabolism have also been implicated in the pathogenesis of contractile dysfunction and heart failure. In experimental models of heart failure the progression from cardiac hypertrophy to ventricular dysfunction is paralleled by a decrease in the expression of genes encoding for enzymes regulating β-oxidation resulting in a shift in myocardial substrate metabolism to primarily glucose use, similar to that seen in the fetal heart.^{99, 107, 108} Paralleling these metabolic changes are the re-expression of fetal isoforms of a variety of contractile and calcium regulatory proteins program. The reactivation of the metabolic fetal gene program may have numerous detrimental consequences on myocardial contractile function ranging from energy deprivation to the inability to process fatty acids leading to accumulation of nonoxidized toxic fatty acid derivatives, resulting in lipotoxicity. Underscoring the importance of this metabolic shift in the pathogenesis of heart failure is the robust discovery and development of novel therapeutics which target specific aspects of cellular metabolism such as the partial fatty acid oxidation antagonists and the insulin sensitizer glucagon-like peptide-1.¹⁰⁹

Both SPECT and PET techniques have documented in humans the metabolic shift associated with cardiomyopathy. For example, SPECT with BMIPP demonstrated reduced myocardial uptake and increased clearance radiotracer in patients with dilated cardiomyopathy compared with controls.¹¹⁰ Moreover, the magnitude of these metabolic abnormalities correlated with other measurements of heart failure severity such as left ventricular size and plasma b-natruietic peptide levels. It appears these abnormalities in BMIPP kinetics reflect the combined effects of reduced fatty acid uptake and oxidation as evidenced by PET with ¹¹C-palmitate studies in a similar patient population. In this same study, myocardial glucose metabolsim was higher in the cardiomyopathic patients compared with controls confirming the metabolic shift.¹¹¹ Similarly, PET has also been used provide mechanistic insights into the myocardial metabolic abnormalitie associated with heart failure. Rapid lowering of fatty acid delivery with acipimox results in reduced fatty acid uptake, MVO₂, and cardiac work and no change cardiac efficiency in normal volunteers.¹⁰⁶ However in patients with nonischemic dilated cardiomyopathy, there is a decrease in myocardial fatty acid uptake and cardiac work, no change in MVO₂ and a decline in efficiency. Although limited by a small sample size, these results appear to reinforce to the central role of loss flexibility in myocardial substrate metabolism in the pathogenesis of heart failure with even minor changes in substrate delivery having detrimental consequences on cardiac energy transduction.

Metabolic imaging can also been used to study the effects of various treatments for cardiomyopathy. For example, treatment with the selective β-blocker metoprolol results in a reduction in oxidative metabolism and an improvement in cardiac efficiency as measured by PET in patients with heart failure.¹¹² Cardiac efficiency has been shown to improve following either exercise training or cardiac resynchronization therapy, implicating improved myocardial energy transduction as a potential mechanism.^{113, 114} Moreover, normalization of the distribution of myocardial glucose metabolism may be partially responsible for the improved energetic following cardiac resynchronization therapy.¹¹⁵ Partial fatty acid oxidation inhibitors have been proposed for the treatment of heart failure based on the theory that decreasing myocardial fatty acid oxidation should increase the oxidation of glucose leading to a more favorable energetic state and improved left ventricular function. In support of this hypothesis is the finding that, the administration of trimetizidine to patients with dilated cardiomyopathy results in a significant improvement in left ventricular ejection fraction.¹⁰⁶ However, the improvement in left ventricular function is paralleled by only a mild decrease in myocardial fatty acid oxidation. Thus it appears the improvement in left ventricular function reflects more than a shift in metabolism and is likely influenced by other factors such as improved whole-body insulin resistance and synergestic effects with β-blockade. In patients with dilated cardiomyopathy, it appears the fraction of myocardial glucose uptake, as measured by PET with FDG predicts the effectiveness of β-blocker therapy.¹¹⁶ Moreover, in patients with ischemic cardiomyopathy the extent of viable myocardium as measured with PET and FDG correlated with the hemodynamic response after cardiac resynchronization therapy suggesting a role for PET in discriminating responders from non-responders to this therapy.¹¹⁷ Thus, metabolic imaging can also be used to predict the response to specific therapies in heart failure patients.

Beyond the Myocardium - Vascular Imaging

Atherosclerosis is a inflammatory process characterized by cycles of intense activity and progression followed by intervals of stabilization. A common result of this process is coronary artery luminal stenosis that compromises myocardial blood flow and induces ischemia during stress. However, commonly the inciting event is acute plaque rupture with thrombosis leading to myocardial infarction and frequently, sudden cardiac death. Based on the premise that glucose metabolism is increased in activated macrophages which are a key component ofatherosclerotic plaque, FDG is being evaluated for the detection of “biologically active” atherosclerosis. Several groups have established that inflamed arterial vessels have increased uptake of FDG as measured by PET. The increased uptake has been noted in animal models of atherosclerosis, and verified in humans with atherosclerosis of the carotid artery and aorta.^118-121 Moreover, a significant correlation between FDG uptake and CD68 cell staining has recently been established. It appears that vascular FDG imaging is fairly reproducible suggesting a role for monitoring of therapy.¹²² Indeed, a decrease in carotid artery FDG uptake is correlative with an increase in plasma high-density lipoprotein levels following statin therapy or life style modifications (Figure 10).^{120, 123} However many questions remain about this technique such as the site of localization of radiotracer (e.g., plaque or smooth muscle), the suitability of the method for evaluating the coronary arteries, and whether the information provides more refined risk stratification compared with other more widely applicable methods or if it alters therapy.

Figure 10.

Serial FDG PET/CT images of 62-y-old male patient. (A) Initial study demonstrates focal site of increased FDG uptake in right common carotid artery. Lesion–to–blood-pool peak SUV ratio was 1.2. (B) On 1-y follow-up study, carotid artery lesion was no longer visible, and peak SUV ratio was reduced to 0.9. Reproduced with permission reference 120.

Future Directions

Despite the success of metabolic imaging in furthering our understanding of disease mechanism and in identifying and evaluating potential novel therapeutics for various forms of cardiovascular disease, advances are needed in several areas. There needs to be continued improvement in instrumentation design for imaging of humans and small animals. For example, advances in PET detector design and post-detector electronics will further increase counting statistics which would improve the ability to perform more complex compartmental modeling permitting more complete characterization of the metabolism of a given substrate. New designs in SPECT technology may permit dynamic data acquisitions permitting quantitative or semi-quantitative measurements of substrate metabolism. For small animal imaging tracer quantification is still difficult but should be facilitated with the new hybrid systems where accurate attenuation correction can be performed. There is a fundamental need for the development of new radiotracers that permit characterization of key metabolic pathways such as uptake, storage or oxidation that are linked to disease manifestations. Moreover, new radiotracers are needed to provide insights into the linkage between substrate metabolism and cell growth, cell survival and energy transduction. Ultimately, for metabolic imaging to achieve clinical relevance beyond viability detection will require the performance of appropriately powered clinical trials designed to answer key questions about the utility of metabolic imaging for diagnosis, risk stratification and monitoring of therapy in specific patient populations. To perform such studies will necessitate the development of radiotracers radiolabeled with either F-18 or I-123 and readily exportable image analysis schemes. However once these challenges are met, metabolic imaging will be positioned as an indispensible tool for the management of the cardiac patient.