Nuclear magnetic resonance imaging-guided phosphorus-31 spectroscopy of the human heart

Evaluation of cardiac energetics by non-invasive 31P magnetic resonance spectroscopy

Alterations in myocardial energy metabolism have been implicated in the pathophysiology of cardiac diseases such as heart failure and diabetic cardiomyopathy. ³¹P magnetic resonance spectroscopy (MRS) is a powerful tool to investigate cardiac energetics non-invasively in vivo, by detecting phosphorus (³¹P)-containing metabolites involved in energy supply and buffering. In this article, we review the historical development of cardiac ³¹P MRS, the readouts used to assess cardiac energetics from ³¹P MRS, and how ³¹P MRS studies have contributed to the understanding of cardiac energy metabolism in heart failure and diabetes. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

In vivo mouse myocardial (31)P MRS using three-dimensional image-selected in vivo spectroscopy (3D ISIS): technical considerations and biochemical validations

(31)P MRS provides a unique non-invasive window into myocardial energy homeostasis. Mouse models of cardiac disease are widely used in preclinical studies, but the application of (31)P MRS in the in vivo mouse heart has been limited. The small-sized, fast-beating mouse heart imposes challenges regarding localized signal acquisition devoid of contamination with signal originating from surrounding tissues. Here, we report the implementation and validation of three-dimensional image-selected in vivo spectroscopy (3D ISIS) for localized (31)P MRS of the in vivo mouse heart at 9.4 T. Cardiac (31)P MR spectra were acquired in vivo in healthy mice (n = 9) and in transverse aortic constricted (TAC) mice (n = 8) using respiratory-gated, cardiac-triggered 3D ISIS. Localization and potential signal contamination were assessed with (31)P MRS experiments in the anterior myocardial wall, liver, skeletal muscle and blood. For healthy hearts, results were validated against ex vivo biochemical assays. Effects of isoflurane anesthesia were assessed by measuring in vivo hemodynamics and blood gases. The myocardial energy status, assessed via the phosphocreatine (PCr) to adenosine 5'-triphosphate (ATP) ratio, was approximately 25% lower in TAC mice compared with controls (0.76 ± 0.13 versus 1.00 ± 0.15; P < 0.01). Localization with one-dimensional (1D) ISIS resulted in two-fold higher PCr/ATP ratios than measured with 3D ISIS, because of the high PCr levels of chest skeletal muscle that contaminate the 1D ISIS measurements. Ex vivo determinations of the myocardial PCr/ATP ratio (0.94 ± 0.24; n = 8) confirmed the in vivo observations in control mice. Heart rate (497 ± 76 beats/min), mean arterial pressure (90 ± 3.3 mmHg) and blood oxygen saturation (96.2 ± 0.6%) during the experimental conditions of in vivo (31)P MRS were within the normal physiological range. Our results show that respiratory-gated, cardiac-triggered 3D ISIS allows for non-invasive assessments of in vivo mouse myocardial energy homeostasis with (31)P MRS under physiological conditions.

Skeletal muscle energetics with PNMR: personal views and historic perspectives

This article reviews historical and current NMR approaches to describing in vivo bioenergetics of skeletal muscles in normal and diseased populations. It draws upon the first author's more than 70 years of personal experience in enzyme kinetics and the last author's physiological approaches. The development of in vivo PNMR jointly with researchers around the world is described. It is explained how non-invasive PNMR has advanced human exercise biochemistry, physiology and pathology. Further, after a brief explanation of bioenergetics with PNMR on creatine kinase, anerobic glycolysis and mitochondrial oxidative phosphorylation, some basic and controversial subjects are focused upon, and the authors' view of the subjects are offered, with questions and answers. Some of the research has been introduced in exercise physiology. Future directions of NMR on bioenergetics, as a part of system biological approaches, are indicated.

Cardiac phosphorus-31 two-dimensional chemical shift imaging in patients with hereditary hemochromatosis

Hemochromatosis is a hereditary iron overload syndrome characterized by increased iron storage, followed by liver cirrhosis and is often associated with restrictive cardiomyopathy. The purpose of this study was to detect alterations of cardiac high-energy phosphate metabolism in patients with hereditary hemochromatosis (HHC) prior to the development of structural heart diseases. Therefore cardiac phosphorus-31 two-dimensional chemical shift imaging ((31)P 2D CSI) was employed. Twenty-four male patients (mean age 47.2 +/- 12 years) homozygous for the C282Y mutation in the hemochromatosis associated HFE gene and twenty-four male healthy volunteers (mean age 47 +/- 11 years) as age-matched controls were included in this study. Using a 1.5-Tesla whole-body magnetic resonance scanner, electrocardiograph-triggered transversal 31P 2D CSI was performed. Left ventricle mean phosphocreatine (PCr) to beta-adenosine triphosphate (beta-ATP) ratios of patients with HHC (1.60 +/- 0.41) were significantly decreased in comparison to healthy volunteers (1.93 +/- 0.36; p = 0.004). Furthermore, we detected moderate, negative correlations between left ventricular PCr to beta-ATP ratios and transferrin saturation, cholesterol, low-density lipoprotein as well as triglyceride. This study shows that 31P 2D CSI permits the detection of alterations of cardiac high-energy phosphate metabolism in patients with HHC, but without any evidence for heart disease. The decreased PCr to beta-ATP ratios in HHC might be caused by mitochondrial impairment due to cardiac iron overload.

Noninvasive measurements of cardiac high-energy phosphate metabolites in dilated cardiomyopathy by using 31P spectroscopic chemical shift imaging

Dilated cardiomyopathy (DCM) is accompanied by an impaired cardiac energy metabolism. The aim of this study was to investigate metabolic ratios in patients with DCM compared to controls by using spectroscopic two-dimensional chemical shift imaging (2D-CSI). Twenty volunteers and 15 patients with severe symptoms (left ventricular ejection fraction, LVEF<30%) and ten patients with moderate symptoms (LVEF>30%) of DCM were investigated. Cardiac 31P MR 2D-CSI measurements (voxel size: 40x40x100 mm3) were performed with a 1.5 T whole-body scanner. Measurement time ranged from 15 min to 30 min. Peak areas and ratios of different metabolites were evaluated, including high-energy phosphates (PCr, ATP), 2,3-diphosphoglycerate (2,3-DPG) and phosphodiesters (PDE). In addition, we evaluated how PCr/ATP ratios correlate with LVEF as an established prognostic factor of heart failure. The PCr/gamma-ATP ratio was significantly decreased in patients with moderate and severe DCM and showed a linear correlation with reduced LVEFs. PDE/ATP ratios were significantly increased only in patients with severe DCM as compared to volunteers. Applying 31P MRS with commonly-available 2D-CSI sequences is a valuable technique to evaluate DCM by determining PCr/ATP ratios noninvasively. In addition to reduced PCr/ATP ratios observed in patients suffering from DCM, significantly-increased PDE/ATP ratios were found in patients with severe DCM.

Impact of aging on cardiac high-energy phosphate metabolism determined by phosphorus-31 2-dimensional chemical shift imaging (31P 2D CSI)

Previous echocardiographic and experimental animal studies have shown that cardiac function, structure, and metabolism change with age. The aim of this study was to evaluate the impact of age on left ventricular high-energy phosphate metabolism. Using a 1.5 Tesla whole-body MR scanner 31P 2D CSI (8 x 8 phase encoding steps, 320 mm field of view) was performed in 76 healthy male volunteers (41.7 +/- 13 years) without any history of coronary heart disease. Fourier interpolation, corrections for T1 saturation effects, the nucleus Overhauser effect, and the blood contamination were applied to the spectroscopic data. The volunteers were divided into two groups, younger (n = 37) and older (n = 39) than 41.7 years. In all volunteers, laboratory specimen were sampled, and transthoracal echocardiography was carried out. Significant differences in left ventricular phosphocreatine (PCr) to beta-adenosine-triphosphate (beta-ATP) ratios (2.16 vs. 1.83, p < 0.001), fasting serum glucose levels (83.3 vs. 98.7 mg/dl, p < 0.001), E/A (1.51 vs. 1.14 p < 0.001), and ejection fraction (EF, 65.3 vs. 59.9%, p = 0.005) were detected between the two groups of volunteers, younger and older than 41.7 years. Moreover, age correlated moderately to well with left ventricular PCr to beta-ATP ratios (r = -0.44), fasting serum glucose levels (r = 0.4), E/A (r = -0.7), left ventricular myocardial mass (r = -0.41), and EF (r = -0.55). In conclusion, our study shows that left ventricular PCr to beta-ATP ratios decrease moderately with age, as suggested by previous experimental animal studies. Additionally, age correlates negatively with E/A, left ventricular myocardial mass, and EF, as reported by previous echocardiography studies. The present study is the first to show the impact of age on left ventricular PCr to beta-ATP values in humans.

Advances in human cardiac 31P-MR spectroscopy: SLOOP and clinical applications

Phosphorus magnetic resonance spectroscopy (31P-MRS) has revealed a lot about the biochemistry of physiological and pathological processes in the heart. Nevertheless, until today, cardiac 31P-MRS has not had any clinical impact, albeit some pioneering studies demonstrated that 31P-MRS can indeed provide diagnostic information. In this paper, the development of techniques for human cardiac 31P-MRS over the past decade is reviewed, and the requirements for a reliable clinical measurement protocol are discussed. Spatial localization with optimal pointspread function (SLOOP) is a new method to achieve spatial localization and absolute quantitation. Its properties are detailed, and preliminary findings in patients with dilated cardiomyopathy or myocardial infarction are presented.

Myocardial high-energy phosphate metabolism in patients with stable chronic dilated cardiomyopathy under a dobutamine-induced prolonged mild workload

BACKGROUND

The myocardial phosphocreatine (PCr) to beta-adenosine triphosphate ratio measured by phosphorus 31 nuclear magnetic resonance spectroscopy, which is analogous to energy reserve, is one of the important clinical predictors in patients with dilated cardiomyopathy (DCM). However, it may vary with the cardiac workload.

METHOD

The myocardial PCr to beta-adenosine triphosphate ratio was measured before and during a 5 and 10 microgram/kg/min infusion of dobutamine in 7 patients with DCM and in 8 normal patients. Dobutamine infusion was kept constant for 50 minutes in each stage. Myocardial contractility and ventricular size were determined by echocardiography with the same protocol.

RESULTS

This ratio was unchanged from 1.5 +/- 0.4 to 1.8 +/- 0.6 in the low-dose stage and stable (1.7 +/- 0.3) in the high-dose stage in patients with DCM. The heart rate and the mean rate of circumferential fiber shortening increased dose dependently both in patients with DCM and in patients without.

CONCLUSION

These results demonstrate that constant loading of dobutamine for hours is tolerated without deterioration of myocardial metabolic function by patients with nonischemic DCM. We concluded that the high-energy phosphate metabolism of stable patients with cardiomyopathy is stable if the workload is temporary and weak. This implies the possibility that mild exercise can be tolerated in patients with heart failure.

Measurement of myocardial pH by saturation transfer in man

Myocardial pH has been shown in animal models to be a sensitive indicator of ischemia. In vivo measurement in humans using 31p magnetic resonance spectroscopy is complicated by the overlap of blood 2,3-diphosphoglycerate peaks with the P(i) peak used for pH measurement. A "saturation transfer" method combined with spatial presaturation of skeletal muscle signal is presented which can obtain spectra from the heart free of contamination of 2,3-DPG signal in which intracellular P(i) resonance can be clearly observed. Application to a group of six normal subjects found that the chemical shift of the intracellular inorganic phosphate peak was 4.95+/-0.06 relative to the phosphocreatine peak. This is equivalent to a pH of 7.11+/-0.05.

Controlled reperfusion after myocardial ischemia in a canine model monitored by two-dimensional phosphorus 31 chemical shift spectroscopic imaging

Phosphorus 31 magnetic resonance spectroscopy at 2 T was used to monitor high-energy phosphate metabolism over a 3-week period in a canine model of myocardial infarction and reperfusion. Twenty animals were divided into two groups: group 1 (n = 11) received intravenous nitroglycerin beginning at the onset of coronary occlusion; group 2 (n = 9) received a 105-minute infusion of superoxide dismutase (SOD) beginning at the onset of reperfusion. A metabolic protective effect was observed (vs controls) with both agents, manifested by a reduction in the degree of pH decline from baseline values and preservation of the adenosine triphosphate/total phosphate ratio during occlusion and reperfusion. Further, both treatments, compared with controls, produced a lower infarct/zone at risk ratio: controls, 1.5 +/- 1.2; nitroglycerin, 0.52 +/- 0.50; and SOD, 0.64 +/- 0.40. The technique of 31P magnetic resonance spectroscopy demonstrated its use for the noninvasive assessment of myocardial metabolism in response to therapeutic intervention.

Magnetic resonance techniques for assessment of myocardial viability

In general, the following three standards for myocardial viability can be used: (a) preserved coronary flow (adequate perfusion); (b) preserved wall motion (systolic wall thickening); and (c) preserved metabolism (metabolic integrity). The current magnetic resonance (MR) techniques provide a great potential to measure all three standards of viability. Adequate perfusion can be assessed by spin-echo MR imaging and/or ultrafast MR imaging, systolic wall thickening by cine MR imaging, and the presence of metabolic integrity can be determined by MR spectroscopy. These noninvasive and versatile techniques have led to an increasing interest and research in recent years. Particular strengths of the MR techniques are: the inherent three-dimensional data acquisition without radiation exposure; the intrinsic soft-tissue contrast that allows tissue characterization; the excellent spatial resolution (in the 1- to 2-mm range), which permits the evaluation of regional abnormalities; multitomographic imaging capabilities that allow acquisition of cardiac images in any plane; the inherent sensitivity to blood and wall motion; and the potential for in vivo measurement of myocardial metabolism using MR spectroscopy. This review article demonstrates that MR techniques might play a growing role in the assessment of myocardial viability.

Cardiac metabolism: a technical spectrum of modalities including positron emission tomography, single-photon emission computed tomography, and magnetic resonance spectroscopy

Noninvasive techniques for the assessment of cardiac metabolism are important for the detection of potentially salvageable tissue in jeopardized areas of the myocardium. The correct identification of hibernating and stunned myocardium in patients with severely depressed cardiac function can have vital therapeutic consequences for the patient. Changes in myocardial fatty acid and glucose metabolism during acute and prolonged ischemia can be traced by positron-emitting or gamma-emitting radiopharmaceuticals. Alternatively, 31P-labeled magnetic resonance spectroscopy can be used for the assessment of high-energy phosphate metabolism. It is not yet clear which modality will emerge as the most useful in the clinical setting. Positron emission tomography (PET) that uses combinations of flow tracers and metabolic tracers offers unique opportunities for quantification and high-resolution static and rapid dynamic studies. Currently, assessment of glucose metabolism with 18F-fluorodeoxyglucose is regarded as the gold standard for myocardial viability and prediction of improvement of impaired contractile function after revascularization. However, preserved oxidative metabolism may be required for potential functional improvement, and therefore assessment of residual oxidative metabolism by 11C-labeled acetate PET may prove to be more accurate than 18F-fluorodeoxyglucose PET, which reflects both anaerobic and oxidative metabolism. Moreover, because fatty acids are metabolized only aerobically, they are excellent candidates for the clinical assessment of myocardial viability and prediction of functional improvement after revascularization. Especially derivatives of fatty acids that are not metabolized but accumulate in the myocyte are attractive for myocardial imaging. Examples are 123I-beta-methyl-p-iodophenyl pentadecanoic acid and 15-(o-123I-phenyl)-pentadecanoic acid. These tracers can be detected by planar scintigraphy and single-photon emission computed tomography, which are more economical and widely available than PET. In addition, 511 keV collimators have been developed recently, making the detection of positron emitters by planar scintigraphy and single-photon emission computed tomography feasible. The experience with 31P-labeled magnetic resonance spectroscopy in humans is still limited. With current magnetic resonance spectroscopic techniques, insufficient spatial resolution is achieved for clinical purposes, but the possibility of serial measurements to monitor rapid changes of phosphate-containing molecules in time makes magnetic resonance spectroscopy very valuable for the research of myocardial metabolism.

Contribution of specific skeletal muscle metabolic abnormalities to limitation of exercise capacity in patients with chronic heart failure: a phosphorus 31 nuclear magnetic resonance study

Several studies of phosphorus 31 (31P) magnetic resonance spectroscopy (MRS) have demonstrated the presence of skeletal muscle metabolic abnormalities during exercise in patients with chronic heart failure (CHF). We studied the contribution of these abnormalities to the limitation of exercise capacity in CHF. In 25 patients (age 57 +/- 2 years, left ventricular ejection fraction [LVEF] 28% +/- 1.6%, peak oxygen consumption (VO2) 16 +/- 1.2 ml/kg/mm) (mean +/- SEM), we studied the calf muscle at rest and during plantar flexion with 31P MRS. The phosphocreatine (PCr) depletion rate was significantly negatively correlated to peak VO2 (r = -0.62, p = 0.001) but not to LVEF. Muscle pH was correlated with the inorganic phosphorus (Pi)/PCr ratio (r = -0.69, p = 0.0001) and with the PCr/adenosine triphosphate beta (ATP beta) ratio (which negatively relates to adenosine diphosphate [ADP] concentration) (r = 0.65, p = 0.00001). Although muscle ATP (ATP/sum of phosphorus [sigma P] remained stable, in 8 patients ATP/sigma P decreased significantly (-15% +/- 4%, p = 0.0002). In this ATP-depleted group, peak VO2 was significantly lower than that of the nondepleted group and PCr depletion more rapid, whereas LVEF did not differ. Skeletal muscle metabolic abnormalities in CHF contribute markedly to the alteration of exercise capacity. Rapid PCr depletion and muscle acidosis are the most relevant abnormalities. ATP depletion and excessive increase in ADP during exercise may contribute further to exercise limitation specifically in patients with more marked CHF.

Saturation correction in human cardiac 31P MR spectroscopy at 1.5 T

This study was conducted to verify the validity of using saturation factors obtained from unlocalized 31P spectra containing both chest wall and heart muscle signals for correcting human heart muscle phosphocreatine/beta-adenosine triphosphate (PCr/beta-ATP) ratios. Saturation factors and T1 relaxation times were determined from 31P magnetic resonance spectra of human chest wall and heart muscle simultaneously in healthy volunteers using one-dimensional spectroscopic imaging in combination with a two-dimensional ISIS sequence by using adiabatic 180 degrees inversion and adiabatic 90 degrees excitation pulses at 1.5 T. Blood corrected saturation factors for PCr/beta-ATP at a TR of 2.4 s were significantly different in heart muscle and chest wall muscle, 1.30 +/- 0.25 and 1.73 +/- 0.31, respectively (p < 0.05). T1 values for PCr and beta-ATP in heart muscle were 4.28 +/- 0.72 and 2.99 +/- 0.52 and in chest wall muscle 6.82 +/- 1.07 and 3.39 +/- 0.48, respectively. The T1(PCr)/T1(beta-ATP) ratios in chest wall and heart muscle were not identical. The mean PCr/beta-ATP ratios in heart and chest wall muscle of six healthy volunteers were 1.23 +/- 0.17 and 3.71 +/- 0.53, respectively.

Direct measurement of spin-lattice relaxation times of phosphorus metabolites in human myocardium

T1 values of phosphorus metabolites visible in human cardiac 31P-MR spectra were determined in 12 volunteers at 1.5 T. Consecutive spectra were acquired with varying pulse repetition time (TR) from 1.6 to 24 s; volume selection was achieved with ISIS. T1's of creatine phosphate (CP), [gamma-P], [alpha-P], and [beta-P]ATP, 2-3 diphosphoglycerate, and phosphodiesters were 6.1 +/- 0.5, 5.4 +/- 0.5, 5.5 +/- 0.5, 5.8 +/- 1.0, 7.6 +/- 1.0, and 5.0 +/- 1.0 s, respectively. CP/ATP ratios showed little change with varying TR; linear regression of CP/ATP vs TR was of borderline significance (r = 0.28, P = 0.06). T1's for CP and ATP were also determined in standard solution (20 mM CP, 10 mM ATP) yielding T1CP of 8.7 +/- 0.2 and T1[gamma-P]-ATP of 9.9 +/- 0.7 s. Thus, T1's for CP and ATP were similar at 1.5 T in both human heart and standard solution. In human cardiac 31P-MR spectra, CP/ATP ratios may need little correction for partial saturation.

Metabolic response of the human heart to inotropic stimulation: in vivo phosphorus-31 studies of normal and cardiomyopathic myocardium

In order to determine if an increase in myocardial oxygen consumption is accompanied by changes in high energy phosphates in normal subjects and patients with dilated cardiomyopathy, phosphorus-31 spectra were acquired under resting conditions and during dobutamine infusion. In seven normal subjects, dobutamine raised the rate-pressure product to 226% of control. The ratio of PCr/ATP was 1.86 +/- 0.17 (mean +/- SE) under resting conditions and 1.90 +/- 0.22 (P = 0.44) with dobutamine infusion. In eight patients with dilated cardiomyopathy, dobutamine raised the rate-pressure product to 161% of control. As in the normal subjects, the ratio of PCr/ATP under resting conditions (1.63 +/- 0.24) was unchanged during dobutamine infusion (1.57 +/- 0.24, P = 0.38). These data indicate that increases in cardiac work do not have a major effect on high energy phosphate concentrations in normal subjects or in patients with clinically compensated dilated cardiomyopathy.

The trouble with spectroscopy papers

Writing a critique and guide for authors of clinical spectroscopy research papers is a likely way of ensuring that one never sees another of one's own papers published in this field. Nevertheless, it is disappointing, though perhaps predictable, that despite its historical foundations in quantitative spectroscopy, the field has its fair share of findings that are not so obviously reconciled. Here is the view of one author, one referee, and one spectroscopy protagonist about what might be expected of a clinical spectroscopy paper. In addition to novelty, the fundamental criteria for acceptance should be that the conclusions are supported by properly and objectively quantified results, and that sufficient experimental detail is provided so that one skilled in the art could reproduce the study and its findings.

Differentiation of reperfused-viable (stunned) from reperfused-infarcted myocardium at 1 to 3 days postreperfusion by in vivo phosphorus-31 nuclear magnetic resonance spectroscopy

Thrombolytic therapy has increased the need for a technique to assess the viability of recently reperfused myocardium. This study examined the ability of in vivo phosphorus-31 (P-31) nuclear magnetic resonance (NMR) spectroscopy to distinguish reperfused-viable (stunned) from reperfused-infarcted myocardium at 6, 30, and 54 hours following coronary artery occlusion in a canine model. A 15-minute occlusion produced reperfused-viable myocardium in five animals and a 360-minute occlusion produced reperfused-infarcted myocardium in six animals. Postreperfusion risk zone myocardial phosphocreatine (PCr) concentration measured by P-31 NMR spectroscopy was significantly depressed throughout the 3-day study period in infarcted but not in viable myocardium (p less than 0.01 between groups, all time points). The postreperfusion ratio of inorganic phosphate (Pi) to PCr concentration, as determined by NMR spectroscopy, was elevated throughout the study period in infarcted but not in viable reperfused myocardium (p less than 0.01 between groups, all time points). Postreperfusion Pi concentration was elevated at 6 hours but not subsequently in reperfused-infarcted myocardium, and was not elevated in reperfused-viable myocardium. Logistic regression models selected PCr concentration and the Pi/PCr ratio as providing the best discrimination between reperfused-viable and reperfused-infarcted myocardium. The accuracy of P-31 NMR variables selected by logistic regression analysis for determining myocardial viability ranged from 97% to 100%.

Quantitation of the extent of acute myocardial infarction by phosphorus-31 nuclear magnetic resonance spectroscopy

Phosphorus-31 nuclear magnetic resonance (P-31 NMR) spectroscopy is able to identify alterations in myocardial high energy phosphate metabolism associated with acute infarction. It was hypothesized that the extent of acute myocardial infarction could be quantitated from changes in the tissue content of inorganic phosphate (Pi), phosphocreatine (PCr) and adenosine triphosphate (ATP) derived from P-31 NMR spectra. Nine isolated, perfused rat hearts were studied at 121.5 MHz. After baseline spectra were obtained, varying locations of either the right or the left coronary artery were occluded without removing the heart from the spectrometer. Spectra were then collected during regional ischemia at 15 and 45 min after occlusion. Phosphate metabolites were quantitated from the baseline and 45-min regional ischemia spectra, times at which the metabolites are at steady state for the normal and ischemic conditions. The heart was removed from the spectrometer, perfused for a total duration of 2 h and sectioned into 2-mm thick slices for triphenyltetrazolium chloride staining. Percent infarct was determined by manual tracing of magnified, digitized images of the stained sections. Coronary blood flow, heart rate and blood pressure were monitored throughout the experiment. Significant linear relations were found between percent infarct (by triphenyltetrazolium chloride staining) and the percent change of beta-ATP (r = -0.74), Pi (r = 0.83) and the PCr/Pi ratio (r = -0.71) at 45 min after coronary occlusion. Coronary flow was also found to correlate significantly with percent infarct (r = -0.70). These results are applicable to in vivo P-31 NMR studies of acute infarction where the volume of interest may include both normal and acutely infarcted myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

31P MRS of myocardial inorganic phosphate using radiofrequency gradient echoes

Determination of the chemical shift and integral of the myocardial intracellular inorganic phosphate (Pi) resonance by 31P magnetic resonance spectroscopy (MRS) is often precluded due to a large overlapping signal from 2,3-diphosphoglycerate (2,3-DPG) from chamber and myocardial blood. This report demonstrates the use of radiofrequency (RF) magnetic field gradient echoes (RFGE) to eliminate signals from 2,3-DPG in flowing blood, while retaining signals from intracellular myocardial Pi, ATP, and phosphocreatine (PCr). The ECG-triggered 31P spectra were acquired from the myocardium of open chest pigs using a Philips Gyroscan 2-T magnetic resonance spectrometer. A 2.5-cm-diameter surface coil attached to the myocardium was used to provide the RF gradient as well as for excitation and detection of signals. Optimal performance of the RFGE pulse sequence was obtained when the RF gradient pulses were centered at peak diastole or peak systole. Under these conditions, 2,3-DPG signals were completely suppressed, and sensitivity was usually sufficient to allow detection of a well-resolved Pi signal. Myocardial pH determined from RFGE experiments was 7.16 +/- 0.10, and the ratio of the integrals of the Pi and ATP resonances (Pi/ATP) was 0.24. The mean signal-to-noise ratio (S/N) for PCr in control spectra acquired in 4 min was 19/1, while the mean S/N for PCr in RFGE-edited spectra acquired in 15 min was 11/1, demonstrating that the present implementation of the RFGE method results in significant loss in sensitivity. These experiments demonstrate that RFGE-editing allows accurate determination of the chemical shift and integral of the Pi resonance in blood-perfused myocardium in situ.

Simultaneous cardiac mechanics and phosphorus-31 NMR spectroscopy during global myocardial ischemia and reperfusion in the intact dog

To investigate the high-energy phosphate metabolic correlates of left ventricular (LV) dysfunction during the onset and recovery from severe, global myocardial ischemia in vivo, seven preinstrumented closed-chest dogs had ECG-gated phosphorus-31 (31P) NMR-spectroscopy (NMR-S) studies performed and LV micromanometer and sonomicrometer data measured before, during, and every 5 min following severe occlusive global myocardial ischemia. Ischemic LV + dP/dtmax fell from 2396 +/- 576 mm Hg/s at baseline to 2185 +/- 478 mm Hg/s (p less than 0.05) and did not normalize until after 30 min of reperfusion. LV ejection fraction (EF) decreased significantly (0.32 +/- 0.07 EF units to 0.12 +/- 0.13 EF units; p less than 0.05) and did not recover by 30 min of reperfusion (0.27 +/- 0.09 units; P less than 0.05 vs baseline). Simultaneous 31P NMR-S studies demonstrated excellent beta-ATP signal-to-noise (10 +/- 4:1). Myocardial acidosis occurred during global ischemia (delta pH = -0.22 +/- 0.23 units; p less than 0.05), with recovery at 30 min of reperfusion. Inorganic phosphate/phosphocreatine ratio (Pi/PCr) increased significantly during ischemia (0.46 +/- 0.07 to 0.61 +/- 0.07; P less than 0.05), with delayed normalization of this ratio at 30 min of reperfusion. beta-ATP peak area did not change during ischemia. Pi/PCr and LV contractility (+dP/dtmax) were significantly correlated at baseline (r = -0.70) and during global ischemia (r = -0.78; p less than 0.01), but not during recovery (r = 0.006; p = NS). Therefore, the simultaneous evaluation of high-fidelity hemodynamic data and topical 31P NMR-S can be performed in the intact state.

Cardiac metabolism during exercise in healthy volunteers measured by 31P magnetic resonance spectroscopy

A technique was devised for individuals to exercise prone in a magnet during magnetic resonance spectroscopy of the heart and phosphorus-31 magnetic resonance spectra of the heart were obtained by the phase modulated rotating frame imaging technique in six healthy volunteers during steady state dynamic quadriceps exercise. During prone exercise heart rate, blood pressure, and total body oxygen consumption were measured at increasing loads and the results were compared with those during Bruce protocol treadmill exercise. During prone exercise with a 5 kg load the heart rate was similar and the systolic and diastolic blood pressures were higher than those during stage 1 of the Bruce protocol. The rate-pressure products were similar but the total body oxygen consumption was lower during prone exercise. There was no difference in the ratio of phosphocreatine to adenosine triphosphate during rest and exercise.Thus during exercise that produced a local cardiac stress equal to or greater than that during stage 1 of the Bruce protocol treadmill exercise, the energy requirements of the normal human myocardium were adequately supplied by oxidative phosphorylation.

Clinical nuclear magnetic resonance spectroscopy: insight into metabolism

Nuclear magnetic resonance (NMR) spectroscopy can nondestructively evaluate changes in metabolites with different disease states, as well as with therapeutic interventions. Animal studies have provided the basis for understanding changes in high-energy phosphates with myocardial ischemia. Studies of graded ischemia due to partial coronary stenosis have shown the sensitivity of the ratio of phosphocreatinine to inorganic phosphate to small reductions in myocardial blood flow and its relation to myocardial function. The application of NMR spectroscopy to humans requires precise localization techniques to avoid acquiring contaminating information from structures around the heart, such as the chest wall and diaphragm. With these localization techniques, metabolic evidence of ischemia has been demonstrated in patients with myocardial infarction and patients with known coronary disease, although the sensitivity of this technique for the diagnosis of inducible ischemia is unknown. At rest, patients with dilated and hypertrophic cardiomyopathies often have an elevated phosphodiester resonance, possibly signifying abnormal breakdown of membrane phospholipids. Increasing oxygen demand in these patients does not usually alter high-energy phosphates, suggesting that oxidative energy metabolism is preserved under these conditions. NMR spectroscopy is a powerful tool to increase understanding of metabolic changes in a variety of pathologic conditions.

Image-guided 31P magnetic resonance spectroscopy of normal and transplanted human kidneys

Image-guided 31-phosphorus magnetic resonance spectroscopy (MRS) was used to obtain spatially localized 31P spectra of good quality from healthy normal human kidneys and from well-functioning renal allografts. A surface coil of 14 cm diameter was used for acquiring phosphorus signals solely from a volume-of-interest located within the kidney. To determine the effects of kidney transplantation on renal metabolism, patients with well functioning allografts were studied. Little or no phosphocreatine in all spectra verifies the absence of muscle contamination, and is consistent with proper volume localization. The intensity ratio of phosphomonoesters (PME) to adenosine triphosphate (ATP) resonances in transplanted kidneys (PME/ATP = 1.1 +/- 0.4) was slightly elevated (P = 0.2) compared to that of healthy normal kidneys (PME/ATP = 0.8 +/- 0.3). The inorganic phosphate (Pi) to ATP ratio was similar in the two groups (Pi/ATP = 1.1 +/- 0.1 in transplanted kidneys vs. 1.2 +/- 0.6 in normal kidneys). Acid/base status, as evidenced from the chemical shift of Pi, was the same in both normal controls and transplanted kidneys. Despite the practical problems produced by organ depth, respiratory movement, and tissue heterogeneity, these results demonstrate that image-guided 31P MR spectra can reliably be obtained from human kidneys.

Evaluation of myocardial viability following ischemic and reperfusion injury using phosphorus 31 nuclear magnetic resonance spectroscopy in vivo

Recovery of myocardial high-energy phosphate (HEP) metabolism after coronary occlusion and reperfusion may vary with ischemic duration and may provide information about the extent of tissue viability. To evaluate the differences between varying durations of ischemia and to attempt to identify metabolic indexes of salvaged viable tissue, intact New Zealand white rabbits underwent either 30 (group 1; n = 8) or 60 (group 2; n = 8) minutes of coronary occlusion followed by reperfusion. HEP metabolism was evaluated with cardiac gated phosphorus 31 (31P) nuclear magnetic resonance (NMR) spectroscopy with a 2.0 T spectrometer. While similar HEP changes were observed during ischemia in both groups, differences in HEP recovery between groups were seen following reperfusion. Group 1 animals demonstrated a gradual decrease in inorganic phosphates (Pi) (p less than 0.05 versus group 2), an immediate recovery of phosphocreatine (PCr) (p = ns versus baseline), and a gradual increase of adenosine triphosphate (ATP) to pre-ischemic levels. Group 2 animals had elevated levels of Pi (p less than 0.05 versus baseline; p less than 0.05 versus group 1), slow recovery of PCr, and continued reduction of ATP (p less than 0.05 versus baseline; p less than 0.05 versus group 1). Group 1 rabbits had a greater extent of viable myocardium than group 2 (77.1 +/- 9.7% of risk area versus 39.4 +/- 9.4%; p less than 0.001). Significant correlations were found between PCr and Pi reperfusion values and myocardial viability (r = 0.59, p less than 0.05; r = 0.73, p less than 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of propranolol on regional cardiac metabolism during ischemia and reperfusion assessed by magnetic resonance spectroscopy

Sixteen anesthetized New Zealand white rabbits were subjected to thoracotomy, and a reversible snare occluder was attached around a large branch of the left circumflex coronary artery. A 1.3 cm. diameter nuclear magnetic resonance (NMR) surface coil was placed adjacent to the myocardium perfused by this vessel. The animals were divided into two groups of eight animals each, treatment and control. The rabbits were studied using a 2.0 T magnetic resonance (MR) spectrometer, and baseline spectra were acquired. The treatment animals then received intravenous propranolol (1.5 mg/kg) and the control animals received an equal volume of saline. Spectra were then acquired during a 20-minute occlusion period and during subsequent reperfusion. Animals in both groups showed expected decreases in phosphocreatine and adenosine triphosphate and an increase in inorganic phosphate during occlusion; these changes reverted toward baseline values with reperfusion. There were no significant differences between the two groups. The myocardium became acidotic during occlusion in both groups, but significantly more so in the control animals: during the first 10 minutes of occlusion pH was 7.30 +/- 0.41 in the treatment group versus 6.55 +/- 0.24 for controls (p = 0.0005). During the second 10 minutes of occlusion pH was 7.05 +/- 0.65 in the treatment group versus 6.24 +/- 0.25 in controls (p = 0.0053). We conclude that attenuation of intracellular acidosis by propranolol during myocardial ischemia was evident by MR spectroscopy in this animal model.

In vivo phosphorus-31 spectroscopic imaging in patients with global myocardial disease

The goals of this study were to determine whether abnormalities in phosphorus metabolism could be noninvasively detected using phosphorus-31 nuclear magnetic resonance spectroscopy in patients with dilated cardiomyopathy and left ventricular hypertrophy, and whether these patient groups could be distinguished from each other based on parameters obtained using this technique. Seventeen patients and 14 control subjects were studied using nuclear magnetic resonance spectroscopy. Spectra were obtained from the human heart at rest using 3-dimensional spectroscopic imaging as a localization technique. Data were acquired over an average volume of 48 cc in 26.3 minutes using a 2 tesla imaging and spectroscopy unit. The ratio of phosphocreatine to adenosine triphosphate was 0.89 +/- 0.88 (mean +/- standard error) in normal subjects and did not differ significantly in patients with dilated cardiomyopathy or left ventricular hypertrophy. A prominent peak in the phosphodiester region was seen much more frequently in patients with dilated cardiomyopathy, resulting in significantly higher ratios of phosphodiester to phosphocreatine (1.28 +/- 0.35) and phosphodiester to adenosine triphosphate (0.79 +/- 0.18) in this group compared to normal subjects (0.33 +/- 0.08 and 0.29 +/- 0.08, respectively). However, the various patient groups could not be reliably distinguished from each other based on spectral patterns. These studies demonstrate the feasibility of performing phosphorus-31 nuclear magnetic resonance spectroscopic imaging in patients with myocardial disease. The initial results indicate that, under resting conditions, the ratio of phosphocreatine to adenosine triphosphate is not consistently altered in patients with severe global cardiomyopathies or hypertrophy. Phosphodiesters are elevated in some patients with dilated cardiomyopathy, a finding that may signify abnormal phospholipid metabolism in this condition.

In vivo phosphorus-31 NMR spectroscopy of abnormal myocardial high-energy phosphate metabolism during cardiac stress in hypertensive-hypertrophied non-human primates

To study the functional and metabolic correlates of left ventricular hypertrophy [LVH] in non-human primates, 7 hypertensive baboons [papio anubis] with 4.6 +/- 0.1 years of hypertension produced by a two-kidney one-clip model, and echocardiographically documented concentric LVH underwent serial phosphorus-31 [P-31] NMR Spectroscopy studies at rest and during inotropic cardiac stress produced by dobutamine infusion [5 micrograms/kg/minute]. Responses in LVH baboons were compared to those in 5 normotensive, sex and weight-matched control animals. The ratio of P-31 NMR-S derived inorganic phosphates [Pi] to phosphocreatine [PCr] was significantly greater at rest in LVH baboons [0.53 +/- 0.06 versus controls = 0.41 +/- 0.17; P less than 0.05]. With dobutamine drug stress, the Pi/PCr ratio rose significantly in LVH baboons [0.77 +/- 0.15 versus 0.56 +/- 0.16; P less than 0.05 at 15 minutes]. Despite hemodynamic recovery, the 5 minute post-dobutamine Pi/PCr ratio remained elevated compared to baseline in LVH baboons only [0.78 +/- 0.16 versus 0.53 +/- 0.06; P less than 0.05]. In pre-instrumented baboons [n = 5], the 'transfer function' of cardiac work [heart rate x LV end-systolic pressure x + dp/dt max] versus Pi/PCr ratio was abnormally shifted rightward and downward [r = 0.80] with LVH as compared to the linearly increasing response in controls. We conclude that in vivo P-31 NMR Spectroscopy studies during dobutamine stress demonstrate reduced PCr stores, delayed metabolic recovery following cessation of inotropic stress, and an abnormal rightward shift in the 'transfer function' in LVH baboons.

Imaging congenital heart disease

When defined in a broad sense, imaging is the most important aspect of modern pediatric cardiovascular medicine. Definition of anatomic defects is now accurately and easily obtained with physical inspection, x-ray technology (including roentgenology, fluoroscopy, and cineangiography), and echocardiography. Echocardiography, with the addition of Doppler and color flow Doppler, is the most important development in clinical cardiac imaging in the past decade. The exciting new areas of "imaging" are in cardiac functional analysis and metabolic evaluation. Viewing the heart at the cellular or biochemical level is the challenge of the future. The new technology offered by computed tomography, positron emission tomography, and nuclear magnetic resonance imaging begins to provide the ability to image the domain of cellular and biochemical function.

Cardiovascular applications of nuclear magnetic resonance spectroscopy

Nuclear magnetic resonance spectroscopy has great potential for defining noninvasively the metabolic status of the heart and skeletal muscle. This technique uses the spin properties of certain nuclei (such as phosphorus-31, hydrogen-1 and carbon-13) to measure high energy phosphates, intracellular pH, lactate and glycogen. Animal studies have formed the basis for human investigations and have demonstrated well-defined changes in high energy phosphates during myocardial ischemia and reperfusion, as well as in cardiomyopathies. Human studies have been limited by issues of sensitivity and localization, although techniques such as rotating frame, depth-resolved surface coil spectroscopy, image-selected in vivo spectroscopy and spectroscopic imaging have been used to acquire phosphorus-31 spectra from the human heart. The few human studies of patients with disease have demonstrated elevated inorganic phosphate peaks after myocardial infarction and abnormal phosphodiester peaks in patients with hypertrophic cardiomyopathy. Studies of patients with heart failure have shown that these patients acidify their peripheral muscles with exercise more easily than do control subjects. Clinical application of nuclear magnetic resonance spectroscopy will depend on technical advances and the demonstration of sensitivity of metabolic changes with disease.