The 31P-NMR stress test: an approach for detecting myocardial ischemia.

31P-NMR spectroscopy has the potential to assess myocardial damage directly and noninvasively by ascertaining the relative abundances of phosphorus-containing compounds relevant to metabolism under stress conditions. Decrease in the PCr/ATP ratio during exercise is an indicator of the level of stress to which the myocardium is subject. This ratio will remain constant under mild to moderate exercise conditions in a healthy subject, but may show a precipitous decrease even under mild exercise when regions of the myocardium are ischemic. The studies examined here indicate that cardiac patients with some forms of ischemia showed a PCr/ATP ratio decrease even under light exercise, while no decrease was observed in patients whose heart disease was known to be nonischemic. Hypertension and nonstenotic chest pain in women can, in some cases, produce a decrease in PCr/ATP ratio. Only the hypertensive patients showed a significant difference in the prestress PCr/ATP ratio when compared with controls. These studies suggest that 31P-NMR spectroscopy before and during mild exercise in the bore of the magnet can be a useful indicator of the presence or absence of an ischemic component to myocardial disorder.

31P-magnetic resonance spectroscopy studies of cardiac transplant patients at rest.

Studies in animal models and patients have suggested that 31P-magnetic resonance spectroscopy (MRS) may be useful in diagnosing transplant rejection, but such studies often are confounded by the late inclusion of patients after transplantation. The present study examined the utility of 31P-MRS in the diagnosis of acute allograft rejection during the first posttransplant month. Thirteen recent heart transplant recipients underwent 57 resting 31P-MRS studies within 24 hr of a biopsy. Subjects lay supine with a 10-cm surface coil placed over the heart. A 1-dimensionsal chemical shift imaging protocol was used to collect spectral information. Spectra from the heart were weighted for distance from the coil and summed before analysis. ANOVA and Duncan's multiple range test were used to analyze the data comparing phosphocreatine (PCr)/ATP ratios with biopsy scores. Transplant patients had significantly lower myocardial PCr/ATP ratios when compared with a normal control group (1.27 +/- 0.27 versus 1.61 +/- 0.22, p < 0.001). However, when the patient group was classified by biopsy score, the expected order of score, 0 > 1 > 2 > 3, was not obtained. Rather, the order was 2 > 0 > 1 > 3. Although the difference between scores 2 and 3 was significant (1.46 versus 1.14, alpha = 0.05 level), the lower three groups were statistically indistinguishable. In addition, the PCr/ ATP ratios were not predictive of future biopsies. Although significantly lower than normal control subjects, resting myocardial PCr/ATP ratios of transplant subjects are not useful in assessing thelevel of rejection. It is suggested that the measurement may be more predictive in mildly exercised myocardium.

1H NMR measurement of triacylglycerol accumulation in the post-ischemic canine heart after transient increase of plasma lipids.

This study tests the hypothesis that increased levels of plasma lipids can accelerate accumulation of myocardial triacylglycerols in post-ischemic but viable myocardium. Two groups of dogs underwent 90 min of left anterior descending coronary artery (LAD) occlusion followed by 240 min of reperfusion. The first group of saline-treated dogs (n = 7) had physiological levels of plasma lipids during reperfusion: a second group treated with Liposyn and heparin (n = 5) experienced increased plasma lipids during reperfusion. The transmural content of triacylglycerols was determined during ischemia and reperfusion using 1H NMR one-dimensional chemical shift imaging (1D CSI), and at the end of reperfusion using Oil Red-O staining and chemical assay. TTC staining was used to identify the extent of irreversibly injured myocardium. Subepicardial and plasma triacylglycerol content, measured both by 1D CSI and chemically, did not change during reperfusion in saline-treated dogs. Infusing dogs with Liposyn and heparin for 90 min during reperfusion transiently elevated their plasma triacylglycerols, which returned to normal levels following Liposyn wash-out. During Liposyn wash-out, myocardial triacylglycerols measured by 1D CSI preferentially increased in the subepicardium of area-at-risk myocardium (P < 0.05). Triacylglycerol content, measured chemically, also increased in area-at-risk compared to non-ischemic subepicardium (P < 0.001). Significant endocardial damage occurred in both groups, but elevated levels of plasma lipids did not increase the size of the area-at-risk. Therefore, elevated plasma lipids caused a preferential accumulation of triacylglycerols in area-at-risk myocardium during reperfusion without exacerbating irreversible ischemic injury. These results are consistent with either inhibited fatty acid oxidation or mis-matched fatty acid extraction and oxidation in area-at-risk myocardium.

1H NMR spectroscopic imaging of myocardial triglycerides in excised dog hearts subjected to 24 hours of coronary occlusion.

BACKGROUND: Myocardial ischemic insult causes depression of fatty-acid beta-oxidation and increased fatty-acid esterification with triglyceride (TG) accumulation. This accumulation has been demonstrated to occur in the territory with diminished blood flow surrounding an infarct, ie, the region at risk. To evaluate whether the extent of TG accumulation in the canine heart after 24 hours of ischemia could be detected, we applied myocardial 1H nuclear magnetic resonance (NMR) spectroscopic imaging (SI). METHODS AND RESULTS: Seven adult mongrel dogs underwent 24 hours of left anterior descending coronary artery occlusion. Postmortem, the hearts were excised and the size and location of the infarct were determined. With a Philips 1.5-T clinical NMR imaging/spectroscopic system, two-dimensional (2D) 1H NMR SI was performed. TG 1H NMR chemical shift images were reconstructed from the frequency domain spectra by numerical integration. A statistically significant (P < .05) increase in TG signal intensity was demonstrated in the region at risk compared with the nonischemic control region. There was an intermediate quantity of TG in the infarct region. Biochemical determination of tissue TG content (milligrams per gram wet weight) in the control, at-risk, and infarct regions confirmed the 1H NMR measurements. Histological evaluation with oil red O staining also demonstrated graded TG accumulation in myocytes. The highest TG levels were found in the at-risk region and the lowest levels in the control region. CONCLUSIONS: By use of 2D 1H NMR SI, the present study confirms and extends previous work that demonstrates preferential accumulation of TG in the reversibly injured myocardium after 24 hours of coronary occlusion. This study provides an important step toward the clinical application of TG imaging. When TG imaging is ultimately possible, resultant data would have diagnostic, prognostic, and therapeutic implications.

Structural studies of NMR detected lipids in myocardial ischemia.

Lipid-induced abnormalities in myocardial function have been implicated in a number of ischemic events including the accumulation of lipids in human myocardium following myocardial infarction. Although animal models have shown the source of these lipids to be triglycerides, the specific species involved has not been identified. In order to better understand the mechanism(s) defining this lipid accumulation, it follows that the identification of the lipids involved may be important in achieving this aim. Therefore, this study examined the use of NMR probes for delineating the biochemical makeup of the increased 1H NMR observed lipid signal following myocardial infarction. Specifically, the present study demonstrated the utility of the spin-echo pulse sequence for the study of alterations in myocardial lipids following ischemic injury. Spin-echo spectra allowed the analysis of subsets of lipids within the large lipid pool inherent in most myocardium. The analyses of the chemical shifts of the lipid resonances provided a simple yet powerful means for deducing lipid class associated with the ischemic injury and suggested the species arises predominantly from saturated lipids. The examination of the CH2/CH3 NMR ratio provided additional information regarding the species involved, however, because the spin-echo technique was utilized, which may distort certain signal intensities, caution must be exercised in interpreting the specific species involved. With this in mind, a tentative assignment has been given to octanoic acid. Finally, a temperature dependence of the lipid signals was noted and determined to be unique for spin-echo lipid.(ABSTRACT TRUNCATED AT 250 WORDS)

Observation of cardiac lipids in humans by localized 1H magnetic resonance spectroscopic imaging.

Different approaches are being explored for the noninvasive observation of myocardial lipids in the human heart by in vivo 1H NMR spectroscopy. One approach is to measure cardiac lipids using a combination of volume selection and 2D gradient phase encoding. From these data sets lipid images can be reconstructed. By comparing these lipid images with 1H MR scout images, it is demonstrated that these signals represent epicardial and pericardial lipid. By selecting a smaller bar-shaped volume combined with 1D phase encoding or by using single volume techniques, it is possible to avoid most of the pericardial and epicardial lipid to obtain myocardial 1H NMR spectra of the human heart showing lipid signals, as well as trimethylamine and (phospho)creatine signals. These measurements demonstrate the feasibility of obtaining 1H NMR spectra of the human myocardium.

Relationship between shortening load, contractility, and myocardial energetics in intact dog.

A canine model was developed to estimate left ventricular wall stress, volumes, contractility, and high-energy phosphate metabolites without the need for major surgery. A percutaneously inserted catheter-tip manometer was used to record high-fidelity left ventricular pressure while gradient echo cinemagnetic resonance (cine-MR) imaging alternated with in vivo 31P-nuclear magnetic resonance (NMR) spectroscopy during pharmacological maneuvers to increase cardiac work. Left ventricular circumferential wall stress, volumes, maximum rate of pressure development (dP/dtmax), and the ratio of phosphocreatine (PCr) to gamma-ATP (PCr/gamma-ATP) were recorded sequentially during control 1, dobutamine infusion, control 2, angiotensin infusion, and control 3 in five anesthetized, closed-chest dogs. PCr/gamma-ATP did not change significantly during controls 1-3, angiotensin, and dobutamine infusion. Left ventricular peak positive dP/dt (+dP/dtmax) increased significantly during dobutamine (3,338 +/- 831 mmHg/s, P < 0.001) but was unchanged during angiotensin (1,818 +/- 317 mmHg/s) and controls 1-3 (1,915 +/- 434 vs. 1,808 +/- 478 vs. 1,859 +/- 414 mmHg/s). However, dobutamine decreased the total systolic stress integral (area under the wall stress-time relationship) and end-diastolic and end-systolic volumes, whereas angiotensin increased these parameters compared with control conditions. The unchanged PCr/gamma-ATP is in accord with the results from other open-chest surface coil 31P-NMR experiments in the normal heart. Our assessment of left ventricular functional parameters provides new information that complements these more invasive studies in which heart rate-pressure product was measured during increases in cardiac work.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo 31P nuclear magnetic resonance spectroscopy of human temporal lobe epilepsy.

We performed localized 31P nuclear magnetic resonance (NMR) 1H-image-guided in vivo spectroscopy to study regional high-energy phosphate levels in the brains of normal controls and in patients with intractable unilateral temporal lobe epilepsy. We did not observe differences in intracellular pH between controls and patients. The phosphocreatine/inorganic phosphate ratio was reduced by 50% in the epileptogenic temporal lobe compared with controls (p less than 0.005) and by 35% when compared with the unaffected contralateral temporal lobe (p less than 0.05). We did not observe differences in the ratio of phosphomonoesters to phosphodiesters between controls and patients. These findings suggest that in vivo 31P NMR spectroscopy yields a distinctive interictal metabolic profile in patients with intractable unilateral temporal lobe epilepsy and may permit noninvasive lateralizing evidence of the seizure focus.

Visualization of altered myocardial lipids by 1H NMR chemical-shift imaging following ischemic insult.

Acute myocardial infarction is associated with an accumulation of lipids. Spectroscopic and chemical-shift imaging strategies which can depict the spatial distribution of these chemical species are evolving. The present study was undertaken to test whether the Dixon method could detect spatially lipids known to accumulate in myocardium after an ischemic insult. Seven dogs underwent a 24-h coronary artery occlusion (LAD = 4, Cx = 3). Post mortem, hearts were removed and imaged ex vivo. Myocardial samples were also evaluated by high-resolution 1H NMR spectroscopy. Lipid images revealed regions of increased signal intensity, in the regions corresponding to the myocardial infarction, particularly in the periphery of the infarction. An increase in mobile lipids was observed by 1H NMR spectroscopy of myocardial samples with moderately reduced blood flow and corresponding to regions with increased signal intensity on the lipid image. This study shows that chemical-shift imaging may be useful for detecting alterations in myocardial lipid levels following an ischemic insult.

NMR spectroscopic assessment of altered myocardial lipid metabolism.

Proton nuclear magnetic resonance (NMR) spectroscopy of myocardial lipids can be used to monitor the metabolic activity of the heart. Alterations in lipid NMR signals occur with disorders such as myocardial ischemic events and postischemic dysfunction ("stunned myocardium"). Proton NMR-derived information may prove useful in evaluating the extent and stage of the ischemic injury and viability of the myocardium.

Demonstration of increased myocardial lipid with postischemic dysfunction ("myocardial stunning") by proton nuclear magnetic resonance spectroscopy.

Histopathologic studies have demonstrated accumulation of lipid droplets in myocardium subjected to greater than or equal to 6 h of ischemic insult. Proton nuclear magnetic resonance (NMR) spectroscopy can provide a noninvasive means to evaluate changes in tissue lipid and, potentially, to characterize the ischemic insult. To determine whether lipids accumulate with a brief ischemic insult, myocardial lipid content was evaluated by 1H NMR spectroscopy of ex-vivo samples from seven dogs in a model of postischemic dysfunction created by 15 min of left anterior descending coronary artery occlusion followed by 3 h of reperfusion. Regional myocardial function was assessed by measuring segment length shortening with use of a pair of ultrasonic crystals placed in the ischemic zone and in the control zone. During the occlusion, all dogs had significant ischemia of the occlusion zone as measured by radiolabeled microspheres (0.08 +/- 0.08 versus 0.88 +/- 0.09 ml/g per min for the control zone), and all dogs developed systolic stretching of the ischemic zone segment. Myocardial lipid content was significantly elevated in the samples from the coronary occlusion zone (p less than 0.02). The increase in lipid signal may result from the ischemia-induced decrease in beta oxidation and resultant accumulation of fatty acyl esters (for example, fatty acids, triglycerides and acylcarnitines). In conclusion, this study shows that myocardium subjected to a brief (approximately 15 min) coronary occlusion followed by 3 h of reperfusion demonstrates a significant increase in NMR-detectable lipid content.

Proton nuclear magnetic resonance relaxation times in severe myocardial ischemia.

Contrast produced by differences in regional proton relaxation times (T1 and T2) provides the potential to assess the extent of myocardial infarction using nuclear magnetic resonance (NMR) imaging. Previous laboratory studies have shown that longitudinal (T1) and transverse (T2) relaxation times are prolonged in acute myocardial infarction, and these prolongations have been attributed entirely to increases in tissue water content. The present study seeks to evaluate the relation between both T1 and T2 and regional myocardial perfusion and water content throughout a wide range of blood flow reduction. The left anterior descending coronary artery and collateral vessels supplying a region of the anterior wall of the left ventricle were ligated in 10 dogs for 4 hours until they were killed. Both water proton and bulk proton relaxation times of myocardial samples from ischemic and control zones were measured at 200 and 20 MHz, respectively. Regions of severe ischemia (flow less than 5% of control) demonstrated no significant alteration in T1 compared with nonischemic myocardium. Greatest T1 and T2 elevations were observed in moderately ischemic myocardium (flow = 5 to 50% of control). The water relaxation behavior differed with the severity of the flow reduction and was not totally dependent on changes in water content. These data suggest that relaxation time alterations are more complex than previously reported in myocardial ischemic insult. In the future, using T1 weighted imaging methods, myocardial ischemic insults associated with severe reductions in blood flow would be anticipated to demonstrate a doughnut pattern with an area of abnormal intensity in the peripheral zone surrounding a central ischemic zone with normal intensity.

Proton NMR spectroscopy in myocardial ischemic insult.

Proton NMR methods can monitor mobile lipids (e.g., fatty acids and glycerides) in intact tissue. Lipids play a major role in cardiac energy production, and elevated levels of myocardial lipids have been reported following an ischemic insult. The present study examines the potential of high-resolution 1H NMR spectroscopy to monitor lipid alterations 24 h following coronary occlusion in dogs, and to correlate these finds with regional myocardial blood flow (RMBF) measured by radiolabeled microspheres. The dogs were killed, and samples of excised myocardium were studied by high-resolution 1H NMR spectroscopic analysis using solvent suppression in combination with the Hahn spin-echo pulse sequence. Mobile lipids levels in myocardium with moderate blood flow reduction (28.6 +/- 7, integral values, arbitrary units; flow 5-50% of control) were significantly elevated compared to the mobile lipid levels in control myocardium (5.3 +/- 8, P less than 0.001) and in myocardium with severe flow reduction (7.2 +/- 10, P less than 0.001; flow less than 5% of control). The mobile lipids in myocardium with severe flow reduction were not elevated significantly relative to control tissue. As anticipated, depression in the level of creatine paralleled the microsphere determined degree of ischemia, i.e., compared to control (9.0 +/- 3); creatine levels were moderately decreased with flows 5-50% of control (5.5 +/- 4, P less than 0.001) and markedly decreased with flows less than 5% of control (1.0 +/- 2, P less than 0.001). This study suggests that high-resolution 1H NMR spectroscopy may be used to evaluate alterations in myocardial lipid levels following an ischemic insult.(ABSTRACT TRUNCATED AT 250 WORDS)

Monitoring the bioenergetics of cardiac allograft rejection using in vivo P-31 nuclear magnetic resonance spectroscopy.

Monitoring human cardiac allograft rejection is currently accomplished by endomyocardial biopsy. Available noninvasive methods for identifying rejection have lacked the necessary sensitivity or specificity, or both, for routine clinical application. In vivo phosphorus-31 (P-31) nuclear magnetic resonance (NMR) spectroscopy has been used for monitoring phosphorus metabolism in both animal models and humans. In the present study this technique was employed as a noninvasive means to assess the bioenergetic processes that occur during cardiac allograft rejection in a rat model. Brown Norway rat hearts were transplanted subcutaneously into the anterior region of the neck of Lewis rat recipients (allografts). Control isografts employed Lewis donors and recipients. Phosphocreatine to inorganic phosphate (PCr/Pi), phosphocreatine to beta-adenosine triphosphate (PCr/ATP beta), beta-adenosine triphosphate to inorganic phosphate (ATP beta/Pi) ratios and pH of the transplanted hearts were monitored using surface coil P-31 NMR spectroscopy (at 4.7 tesla) daily for 7 days. To allow recovery from the compromise induced by the surgical procedure, the measurements obtained on day 2 were taken as a baseline. PCr/Pi was unchanged or increased in the isografts but decreased continually in allografts, with the difference becoming significant by day 4 when compared with levels in day 2 allografts (p less than 0.005) and by day 3 when compared with levels in the isograft group (p less than 0.05). PCr/ATP beta in isografts did not change throughout the study; however, allografts demonstrated a significant decrease as early as day 3 (p less than 0.01), although a significant difference between isografts and allografts did not become manifest until day 4 (p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Potential approaches to evaluating the cardiovascular system using NMR.

The current status and some of the future possibilities for nuclear magnetic resonance imaging of the cardiovascular system have been described. With many of these possibilities there is overlap with existing techniques. For example, functional analysis of the left ventricle can be obtained using either echocardiography or radionuclide techniques. With current instrumentation and current costs, these conventional techniques could frequently provide a more cost-effective approach for morphologic and functional assessment of the cardiovascular system. Nevertheless, because of the excellent resolution, the inherent contrast, the sensitivity to blood motion, the three-dimensional nature, and the lack of ionizing radiation, the cardiovascular morphologic imaging capabilities of NMR may provide justification for such applications. However, for NMR to achieve its most important status as a cardiovascular imaging technique, some of its unique possibilities will need to be developed. These include the ability to reproducibly depict the proximal coronary arteries, to define regional myocardial blood flow distribution, to evaluate regional high energy phosphate or other metabolic activity; and to characterize myocardial disease using proton T1 and T2 alterations.

Evaluation of the cardiovascular system using NMR.

The current status and suggestions of the future potential for nuclear magnetic resonance imaging of the cardiovascular system are described. With many of the potential applications, there is overlap with existing methods. For example, imaging of the left ventricle can be accomplished with either echocardiography or radionuclide techniques with adequate evaluation of the left ventricular function. At current costs these conventional techniques may provide a more cost-effective approach for morphologic and functional assessment of the cardiovascular system. For this type of imaging, the advantages of NMR include its excellent resolution, the inherent tissue contrast, the sensitivity to blood motion, the 3-dimensional measure, and the lack of ionizing radiation. Because of these, NMR could provide an important adjunct for evaluation of the cardiovascular system. However for NMR to achieve its full promise as a cardiovascular imaging technique, some of its unique potentials need to be developed. These include: the ability to reliably image at least the proximal coronary arteries, the ability to delineate regional myocardial blood flow distribution, the ability to evaluate regional metabolic activity such as high-energy phosphate metabolites, and the ability to characterize myocardial disease using proton T1 and T2 alterations.