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Nuclear Magnetic Resonance and Cardiovascular Surgery
Symposium on the Latest Advances in Cardiac Surgery
Nuclear Magnetic Resonance and Cardiovascular Surgery Glenn]. R. Whitman, M.D.,* and Alden H. Harken, M.D.t
Cardiac surgical diagnosis currently relies almost entirely on invasive angiography for delineation of cardiac and vascular anatomy and function. For the past few decades, this modality appropriately has been the gold standard for evaluation of cardiovascular anatomy. However, the risks inherent in arteriography are well known. 10 Recently, a totally noninvasive technique has evolved that promises not only to elucidate cardiovascular anatomy but to reveal information about the metabolic state of the tissue being examined. This technique, known as nuclear magnetic resonance (NMR), provides images that are based not on electron density but on the magnetic properties of atoms such as hydrogen (H-1) and phosphorus (P-31). The last decade has produced numerous studies dealing with the application of NMR to physiology and medicine. NMR was first described in 1946 by Purcell22 at Harvard and Bloch 1 at Stanford. Only 6 years later, the first reports of NMR application to biology appeared, 27 and in 1955 data derived from studies of human tissue were published. 21 Recently developed methods for presenting NMR in pictorial form 17 now provide physicians with a powerful diagnostic tool. This article examines the principles and ·applications of NMR as it pertains to cardiovascular disease and pathophysiology. PRINCIPLES AND APPLICATIONS OF NUCLEAR MAGNETIC RESONANCE IMAGING Any atom with an odd atomic mass (protons plus neutrons) behaves like a spinning electric charge. This atom can then be thought of as a tiny bar magnet with a north and a south pole. When exposed to a strong external magnetic field (a typical NMR field is 100,000 times the strength of the
*Resident, Department of Surgery, University of Colorado Health Sciences Center, Denver, Colorado tProfessor and Chairman, Department of Surgery, University of Colorado Health Sciences Center, Denver, Colorado
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earth's magnetic field), these polar nuclei are aligned along its axis (Fig. lA). To image, the NMR magnet pulses a perpendicular magnetic field for an instant (approximately 50 to 100 microseconds), tipping the nuclei out of their original orientation (Fig. lB). When this perpendicular pulse ceases, each proton is exposed again only to the original external magnetic field. In realigning itself with this field, the nucleus does not simply flip back up, but rather spirals or precesses as it becomes reoriented (Fig. lC). This precessional motion is responsible for the emission of a radiofrequency signal. After each perpendicular pulse, the NMR unit "listens" for this signal and then pulses again. In this fashion, data are gathered for computer processing and image production. As with computed tomographic scanning, NMR images are a series of sequential slices obtained through the patient; however, unlike the CT scan, magnetic resonance images can be generated in coronal and sagittal, as well as transverse planes. Image contrast depends upon the strength of the radio-frequency signal received by the NMR unit, which in turn depends upon the tissue concentration of protons, local magnetic properties of the tissue, and blood flow. A thorough explanation of the production of image contrast from blood flow is complicated15• 16 and beyond the scope of this article; however, the following is a simple explanation for the phenomenon. When the perpendicular field is pulsed into the patient, the protons in both the blood and blood vessel walls are tipped perpendicular to the external magnetic field (Fig. 2A and B). As blood and vessel-wall protons realign in the external field's axis, the tipped blood protons are washed downstream (Fig. 2C). Thus, there is a strong signal returned from the vessel wall and surrounding tissues and a much reduced signal from the blood. Indeed, the greater the blood flow, the better the resolution at the meeting point of the blood and its surrounding surfaces. Unlike x-ray films, which perceive blood and A
B
c
Figure 1. The NMR experiment. a, When subjected to a magnetic field B0 , the nuclear spins of atoms with odd atomic mass align with the magnetic field, producing a net magnetization vector M0. b, M0, the vector of aligned spins, is displaced 90° by a radio frequency pulse B1. c, Once the RF is discontinued, an RF signal is emitted (output) as the spins once again realign with the main magnetic field.
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c::::====::> polar atoms In N-S axis
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"tipped" blood protons waehecl downati8MI
Figure 2. Vascular imaging using NMR. a, This figure represents a transverse slice of aorta being scanned by the NMR magnet. Note the uniform alignment of vessel wall and blood protons with the north-south external magnetic field. b, The short, perpendicular pulse tips all protons into the east-west plane. c, During signal acquisition, the vessel wall protons "precess" as they realign themselves, generating a strong signal. The perpendicularly oriented intravascular protons, however, are being washed downstream during this time and are being replaced by north-south-oriented protons. These new protons are already aligned with the external field and therefore do not "precess" or give off an NMR signal. Thus, the phenomenon of flow itself produces the contrast seen between intravascular blood and the surrounding stationary tissue (see Figs. 3 to 5).
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surrounding tissue as having similar density, NMR is uniquely capable of separating flowing blood from stationary tissue. Anatomic Imaging Until recently, magnetic resonar.ce imaging within the thorax was limited by movement associated with respiration and the cardiac cycle. Regardless, left veutricular aneurysms, pericardia} effusions, and even bicuspid aortic valves were well delineated. In the case of cardiac structure, detail has been improved orders of magnitude by "gating" scans to the electrocardiogram. In this method, the acquisition of the magnetic resonance signal is timed to the electrocardiogram. Therefore, the signal can be received at the same point in the cardiac cycle, when the heart is in the same spatial position. This technique has improved anatomic resolution vastly. Recently, Higgins et al. displayed spectacular results from "gating" signal acquisition to the electrocardiogram. 11 They evaluated 32 patients with congenital heart disease who had been studied also by angiography or echocardiography or both. Using multisectional transverse images extending throughout the entire thorax, they correctly defined 11 great vessel abnormalities along with their ventricular relationships, 10 of 11 ventricular septal defects, and 6 of 6 atrial septal defects. Anomalous pulmonary venous and systemic connections were identified. Furthermore, they showed excellent ventricular anatomy in transposition and the Eisenmenger syndrome. Left Ventricular Function For those physicians interested in cardiac function, it was immediately apparent that one could use "electrocardiogram-gating" to stop the heart at any point in its cycle. By generating stop-action images in diastole and systole (Fig. 3), a noninvasive, anatomic assessment of ventricular ejection fraction could be obtained. In fact, this method may be more accurate than the computed ejection fractions obtained from angiographic ventriculography as derived from multiple assumptions using the planimetric method. Furthermore, coronal, sagittal, and transverse anatomic cardiac-gated images may give vastly superior ventricular wall-motion analyses.
Figure 3. Magnetic resonance image along the long axis of the left ventricle. a, The heart in diastole (arrow is within left atrium). Note the pulmonary artery above the arrow as well as the aortic arch with the takeoff of the left common carotid. b, The heart in systole. Note the thickened ventricular wall as well as a closed mitral valve.
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Vascular Imaging Frequently, the cardiac surgeon is faced with superior mediastinal abnormalities that are difficult to assess with even the most sophisticated techniques. Conventional radiographic examination may be nondiagnostic. 8• 25 Contrast-enhanced computerized axial tomography may distinguish vascular from neoplastic pathology in this area, but it is not always successful. As explained previously, vascular magnetic resonance images can utilize the fact that protons in flowing blood may have left the area being scanned and thus emit virtually no signal. The stationary vascular wall is easily seen in juxtaposition to this blood, and excellent vascular image resolution results (Fig. 4). As with mediastinal vessels, aortic anatomy can be assessed similarly (Fig. 5). Blood Flow In the past, images of blood flow have been generated by the distribution of injected radiopaque contrast materials. Measuring dilution of the electron-dense dyes is very indirect; however, magnetic resonance imaging allows quantification of proton movement within the blood itself. Previously, magnetic resonance attempts to evaluate blood flow quantitatively have faced problems resulting from limited variations in pulse sequences used to generate images, as well as the pulsatile nature of the blood flow itself. Measurements of blood flow based on the decrease in signal intensity caused by fresh blood filling the image plane (see Fig. 2) are limited to
Figure 4. a, Chest radiograph. widened superior mediastinum (arrow). b, Unenhanced CT. Note that the etiology of the increased soft-tissue density cannot be determined (arrow). c, NMR image. Only low-signal tubular structures representing vessels are seen within the normal high-signal right superior mediastinal fat (curved arrow). (CT and NMR images courtesy of Dr. H. Kresse! and Dr. W. Gefter, Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania.)
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Figure 5. Magnetic resonance images designed to delineate aortic anatomy. a, The aorta at the level of its entrance into the abdomen (arrowhead). ote the excellent resolution of the hepatic venous system. b, The abdominal aorta (arrowhead) in a patient with severe atherosclerosis with luminal narrowing. c, A coronal section displaying a longitudinal slice of the abdominal aorta (long arrow). One can see the left pulmonary artery (short arrow) as well as the aortic arch (arrowhead) in cross section. ( MR images courtesy of Prof. H. Kutzim and Dr. U. Buschcieweke, uclear Medizinische Abteilung, University of Cologne, West Germany. )
velocities less than 10 em per sec. 16• 28 Recently, Feinberg et al. have described a new technique: Signals from blood exposed to a perpendicular excitation pulse are picked up by a spatially separate planar volume located distally along the artery in question. 4 Thus, by varying either the time of signal acquisition or the distance between the excitation volume and the acquisition volume, signal intensity within the vessel can be maximized. In so doing, blood velocities up to 21 em per sec can be quantified. These same authors have been able to demonstrate clearly concentric rings of similar velocity within the carotid artery, and they have shown blood velocity to be proportional to radial position within the artery. The technology for assessing luminal patency and flow within vessels as small as coronary arteries has already been developed. The ability to evaluate the effect of atherosclerotic plaques and stenosis on blood velocity as well as turbulence may soon be available clinically. Relaxation Times and the Imaging of Myocardial Ischemia After the nucleus is tipped out of its original axis (the z axis) by the perpendicular magnetic excitation pulse, BI> two relaxation times describe the return of M0 , the net magnetization vector of the aligned nuclei, to equilibrium along the z axis: Tt. the longitudinal (or "spin-lattice") relaxation time, and T 2 , the transverse (or "spin-spin") relaxation time. T 1 characterizes the time required for M0 to return to equilibrium in the z axis after the excitation pulse, that is, to point once again in the z direction as determined by the external magnetic field, B0 • One can regard T 1 as a mea-
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sure of the ease with which the tipped nuclei can give up their absorbed energy (which resulted from the excitation pulse) to the nuclei comprising the surrounding lattice structure. In general, typical T 1 values range from 0.1 to 3 sec. The spin-spin relaxation time, T2 , should be viewed as that time constant that describes the decay of the magnetic vector, M,,y, which lies in the x,y plane. Mter the excitation pulse, each nucleus feels not only B0 , the external static field that causes realignment, but also local fields caused by nearby nuclei with magnetic properties. Therefore, each precessing nucleus encounters a slightly different magnetic field and consequently precesses at a slightly different frequency. One can think of the M0 vector as "fanning out" while the spinning nuclei go out of phase with each other. M,,Y' the projection of the vector M0 onto the x,y plane, therefore will get smaller and smaller as M0 fans out until M,,y disappears, at which time the signal is no longer picked up by the NMR unit. T 2 values are generally less than 100 msec. Clinically, different pulsing techniques can emphasize different relaxation times in neighboring tissue. For example, after a 90 degree perpendicular pulse, approximately 95 per cent of the original magnetization vector will recover within three T 1 times. If another 90 degree radio-frequency pulse is applied after M0 has had time to relax fully, the resultant NMR signal is of maximum intensity. However, if an excitation pulse is applied before full spin-lattice relaxation has occurred, the NMR signal following the second pulse will be reduced. This fact may be used in imaging to em.vhasize T 1 variations in tissue. In a similar fashion, T 2 variations may be emphasized to generate contrast between adjacent tissues. Investigators have shown that during ischemia myocardial proton relax~tion parameters change. 2 · 12 Demonstrated in ex vivo tissue biopsies, prolongation of T 1 and T 2 relaxation times have ranged from 3 to 25 per cent, sometimes even less for acutely ischemic tissue. 19• 30 As a result, clinical in vivo NMR imaging may be too ins.ensitive, despite T 1 or T 2 weighted images, to detect ischemic or infarcted myocardium. Recently, intravenous administration of paramagnetic ions (atoms with unpaired electrons) has been used to accentuate the difference in relaxation times induced by ischemia. These ions distribute themselves with flow and thereby diminish the T 1 and T 2 values of normal, perfused tissue to a much greater degree than ischemic tissue. In this way, T 1 and T 2 differences between normal and ischemic tissue may be increased as much as 100 per cent. Currently being evaluated in European clinical trials, these benign contrast agents combined with gated myocardial imaging may provide exact delineation of ischemic or infarcted myocardium. Phosphorus-31 Because of the high concentration (100 molar) and natural abundance (99. 98 per cent) of protons, these particles are the most easily seen nuclei of biologic interest. Protons are the perfect nuclei for investigation in large clinically affordable magnets with field strengths of 0.1 to 1. 5 tesla. Recently, however, detection of the Phosphorus-31 (P-31) nucleus has been a major focus of in vivo NMR experiments. Because of the low concentrations of phosphorus-containing molecules (approximately 1 to 10 millimolar),
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stronger magnets (1.5 to 8 tesla) must be used to pick up a signal. P-31 NMR has not yet reached technologic sophistication in image production; thus, tissue scanning produces spectra and not pictures. The major signals in P-31 NMR of cardiac tissue are from the phosphates associated with adenosine triphosphate (three resonance peaks), adenosine diphosphate (two resonance peaks, both obscured by adenosine triphosphate), phosphocreatine, inorganic phosphate, and phosphorylated sugars (Fig. 6). The ability to follow changes noninvasively in adenosine triphosphate and phosphocreatine concentration permits P-31 NMR to study cellular energy metabolism. To date, the most complete and detailed studies have been reported in skeletal and cardiac muscle. 5-7• 9• 14 pH The measurement of intracellular pH classically has involved either the use of pH-sensitive microelectrodes 29 or an analysis of the distribution
PCr
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NMR PHOSPHATE PROFLE Figure 6. Phosphorus-31 nuclear magnetic resonance spectrum obtained from a rabbit neart. The spectrum was obtained over 5 minutes with 300 pulses. Labeled peaks from left to right are P;, inorganic phosphate; PCr, phosphocreatine; and the-y, a, and ~ phosphates of ATP, adenosine triphosphate. By measuring changes in the areas under the peaks, one can follow relative changes in the concentrations of the respective phosphorylated compounds. The bottom line, is a calibration scale, with PCr designated as the reference frequency set to 0 ppm. Each tick represents 5 ppm.
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of weak acids or bases. 3 Since the first report by Moon and Richards in 1973, 20 P-31 NMR has proved to be a useful noninvasive alternative. The resonant frequency of inorganic phosphate (Pi) is exquisitely sensitive to pH changes in ranges near the pK values of its protons. In fact, one can assign phosphate resonant frequency changes to the pH of the tissue being scanned. Roos and Boren have suggested that the extraordinary precision of this method for intact tissue makes it superior to the microelectrode or weak base distribution method for measuring pH. 23 This capability of P-31 NMR has enabled the intracellular pH of cardiac muscle to be determined. Investigations have shown that after the onset of global ischemia at 37°C, cardiac tissue pH falls. 5• 6· 13• 14• 33 Furthermore, by following the drop in pH, P-31 NMR can be used as a noninvasive measure of anaerobic glycolysis. Flaherty and coworkers have analyzed the effect of hypothermia and cardioplegia in the isolated rabbit heart. 5 After 60 minutes of global ischemia in hypothermic hearts (24°C), they found the pH to be 6.09 + 0.12, as compared with 6.79 + 0.03 for hypothermic hearts given three doses of potassium cardioplegia (10°C). Thus, there is evidence that hypothermia with dosed cardioplegia decreases the energy requirements of the cell, as indicated by the higher pH reflecting a lower anaerobic glycolytic rate. Interestingly, if one examines the linewidth of the inorganic phosphate signal, it invariably is wider than that for phosphocreatine. Linewidth is defined as the width of a peak at one-half its height. Experiments have shown that the peak generated by inorganic phosphate is really a summation of several peaks, 26 each slightly staggered and thus reflecting inhomogeneities in intracellular pH. Early in the genesis of P-31 evaluation of myocardial ischemia, Hollis and coworkers employed this concept to evaluate myocardial blood supplyY They scanned an isolated perfused rabbit heart subjected to regional ischemia by ligation of the left anterior descending artery. Before ligation they found only one inorganic phosphate peak, indicative of a pH of 7. 2. After ligation they found two inorganic phosphate peaks, one corresponding to a pH of 1:2 and the other corresponding to a pH of 6.6. Theoretically, the more acidic peak was generated by the phosphate in the ischemic myocardium. The proportion of acidic phosphorus relative to the total phosphorus concentration could indicate the amount of myocardium at risk for infarction. Tissue Metabolism Whitman and coworkers have followed the time course of high energy phosphate changes during global ischemia and reperfusion in the guinea pig and rabbit heart and have correlated this with recovery of left ventricular function. 32 It appears that phosphocreatine falls rapidly in ischemic tissue, buffering the fall in adenosine triphosphate. As a result, adenosine triphosphate falls more slowly. Upon reperfusion, the final recovery of left ventricular function is linearly related to end-ischemic adenosine triphosphate concentration. Flaherty and coworkers examined the efficacy of potassium cardioplegia in a similar model. 5 They showed adenosine triphosphate concentration to be higher after an ischemic insult if hypothermia and multiple-dose potassium cardioplegia were used. After
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45 minutes of reperfusion, they, too, found recovery of left ventricular function to be dependent upon the degree to which adenosine triphosphate was preserved during ischemia. Human Applications Applying the unique ability of NMR to follow metabolism of intact human tissue, Ross and coworkers investigated a case of suspected McArdle's syndrome. 24 This syndrome is characterized by a lack of glycogen phosphorylase activity in skeletal muscle. 18 Clinically, the diagnosis is suggested when exercise fails to produce lactic acid. After exercise, Ross and coworkers were able to demonstrate a larger decrease in phosphocreatine concentration in their patient over that of controls, but without a decrease in pH, noninvasively demonstrating abnormal absence of lactate production. 24 The authors speculated that NMR could be valuable both for diagnosis and for monitoring the response to therapy of peripheral muscle disorders. Recently, Whitman et al. have applied P-31 NMR to the evaluation of a pediatric cardiomyopathy. 31 An 8-month-old girl presented with an undiagnosed cardiomyopathy with massive cardiomegaly and congestive heart failure. P-31 NMR evaluation demonstrated a metabolic disorder in both her myocardium and skeletal muscle. Manipulation of serum substrate availability revealed that intravenous glucose or oral carbohydrate loading raised her myocardial high-energy phosphate content significantly. This study demonstrates the feasibility of using P-31 NMR to evaluate the biochemistry of the human myocardium in vivo and to diagnose a metabolic abnormality. Furthermore, it provides an avenue for the application ofP-31 NMR to optimize therapy for human disease. CONCLUSION Nuclear magnetic resonance appears to add an entirely new dimension to present-day diagnostic modalities. Because of its ability to distinguish stationary tissue from flowing blood, clinically available noninvasive proton imaging may produce anatomic detail as accurate as angiography for the evaluation of myocardial function and anatomy. Although specific atherosclerotic lesions may not be discernible, the patency of native vessels, as well as saphenous vein bypass grafts, should be easily discernible and NMR may become the procedure of choice for evaluation of patients who have had myocardial revascularization or percutaneous coronary dilatation. Similarly, NMR lends itself perfectly to diagnosis of peripheral vascular lesions, again with the added safety of a noninvasive procedure. Certainly one of the most fascinating capabilities of NMR is its ability to evaluate muscle bioenergetics, that is, to assay tissue oxygen supply and demand noninvasively. Although magnets capable of imaging phosphocreatine or adenosine triphosphate content are expensive, experimental, and in the early stages of development, it is through just such efforts that medicine truly may be able to understand and advance its treatment of congenital and ischemic heart disease.
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REFERENCES l. Bloch, F.: Nuclear induction. Physiol. Rev., 70:46~74, 1946. 2. Brady, T. J., Goldman, M. R., Pykett, I. L., et al.: Proton nuclear magnetic resonance imaging of regionally ischemic canine hearts: Effect of paramagnetic signal enhancement. Radiology, 144:343-347, 1982. 3. Cohen, R. D., and lies, R. A.: Intracellular pH: Measurement, control and metabolic interrelationships. CRC Crit. Rev. Clin. Lab. Sci., 6:101-143, 1975. 4. Feinberg, D. A., Crooks, L., Hoenninger, J., III, et al.: Pulsatile blood velocity in human arteries displayed by magnetic resonance imaging. Radiology, 153:177-180, 1984. 5. Flaherty, J. T., Weisfeldt, M. L., Buckley, B. H., et al.: Mechanism of ischemic myocardial cell damage assessed by P-31 nuclear magnetic resonance. Circulation, 65:561-571, 1982. 6. Gadian, D. G., Hoult, D. I., Radda, G. K., et al.: Phosphorous nuclear magnetic resonance studies on normoxic and ischemic cardiac tissue. Proc. Natl. Acad. Sci. U.S.A., 73:4446-4448, 1976. 7. Gadian, D. G., Radda, G. K., Richards, R. E., and Seeley, P. J.: P-31 NMR in living tissue: The road from a promising to an important tool in biology. In Shulman, R. G. (ed.): Biological Applications of Magnetic Resonance. New York, Academic Press, 1979. 8. Condos, B.: The roentgen image of the subclavian artery in the pulmonary apex. A. J. R., 86:1058-1062, 1961. 9. Grove, T. H., Ackerman, J. J. H., Radda, G. K., and Bowe, P. J.: Analysis of rat heart in vivo by phosphorous nuclear magnetic resonance. Proc. Nat!. Acad. Sci. U.S.A., 77:299-302, 1980. 10. Helle], S. J., Adams, D. F., and Abrams, H. L.: Complications of angiography. Radiology, 138:273-281, 1981. 11. Higgins, C. B., Byrd, B. F., III, and Farmer, D.: MR imaging of congenital heart disease. Radiology, 153(P):172, Abst. 517, 1984. 12. Higgins, C. B., Herfkens, R., Lipton, M. J., et al.: Nuclear magnetic resonance imaging of acute myocardial infarction in dogs: Alterations in magnetic resonance times. Am. J. Cardiol., 52:184-188, 1983. 13. Hollis, D. P., Nunnally, R. L., Jacobus, W. E., and Taylor, G. J.: Detection of regional ischemia in perfused beating hearts by phosphorous nuclear magnetic resonance. Biochem. Biophys. Res. Commun., 75:1086-1091, 1977. 14. Jacobus, W. E., and Weisfeldt, M. L.: Intracellular pH and myocardial function in normal and ischemic hearts: A P-31 NMR Study (abstract). Biophys. J., 33:33, 1981. 15. Kaufman, L., Crooks, L. E., Sheldon, P., el al.: The potential impact of nuclear magnetic resonance imaging on cardiovascular diagnosis. Circulation, 67:251-257, 1983. 16. Kaufman, L., Crooks, L. E., Sheldon, P. E., et al.: Evaluation of NMR imaging for detection and quantification of obstructions in vessels. Invest. Radio!., 17:554-560, 1982. 17. Lauterbor, P. L.: Image formation by induced local interactions: Examples employing nuclear magnetic resonance. Nature, 242:190-191, 1973. 18. McArdle, B.: Myopathy due to a defect in muscle glycogen breakdown. Clin. Sci., 10:13-35, 1951. 19. McNamara, M. T., Higgins, C. B., Ehman, R. L., et al.: Acute myocardial ischemia: Magnetic resonance contrast enhancement with gadolinium-DPTA. Radiology, 153:157-163, 1984. 20. Moon, R. B., and Richards, J. H.: Determination of intracellular pH by P-31 nuclear magnetic resonance. J. Bioi. Chern., 248:7276-7278, 1973. 21. Odeblad, E., Bhar, B. N., and Lindstrom, G.: Proton magnetic resonance of human red blood cells in heavy water exchange experiments. Arch. Biochem. Biophys., 63:221, 1956. 22. Purcell, E. M., Towney, H. C., and Pound, R. J.: Resonance absorption by nuclear magnetic moments in a solid. Phys. Rev., 69:37-38, 1946. 23. Roos, A., and Boron, W. F.: Intracellular pH. Physiol. Rev., 61:296, 1981. 24. Ross, B. D., Radda, G. K., Gadian, D. G., et al.: Examination of a case of suspected McArdle's syndrome by P-31 nuclear magnetic resonance. N. Engl. J. Med., 304:1338-1342, 1981.
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25. Sandler, C. M., Toombs, B. D., and Lester, R. G.: Buckling of the left common carotid artery simulating mediastinal neoplasm. A. J. R., 133:312-313, 1979. 26. Seeley, P. J., Busby, S. J. W., Gadian, D. G., et al.: A new approach to metabolite compartmentation in muscle. Biochem. Soc. Trans., 4:62-64, 1976. 27. Shaw, T. M., Ericksen, G. H., and Kunsman, C. H.: Proton magnetic resonance absorption and water content format of biological materials. Phys. Rev., 85:708, 1952. 28. Singer, J. R., and Crooks, L. E.: Nuclear magnetic resonance blood flow measurements in the human brain. Science, 221:654-656, 1983. 29. Waddell, W. J., and Bates, R. G.: Intracellular pH. Physiol. Rev., 49:285-329, 1969. 30. Wesbey, G. E., Higgins, C. B., McNamara, M. T., et al.: Effect of gadolinium-DTPA on the magnetic relaxation times of normal and infarcted myocardium. Radiology, 153:165-169, 1984. 31. Whitman, G. J. R., Chance, B., Clark, B. J., et al.: Use of P-31 NMR in diagnosis and therapy of a familial cardiomyopathy. J. Am. Coli. Cardiol., 5:745-749, 1985. 32. Whitman, G., Kieval, R., Seeholzer, S., et al.: Recovery of left ventricular function following graded cardiac ischemia as predicted by P-31 nuclear magnetic resonance of the postischemic myocardium. Surgery, 97:428-435, 1985. 33. Whitman, G., Kieval, R., Wetstein, L., et al.: The relationship between global myocardial ischemia, left ventricular function, myocardial redox state, and high energy phosphate profile. J. Surg. Res., 35:332-339, 1983.
ct. Alden H.
Harken, M.D. Department of Surgery University of Colorado Health Sciences Center 4200 East Ninth Avenue Denver, Colorado 80262
Nuclear Magnetic Resonance and Cardiovascular Surgery Glenn]. R. Whitman, M.D.,* and Alden H. Harken, M.D.t
Cardiac surgical diagnosis currently relies almost entirely on invasive angiography for delineation of cardiac and vascular anatomy and function. For the past few decades, this modality appropriately has been the gold standard for evaluation of cardiovascular anatomy. However, the risks inherent in arteriography are well known. 10 Recently, a totally noninvasive technique has evolved that promises not only to elucidate cardiovascular anatomy but to reveal information about the metabolic state of the tissue being examined. This technique, known as nuclear magnetic resonance (NMR), provides images that are based not on electron density but on the magnetic properties of atoms such as hydrogen (H-1) and phosphorus (P-31). The last decade has produced numerous studies dealing with the application of NMR to physiology and medicine. NMR was first described in 1946 by Purcell22 at Harvard and Bloch 1 at Stanford. Only 6 years later, the first reports of NMR application to biology appeared, 27 and in 1955 data derived from studies of human tissue were published. 21 Recently developed methods for presenting NMR in pictorial form 17 now provide physicians with a powerful diagnostic tool. This article examines the principles and ·applications of NMR as it pertains to cardiovascular disease and pathophysiology. PRINCIPLES AND APPLICATIONS OF NUCLEAR MAGNETIC RESONANCE IMAGING Any atom with an odd atomic mass (protons plus neutrons) behaves like a spinning electric charge. This atom can then be thought of as a tiny bar magnet with a north and a south pole. When exposed to a strong external magnetic field (a typical NMR field is 100,000 times the strength of the
*Resident, Department of Surgery, University of Colorado Health Sciences Center, Denver, Colorado tProfessor and Chairman, Department of Surgery, University of Colorado Health Sciences Center, Denver, Colorado
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earth's magnetic field), these polar nuclei are aligned along its axis (Fig. lA). To image, the NMR magnet pulses a perpendicular magnetic field for an instant (approximately 50 to 100 microseconds), tipping the nuclei out of their original orientation (Fig. lB). When this perpendicular pulse ceases, each proton is exposed again only to the original external magnetic field. In realigning itself with this field, the nucleus does not simply flip back up, but rather spirals or precesses as it becomes reoriented (Fig. lC). This precessional motion is responsible for the emission of a radiofrequency signal. After each perpendicular pulse, the NMR unit "listens" for this signal and then pulses again. In this fashion, data are gathered for computer processing and image production. As with computed tomographic scanning, NMR images are a series of sequential slices obtained through the patient; however, unlike the CT scan, magnetic resonance images can be generated in coronal and sagittal, as well as transverse planes. Image contrast depends upon the strength of the radio-frequency signal received by the NMR unit, which in turn depends upon the tissue concentration of protons, local magnetic properties of the tissue, and blood flow. A thorough explanation of the production of image contrast from blood flow is complicated15• 16 and beyond the scope of this article; however, the following is a simple explanation for the phenomenon. When the perpendicular field is pulsed into the patient, the protons in both the blood and blood vessel walls are tipped perpendicular to the external magnetic field (Fig. 2A and B). As blood and vessel-wall protons realign in the external field's axis, the tipped blood protons are washed downstream (Fig. 2C). Thus, there is a strong signal returned from the vessel wall and surrounding tissues and a much reduced signal from the blood. Indeed, the greater the blood flow, the better the resolution at the meeting point of the blood and its surrounding surfaces. Unlike x-ray films, which perceive blood and A
B
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Figure 1. The NMR experiment. a, When subjected to a magnetic field B0 , the nuclear spins of atoms with odd atomic mass align with the magnetic field, producing a net magnetization vector M0. b, M0, the vector of aligned spins, is displaced 90° by a radio frequency pulse B1. c, Once the RF is discontinued, an RF signal is emitted (output) as the spins once again realign with the main magnetic field.
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Figure 2. Vascular imaging using NMR. a, This figure represents a transverse slice of aorta being scanned by the NMR magnet. Note the uniform alignment of vessel wall and blood protons with the north-south external magnetic field. b, The short, perpendicular pulse tips all protons into the east-west plane. c, During signal acquisition, the vessel wall protons "precess" as they realign themselves, generating a strong signal. The perpendicularly oriented intravascular protons, however, are being washed downstream during this time and are being replaced by north-south-oriented protons. These new protons are already aligned with the external field and therefore do not "precess" or give off an NMR signal. Thus, the phenomenon of flow itself produces the contrast seen between intravascular blood and the surrounding stationary tissue (see Figs. 3 to 5).
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surrounding tissue as having similar density, NMR is uniquely capable of separating flowing blood from stationary tissue. Anatomic Imaging Until recently, magnetic resonar.ce imaging within the thorax was limited by movement associated with respiration and the cardiac cycle. Regardless, left veutricular aneurysms, pericardia} effusions, and even bicuspid aortic valves were well delineated. In the case of cardiac structure, detail has been improved orders of magnitude by "gating" scans to the electrocardiogram. In this method, the acquisition of the magnetic resonance signal is timed to the electrocardiogram. Therefore, the signal can be received at the same point in the cardiac cycle, when the heart is in the same spatial position. This technique has improved anatomic resolution vastly. Recently, Higgins et al. displayed spectacular results from "gating" signal acquisition to the electrocardiogram. 11 They evaluated 32 patients with congenital heart disease who had been studied also by angiography or echocardiography or both. Using multisectional transverse images extending throughout the entire thorax, they correctly defined 11 great vessel abnormalities along with their ventricular relationships, 10 of 11 ventricular septal defects, and 6 of 6 atrial septal defects. Anomalous pulmonary venous and systemic connections were identified. Furthermore, they showed excellent ventricular anatomy in transposition and the Eisenmenger syndrome. Left Ventricular Function For those physicians interested in cardiac function, it was immediately apparent that one could use "electrocardiogram-gating" to stop the heart at any point in its cycle. By generating stop-action images in diastole and systole (Fig. 3), a noninvasive, anatomic assessment of ventricular ejection fraction could be obtained. In fact, this method may be more accurate than the computed ejection fractions obtained from angiographic ventriculography as derived from multiple assumptions using the planimetric method. Furthermore, coronal, sagittal, and transverse anatomic cardiac-gated images may give vastly superior ventricular wall-motion analyses.
Figure 3. Magnetic resonance image along the long axis of the left ventricle. a, The heart in diastole (arrow is within left atrium). Note the pulmonary artery above the arrow as well as the aortic arch with the takeoff of the left common carotid. b, The heart in systole. Note the thickened ventricular wall as well as a closed mitral valve.
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Vascular Imaging Frequently, the cardiac surgeon is faced with superior mediastinal abnormalities that are difficult to assess with even the most sophisticated techniques. Conventional radiographic examination may be nondiagnostic. 8• 25 Contrast-enhanced computerized axial tomography may distinguish vascular from neoplastic pathology in this area, but it is not always successful. As explained previously, vascular magnetic resonance images can utilize the fact that protons in flowing blood may have left the area being scanned and thus emit virtually no signal. The stationary vascular wall is easily seen in juxtaposition to this blood, and excellent vascular image resolution results (Fig. 4). As with mediastinal vessels, aortic anatomy can be assessed similarly (Fig. 5). Blood Flow In the past, images of blood flow have been generated by the distribution of injected radiopaque contrast materials. Measuring dilution of the electron-dense dyes is very indirect; however, magnetic resonance imaging allows quantification of proton movement within the blood itself. Previously, magnetic resonance attempts to evaluate blood flow quantitatively have faced problems resulting from limited variations in pulse sequences used to generate images, as well as the pulsatile nature of the blood flow itself. Measurements of blood flow based on the decrease in signal intensity caused by fresh blood filling the image plane (see Fig. 2) are limited to
Figure 4. a, Chest radiograph. widened superior mediastinum (arrow). b, Unenhanced CT. Note that the etiology of the increased soft-tissue density cannot be determined (arrow). c, NMR image. Only low-signal tubular structures representing vessels are seen within the normal high-signal right superior mediastinal fat (curved arrow). (CT and NMR images courtesy of Dr. H. Kresse! and Dr. W. Gefter, Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania.)
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Figure 5. Magnetic resonance images designed to delineate aortic anatomy. a, The aorta at the level of its entrance into the abdomen (arrowhead). ote the excellent resolution of the hepatic venous system. b, The abdominal aorta (arrowhead) in a patient with severe atherosclerosis with luminal narrowing. c, A coronal section displaying a longitudinal slice of the abdominal aorta (long arrow). One can see the left pulmonary artery (short arrow) as well as the aortic arch (arrowhead) in cross section. ( MR images courtesy of Prof. H. Kutzim and Dr. U. Buschcieweke, uclear Medizinische Abteilung, University of Cologne, West Germany. )
velocities less than 10 em per sec. 16• 28 Recently, Feinberg et al. have described a new technique: Signals from blood exposed to a perpendicular excitation pulse are picked up by a spatially separate planar volume located distally along the artery in question. 4 Thus, by varying either the time of signal acquisition or the distance between the excitation volume and the acquisition volume, signal intensity within the vessel can be maximized. In so doing, blood velocities up to 21 em per sec can be quantified. These same authors have been able to demonstrate clearly concentric rings of similar velocity within the carotid artery, and they have shown blood velocity to be proportional to radial position within the artery. The technology for assessing luminal patency and flow within vessels as small as coronary arteries has already been developed. The ability to evaluate the effect of atherosclerotic plaques and stenosis on blood velocity as well as turbulence may soon be available clinically. Relaxation Times and the Imaging of Myocardial Ischemia After the nucleus is tipped out of its original axis (the z axis) by the perpendicular magnetic excitation pulse, BI> two relaxation times describe the return of M0 , the net magnetization vector of the aligned nuclei, to equilibrium along the z axis: Tt. the longitudinal (or "spin-lattice") relaxation time, and T 2 , the transverse (or "spin-spin") relaxation time. T 1 characterizes the time required for M0 to return to equilibrium in the z axis after the excitation pulse, that is, to point once again in the z direction as determined by the external magnetic field, B0 • One can regard T 1 as a mea-
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sure of the ease with which the tipped nuclei can give up their absorbed energy (which resulted from the excitation pulse) to the nuclei comprising the surrounding lattice structure. In general, typical T 1 values range from 0.1 to 3 sec. The spin-spin relaxation time, T2 , should be viewed as that time constant that describes the decay of the magnetic vector, M,,y, which lies in the x,y plane. Mter the excitation pulse, each nucleus feels not only B0 , the external static field that causes realignment, but also local fields caused by nearby nuclei with magnetic properties. Therefore, each precessing nucleus encounters a slightly different magnetic field and consequently precesses at a slightly different frequency. One can think of the M0 vector as "fanning out" while the spinning nuclei go out of phase with each other. M,,Y' the projection of the vector M0 onto the x,y plane, therefore will get smaller and smaller as M0 fans out until M,,y disappears, at which time the signal is no longer picked up by the NMR unit. T 2 values are generally less than 100 msec. Clinically, different pulsing techniques can emphasize different relaxation times in neighboring tissue. For example, after a 90 degree perpendicular pulse, approximately 95 per cent of the original magnetization vector will recover within three T 1 times. If another 90 degree radio-frequency pulse is applied after M0 has had time to relax fully, the resultant NMR signal is of maximum intensity. However, if an excitation pulse is applied before full spin-lattice relaxation has occurred, the NMR signal following the second pulse will be reduced. This fact may be used in imaging to em.vhasize T 1 variations in tissue. In a similar fashion, T 2 variations may be emphasized to generate contrast between adjacent tissues. Investigators have shown that during ischemia myocardial proton relax~tion parameters change. 2 · 12 Demonstrated in ex vivo tissue biopsies, prolongation of T 1 and T 2 relaxation times have ranged from 3 to 25 per cent, sometimes even less for acutely ischemic tissue. 19• 30 As a result, clinical in vivo NMR imaging may be too ins.ensitive, despite T 1 or T 2 weighted images, to detect ischemic or infarcted myocardium. Recently, intravenous administration of paramagnetic ions (atoms with unpaired electrons) has been used to accentuate the difference in relaxation times induced by ischemia. These ions distribute themselves with flow and thereby diminish the T 1 and T 2 values of normal, perfused tissue to a much greater degree than ischemic tissue. In this way, T 1 and T 2 differences between normal and ischemic tissue may be increased as much as 100 per cent. Currently being evaluated in European clinical trials, these benign contrast agents combined with gated myocardial imaging may provide exact delineation of ischemic or infarcted myocardium. Phosphorus-31 Because of the high concentration (100 molar) and natural abundance (99. 98 per cent) of protons, these particles are the most easily seen nuclei of biologic interest. Protons are the perfect nuclei for investigation in large clinically affordable magnets with field strengths of 0.1 to 1. 5 tesla. Recently, however, detection of the Phosphorus-31 (P-31) nucleus has been a major focus of in vivo NMR experiments. Because of the low concentrations of phosphorus-containing molecules (approximately 1 to 10 millimolar),
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stronger magnets (1.5 to 8 tesla) must be used to pick up a signal. P-31 NMR has not yet reached technologic sophistication in image production; thus, tissue scanning produces spectra and not pictures. The major signals in P-31 NMR of cardiac tissue are from the phosphates associated with adenosine triphosphate (three resonance peaks), adenosine diphosphate (two resonance peaks, both obscured by adenosine triphosphate), phosphocreatine, inorganic phosphate, and phosphorylated sugars (Fig. 6). The ability to follow changes noninvasively in adenosine triphosphate and phosphocreatine concentration permits P-31 NMR to study cellular energy metabolism. To date, the most complete and detailed studies have been reported in skeletal and cardiac muscle. 5-7• 9• 14 pH The measurement of intracellular pH classically has involved either the use of pH-sensitive microelectrodes 29 or an analysis of the distribution
PCr
ATP
NMR PHOSPHATE PROFLE Figure 6. Phosphorus-31 nuclear magnetic resonance spectrum obtained from a rabbit neart. The spectrum was obtained over 5 minutes with 300 pulses. Labeled peaks from left to right are P;, inorganic phosphate; PCr, phosphocreatine; and the-y, a, and ~ phosphates of ATP, adenosine triphosphate. By measuring changes in the areas under the peaks, one can follow relative changes in the concentrations of the respective phosphorylated compounds. The bottom line, is a calibration scale, with PCr designated as the reference frequency set to 0 ppm. Each tick represents 5 ppm.
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of weak acids or bases. 3 Since the first report by Moon and Richards in 1973, 20 P-31 NMR has proved to be a useful noninvasive alternative. The resonant frequency of inorganic phosphate (Pi) is exquisitely sensitive to pH changes in ranges near the pK values of its protons. In fact, one can assign phosphate resonant frequency changes to the pH of the tissue being scanned. Roos and Boren have suggested that the extraordinary precision of this method for intact tissue makes it superior to the microelectrode or weak base distribution method for measuring pH. 23 This capability of P-31 NMR has enabled the intracellular pH of cardiac muscle to be determined. Investigations have shown that after the onset of global ischemia at 37°C, cardiac tissue pH falls. 5• 6· 13• 14• 33 Furthermore, by following the drop in pH, P-31 NMR can be used as a noninvasive measure of anaerobic glycolysis. Flaherty and coworkers have analyzed the effect of hypothermia and cardioplegia in the isolated rabbit heart. 5 After 60 minutes of global ischemia in hypothermic hearts (24°C), they found the pH to be 6.09 + 0.12, as compared with 6.79 + 0.03 for hypothermic hearts given three doses of potassium cardioplegia (10°C). Thus, there is evidence that hypothermia with dosed cardioplegia decreases the energy requirements of the cell, as indicated by the higher pH reflecting a lower anaerobic glycolytic rate. Interestingly, if one examines the linewidth of the inorganic phosphate signal, it invariably is wider than that for phosphocreatine. Linewidth is defined as the width of a peak at one-half its height. Experiments have shown that the peak generated by inorganic phosphate is really a summation of several peaks, 26 each slightly staggered and thus reflecting inhomogeneities in intracellular pH. Early in the genesis of P-31 evaluation of myocardial ischemia, Hollis and coworkers employed this concept to evaluate myocardial blood supplyY They scanned an isolated perfused rabbit heart subjected to regional ischemia by ligation of the left anterior descending artery. Before ligation they found only one inorganic phosphate peak, indicative of a pH of 7. 2. After ligation they found two inorganic phosphate peaks, one corresponding to a pH of 1:2 and the other corresponding to a pH of 6.6. Theoretically, the more acidic peak was generated by the phosphate in the ischemic myocardium. The proportion of acidic phosphorus relative to the total phosphorus concentration could indicate the amount of myocardium at risk for infarction. Tissue Metabolism Whitman and coworkers have followed the time course of high energy phosphate changes during global ischemia and reperfusion in the guinea pig and rabbit heart and have correlated this with recovery of left ventricular function. 32 It appears that phosphocreatine falls rapidly in ischemic tissue, buffering the fall in adenosine triphosphate. As a result, adenosine triphosphate falls more slowly. Upon reperfusion, the final recovery of left ventricular function is linearly related to end-ischemic adenosine triphosphate concentration. Flaherty and coworkers examined the efficacy of potassium cardioplegia in a similar model. 5 They showed adenosine triphosphate concentration to be higher after an ischemic insult if hypothermia and multiple-dose potassium cardioplegia were used. After
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45 minutes of reperfusion, they, too, found recovery of left ventricular function to be dependent upon the degree to which adenosine triphosphate was preserved during ischemia. Human Applications Applying the unique ability of NMR to follow metabolism of intact human tissue, Ross and coworkers investigated a case of suspected McArdle's syndrome. 24 This syndrome is characterized by a lack of glycogen phosphorylase activity in skeletal muscle. 18 Clinically, the diagnosis is suggested when exercise fails to produce lactic acid. After exercise, Ross and coworkers were able to demonstrate a larger decrease in phosphocreatine concentration in their patient over that of controls, but without a decrease in pH, noninvasively demonstrating abnormal absence of lactate production. 24 The authors speculated that NMR could be valuable both for diagnosis and for monitoring the response to therapy of peripheral muscle disorders. Recently, Whitman et al. have applied P-31 NMR to the evaluation of a pediatric cardiomyopathy. 31 An 8-month-old girl presented with an undiagnosed cardiomyopathy with massive cardiomegaly and congestive heart failure. P-31 NMR evaluation demonstrated a metabolic disorder in both her myocardium and skeletal muscle. Manipulation of serum substrate availability revealed that intravenous glucose or oral carbohydrate loading raised her myocardial high-energy phosphate content significantly. This study demonstrates the feasibility of using P-31 NMR to evaluate the biochemistry of the human myocardium in vivo and to diagnose a metabolic abnormality. Furthermore, it provides an avenue for the application ofP-31 NMR to optimize therapy for human disease. CONCLUSION Nuclear magnetic resonance appears to add an entirely new dimension to present-day diagnostic modalities. Because of its ability to distinguish stationary tissue from flowing blood, clinically available noninvasive proton imaging may produce anatomic detail as accurate as angiography for the evaluation of myocardial function and anatomy. Although specific atherosclerotic lesions may not be discernible, the patency of native vessels, as well as saphenous vein bypass grafts, should be easily discernible and NMR may become the procedure of choice for evaluation of patients who have had myocardial revascularization or percutaneous coronary dilatation. Similarly, NMR lends itself perfectly to diagnosis of peripheral vascular lesions, again with the added safety of a noninvasive procedure. Certainly one of the most fascinating capabilities of NMR is its ability to evaluate muscle bioenergetics, that is, to assay tissue oxygen supply and demand noninvasively. Although magnets capable of imaging phosphocreatine or adenosine triphosphate content are expensive, experimental, and in the early stages of development, it is through just such efforts that medicine truly may be able to understand and advance its treatment of congenital and ischemic heart disease.
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REFERENCES l. Bloch, F.: Nuclear induction. Physiol. Rev., 70:46~74, 1946. 2. Brady, T. J., Goldman, M. R., Pykett, I. L., et al.: Proton nuclear magnetic resonance imaging of regionally ischemic canine hearts: Effect of paramagnetic signal enhancement. Radiology, 144:343-347, 1982. 3. Cohen, R. D., and lies, R. A.: Intracellular pH: Measurement, control and metabolic interrelationships. CRC Crit. Rev. Clin. Lab. Sci., 6:101-143, 1975. 4. Feinberg, D. A., Crooks, L., Hoenninger, J., III, et al.: Pulsatile blood velocity in human arteries displayed by magnetic resonance imaging. Radiology, 153:177-180, 1984. 5. Flaherty, J. T., Weisfeldt, M. L., Buckley, B. H., et al.: Mechanism of ischemic myocardial cell damage assessed by P-31 nuclear magnetic resonance. Circulation, 65:561-571, 1982. 6. Gadian, D. G., Hoult, D. I., Radda, G. K., et al.: Phosphorous nuclear magnetic resonance studies on normoxic and ischemic cardiac tissue. Proc. Natl. Acad. Sci. U.S.A., 73:4446-4448, 1976. 7. Gadian, D. G., Radda, G. K., Richards, R. E., and Seeley, P. J.: P-31 NMR in living tissue: The road from a promising to an important tool in biology. In Shulman, R. G. (ed.): Biological Applications of Magnetic Resonance. New York, Academic Press, 1979. 8. Condos, B.: The roentgen image of the subclavian artery in the pulmonary apex. A. J. R., 86:1058-1062, 1961. 9. Grove, T. H., Ackerman, J. J. H., Radda, G. K., and Bowe, P. J.: Analysis of rat heart in vivo by phosphorous nuclear magnetic resonance. Proc. Nat!. Acad. Sci. U.S.A., 77:299-302, 1980. 10. Helle], S. J., Adams, D. F., and Abrams, H. L.: Complications of angiography. Radiology, 138:273-281, 1981. 11. Higgins, C. B., Byrd, B. F., III, and Farmer, D.: MR imaging of congenital heart disease. Radiology, 153(P):172, Abst. 517, 1984. 12. Higgins, C. B., Herfkens, R., Lipton, M. J., et al.: Nuclear magnetic resonance imaging of acute myocardial infarction in dogs: Alterations in magnetic resonance times. Am. J. Cardiol., 52:184-188, 1983. 13. Hollis, D. P., Nunnally, R. L., Jacobus, W. E., and Taylor, G. J.: Detection of regional ischemia in perfused beating hearts by phosphorous nuclear magnetic resonance. Biochem. Biophys. Res. Commun., 75:1086-1091, 1977. 14. Jacobus, W. E., and Weisfeldt, M. L.: Intracellular pH and myocardial function in normal and ischemic hearts: A P-31 NMR Study (abstract). Biophys. J., 33:33, 1981. 15. Kaufman, L., Crooks, L. E., Sheldon, P., el al.: The potential impact of nuclear magnetic resonance imaging on cardiovascular diagnosis. Circulation, 67:251-257, 1983. 16. Kaufman, L., Crooks, L. E., Sheldon, P. E., et al.: Evaluation of NMR imaging for detection and quantification of obstructions in vessels. Invest. Radio!., 17:554-560, 1982. 17. Lauterbor, P. L.: Image formation by induced local interactions: Examples employing nuclear magnetic resonance. Nature, 242:190-191, 1973. 18. McArdle, B.: Myopathy due to a defect in muscle glycogen breakdown. Clin. Sci., 10:13-35, 1951. 19. McNamara, M. T., Higgins, C. B., Ehman, R. L., et al.: Acute myocardial ischemia: Magnetic resonance contrast enhancement with gadolinium-DPTA. Radiology, 153:157-163, 1984. 20. Moon, R. B., and Richards, J. H.: Determination of intracellular pH by P-31 nuclear magnetic resonance. J. Bioi. Chern., 248:7276-7278, 1973. 21. Odeblad, E., Bhar, B. N., and Lindstrom, G.: Proton magnetic resonance of human red blood cells in heavy water exchange experiments. Arch. Biochem. Biophys., 63:221, 1956. 22. Purcell, E. M., Towney, H. C., and Pound, R. J.: Resonance absorption by nuclear magnetic moments in a solid. Phys. Rev., 69:37-38, 1946. 23. Roos, A., and Boron, W. F.: Intracellular pH. Physiol. Rev., 61:296, 1981. 24. Ross, B. D., Radda, G. K., Gadian, D. G., et al.: Examination of a case of suspected McArdle's syndrome by P-31 nuclear magnetic resonance. N. Engl. J. Med., 304:1338-1342, 1981.
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25. Sandler, C. M., Toombs, B. D., and Lester, R. G.: Buckling of the left common carotid artery simulating mediastinal neoplasm. A. J. R., 133:312-313, 1979. 26. Seeley, P. J., Busby, S. J. W., Gadian, D. G., et al.: A new approach to metabolite compartmentation in muscle. Biochem. Soc. Trans., 4:62-64, 1976. 27. Shaw, T. M., Ericksen, G. H., and Kunsman, C. H.: Proton magnetic resonance absorption and water content format of biological materials. Phys. Rev., 85:708, 1952. 28. Singer, J. R., and Crooks, L. E.: Nuclear magnetic resonance blood flow measurements in the human brain. Science, 221:654-656, 1983. 29. Waddell, W. J., and Bates, R. G.: Intracellular pH. Physiol. Rev., 49:285-329, 1969. 30. Wesbey, G. E., Higgins, C. B., McNamara, M. T., et al.: Effect of gadolinium-DTPA on the magnetic relaxation times of normal and infarcted myocardium. Radiology, 153:165-169, 1984. 31. Whitman, G. J. R., Chance, B., Clark, B. J., et al.: Use of P-31 NMR in diagnosis and therapy of a familial cardiomyopathy. J. Am. Coli. Cardiol., 5:745-749, 1985. 32. Whitman, G., Kieval, R., Seeholzer, S., et al.: Recovery of left ventricular function following graded cardiac ischemia as predicted by P-31 nuclear magnetic resonance of the postischemic myocardium. Surgery, 97:428-435, 1985. 33. Whitman, G., Kieval, R., Wetstein, L., et al.: The relationship between global myocardial ischemia, left ventricular function, myocardial redox state, and high energy phosphate profile. J. Surg. Res., 35:332-339, 1983.
ct. Alden H.
Harken, M.D. Department of Surgery University of Colorado Health Sciences Center 4200 East Ninth Avenue Denver, Colorado 80262