Spatial localization in NMR spectroscopy in vivo

Spatial localization techniques are necessary for in vivo NMR spectroscopy involving heterogeneous organisms. Localization by surface coil NMR detection alone is generally inadequate for deep-lying organs due to contaminating signals from intervening surface tissues. However, localization to preselected planar volumes can be accomplished using a single selective excitation pulse in the presence of a pulsed magnetic field gradient, yielding depth-resolved surface coil spectra (DRESS). Within selected planes, DRESS are spatially restricted by the surface coil sensitivity profiles to disk-shaped volumes whose radii increase with depth, notwithstanding variations in the NMR signal density distribution. Nevertheless, DRESS is a simple and versatile localization procedure that is readily adaptable to spectral relaxation time measurements by adding inversion or spin-echo refocusing pulses or to in vivo solvent-suppressed spectroscopy of proton (1H) metabolites using a combination of chemical-selective RF pulses. Also, the spatial information gathering efficiency of the technique can be improved to provide simultaneous acquisition of spectra from multiple volumes by interleaving excitation of adjacent planes within the normal relaxation recovery period. The spatial selectivity can be improved by adding additional selective excitation spin-echo refocusing pulses to achieve full, three-dimensional point resolved spectroscopy (PRESS) in a single excitation sequence. Alternatively, for samples with short spin-spin relaxation times, DRESS can be combined with other localization schemes, such as image-selected in vivo spectroscopy (ISIS), to provide complete gradient controlled three-dimensional localization with a reduced number of sequence cycles.

A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from 1-100 MHz: dependence on tissue type, NMR frequency, temperature, species, excision, and age

The longitudinal (T1) and transverse (T2) hydrogen (1H) nuclear magnetic resonance (NMR) relaxation times of normal human and animal tissue in the frequency range 1-100 MHz are compiled and reviewed as a function of tissue type, NMR frequency, temperature, species, in vivo versus in vitro status, time after excision, and age. The dominant observed factors affecting T1 are tissue type and NMR frequency (V). All tissue frequency dispersions can be fitted to the simple expression T1 = AVB in the range 1-100 MHz, with A and B tissue-dependent constants. This equation provides as good or better fit to the data as previous more complex formulas. T2 is found to be multicomponent, essentially independent of NMR frequency, and dependent mainly on tissue type. Mean and raw values of T1 and T2 for each tissue are tabulated and/or plotted versus frequency and the fitting parameters A, B and the standard deviations determined to establish the normal range of relaxation times applicable to NMR imaging. The mechanisms for tissue NMR relaxation are reviewed with reference to the fast exchange two state (FETS) model of water in biological systems, and an overview of the dynamic state of water and macromolecular hydrogen compatible with the frequency, temperature, and multicomponent data is postulated. This suggests that 1H tissue T1 is determined predominantly by intermolecular (possibly rotational) interactions between macromolecules and a single bound hydration layer, and the T2 is governed mainly by exchange diffusion of water between the bound layer and a free water phase. Deficiencies in measurement techniques are identified as major sources of data irreproducibility.

Fluorine (19F) MRS and MRI in biomedicine

Shortly after the introduction of (1)H MRI, fluorinated molecules were tested as MR-detectable tracers or contrast agents. Many fluorinated compounds, which are nontoxic and chemically inert, are now being used in a broad range of biomedical applications, including anesthetics, chemotherapeutic agents, and molecules with high oxygen solubility for respiration and blood substitution. These compounds can be monitored by fluorine ((19)F) MRI and/or MRS, providing a noninvasive means to interrogate associated functions in biological systems. As a result of the lack of endogenous fluorine in living organisms, (19)F MRI of 'hotspots' of targeted fluorinated contrast agents has recently opened up new research avenues in molecular and cellular imaging. This includes the specific targeting and imaging of cellular surface epitopes, as well as MRI cell tracking of endogenous macrophages, injected immune cells and stem cell transplants.

A review of 1H nuclear magnetic resonance relaxation in pathology: are T1 and T2 diagnostic?

The longitudinal (T1) and transverse (T2) proton (1H) nuclear magnetic resonance (NMR) relaxation times of pathological human and animal tissues in the frequency range 1-100 MHz are archived, reviewed, and analyzed as a function of tissue of origin, NMR frequency, temperature, species, and in vivo versus in vitro status. T1 data from specific disease states of the bone, brain, breast, kidney, liver, muscle, pancreas, and spleen can be characterized by simple dispersions of the form T1 = AvB in the range 1-100 MHz with A and B empirically determined pathology-dependent constants. Pathological tissue T2 values are essentially independent of NMR frequency. Raw relaxation data, best-fit T1 parameters A and B, and the mean T2 values, are tabulated along with standard deviations and sample size to establish the normal range of pathological tissue relaxation times applicable to NMR imaging or in vitro NMR examination. Statistical analysis of relaxation data, assumed independent, reveals that most tumor and edematous tissue T1 values and some breast, liver, and muscle tumor T2 values are significantly elevated (p greater than or equal to 0.95) relative to normal, but do not differ significantly from other tumors and pathologies. Statistically significant abnormalities in the T1 values of some brain, breast, and lung tumors, and most pathological tissue T2 values could not, however, be demonstrated in the presence of large statistical errors. Both T1 and T2 in uninvolved tissue from tumor-bearing animals or organs do not demonstrate statistically significant differences from normal when considered as a group, suggesting no appreciable systemic effects associated with the presence of tumors compared to the statistical uncertainty. Statistical prediction analysis for both T1 and T2 indicates that of all the tissues studied, only liver hepatoma can be reliably distinguished from normal liver based on a single T1 measurement (p greater than or equal to 0.95) given the scatter in the current published data. Indeed, data scatter, not easily attributable to temperature, species, in vivo versus in vitro status, the inclusion of implanted or chemical induced tumors, or the possible existence of multiple component relaxation, is recognized as the major factor inhibiting the diagnostic utility of quantitative NMR relaxation measurements. Malignancy indexes that combine T1 and T2 data as a diagnostic indicator suffer similar problems of uncertainty. The literature review reveals a dearth of information on the temperature and frequency dependence of pathological tissue relaxation and the possible existence of multiple relaxation components.(ABSTRACT TRUNCATED AT 400 WORDS)

RF magnetic field penetration, phase shift and power dissipation in biological tissue: implications for NMR imaging

The magnetic field penetration, phase shift and power deposition in planar and cylindrical models of biological tissue exposed to a sinusoidal time-dependent magnetic field have been investigated theoretically over the frequency range 1 to 100 MHz. The results are based on measurements of the relative permittivity and resistivity dispersions of a variety of freshly excised rat tissue at 37 and 25 degrees C, and are analysed in terms of their implications for human body nuclear magnetic resonance (NMR) imaging. The results indicate that at NMR operating frequencies much greater than about 30 MHz, magnetic field amplitude and phase variations experienced by the nuclei may cause serious distortions in an image of a human torso. The maximum power deposition envisaged during an NMR imaging experiment on a human torso is likely to be comparable to existing long-term safe exposure levels, and will depend ultimately on the imaging technique and NMR frequency employed.

ATP flux through creatine kinase in the normal, stressed, and failing human heart

The heart consumes more energy per gram than any other organ, and the creatine kinase (CK) reaction serves as its prime energy reserve. Because chemical energy is required to fuel systolic and diastolic function, the question of whether the failing heart is "energy starved" has been debated for decades. Despite the central role of the CK reaction in cardiac energy metabolism, direct measures of CK flux in the beating human heart were not previously possible. Using an image-guided molecular assessment of endogenous ATP turnover, we directly measured ATP flux through CK in normal, stressed, and failing human hearts. We show that cardiac CK flux in healthy humans is faster than that estimated through oxidative phosphorylation and that CK flux does not increase during a doubling of the heart rate-blood pressure product by dobutamine. Furthermore, cardiac ATP flux through CK is reduced by 50% in mild-to-moderate human heart failure (1.6 +/- 0.6 vs. 3.2 +/- 0.9 micromol/g of wet weight per sec, P <0.0005). We conclude that magnetic resonance strategies can now directly assess human myocardial CK energy flux. The deficit in ATP supplied by CK in the failing heart is cardiac-specific and potentially of sufficient magnitude, even in the absence of a significant reduction in ATP stores, to contribute to the pathophysiology of human heart failure. These findings support the pursuit of new therapies that reduce energy demand and/or augment energy transfer in heart failure and indicate that cardiac magnetic resonance can be used to assess their effectiveness.

Human in vivo NMR spectroscopy in diagnostic medicine: clinical tool or research probe?

In this critical review of human in vivo nuclear magnetic resonance (NMR) spectroscopy, the questions of which chemical species can be detected and with what sensitivity, their biochemical significance, and their potential clinical value are addressed. The current in vivo detectability limit is about 10(-6) of that of tissue water protons, necessitating a 1-10 cm3-volume of tissue and approximately 10-minute averaging time. This permits access to fats, membrane lipid metabolism, high-energy phosphate metabolism, glycogen, some neurotransmitters and metabolites in the citric acid cycle, and artificially introduced fluorocompounds. While hydrogen-31, phosphorus-31, carbon-13, sodium-23, and fluorine-19 in vivo results are discussed, the majority of patient studies use P-31 NMR spectroscopy. Here results from metabolic and ischemic disorders substantiate a case for spectroscopy as a diagnostic modality. The use of a broad range of spatial localization strategies is justifiable on the basis of the location and size of the pathologic condition and of NMR sensitivity. Abnormalities in spectra from many other disorders, most notably cancer, and improvements are often observed early in the course of successful therapy. Yet the potential impact of these results on clinical diagnosis and therapeutic monitoring is not always well understood, and many questions remain. Neurotransmitters and citric acid cycle metabolites exhibit high H-1 NMR sensitivities and represent major untapped potential for human clinical spectroscopy research. Studies evaluating spectroscopy in the context of existing modalities are needed. The unique ability of spectroscopy to provide noninvasive information about tissue chemistry in patients bodes well for its impact on clinical research and disease diagnosis.

Tissue sodium concentration in human brain tumors as measured with 23Na MR imaging

PURPOSE

To use combined proton (1H) and sodium 23 (23Na) magnetic resonance (MR) imaging to noninvasively quantify total tissue sodium concentration and to determine if concentration is altered in malignant human brain tumors.

MATERIALS AND METHODS

Absolute tissue sodium concentration in malignant gliomas was measured on quantitative three-dimensional 23Na MR images with tissue identification from registered 1H MR images. Concentration was determined in gray matter (GM), white matter (WM), cerebrospinal fluid (CSF), and vitreous humor in 20 patients with pathologically proven malignant brain tumors (astrocytoma, n = 17; oligodendroglioma, n = 3) and in nine healthy volunteers. Sodium concentration in tumors and edema was determined from 23Na image signal intensities in regions that were contrast material enhanced on T1-weighted 1H images (tumors) or regions that were only hyperintense on fluid-attenuated inversion recovery (FLAIR) 1H images (edema). Sodium concentrations were measured noninvasively from 23Na images obtained with short echo times (0.4 msec) by using external saline solution phantoms for reference. Differences in mean sodium concentration of all healthy tissue and lesions in patients were tested with a paired t test. Concentration in uninvolved tissues in patients was compared with that in the same tissue types in the volunteers with an independent samples two-tailed t test.

RESULTS

Mean concentration (in millimoles per kilogram wet weight) was 61 +/- 8 (SD) for GM, 69 +/- 10 for WM, 135 +/- 10 for CSF, 113 +/- 14 for vitreous humor, 103 +/- 36 for tumor, 68 +/- 11 for unaffected contralateral tissue, and 98 +/- 12 for FLAIR hyperintense regions surrounding tumors. Significant differences (P <.002) in sodium concentration were demonstrated by using a t test for both tumors and surrounding FLAIR hyperintense tissues versus GM, WM, CSF, and contralateral brain tissue.

CONCLUSION

23Na MR imaging with short echo times can be used to quantify absolute tissue sodium concentration in patients with brain tumors and shows increased sodium concentration in tumors relative to that in normal brain structures.

Regional myocardial metabolism of high-energy phosphates during isometric exercise in patients with coronary artery disease

BACKGROUND

The maintenance of cellular levels of high-energy phosphates is required for myocardial function and preservation. In animals, severe myocardial ischemia is characterized by the rapid loss of phosphocreatine and a decrease in the ratio of phosphocreatine to ATP.

METHODS

To determine whether ischemic metabolic changes are detectable in humans, we recorded spatially localized phosphorus-31 nuclear-magnetic-resonance (31P NMR) spectra from the anterior myocardium before, during, and after isometric hand-grip exercise.

RESULTS

The mean (+/- SD) ratio of phosphocreatine to ATP in the left ventricular wall when subjects were at rest was 1.72 +/- 0.15 in normal subjects (n = 11) and 1.59 +/- 0.31 in patients with nonischemic heart disease (n = 9), and the ratio did not change during hand-grip exercise in either group. However, in patients with coronary heart disease and ischemia due to severe stenosis (greater than or equal to 70 percent) of the left anterior descending or left main coronary arteries (n = 16), the ratio decreased from 1.45 +/- 0.31 at rest to 0.91 +/- 0.24 during exercise (P less than 0.001) and recovered to 1.27 +/- 0.38 two minutes after exercise. Only three patients with coronary heart disease had clinical symptoms of ischemia during exercise. Repeat exercise testing in five patients after revascularization yielded values of 1.60 +/- 0.20 at rest and 1.62 +/- 0.18 during exercise (P not significant), as compared with 1.51 +/- 0.19 at rest and 1.02 +/- 0.26 during exercise before revascularization (P less than 0.02).

CONCLUSIONS

The decrease in the ratio of phosphocreatine to ATP during hand-grip exercise in patients with myocardial ischemia reflects a transient imbalance between oxygen supply and demand in myocardium with compromised blood flow. Exercise testing with 31P NMR is a useful method of assessing the effect of ischemia on myocardial metabolism of high-energy phosphates and of monitoring the response to treatment.

A signal-to-noise calibration procedure for NMR imaging systems

A nuclear magnetic resonance (NMR) imaging system signal-to-noise calibration technique based on an NMR projection of distilled water in a cylindrical bottle is proposed. This measurement can characterize any arrangement of rf coils in any magnetic field as signal to noise per ml times root Hz. Inductive losses in a typical patient must be included in the calibration, and such losses can be simulated in a particular system by an externally attached resistor(s) appropriate to that system. Alternatively, an rf inductive damping phantom consisting of a conducting loop of wire containing an appropriate resistor is suggested that can be inserted into any NMR imaging coil to simulate subject Q damping. The same resistor can be used, independent of the details of the coil construction. Furthermore, if the loop inductance is tuned out at each frequency with a series capacitor, then the same loop resistance will serve for all frequencies as a good approximation to human subject damping. This "projection method" signal-to-noise ratio is related to the conventional signal-to-noise ratio measured from a Lorentzian-shaped spectral line as psi P = psi L [2/T2]1/2, where psi stands for signal-to-noise ratio, subscripts P and L stand, respectively, for the projection and "Lorentzian" methods, and T2 is the transverse relaxation time of the spectral line used in the Lorentzian method.

Signal, noise, and contrast in nuclear magnetic resonance (NMR) imaging

Calculations of the sensitivity of the saturation recovery and inversion recovery pulse sequences used in nuclear magnetic resonance imaging show the former to be superior in discriminating between tissues with the same proton density but different T1's. Two other pulse sequences, which are combinations of the above, have also been analyzed. These have lower T1 discrimination sensitivity, but other considerations, such as self-normalization, may still make them attractive. The calculations are only valid for selective excitation pulse sequences in which the selected slice profiles are approximately rectangular, and thus a sin(bt)/t radiofrequency excitation is desirable. In order to ensure that the saturation recovery sequence gives valid results for pulse repetition times comparable to or shorter than T2, it is necessary to destroy the coherence between pulse applications. For this purpose we use a series of "spoiler" gradient pulses between pulse trains. The saturation recovery pulse sequence also has the advantage that, by the correct choice of interpulse spacing, sensitivity close to the optimum T1 discrimination can be achieved over a wide range of T1 values. This has the potential advantage to the clinician of simplifying his choice of parameters for imaging.

Altered myocardial high-energy phosphate metabolites in patients with dilated cardiomyopathy

Myocardial high-energy phosphate metabolism in patients with dilated cardiomyopathy (DCM) of ischemic or idiopathic etiology was assessed at rest by one-dimensional phase-encoded 31P-nuclear magnetic resonance (NMR) spectroscopy studies performed in conjunction with 1H imaging in 20 patients with DCM and in 12 normal volunteers. The measured values of anterior myocardial phosphocreatine/beta-adenosine triphosphate (PCr/beta-ATP), corrected for partial saturation and contamination of the spectra by blood metabolites, averaged 1.80 +/- 0.06 (mean +/- SE) in normal volunteers and 1.46 +/- 0.07 in the patients overall, a highly significant (p less than 0.001) decrease. In patients with DCM accompanied by coronary artery disease (n = 9), the PCr/beta-ATP ratio averaged 1.53 +/- 0.07, while in those with DCM alone it was 1.41 +/- 0.12 (n = 11), a value that was not significantly different. There was no significant correlation (r = 0.34) between myocardial PCr/ATP ratio and left ventricular ejection fraction in patients. These studies demonstrate that myocardial PCr/ATP ratios are reduced at rest in human ischemic and idiopathic dilated cardiomyopathy.

Elevated tissue sodium concentration in malignant breast lesions detected with non-invasive 23Na MRI

BACKGROUND

The hypothesis that physiological and biochemical changes associated with proliferating malignant tumors may cause an increase in total tissue sodium concentration (TSC) was tested with non-invasive, quantitative sodium ((23)Na) magnetic resonance imaging (MRI) in patients with benign and malignant breast tumors.

METHODS

(23)Na and (1)H MRI of the breast was performed on 22 women with suspicious breast lesions (> or =1 cm) at 1.5 Tesla. A commercial proton ((1)H) phased array breast coil and custom solenoidal (23)Na coil were used to acquire (1)H and (23)Na images during the same MRI examination. Quantitative 3-dimensional (23)Na projection imaging was implemented with negligible signal loss from MRI relaxation, or from radio-frequency field inhomogeneity, in less than 15 min. Co-registered (1)H and (23)Na images permitted quantification of TSC in normal and suspicious tissues on the basis of (1)H MRI contrast enhancement and anatomy, with histology confirmed by biopsy.

RESULTS

Sodium concentrations were consistently elevated in (N = 19) histologically proven malignant breast lesions by an average of 63% compared to glandular tissue. The increase in sodium concentration in malignant tissue was highly significant compared to unaffected glandular tissue (P < 0.0001, paired t-test), adipose tissue, and TSC in three patients with benign lesions.

CONCLUSION

Elevated TSC in breast lesions measured by non-invasive (23)Na MRI appears to be a cellular-level indicator associated with malignancy. This method may have potential to improve the specificity of breast MRI with only a modest increase in scan time per patient.

High resolution intravascular MRI and MRS by using a catheter receiver coil

Potentially important diagnostic information about atherosclerosis can be obtained by using magnetic resonance imaging and spectroscopy techniques. Because critical vessels such as the aorta, coronary arteries, and renal arteries are not near the surface of the body, surface coils are not adequate to increase the data quality to desired levels. A few catheter MR receiver coil designs have been proposed for imaging the walls of large blood vessels such as the aorta. These coils have limited longitudinal coverage and they are too thick to be placed into small vessels. A flexible, long and narrow receiver coil that can be placed on the tip of a catheter and will enable multi-slice high resolution imaging of small vessels has been developed. The authors describe the theory of the coil design technique, derive formulae for the signal-to-noise ratio characteristics of the coil, and show examples of high resolution cross-sectional images from isolated human aortas acquired by using this catheter coil. In addition, high resolution in vivo rabbit aorta images were obtained as well as a set of spatially resolved chemical shift spectra from a dog circumflex coronary artery.

Altered creatine kinase adenosine triphosphate kinetics in failing hypertrophied human myocardium

BACKGROUND

The progression of pressure-overload left ventricular hypertrophy (LVH) to chronic heart failure (CHF) may involve a relative deficit in energy supply and/or delivery.

METHODS AND RESULTS

We measured myocardial creatine kinase (CK) metabolite concentrations and adenosine triphosphate (ATP) synthesis through CK, the primary energy reserve of the heart, to test the hypothesis that ATP flux through CK is impaired in patients with LVH and CHF. Myocardial ATP levels were normal, but creatine phosphate levels were 35% lower in LVH patients (n = 10) than in normal subjects (n = 14, P < 0.006). Left ventricular mass and CK metabolite levels in LVH were not different from those in patients with LVH and heart failure (LVH+CHF, n = 10); however, the myocardial CK pseudo first-order rate constant was normal in LVH (0.36 +/- 0.04 s(-1) in LVH versus 0.32 +/- 0.06 s(-1) in normal subjects) but halved in LVH+CHF (0.17 +/- 0.06 s(-1), P < 0.001). The net ATP flux through CK was significantly reduced by 30% in LVH (2.2 +/- 0.7 micromol x g(-1) x s(-1), P = 0.011) and by a dramatic 65% in LVH+CHF (1.1 +/- 0.4 micromol x g(-1) x s(-1), P < 0.001) compared with normal subjects (3.1 +/- 0.8 micromol x g(-1) x s(-1)).

CONCLUSIONS

These first observations in human LVH demonstrate that it is not the relative or absolute CK metabolite pool sizes but rather the kinetics of ATP turnover through CK that distinguish failing from nonfailing hypertrophic hearts. Moreover, the deficit in ATP kinetics is similar in systolic and nonsystolic heart failure and is not related to the severity of hypertrophy but to the presence of CHF. Because CK temporally buffers ATP, these observations support the hypothesis that a deficit in myofibrillar energy delivery contributes to CHF pathophysiology in human LVH.

Noninvasive study of high-energy phosphate metabolism in human heart by depth-resolved 31P NMR spectroscopy

Phosphorus-31 nuclear magnetic resonance (NMR) spectra showing the relative concentrations of high-energy phosphate metabolites have been recorded noninvasively from the human heart in vivo. Spectral data were spatially localized by combining a pulsed magnetic field gradient with surface NMR excitation-detection coils. The location of the selected spectral region was determined by conventional proton NMR imaging immediately before examination by phosphorus-31 NMR spectroscopy.

Anatomy and metabolism of the normal human brain studied by magnetic resonance at 1.5 Tesla

Proton magnetic resonance (MR) images were obtained of the human head in magnetic fields as high as 1.5 Tesla (T) using slotted resonator high radio-frequency (RF) detection coils. The images showed no RF field penetration problems and exhibited an 11 (+/- 1)-fold improvement in signal-to-noise ratio over a .12-T imaging system. The first localized phosphorus 31, carbon 13, and proton MR chemical shift spectra recorded with surface coils from the head and body in the same instrument showed relative concentrations of phosphorus metabolites, triglycerides, and, when correlated with proton images, negligible lipid (-CH2-) signal from brain tissue on the time scale of the imaging experiment. Sugar phosphate and phosphodiester concentrations were significantly elevated in the head compared with muscle. This method should allow the combined assessment of anatomy, metabolism, and biochemistry in both the normal and diseased brain.

Proton magnetic resonance spectroscopic imaging of human breast cancer: A preliminary study

PURPOSE

To investigate the diagnostic value of proton magnetic resonance spectroscopic imaging (MRSI) in patients with breast lesions.

MATERIALS AND METHODS

Eighteen patients underwent breast MRSI and MRI at 1.5 T. Contrast-enhanced MR was used to identify the lesion, after which single-slice MRSI (TR/TE = 2000/272 msec, 10-mm slice thickness) was performed. Water, lipid, and choline (Cho) images were reconstructed from MRSI data. The area of the Cho was measured in the lesion and expressed relative to the background noise level (signal-to-noise ratio (SNR)), measured between 7.0 and 9.0 ppm. Cho SNRs were compared between benign and malignant lesions as determined by histopathology.

RESULTS

Three cases were considered technical failures on MRSI. Of the remaining 15 cases, on histopathology, eight were classified as malignant carcinoma and seven were benign. The Cho SNR from malignant tissue was significantly elevated compared to benign tissue (6.2 +/- 2.1 vs. 2.4 +/- 0.7, P < 0.0008).

CONCLUSIONS

MRSI measurements of Cho are feasible in the human breast, and the SNR for Cho was significantly different between benign and malignant lesions. The potential advantages of MRSI over SV spectroscopy include the ability to assess multiple lesions as well as tissue with normal MRI appearance, as well as to perhaps gauge lesion borders and infiltration into surrounding tissue.

Non-invasive magnetic-resonance detection of creatine depletion in non-viable infarcted myocardium

BACKGROUND

Preserved energy metabolism is essential for myocardial viability and the creatine kinase reaction is central to energy production and reserve. Although the appearance of myocardial creatine kinase enzyme in the blood is widely used to diagnose cardiac necrosis, there are no non-invasive ways to measure local creatine concentrations in the healthy and diseased human heart.

METHODS

We measured total myocardial creatine by spatially-localised, water-suppressed hydrogen magnetic-resonance spectroscopy (1H-MRS) on a clinical (1.5 T) magnetic-resonance-imaging system in ten healthy volunteers (controls) and ten patients with a history of myocardial infarction. We validated this technique by comparison of 1H-MRS values of creatine with biopsy assays in an animal model of infarction.

FINDINGS

Total creatine was measured in the posterior and anterior left ventricle and septum, and was significantly lower in regions of infarction (10 [9] SD micromol/g wet weight) than in non-infarcted regions (26 [11] micromol/g, p=0.001) of myocardium in patients or in the myocardium of healthy controls (28 [6] micromol/g, p<0.0001).

INTERPRETATION

Spatially localised 1H-MRS can be used to measure total creatine non-invasively throughout the human heart. The detection of regional creatine depletion may provide a metabolic means to distinguish healthy from infarcted non-viable myocardium.

Estimating radiofrequency power deposition in body NMR imaging

Simple theoretical estimates of the average, maximum, and spatial variation of the radiofrequency power deposition (specific absorption rate) during hydrogen nuclear magnetic resonance imaging are deduced for homogeneous spheres and for cylinders of biological tissue with a uniformly penetrating linear rf field directed axially and transverse to the cylindrical axis. These are all simple scalar multiples of the expression for the cylinder in an axial field published earlier (Med. Phys. 8, 510 (1981]. Exact solutions for the power deposition in the cylinder with axial (Phys. Med. Biol. 23, 630 (1978] and transversely directed rf field are also presented, and the spatial variation of power deposition in head and body models is examined. In the exact models, the specific absorption rates decrease rapidly and monotonically with decreasing radius despite local increases in rf field amplitude. Conversion factors are provided for calculating the power deposited by Gaussian and sinc-modulated rf pulses used for slice selection in NMR imaging, relative to rectangular profiled pulses. Theoretical estimates are compared with direct measurements of the total power deposited in the bodies of nine adult males by a 63-MHz body-imaging system with transversely directed field, taking account of cable and NMR coil losses. The results for the average power deposition agree within about 20% for the exact model of the cylinder with axial field, when applied to the exposed torso volume enclosed by the rf coil. The average values predicted by the simple spherical and cylindrical models with axial fields, the exact cylindrical model with transverse field, and the simple truncated cylinder model with transverse field were about two to three times that measured, while the simple model consisting of an infinitely long cylinder with transverse field gave results about six times that measured. The surface power deposition measured by observing the incremental power as a function of external torso radius was comparable to the average value. This is consistent with the presence of a variable thickness peripheral adipose layer which does not substantially increase surface power deposition with increasing torso radius. The absence of highly localized intensity artifacts in 63-MHz body images does not suggest anomalously intense power deposition at localized internal sites, although peak power is difficult to measure.

Human in vivo phosphate metabolite imaging with 31P NMR

Phosphorus (31P) spectroscopic images showing the distribution of high-energy phosphate metabolites in the human brain have been obtained at 1.5 T in scan times of 8.5 to 34 min at 27 and 64 cm3 spatial resolution using pulsed phase-encoding gradient magnetic fields and three-dimensional Fourier transform (3DFT) techniques. Data were acquired as free induction decays with a quadrature volume NMR detection coil of a truncated geometry designed to optimize the signal-to-noise ratio on the coil axis on the assumption that the sample noise represents the dominant noise source, and self-shielded magnetic field gradient coils to minimize eddy-current effects. The images permit comparison of metabolic data acquired simultaneously from different locations in the brain, as well as metabolite quantification by inclusion of a vial containing a standard of known 31P concentration in the image array. Values for the NMR visible adenosine triphosphate in three individuals were about 3 mM of tissue. The ratio of NMR detectable phosphocreatine to ATP in brain was 1.15 +/- 0.17 SD in these experiments. Potential sources of random and systematic error in these and other 31P measurements are identified.

Assessment of pharmacological treatment of myocardial infarction by phosphorus-31 NMR with surface coils

Phosphorus-31 nuclear magnetic resonance (NMR) measurements with small surface coils have been used to observe phosphorus metabolism of perfused hearts within localized regions. The method allows for direct, noninvasive, sequential assessment of the altered regional metabolism resulting from myocardial infarction and its response to drug treatment, which cannot be observed by conventional techniques.

MR spectroscopy of the human heart: the status and the challenges

Noninvasive measurements of high-energy phosphate metabolism in the anterior myocardium of heart patients are now possible with image-guided, localized nuclear magnetic resonance (MR) spectroscopy. The results, reviewed herein, are largely consistent with those of prior animal studies. Quantification with phosphorus-31 MR yields normal phosphocreatine (PCr) and adenosine triphosphate (ATP) concentrations of about 11 and 6 mumol per gram wet weight, respectively, with a PCr/ATP ratio of around 1.8. Studies of patients with hypertrophic and dilated cardiomyopathy, left ventricular hypertrophy, valve disease, transplanted hearts, myocardial infarction, or reversible ischemia reveal abnormalities in the PCr/ATP ratio and/or the metabolite concentrations. Differences in reported findings for cardiomyopathies might be attributable to statistical sensitivity and the presence of heart failure. The technique might find use in the clinic for identifying failure when other factors complicate diagnosis. The PCr/ATP ratio is often reduced in transplanted hearts but is not a reliable predictor of histologic rejection involving myocyte necrosis. In myocardial infarction, metabolite levels may be reduced while the remaining PCr and ATP signals likely reflect surrounding surviving tissue. Stress-test studies of anterior myocardial ischemia produce transient reductions in the PCr/ATP ratio, which appear to be specific for ischemic disease. This may lead to a new way of assessing ischemia, particularly if the technology can gain access to a larger portion of the heart. Cardiac spectroscopy with nuclei other than P-31 shows promise.

Phosphate metabolite imaging and concentration measurements in human heart by nuclear magnetic resonance

Cardiac-gated phosphorus (31P) nuclear magnetic resonance (NMR) spectroscopic imaging with surface coils resolves in three dimensions the spatial distribution of high energy phosphate metabolites in the human heart noninvasively. 31P spectra derive from 6- to 14-cm3 volumes of myocardium in the anterior left ventricle, septum, and apex, at depths of up to about 8 cm from the chest, as identified by proton (1H) NMR anatomical images acquired without moving the subject. Spectroscopic images are acquired in 9 to 21 min at 1.5 T. Metabolite concentrations are quantified with reference to a standard located outside the chest, yielding normal in vivo concentrations of phosphocreatine and adenosine triphosphate of about 11.0 +/- 2.7 (SD) and 6.9 +/- 1.6 mumol/g of wet heart tissue, respectively. High energy phosphate contents did not vary significantly with location in the normal myocardium, but 2,3-diphosphoglycerate signals from blood varied with subject and location.