W.P Aue

Volume-selective excitation. A novel approach to topical NMR

Topical nuclear magnetic resonance (TNMR) is a noninvasive and nonhazardous new technique to obtain high-resolution NMR spectra from a restricted region within a living system. It allows the collection of detailed information about molecular structure, concentration, kinetics, and metabolism in vivo. For detailed TNMR studies of a complex three-dimensional sample such as that of an animal, a precise method for volume selection is crucial. To give the rationale for the development of a new technique, the methods which are presently available to select a sensitive volume are compared briefly. In principle, TNMR spectra can be obtained indirectly via chemical-shift imaging (l-6), a method to investigate the distribution of various chemical compounds throughout the sample, although at significantly reduced sensitivity. To optimize sensitivity, as well as to simplify the experiment and data handling, it is advantageous to use a one-dimensional method to measure a TNMR spectrum. A number of different techniques have been described to restrict the size of the sensitive volume to a predetermined region of interest. They alI rely either on focusing the static magnetic field B0 or on localizing the rf field Br . The methods with static focusing of B0 (7, 8) make use of the fact that highresolution NMR spectra can be obtained only in a volume with high B0 homogeneity. Outside this region, the spectral lines are very much broadened and therefore do not contribute much signal. At the present time, the position of the focused Bo is restricted to the center of the magnet system, which then requires the object under investigation to be moved for every new volume element of interest. Dynamic focusing of BO has also been proposed (9, 10). In this, a steady-state free-precession experiment is performed under the influence of slowly varying linear BO gradients, which eliminate signal contributions from volumes with time dependent Bo. Although this approach allows the selective volume to be moved easily, it su8ers from ill defined boundaries of the sensitive volume and corresponding lineshape problems. The most common method to localize the rf field is the use of a surface rf coil (II). As the name implies, its application is normally restricted to the surface of samples, although efforts have been taken to push the sensitive volume deeper inside the sample by means of special pulse sequences (12, 13). The inherent drawbacks of the methods mentioned above provided the impetus to develop volume-selective excitation (VSE) to localize the sensitive volume. VSE, for which a possible pulse and gradient sequence is given in Fig. 1, is actually a

Phosphocreatine content and intracellular pH of calf muscle measured by phosphorus NMR spectroscopy in occlusive arterial disease of the legs

Abstract. Energy metabolism of calf muscle was assessed non‐invasively by phosphorus (31P) NMR spectroscopy in eleven patients with symptomatic arterial occlusion and in seven matched controls. Phosphocreatine (PCr) content and pH values decreased during non‐ischaemic foot exercise to lower values in severely afflicted patients but in all patients, as a group, they were not significantly decreased compared to controls. In contrast, recovery from ischaemic exercise (arterial occlusion by a tourniquet) demonstrated significant differences between patients and controls. Intracellular pH and PCr recovered more slowly in patients than in controls; PCr recovery proceeded exponentially with a recovery half‐time of 203 ± 74 s in patients compared to 36±7 ± 5±5 s in controls (P<0±02). Phosphocreatine (PCr) recovery after ischaemic exercise correlated significantly with the degree of arterial stenoses as assessed by Doppler ultrasound (r= 0±739, P= 0±019) and by angiography (r= 0±885, P= 0±005), suggesting that the degree of large vessel stenoses limits the postischaemic increase in mitochondrial oxidative phosphorylation. Reactive blood flow after ischaemia failed to correlate with PCr recovery or with the degree of arterial stenoses. Phosphorus (31P) NMR spectroscopy provides, therefore, quantitative parameters of muscle energy metabolism in patients with peripheral arterial occlusions.

Effects of the anti-cancer drug adriamycin on the energy metabolism of rat heart as measured by in vivo 31P-NMR and implications for adriamycin-induced cardiotoxicity

In vivo 31P-NMR was used to measure the effects of the anti-tumor drug adriamycin on the energy metabolism of rat heart. The exclusive acquisition of NMR signal from cardiac muscle was assured by positioning a solenoidal radio-frequency NMR coil around the heart. Appropriate control experiments verified that 31P-NMR spectra solely originated from this organ. Acute effects occurring shortly after adriamycin administration are expressed in 31P spectra as a dose-dependent decline in the cardiac levels of phosphocreatine, after which stabilization at a new steady-state level occurs. These acute effects of a single dose are complete in 30-60 min and no significant further changes take place within 150 min after drug introduction. Longer-term effects of single high doses and of multiple lower doses were measured up to a week after the initiation of treatment. It seemed that at a total dose of 20 mg/kg, drug-induced interference with cardiac energy metabolism was more pronounced than at the same dose in the acute phase. These 31P-NMR data demonstrate that adriamycin treatment is accompanied by a decrease of the cardiac phosphocreatine/ATP ratio which might be an expression of the well-established cardiotoxicity of the drug.

Radiofrequency resonators for high-field imaging and double-resonance spectroscopy

Abstract Resonators of the type first described by Alderman and Grant have been built for a 1.9 T magnet with a 24 cm clear bore. The probe dimensions and characteristics for three 1H resonators (80 MHz) with differing inner diameters are given. Two insert structures were tuned for the 31P resonant frequency of 32.4 MHz, one a resonator similar to the 1H resonators and the other a geometrically optimized Helmholtz structure. These inserts were utilized with the 15 cm diameter 1H resonator as double-resonance probes. The probes were applied to a variety of in vivo imaging and spectroscopy applications. Efficient 1H decoupling in a rats liver, large-dimension imaging of a human leg, and combined imaging and spectroscopy with volume-selective excitation of a rat are all demonstrated. The resonators are shown to have good rf homogeneity and high Q and to generate minimal electric fields.

NMR imaging and volume selective spectroscopy with a single surface coil

A method is presented to combine 1H NMR imaging and localized in vivo 31P NMR spectroscopy by means of a single surface coil. The method uses linear B0 gradients and frequency-selective rf pulses to define the imaging plane and to localize precisely the origin of the high-resolution spectrum. The surface coil serves as transmitter and receiver. Experiments are shown, where 1H images and volume-selective 31P spectra of the same object are taken. Simple experimental setup, high sensitivity, and easy data handling render the method favorable for in vivo NMR investigations.

Cerebral metabolic studies in situ by 31P-nuclear magnetic resonance after hypothermic circulatory arrest.

ABSTRACT. Cerebral high energy phosphates were studied in the intact rabbit brain using nuclear magnetic resonance spectroscopy. The effect of hypothermia on degradation kinetics in total ischemia due to circulatory arrest was examined, measuring phosphocreatine, adenosine triphosphate, and inorganic phosphate as a function of time at three different temperatures (35, 24, 21° C). Phosphocreatine- and ATP-decays followed single exponential functions at all three temperatures. The half-life times increased by approximately a factor of three upon lowering the temperature from 35 to 21° C with activation energies of 15–20 kcal/mol, which corresponds to values of Q10 between 2.4 and 3.2. In the temperature range studied, no critical temperature was found below which metabolism would stop completely. We conclude that nuclear magnetic resonance spectroscopy allows, in the intact animal, quantitative assessment of the influence of hypothermia on energy metabolism in the brain. This influence is a major concern in the field of cardiac surgery in infants and children who are often operated in total circulatory arrest under deep hypothermia.

DEGRADATION KINETICS OF HIGH ENERGY PHOSPHATES IN THE RABBIT BRIN USING NMR-SPECTROSCOPY

Others have shown by NMR spectroscopy that ischaemia produces a degradation of phosphocreatine (PCr) and ATP. We tested the feasibility of NMR spectroscopy for gaining kinetic data on the degradation of PCr and ATP at different body temperatures. Anaesthesized rabbits were ventilated and cooled externally to 24° C and 21° C respectively. NMR spectra of 31P phosphates were recorded using a 1.9 Tesla, 24 cm bore superconducting magnet. Degradation kinetics of the compounds were measured after cardiac arrest at 35°, 24° and 21°.The decay of PCr and ATP followed a single exponential function, indicating first order kinetics. The half time values (T/2) are given in the table (4 animals per temperature; SD)Temperature dependency follows the Arrhenius law for chemical kinetics. From the Arrhenius plot the activation energies were calculated to be 16.7 ± 2.8 kcal/mol for PCr and 14.6 ± 2.1 kcal/mol for ATP. These energies are typical for enzyme catalyzed reactions. Our data show that in situ NMR spectroscopy can be used to study the kinetics of degradation of high energy phosphates. This method could be used to investigate how far hypothermia can protect the brain during operations in circulatory arrest and deep hypothermia.