S Müller

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.

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.

In vivo NMR imaging of deuterium

D2O is used as a contrast agent for studying anatomical images and flow in vivo by deuterium NMR. A deuterium image of the head of a living rat after administration of D2O (5% v/v) in the drinking water is shown. It was obtained in 14 min with a surface coil and has a spatial resolution of about one millimeter. The application of D2O as a tracer is discussed and the inflow of heavy water into the brain of a rat is recorded in a time series of deuterium images. Spatially resolved inflow time constants have been determined.

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.