J. S. Leigh

A new double-tuned probed for concurrent 1H and 31P NMR☆

Abstract A new method for double tuning NMR probes, utilizing a parallel LC trap in series with the usual tuning capacitor, is described. This method of probe tuning provides both resonances to a single input, creating an ideal probe for concurrent 31 P and 1 H NMR. The sensitivity of this probe at 31 P is not appreciably different from single-tuned probes. This circuit design has the advantages of requiring fewer reactive components and is easily generalized to more than two resonances.

Acute intestinal ischemia studies by phosphorus nuclear magnetic resonance spectroscopy.

31P nuclear magnetic resonance (NMR) spectroscopy has been used to follow the metabolism of acutely ischemic rat small intestine and its recovery after reversal of ischemia. Loops of small intestine were subjected to occlusive external pressure for up to 60 minutes, followed by a recovery period. The depletion of PCr and ATP is rapid and complete within 20 minutes. Recovery from ischemia is also rapid but with recovery ATP levels lower than initial values after prolonged ischemic periods. Intestinal shock was avoided. Clinical recovery correlated with shorter ischemic periods. 31P NMR spectroscopy thus appears to be a suitable technique for studying the effects of pharmacological agents and other treatments for amelioration of ischemic effects on the bowel.

The orientation of a heme of cytochrome c oxidase in submitochondrial particles

The electron paramagnetic resonance of the low spin signal from oxidized cytochrome c oxidase has been studied in oreinted multilayers of submitochondrial and electron transport particles. Measurements of the angular variation of the EPR spectra with the multilayer plane orientation allow the determination of the heme orientation in the multilayer. The heme normal lies in the membrane plane and the y-axis of the heme makes an angle of 30 degrees with the membrane normal. Analysis of the line shape reveals the presence of mosaic spread in the multilayer almost half of which is attributable to deviations of protein orientation within the membrane.

The application of overcoupled tank circuits to NMR probe design

In viva NMR measurements from many different nuclei are now being used to study physiologic and medical problems (e.g., ‘H, 19F, 3’P, 23Na, 7Li, and 13C). Concurrent acquisition of in vivo NMR data from more than one nucleus increases the information available from an experiment without requiring extra data collection time, while eliminating many of the problems of biological variability. The development of multinuclear probes capable of performing with high sensitivity is a prerequisite to collecting NMR data concurrently from any set of nuclei. We have already reported on double-tuned probes that will perform at both ‘H and “P with high sensitivity (1, 2). Simple extension of our existing design has provided a useful probe to collect 23Na “P and ‘H in vivo (3). However, this method of probe tuning does not accommodate closely spaced resonances well, resulting in a probe with somewhat reduced sensitivity at either 23Na or “P. Applying the same technique to tuning nuclei even closer in frequency, such as 19F and ‘H, would result in a large sacrifice in sensitivity at both resonances. Since many of the nuclei of interest resonate at relatively closely spaced frequencies (Table I), the development of other techniques to tune probes is necessary. We propose a method of probe tuning that utilizes coupled “tank” circuits to “split” a resonance, and thus is ideal for closely spaced nuclei. A useful NMR probe is created by using an LC tank circuit with an impedance matching network on the input. This circuit will have a single resonance at the resonant frequency of the tank circuit, which can be split into two resonances by strongly coupling this tank circuit to another tank circuit on resonance with the sample-coil tank circuit. Several well described methods exist to achieve this (i.e., transformer coupling, inductive coupling, and capacitive coupling), but we found capacitive coupled tank circuits to be particularly convenient (Fig. la) (4-6). In general three conditions of coupling exist: undercoupled, critically coupled, and overcoupled. The overcoupled case will yield two distinct resonances so only this will be described in detail.

Exact solutions of the bloch equations with n-site chemical exchange

Abstract Exact solutions of the modified Bloch equations are obtained in the case of n-site chemical exchange. Solutions are given with initial data corresponding to arbitrary tip angles of the z component of the magnetization. These results are specialized for saturation-recovery and inversion-recovery experiments in two-site chemical exchange.

An approach to the problem of metabolic heterogeneity in brain: ischemia and reflow after ischemia.

We have proposed that tissue metabolic failure during hypoxia or ischemia is related to the microheterogeneous distribution of tissue oxygen and not to failure of the creatine kinase equilibrium. This theory is based on the concept that sharp oxygen gradients exist in rapidly metabolizing tissue and that shifts in these gradients can place specific cells at risk for metabolic death while relatively adjacent cells escape unharmed; cells that are unharmed meet the steady-state requirements (V less than Vmax), those at risk do not (V greater than Vmax). Though it would seem that confirmation of such a hypothesis would require metabolic delineation at a high resolution, we have shown how 31P MRS provides information supporting this hypothesis. This possible use of MR spectroscopy to define microheterogeneous events suggests further clinical possibilities for this instrument in defining the rate of cell loss and the response to therapeutic interventions.

A model for the simulation of the EPR spectra of chromophores in partially oriented membrane multilayers

Abstract The EPR spectrum of membrane-bound chromophores in oriented membrane multilayers can be simulated by a model which assumes a rhombic g tensor, an angle-dependent linewidth, and a fixed orientation of the center relative to the membrane normal. Membrane multilayers display no order with respect to rotation about the membrane normal. The remaining order- is subject to mosaic spread. Application of appreciable mosaic spread causes the line positions to move from their expected values to the principal g values and the lineshapes change from derivative to absorption-like powder spectra.

Magnetization transfer imaging of the brain: A quantitative comparison of results obtained at 1.5 and 4.0 t

The preliminary results of magnetization transfer (MT) imaging on a whole body 4.0 T system are presented. Cooked egg phantoms and several volunteers were imaged on 1.5 and 4.0 T magnets interfaced to GE Signa scanners. The MT ratio (MTR), signal difference to noise ratio (SDNR), and contrast parameters were measured at both fields and compared. Furthermore, single‐shot Z‐spectroscopy was used to characterize the frequency dependence of the MT phenomenon. The results show that MT imaging can be safely performed at 4.0 T without exceeding limitations of radio frequency power. The MT effect is more pronounced at the higher field, leading to better quality images with higher contrast and SDNR. The Z‐spectra are not markedly different at the higher field although the MTR is greater. The potential applications of this technique to study neurodegenerative diseases, as well as, perfusion imaging and angiography are discussed. J. Magn. Reson. Imaging 1999;10:527–532.

Effect of dysprosium on the spin-lattice relaxation time of cytochrome c and cytochrome a.

The progressive power saturation of the electron paramagnetic resonance of horse heart cytochrome c and solubilized bovine heart cytochrome oxidase has been monitored at low temperature in the presence of the relaxing agent, dysprosium. The saturation of the EPR signal of cytochrome c is relieved even at 6 K. With increasing temperature the effect is enhanced as the relaxation time of the dysprosium becomes shorter; however, the intrinsic spin-lattice relaxation time, T1, for cytochrome c decreases even more rapidly with increasing temperature. T1 for cytochrome c can be described by an intrinsic component, a component which is proportional to the concentration of dysprosium and a third component due to local binding which is independent of dysprosium concentration. The cytochrome a component of cytochrome oxidase is also affected by dysprosium. In the presence of cytochrome oxidase, T1 for cytochrome c is almost unaffected by dysprosium, indicating that access to the cytochrome c heme is blocked by the binding of c to oxidase. Based on the concentration-dependent effect of dysprosium on the lifetime of cytochrome c, it is possible to make distance estimates from the EPR active center to Dy3+. Dysprosium is therefore useful for determining the spatial relationships among paramagnetic enzyme components in a quantitative way.

Nuclear magnetic resonance: An overview of its spectroscopic and imaging applications in pediatric patients

Magnetic resonance (MR) is a technique that permits the noninvasive evaluation of morphologic features and function based on the distribution of protons and other selected elements. We present a basic description of MR and illustrate both 31P-MR spectroscopy and proton imaging applications in pediatric patients. The applications of these techniques are diverse, and are presented concisely in an attempt to give pediatricians an overview of this new technology and its potential role in patient management.