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Biomedical subjects

D I Hoult

Publications and source records attributed to D I Hoult.

16 recordsLinked to original sources
PubMed1990

A high-sensitivity, high-B1 homogeneity probe for quantitation of metabolites.

Accurate quantitation of metabolites in biological samples of irregular shape and inhomogeneous composition is generally acknowledged to be difficult. The difficulties are less pronounced with a probe having excellent B1 field homogeneity, high sensitivity, and a resonant frequency independent of sample size and composition. A prototype probe that aims to fulfill these requirements in wide-bore horizontal magnets is described. It comprises four separate tuned rings on a spherical surface which give a B1 field that is flat to +/- 1% over the design volume. Inter-ring coupling, and to a fifth ring used for matching, is by induction, and the mathematics of the tuning of the system are derived. It is shown that resonant frequency variation with sample size is negligible, and that the sensitivity closely approaches the theoretical limit.

Animals↗
PubMed1990

Elimination of signal strength dependency upon coil loading--an aid to metabolite quantitation when the sample volume changes.

The phase, height, and width of NMR spectral lines from a constant number of nuclei are frequently dependent upon changes in probe Q factor and tuning, caused by motion of the sample and/or changes in its electrical conductivity and size. Thus quantitation of metabolites in biological samples can be difficult. When probe tuning and matching are rendered independent by the use of a tuned coupling coil for matching, use of a very low input impedance preamplifier can virtually eliminate such dependencies in the received signal. Likewise, by reciprocity, the use of a low output impedance transmitter greatly reduces the dependence of pulse flip angle upon sample characteristics. Experimental results verifying these statements are presented, and the time course of signals from a swelling, perfused pig heart and an external reference are plotted.

Animals↗
PubMed1986

The field dependence of NMR imaging. I. Laboratory assessment of signal-to-noise ratio and power deposition.

A method is proposed for measuring on the bench the NMR signal-to-noise ratio of rf probes, (over the range 1-100 MHz) and also the power deposited in patients during the imaging experiment. The technique is based on the principle of reciprocity, in that a direct relationship exists between the magnetic field generated (upon transmission) by a matched probe coil and the signal-to-noise ratio delivered by the same coil when used as a receiver. The construction and use of a calibrated sense coil for measuring the field is described, and the precautions and theory necessary for accurate measurement and understanding are outlined. Finally, the method is verified by comparison with a direct spectral measure of sensitivity obtained from a small doped water sample placed in NMR imaging equipment.

Calibration↗
PubMed1986

The field dependence of NMR imaging. II. Arguments concerning an optimal field strength.

Some of the factors involved in the choice of field strength for NMR imaging are examined. The influences of relaxation times and chemical shift upon image quality and signal-to-noise ratio are highlighted, and power deposition is introduced as a significant factor which may limit the flexibility and information available at higher fields as long as 180 degrees echo pulses continue to be necessary. Chemical-shift imaging is examined and found wanting as a means of coping with chemical-shift artifacts, and the use of multiple echoes (albeit with research) in conjunction with multiple-slice techniques is advocated as representing an efficient data-gathering scheme which can improve image signal-to-noise ratio. With such use, a medium field strength (0.5-1 T) is presented as representing, for general purpose imaging of head and torso, the best current compromise when imaging time is of major importance, with the important caveat that new techniques may always invalidate this conclusion.

Fourier Analysis↗
PubMed1984

Magnet field profiling: analysis and correcting coil design.

A full mathematical framework for the analysis and production of localized magnetic field profiles is presented. Of primary use in the production of highly homogeneous fields for nuclear magnetic resonance studies, the paper details the analysis of fields in terms of spherical harmonics, describes how field plotting in the appropriate manner may be used to obtain a direct measure of which harmonics are present, and shows how to combine basic "building blocks" to produce the various lower-order zonal and tesseral harmonics. "Building blocks" described include coils, arcs, and sinusoids of current as well as rings and arcs of steel. The use of shaped magnets is also briefly mentioned. Attention is drawn to the presence, in high-order designs, of possibly dominant lower orders of harmonics created by errors in fabrication. The goal of the paper is to present a design philosophy, backed by the appropriate mathematics, which is applicable to the variety of situations encountered in magnet design. Practical examples of correcting coils and "shims" are also given.

Humans↗
PubMed1984

Quadrature detection in the laboratory frame.

A theory of quadrature detection in the laboratory frame is developed. It is shown that the geometry of the two orthogonal coil systems needed for quadrature detection is radically different from that used with saddle-shaped coils, and that the homogeneity of the B1 field produced upon transmission is marginally better. The opposing quadrature phase shifts needed for transmission and reception are emphasized, and the use of a quadrature hybrid is advocated as a simple and inexpensive means of interfacing the transmitter, probes, and preamplifier. Experimental results are presented which confirm the theoretical predictions, and show that up to a 40% improvement in sensitivity and a twofold reduction in transmitter power are possible, particularly in those instances where the sample is conductive--namely, imaging of humans and in vivo spectroscopy.

Humans↗
PubMed1984

A parallel algorithm for rotating-frame zeugmatography.

An algorithm, which utilizes a high degree of parallel processing, has been developed for two-dimensional rotating-frame zeugmatography so that a picture of 256 X 256 pixels can be generated with a minicomputer system 2 sec after data accumulation. An array processor is employed as a second processor and, the refresh memory of a display unit is used as a fast data storage entity. Approximately half the calculations are carried out in the array processor during the nuclear relaxation period. Data movement to and from the display memory occurs largely during, and in parallel with, the array processing. The sequence can be run at a repetition rate of 35 msec per free induction decay and the algorithm can be implemented in any two-dimensional experiment of the Fourier transform genre.

Data Display↗
PubMed1980

Rotating frame zeugmatography.

The proton sensitivity obtainable at 5 MHz from a baby is sufficiently large to suggest the possibility of obtaining images with millimetre solution in seconds. Such goals present difficulties; for example, the high quality factor of the receiving coil necessary for good sensitivity limits the receiver bandwidth and leads to long pulse recovery times, and large, rapidly switched field gradients require power engineering. Solutions to these problems are being pursued at the National Institutes of Health (N.I.H.), where a high speed imaging system is under construction. For example, bandwidth and recovery time problems may be resolved with the aid of a low noise preamplifier with imaginary gain and Miller feedback, large gradients may be generated with a magnet comprising two rotatable hemispherical windings and switching of gradients may be eliminated by performing zeugmatography in the rotating frame. It is upon this latter aspect of the design that the present article concentrates.

Magnetic Resonance Spectroscopy↗
PubMed1976

Phosphorus nuclear magnetic resonance studies on normoxic and ischemic cardiac tissue.

The intact heart of a young rat was excised rapidly and cooled to 0 degree C; its energy-rich compounds were examined by 31P Fourier Transform nuclear magnetic resonance. The heart showed the characteristic spectrum of sugar phosphates, inorganic phosphate, phosphocreatine, and magniesium phates, inorganic phosphate, phosphocreatine, and magnesium ATP, characteristics of the energizing state of the nonbeating tissue. Warming to 30 degrees C imposes an energy load upon the heart consistent with short-term resumption of beating, concomitant intracellular acidosis, and decomposition of all detectable energy-rich compounds. The intracellular acidity causes a shift from pH 7.0 to 6.0. The effects of possible interferences with this pH measurement are considered. The method appears to have wide usefulness in cardiac infarct models for detecting the fraction of the total volume occupied by the infarct and for studying the effect of various proposed therapies upon this infarcted volume.

Adenosine Triphosphate↗
PubMedDate not supplied

Selective population inversion in NMR.

Population inversion of a selected region of a spectrum is a concept which has wide application in both NMR spectroscopy and imaging. While inversion of population at any one frequency is a trivial matter, ensuring an accurate inversion over a specified bandwidth, with negligible perturbation of the magnetization outside that bandwidth, is a major problem. However, by using as a driving function a complex radiofrequency (r.f.) pulse with an envelope of the form (sech beta t)1+5i where 1/beta is the temporal width and t is time, we have found that above a critical r.f. power threshold, magnetization is accurately inverted over a very sharply defined bandwidth, while outside that region, magnetization is returned to its initial position, and population is unaffected. Within the broad limits imposed by our equipment, we have also discovered that the phenomenon is independent of the incident r.f. power.

Magnetic Resonance Spectroscopy↗
PubMedDate not supplied

Frequency shift artifacts in MR imaging.

Chemical shifts may be expressed as distortions and displacements in magnetic resonance (MR) images. Specifically, in two-dimensional Fourier transform reconstructions such shifts produce visible displacements in the direction of frequency encoding. This is readily observable at 0.26 T with phantoms comprised of in vitro solutions with known chemical shifts and human tissues with disparate fat content. Moreover, frequency shift artifacts are visible in routine abdominal scanning at the interfaces of structures of differing fat content. Two common examples of this involve the vertebral body and intervertebral disk and the kidney and surrounding retroperitoneal fat. Without appropriate changes in gradients, such distortions may be expected to increase with increasing magnetic field strength.

Humans↗
PubMedDate not supplied

Hyperthermia system combined with a magnetic resonance imaging unit.

Magnetic resonance imaging (MRI) has recently been proposed as a method to monitor, noninvasively, temperature, blood flow, and cell metabolism during oncologic hyperthermia (HT). To heat and "image" simultaneously, it is necessary to combine a HT device and a MRI unit. As a demonstrative example of the problems associated with implementing such a system, a mini-annular phased array hyperthermia applicator was combined with a 0.5-T whole body MRI unit. With the aid of filters, baluns, and switches, the HT applicator and the MRI unit were made compatible. The overall system was tested using a muscle-equivalent, cylindrically shaped polyacrylamide gel phantom. No interference between the HT device and the MRI unit was observed. Noninvasive temperature images, with a resolution better than 1 degree C/cm, were obtained from images of molecular diffusion recorded before and during heating.

Humans↗