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.

The current status of NMR imaging.

A brief account is given of the current status of nuclear-magnetic-resonance (NMR) imaging for medical purposes. The procedures in present use for two- and three-dimensional NMR imaging are outlined and examples given. The quality of images approaches that of computerized tomography X-ray scans and demonstrates superior tissue discrimination and pathological discrimination. Guidelines for safe operation are discussed. Clinical trials of commercial NMR scanners at an advanced stage.

Low inductance transverse gradient system of restricted length.

A new transverse gradient coil assembly of restricted length is presented. The coil is symmetric, has the advantage of simplicity, generates a remarkably large volume of uniform transverse gradient field, features very low inductance, and can therefore be suitable for applications requiring fast switched gradients. A prototype coil has been constructed to check computer simulations and to compare measured parameters of the system with those expected. Coils of this type may be used for MRI of the human head, and of animals, and for NMR microimaging.

Display of cross sectional anatomy by nuclear magnetic resonance imaging. 1978.

High definition cross-sectional images produced by a new nuclear magnetic resonance (NMR) technique are shown. The images are a series of thin section scans in the coronal plane of the head of a rabbit. The NMR images are derived from the distribution of the density of mobile hydrogen atoms. Various tissue types can be distinguished and a clear registration of gross anatomy is demonstrated. No known hazards are associated with the technique.

Display of cross sectional anatomy by nuclear magnetic resonance imaging.

High definition cross-sectional images produced by a new nuclear magnetic resonance (NMR) technique are shown. The images are a series of thin section scans in the coronal plane of the head of a rabbit. The NMR images are derived from the distribution of the density of mobile hydrogen atoms. Various tissue types can be distinguished and a clear registration of gross anatomy is demonstrated. No known hazards are associated with the technique.

An in vivo study of the fore-arm and hand by thin section NMR imaging.

A series of transverse thin section NMR images of the living fore-arm and hand is compared with the morphology of corresponding cadaver sections. Each image is derived from the distribution of the density of mobile protons within the section. The contrast and spatial resolution achieved indicate the potential information that future whole-body systems could provide.

Threshold photoelectron-photoion coincidence spectroscopy study of ChCl2F+, CHClF2+ and CH2ClF+: steric influence of the chlorine, fluorine and hydrogen atoms.

The threshold photoelectron spectrum and threshold photoelectron-photoion coincidence spectra of CHCl2F, CHClF2 and CH2ClF are reported in the range 11.3-24.8 eV. Tunable photoionizing radiation with a resolution of 0.3 nm is provided from a synchrotron source with a vacuum-UV monochromator. The coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Energetic comparisons suggest that the major products of the titled molecules dissociate in a similar manner at low photon energy, with the parent and first fragment ion, corresponding to cleavage of the weakest bond, appearing at their thermochemical thresholds. The second major ion, corresponding to cleavage of the second weakest bond, is formed ca. 1 eV higher than its predicted threshold, this disparity implying state-selected dissociation. CHCl2F and CHClF2 fragment in a similar manner at higher photon energies, with minor ions formed by the cleavage of three bonds possessing lower appearance energies than fragment ions formed by the cleavage of two bonds. CH2ClF displays the more expected behaviour, namely sequential bond cleavage as the photon energy increases. These observations can be rationalised in terms of the height of the barrier on the exit channel, as determined by the steric bulk of the leaving group. For the three titled molecules, mean translational kinetic energy releases have also been measured into the channels involving C-F or C-Cl bond fission. These data infer that impulsive dissociations occur at lower energy, with a trend towards statistical behaviour with increasing photon energy. Competition between statistical and impulsive processes is observed, for example C-Cl vs. C-F bond cleavage in CHCl2F+ and CHClF2+.

N.m.r. imaging of intact biological systems.

Internal images of structured objects may be obtained with n.m.r. by labelling component parts with different magnetic field strengths and therefore recognizably different n.m.r. frequencies. A linear field gradient generates a one-dimensional projection of nuclear density and a variety of techniques are employed to manipulate this one-dimensional probe to yield internal images in two and three dimensions. In the past few years, n.m.r. imaging, sometimes also called zeugmatography or spin mapping, has been applied progressively to provide proton images of small phantoms, fruit, vegetables and small animals, and finally to in vivo imaging of the human body; it promises to provide a valuable means of interior investigation of intact biological systems generally. For medical imaging the method is non-invasive, does not use ionizing radiations, appears to be without hazard and penetrates bony cavities without attenuation. Furthermore, other n.m.r. parameters, for example, relaxation times and fluid flow, may also be mapped; there is evidence that the relaxation times from tumours are significantly longer than those from corresponding normal tissue. Effort to date has mostly been concentrated on proton n.m.r., but some work has been done with other nuclei. Three examples are shown of n.m.r. images of intact biological systems: a fruit, an animal and a human system. The discussion includes the quantitative nature of the images, tissue discrimination, the relation between the resolution in the image and image acquisition time, attenuation and phase shift of the r.f. field in the biological tissue, and magnets suitable for n.m.r. imaging. In principle, all conventional n.m.r. techniques can be combined with n.m.r. methods in order to investigate heterogeneous systems. Overhauser imaging is briefly discussed.

Passive magnetic screening.

It is shown that a passive magnetic shield for a 1.5-T whole-body magnet requires about 20 tons of iron. Moreover, to first order, the amount of shielding material is independent of the radius of the shield. The choice between a thick shield fitting tightly round the magnet and a thinner shield of larger radius is determined by considerations of available space and the need for the highest uniformity of field in the bore. Very high permeability materials such as mu-metal are useful only in special circumstances. Multiple shields are valuable if a high degree of shielding is required, but the spacing between the shields needs careful attention. Although exact reciprocity of internal and external shielding is not found in the general case, the degree of shielding will be of the same order in both cases. The complete behavior of cylindrical shields around superconducting magnets can be determined by analytical solution of Maxwell's equations; for less regular shapes, solutions may be determined numerically by computer.

The Wellcome Foundation lecture, 1981. Nuclear magnetic resonance imaging in medicine: physical principles.

In recent years nuclear magnetic resonance (n.m.r.) has become a means of providing excellent images of the interior of the human body which are proving useful in medical practice. The development of n.m.r. imaging, much of which was pioneered in Britain, is outlined. Proton image resolution of human anatomy is comparable with X-ray computed tomography images, but without the hazard of ionizing radiation. There is improved soft tissue discrimination and pathological contrast through the basic imaging parameters of the proton density and the relaxation times T1 and T2, whose differences from one tissue to another are exploited by use of appropriate radiofrequency pulse sequences. Images may be obtained directly of transverse, coronal and sagittal sections of the head and body. Single slices or multiple slices may be imaged and imaging may be done in three dimensions. The lecture describes the more important imaging techniques and gives illustrative examples of images obtained. The efficient use of time in n.m.r. imaging is discussed, particularly mentioning the multiecho-multislice procedure and the development of real-time n.m.r. imaging. Magnetic field strengths in current use for proton n.m.r. imaging range from 0.02 to 2 T. At the lower end of the range resistive magnets are used, while for higher fields superconducting magnets are needed. A considerable improvement in image quality is obtained by use of special receiver coils.

An introduction to nuclear magnetic resonance in biomedicine.

In this paper the author illustrates the historical aspects of the development, first, of the fundamental principles of nuclear magnetic resonance and, second, the extension of these principles to magnetic resonance imaging and in vivo spectroscopy.

Nuclear magnetic resonance and the brain.

The first successful demonstrations of nuclear magnetic resonance (NMR) in bulk matter were reported in 1946 (Bloch, Hansen and Packard 1946; Purcell, Torrey and Pound 1946). Since then NMR has become a widespread technique for investigating matter of all kinds. In the 1970's NMR was applied to living systems, including man, in 2 distinct approaches. One application was in the production of images (Lauterbur 1973), called Magnetic Resonance Imaging or MRI, and the other in the production of NMR spectra (Moon and Richards 1973; Hoult et al. 1974), called Magnetic Resonance Spectroscopy or MRS. By appropriate manipulation of the NMR signal an NMR image may be generated. This can be a 2D image of a single slice, or a set of 2D images of parallel slices, or a 3D image. 2D images may be obtained directly in any orientation, axial, coronal, sagittal. The method uses no ionizing radiation and is inherently safe. It is non-invasive, although paramagnetic solutions may be injected intravenously to improve contrast. MRI images observed in normal clinical practice are maps of the NMR signals from water and fat in the tissues; they depend on proton density, but also significantly on the relaxation times T1 and T2. Images can be provided of flow (MR angiography) and diffusion (free, restricted or anisotropic). Images are typically 512 x 512 pixels with spatial resolution of about 0.5 mm. The images can be correlated with anatomical structures and indeed MRI is a primary source of such structures with localization precision of 0.5 mm as in CT.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular motions in solid estradiol studied by nuclear magnetic resonance spectroscopy.

The temperature dependence of the proton nuclear magnetic resonance (NMR) relaxation time T1 and second moment M2 of polycrystalline beta-estradiol hemihydrate (1,3,5-estratriene-3,17 beta-diol) is measured at frequencies of 14 and 25 MHz. Below 260 K relaxation is found to be dominated by C3 reorientation of the single methyl group in each molecule, characterized by an activation energy of 9.4 +/- 0.1 kJ/mol. Above 260 K another relaxation mechanism becomes evident, characterized by an activation energy of 22 +/- 2 kJ/mol and ascribed to motion of the water molecules in the solid.

Molecular motion in solid all-trans retinoic acid (vitamin A acid) by proton NMR.

An investigation of the molecular dynamics of all-trans retinoic acid (vitamin A) has been carried out in the solid state by proton NMR. Measurements were made of T1 at 14 and 25 MHz and of second moment at 25 MHz in the temperature range 55 to 420 K for both monoclinic and triclinic forms. In the monoclinic state relaxation was attributed to methyl group reorientation below 250 K and to conformational motion of the cyclohexene ring above this temperature. Activation energies and time constants were derived for the motions. In the triclinic state the same motions were active, but the cyclohexene ring motion was less restricted and some of the methyl group motions were more restricted indicating that hindrances to the motions were at least partly determined by the intermolecular packing arrangements.

Transverse gradient coil with circle current paths.

The paper describes a new type of transverse magnetic field gradient coil for applications in MRI and MRS. It uses full circle current paths rather than current arcs and has a particularly simple unit construction of symmetric form. It features a large volume of uniform transverse field gradient, high efficiency and low inductance for rapid switching. A prototype coil has been constructed and evaluated to check computer simulations. Two smaller sets have subsequently been made for NMR microscopy. This coil design may be used for all sizes.

1H NMR study of lithium D-lactate.

Polycrystalline D-lactic acid lithium salt [(R)-2-hydroxypropanoic acid lithium salt, lithium D-lactate] has been investigated by pulsed proton magnetic resonance methods between 77 and 300 K at 25 MHz. The main relaxation mechanism is methyl rotation; the motion is characterized by an activation energy Ea = 14.5 +/- 0.5 kJ/mol and time factor tau 0 = (1.5 +/- 0.5) x 10(-13) s. The activation energy is higher than the potential barrier obtained by ESR and ENDOR techniques for methyl rotation in the lactate radical. The methyl rotation is also responsible for a reduction of the dipolar second moment. Below 100 K the reduction of the dipolar second moment is ascribed to quantum-mechanical tunneling; an excitation energy of 6.1 +/- 1 kJ/mol is derived from a contribution to the spin-lattice relaxation times from the tunneling.

Molecular dynamics in solid riboflavin as studied by 1H NMR.

Spin-lattice relaxation times Tl and Tld as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B2) in the temperature range 55-350 K. The broad and flat Tl minimum observed at low temperatures is attributed to the motion of two nonequivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. An additional mechanism of relaxation in the high temperature region is provided by the motion of a proton in one of the hydroxyl groups. The Davidson-Cole distribution of correlation times for this motion is assumed.

Magnetic shielding of magnetic resonance systems.

Basic theoretical concepts of static magnetic shielding are summarized. Expressions describing the magnetic field produced by a solenoidal coil confined coaxially inside a long thin ferromagnetic cylinder of constant permeability are derived. Conditions for the optimum arrangement of a magnetic screen for whole-body NMR systems are discussed.