John R. Mallard

NUCLEAR MAGNETIC RESONANCE TOMOGRAPHIC IMAGING IN LIVER DISEASE

The non-invasive diagnostic technique of whole-body nuclear magnetic resonance (NMR) imaging was evaluated in 30 patients with established liver disease and 20 patients without liver disease. Comparison with diagnostic ultrasound and radionuclide liver scan shows that NMR easily differentiates malignant tumours from benign cystic lesions and provides useful information in patients with cirrhosis and metastatic deposits. In the demonstration of space-occupying lesions in the liver, NMR is as sensitive as ultrasound and more so than radionuclide liver scans when the metastases are less than 1.5 cm in diameter. In the demonstration of cirrhosis it is more sensitive than both ultrasound and radionuclide liver scan. The specificity of NMR is superior to both ultrasound and radionuclide liver scan, both of which only demonstrate the presence of lesions, whereas NMR tomographic imaging based on the proton spin-lattice time (T1) of tissue accurately indicates the nature of the lesion.

Field-Cycled Proton-Electron Double-Resonance Imaging of Free Radicals in Large Aqueous Samples

We have recently published a new method of imaging free radicals in aqueous solutions called proton-electron double-resonance imaging (PEDRI) ( I ). In this technique a conventional proton NMR image is collected while the EPR resonance of a free radical solute is irradiated. If the EPR irradiation has sufficient power, the NMR signal from those protons being relaxed by the paramagnetic solute is enhanced, and the parts of the sample containing free radical exhibit greater intensity in the final image. Unlike EPR imaging (2) the sample size in PEDRI is not constrained by magnetic field gradient requirements. In this Communication we present the first results of an extension of PEDRI which uses magnetic field cycling, allowing larger samples to be imaged with lower levels of applied radiofrequency power. PEDRI is an imaging version of a dynamic nuclear polarization experiment (35). The enhancement of the NMR signal upon irradiation of the EPR of the solute may be written empirically as

NMR imaging: image recovery under magnetic fields with large non-uniformities.

In the field of NMR imaging using selective excitation techniques, a procedure is described whereby image information can be recovered with acceptable fidelity in the presence of large magnetic field non-uniformities. It entails only the running of a reference uniform sample under the same conditions as the unknown and performing a kind of deconvolution of signals. By this means, a number of different kinds of distortion can be compensated, without requiring any detailed knowledge of the particular magnetic field non-uniformity involved.

Nuclear magnetic resonance pulse sequence and discrimination of high- and low-fat tissues

Abstract Signal size compared to independently measured T 1 is described for various pulse sequences on the Aberdeen Mk II nuclear magnetic resonance imager. The ability of these sequences to discriminate between certain tissue types, and in particular between adipose tissue and muscle, is discussed. Inversion recovery, with a t interval of 200 ms, gives the best discrimination for this purpose, with a contrast ratio of 6 between fat and muscle. Other image types, and especially T 1 , give better contrast for low-lipid soft tissues such as liver and spleen.

Errors in temperature measurement by thermocouple probes during ultrasound induced hyperthermia

A major problem in the use of localised hyperthermia for treatment of malignant tumours is to obtain an accurate measurement of the temperature in the tissue being treated. Thermocouple probes have generally been employed for measuring temperature elevation during ultrasound irradiation. However, when small objects such as thermocouples are in an ultrasound field in a medium such as tissue, viscous forces acting between the object and the tissue will cause an additional local rise in temperature (Fry & Fry, 1954a, b; Dunn, 1962; Hynynen et al, 1982). This will produce an error in any measurement of tissue temperature with invasive probes. The magnitude of the temperature elevation resulting from shear viscosity has been measured for 50 μm thermocouples in tumour tissue.

A radioisotope scanner for rectilinear, arc, transverse section and longitudinal section scanning: (ASS—the Aberdeen Section Scanner)

Abstract The authors describe the construction and operation of the Aberdeen Section Scanner (ASS) with a few clinical examples. The scanner is a dual headed device which can perform conventional rectilinear scans and the more recently developed section or tomographic scans. A punched paper-tape output from both heads is incorporated in the system so that count and position information may be fed into a PDP 81 computer enabling subsequent image processing and display. The same computer and display unit are optionally “on-line” to a Nuclear Enterprises gamma camera. The link to the ASS means that the advantages of computerized image processing are available for scans carried out with the new rectilinear scanner. Various design features are discussed together with the storage and display facilities. The routine operation of the scanner and the initial clinical results are also described.

Free radicals imaged in vivo in the rat by using proton-electron double-resonance imaging

A new technique called proton—electron double-resonance imaging is described for imaging free radicals in aqueous samples. The method is a combination of proton NMR imaging with nuclear electron double resonance. The results of using this technique to image free radicals in vivo in the rat are presented. Rats were injected intravenously with a nitroxide free radical solution and a series of images was obtained from which the clearance of the free radical through the liver and kidneys could be observed.

Design of ultrasonic transducers for local hyperthermia

Abstract Computer simulations of the interdependence of different pairs of the parameters of spherically focusing ultrasonic transducers were performed, taking the ultrasonic attenuation in biological tissue into consideration. The maximum distance and gain of the last intensity maximum were calculated with different frequencies between 0.3 and 5 MHz and frequencies around 1 MHz were found to be most useful for hyperthermia. The relationships between the distance of the last axial maximum and the radius of curvature, as well as between the intensity gain and radius of curvature with different diameters of ultrasonic transducers operating with a frequency of 1 MHz were calculated. Also the effect of thermal conduction on the temperature distribution was discussed.

Lyoluminescence dosimetry with some saccharides.

Lyoluminescent properties of mannose, glucose monohydrate, sucrose and trehalose dihydrate were investigated from the point of view of suitability as dosimetric materials in a new system of solid state dosimetry. Distilled water was used as a solvent. With a reader equipped with an uncooled PM tube it was possible to measure doses of X- and gamma-rays in the range from a few rad to about 100 krad. Various experimental factors which could affect the dose measurements using lyoluminescence are discussed, including the energy response for X-rays. Some preliminary information on the response of saccharides to fast neutrons is also included.

Particulate oral NMR contrast agents

Insoluble paramagnetic compounds in suspension can be used to achieve visualization of the gastrointestinal system on magnetic resonance imaging (NMR). Particulate preparations of these agents decrease the T1 and T2 of solutions to which they are added. Gadolinium oxalate, a prototype of these particulate agents, was evaluated in vitro and in vivo (in rabbits) by NMR imaging. The effect of this compound upon T1 and T2 in vitro was also quantitated by NMR spectroscopy. Opacification of the upper gastrointestinal tract was achieved with gadolinium oxalate following oral administration. The colon was visualized following rectal administration.