A.A Maudsley

In vivo NMR imaging of sodium-23 in the human head

Surgicale report the first clinical nuclear magnetic resonance (NMR) images of cerebral sodium distribution in normal volunteers and in patients with a variety of pathological lesions. We have used a 1.5 T NMR magnet system. When compared with proton distribution, sodium, shows a greater vriation in its concentration from tissue to tissue and from normal to pathological conditions. Image contrast calculated on the basis of sodium concentration is 7 to 18 times greater than that of proton spin density. Normal images emphasize the extracellular compartments. In the clinical studies, areas of recent or old cerebral infarction and tumors show a pronounced increase of sodium content (300–400%). Actual measurements of image density values indicate that there is probably a further accentuation of the contrast by the increased “NMR visibility” of sodium in infarcted tissue. Sodium imaging may prove to be a more sensitive menas for early detection of some brain disorders than other imaging methods.

Magnetic field measurement by NMR imaging

Measurement of magnetic field distributions is performed using a method based on the nuclear magnetic resonance (NMR) technique of Fourier imaging. By selecting objects which limit the spatial distribution of the NMR-observable nuclei, images can be obtained which allow easy interpretation of the magnetic field distribution and facilitate rapid shimming of magnets. Results are presented showing the effect of a shim coil set of a superconducting solenoid magnet of the type use for NMR imaging. The application of this type of measurement to the rapid shimming of similar magnet designs is analysed. The effects of steel plates close to the magnet are also observed.

Dynamic range improvement in NMR imaging using phase scrambling

Abstract Data collection efficiency in NMR imaging is impaired if the dynamic range of the receiver system is limited in comparison with that of the observed signal. This situation may occur in high-resolution proton imaging of large objects at high magnetic field strengths. The efficiency with which information is received can be increased by reducing the peak amplitude of the spin response by varying the phase distribution of the excited spins. This phase scrambling technique may be implemented using tailored RF excitation or by dephasing using nonlinear magnetic field gradients and can be applied in all dimensions of an acquired data set, providing a significant reduction in the dynamic range requirements of the detection electronics. Experimental results using 2D Fourier imaging have obtained up to 25 dB reduction in peak signal intensities. Image signal-to-noise ratios improved up to a factor of 6, with actual values dependent on experimental conditions. Simulation studies show that computational noise introduced during Fourier transformation is significantly reduced when phase scrambling is employed.

Sodium nuclear magnetic resonance imaging of myocardial tissue of dogs after coronary artery occlusion and reperfusion

Nuclear magnetic resonance (NMR) imaging techniques have been applied to the observation of tissue sodium-23 in normal and ischemic canine myocardium. To produce a region of ischemia and infarction in the myocardium, in six dogs a coronary artery was subjected to 1 hour of surgical occlusion followed by 1 hour of reperfusion. The dogs were then killed and sodium-23 NMR images of the excised hearts were obtained using a high field NMR imaging system. These images were compared with tissue sodium contents measured by flame photometry. The regions of ischemic damage were clearly visible as areas of increased sodium NMR signal on the three-dimensional images. A good correspondence was found between the relative intensity of the sodium signals and the sodium contents of normal myocardium and myocardium subjected to coronary artery occlusion and reperfusion. The data suggest the feasibility of NMR sodium imaging to detect the location and extent of myocardial damage in patients with coronary artery disease.