Generation of the Magnetic Focus for Noninvasive Measurements of Biological Informations by NMR

Abstract

Previously we proposed a new application of NMR to measure biological materials in human body noninvasively, using the magnetic focusing technique on the measuring target (MT).
The present paper considers how to generate the magnetic focus and demonstrates this technique by using the low-field NMR spectrometer.
In the first parts of this paper, the signal error due to the fringe region of MT is evaluated theoretically and the constant field contours for several arrangements of multiple coils are calculated so as to generate the magnetic focus.
Simple results are obtained analytically for the axisymmetric field especially. Furthermore, a new approach which enlarges the zero-flux region is proposed to achieve a high S/N ratio.
In the latter parts, the construction of 100kHz NMR spectrometer, whose static field H₀ is generated by the main solenoid 70cm long and 40cm in diameter with two compensation windings, is described. The proton resonance signals from 1cm³ sample water are obtained with a S/N ratio of 15, which is more than ten times better than the value estimated from the relation S/N∝H₀^2/3.
By using the separate samples under the gradient field, a separate signal corresponding quantitatively to each sample is obtained. The signal from a certain specified area near the surface of a large sample has a shifted resonant frequency and an emphasized magnitude when the magnetic focusing technique is applied.