REGION SELECTION IN NUCLEAR MAGNETIC RESONANCE INSPECTION

摘要

A method and apparatus is described for selecting a region of interest within an object whose nuclear magnetic resonance is to be investigated. Selection is achieved by locating the object within a zone within a generally uniform intense magnetic field, adding two or more magnetic fields so as to introduce a magnetic field gradient across the zone so that different regions of an object located within the zone are influenced by different magnetic field intensities so as to possess differing magnetic resonance frequencies. The magnetic fields are typically produced by electric currents in appropriately positioned coils and by varying the relative values of the currents so the resulting magnetic field gradient can be positioned and rotated of otherwise displaced within the zone. By following each incremental repositioning of the magnetic field gradient with an RF signal having a broad spectrum of finite energy frequency components and a defined band of zero of very low energy frequency components, so a region of the material within the zone can be affected so that the nuclea spin orientations in that region are aligned whilst those outside the region are random and by stepwise rotating the gradient direction and applying RF signals at each step so a region is defined with its axis coinciding with the axis of rotation of the magnetic field gradient.