Answer to Question #165948 in Physics for Sam

Question

Describe how MRI work  with a labelled diagram     

Answer 1

An MRI scanner is made up of four components: the magnet, gradient coils, r.f. transmitter and receiver, and the computer. 

 The Magnet

The magnet is the most expensive part of the whole scanner. The earliest systems were based around water-cooled resistive magnets, and for particular applications it is possible to use permanent magnets, but the majority of modern scanners use superconducting magnets. The reason for this is the high fields now desirable for MRI. Whole body resistive and permanent magnets are limited to around 0.3 T field strength, before their weight becomes prohibitively large. Superconducting magnets are able to generate much larger fields, and there are a number of 4.0 T whole body scanners now available. These magnets are constructed from materials such as NbTi alloy, which below a critical temperature of about 9 K loses its resistivity. Once started the current will flow in the coils indefinitely, provided that the temperature is kept below the critical temperature by cooling with liquid helium. The fields from such magnets are very stable with time, which is essential for an MRI system.

The Gradient Coils

The requirement of the gradient coils are twofold. First they are required to produce a linear variation in field along one direction, and secondly to have high efficiency, low inductance and low resistance, in order to minimise the current requirements and heat deposition.

Linear variation in field along the direction of the field (traditionally labelled the z-axis) is usually produced by a Maxwell coil. This consists of a pair of coils separated by 1.73 times their radius. Current flows in the opposite sense in the two coils, and produces a very linear gradient.

To produce a linear gradient in the other two axes requires wires running along the bore of the magnet. This is best done using a saddle-coil, such as the Golay coil. This consists of four saddles running along the bore of the magnet which produces a linear variation in Bz along the x or y axis, depending on the axial orientation. This configuration produces a very linear field at the central plane, but this linearity is lost rapidly away from this. In order to improve this, a number of pairs can be used which have different axial separations. If a gradient is required in an axis which is not along x,y or z, then this is achievable by sending currents in the appropriate proportions to Gx, Gy and Gz coils. If for example a transverse gradient G at an angle q to the x-axis is required, then a gradient Gcosq should be applied in the x direction, and Gsinq in y.