Making an Image: Locating and Encoding Signals in Space

Abstract

Single MR echoes produced by rf pulses alone cannot be used to produce an image as they do not contain any information about position. This information is introduced by using the gradient coils described in to generate magnetic field gradients. When a gradient is applied in a particular direction, this causes the strength of the magnetic field and hence, the Larmor frequency to depend on position along that direction. MR echoes that are generated and measured in the presence of magnetic field gradients therefore contain spatial information that can be used to build up an image. The sections that follow describe the most commonly used method to build up a cross-sectional image (or image slice) using rf pulses and gradient magnetic fields.