Introduction

By now you are probably a regular user of the MR scanner and are familiar with the appearance of images, in terms of brightness or contrast (as seen in chapter 3) and with the digital nature of the image, as pixels or voxels (chapters 4 and 5). In this chapter we will examine how the scanner produces these voxels from MR signals.

An understanding of the image-formation process is particularly helpful for obtaining the optimum diagnostic information from an examination, modifying or creating new protocols, recognizing common image artefacts and taking measures to overcome or avoid them. It will also help as a basis for understanding the diverse data-acquisition strategies examined in chapter 12. It should be stressed here that understanding image formation in MRI is neither simple nor obvious and most people struggle to conceptualize it. There are a number of ways of understanding this and what matters is that you find a way that makes sense to you. Persistent students also find that eventually the penny always drops, a light bulb inside their brain suddenly switches on and usually, like the current in a superconducting magnet, it stays on forever.

In this chapter you will see that:

• magnetic field gradients form the basis of MR signal localization;

• 2D slices are produced by the combination of an excitation RF pulse and simultaneous slice-select gradient;

• the in-plane MR signal is encoded in terms of the spatial frequencies of the object using phase-encoding and frequency-encoding gradients;

• we collect or sample every spatial frequency that can exist within the image before we Fourier transform these data (known as ‘k-space’) to produce the image directly;

• inadequate or erroneous k-space sampling leads to certain image artefacts.