Introduction

Now that you have a basic idea of what MR images can show, we need to look a little more closely at the different types of contrast that are produced. We will use a very simple classification of the body tissues, which will be good enough to describe the basic appearances:

• fluids — cerebrospinal fluid (CSF), synovial fluid, oedema;

• water-based tissues—muscle, brain, cartilage, kidney;

• fat-based tissues—fat, bone marrow.

Fat-based tissues have some special MR properties, which can cause artefacts. Fluids are separated from other water-based tissues because they contain very few cells and so have quite different appearances on images. (Flowing fluids are rather complicated and their appearance depends on many factors including their speed; they will be dealt with in detail in chapter 13.) Pathological tissues frequently have either oedema or a proliferating blood supply, so their appearance can be a mixture of water-based tissues and fluids.

The various tissues have different signal intensities, or brightness, on MR images. The differences are described as the image contrast, and allow us to see the boundaries between tissues. For example, if a tumour is bright and brain tissue is mid-grey, we can detect the extent of the tumour (figure 3.1(a)).