Introduction

By using the mono-exponential curve described in the previous sections, we use a single compartment tissue model. This means that we assume that all tissue components contained in a voxel have the same T₂ relaxation time. However, in practice, the assumption of T₂ decay uniformity within a voxel can result in inaccurate fittings, as a voxel may contain different tissue compartments with different T₂ times. For example, a voxel at a tissue boundary inside the brain can contain both cerebrospinal fluid and gray matter, a phenomenon which is also commonly referred to as the partial volume effect. In such cases, it would be preferable to compartmentalize different tissues inside a single voxel: this is made possible with multi-exponential T₂ mapping, where we consider the T₂ relaxation contribution of each tissue compartment within a voxel. The multi-exponential T₂ mapping method, which will be described in this section, can be useful in many applications, such as myelin water fraction imaging which will be explained further in the section on applications.