Synonyms: T2-weighted gradient-echo, T2-GE, T2*, T2-star (Fig. 1.1).

T2- and T1-weighted gradient-echo sequences are very sensitive to magnetic susceptibility artifacts.

Methaemoglobin contains iron, which induces artifacts. These sequences are therefore widely used to detect a haemorrhagic component within a zone of infarction or old haemorrhage. Blood has a markedly hypointense signal. This sequence is also useful to demonstrate intravascular thrombus, particularly venous thrombus in the context of cerebral venous thrombosis.

Synonym: T2 and T1-SE (Fig. 1.2)

In addition to demonstrating signs of ischaemia about 3 h after onset of symptoms, as hypointensity on T1 and hyperintensity on T2, these sequences may also visualize arterial thrombus, as blood is markedly hypointense (flow void), while arterial occlusion gives an iso-intense or hyperintense signal (particularly useful for middle cerebral arteries).

TOF is an unenhanced T1-weighted angiography sequence. The vascular signal is favoured over that of surrounding tissues. The tissue signal is saturated and the blood flow signal entering the volume of interest is not saturated, thereby allowing reconstruction of the vessels.

Time-of-flight sequences may present artifacts due to slow or turbulent flow, which is why it is preferable to use contrast-enhanced T1-weighted volume-rendering sequences to explore neck vessels. Signal acquisition is performed during the first pass of the contrast agent bolus in the vessels explored. The contrast agent decreases the T1 relaxation time by a factor of 10, allowing a short repetition time and rapid volume acquisition for three-dimensional reconstructions.

Diffusion-weighted MRI allows exploration of random micro-movements of extracellular water molecules in a medium (Fig. 1.5). These movements depend on the obstacles encountered by water molecules in the medium. In the body, these obstacles essentially consist of proteins, cellular debris or nerve fibres. Free diffusion is said to be isotropic, i.e. water molecules diffuse freely in all directions in a pure medium such as CSF (Fig. 1.6). Isotropic diffusion can be restricted by the presence of an obstacle (protein or cells, etc.), as in the case of an abscess (Fig. 1.7) or a highly cellular lesion (lymphoma) (Fig. 1.8). Finally, diffusion can be anisotropic, for example when water molecules diffuse along nerve fibres (Fig. 1.9). In that case, water molecules no longer move freely in all directions, but preferably diffuse in the direction of the nerve fibres with more limited diffusion in the direction perpendicular to the nerve fibres.