T1-weighted scans are a standard basic scan, in particular differentiating fat from water – with water darker and fat brighter use a gradient echo (GRE) sequence, with short TE and short TR. This is one of the basic types of MR contrast and is a commonly run clinical scan. The T1 weighting can be increased (improving contrast) with the use of an inversion pulse as in an MP-RAGE sequence. Due to the short repetition time (TR) this scan can be run very fast allowing the collection of high resolution 3D datasets. A T1 reducing gadolinium contrast agent is also commonly used, with a T1 scan being collected before and after administration of contrast agent to compare the difference. In the brain T1-weighted scans provide good gray matter/white matter contrast; in other words, T1-weighted images highlight fat deposition.
T2-weighted scans are another basic type. Like the T1-weighted scan, fat is differentiated from water – but in this case fat shows darker, and water lighter. They are therefore particularly well suited to imaging edema. On brain scans cerebral white matter (fat containing) therefore shows as darker than the grey matter. T2-weighted scans use a spin echo (SE) sequence, with long TE and long TR. They have long been the clinical workhorse as the spin echo sequence is less susceptible to inhomogeneities in the magnetic field.
T*2 (pronounced “T 2 star”) weighted scans use a gradient echo (GRE) sequence, with long TE and long TR. The gradient echo sequence used does not have the extra refocusing pulse used in spin echo so it is subject to additional losses above the normal T2 decay (referred to as T2′), these taken together are called T*2. This also makes it more prone to susceptibility losses at air/tissue boundaries, but can increase contrast for certain types of tissue, such as venous blood.
Spin density weighted MRI
Spin density, also called proton density, weighted scans try to have no contrast from either T2 or T1 decay, the only signal change coming from differences in the amount of available spins (hydrogen nuclei in water). It uses a spin echo or sometimes a gradient echo sequence, with short TE and long TR.
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