T2 weighting is a technique in magnetic resonance imaging to obtain a different set of contrast elements during the scan. It is performed by waiting for a particular interval after the tissue is subjected to the magnetic field before actually taking the scan.
MRI works by applying a very strong magnetic field to the tissue being studied. This induces a magnetic field in the tissue which can then be picked up and translated into a scan. However, this magnetic field dissipates very quickly. Moreover, the rate of dissipation is dependent on the type of tissue being scanned.
In T2 weighting, the technician times the scan of the magnetic field to occur at an interval where about 63% of the magnetic force has dissipated. Typically, this interval is calculated in thousandths of a second. As such compared to T1 weighting (where the scan is nearly simultaneous with the magnetic pulse), the magnetic field in the tissue is not as strong. However, since different tissues will have different T2 signatures at this stage, different tissues appear as different shades of white and black in the scan compared to a T1 scan.
For example, in a T1 scan of the brain, both empty space and space filled with cerebro-spinal fluid will appear dark compared to the brain matter. However, in a T2 scan, the cerebro-spinal fluid appears very bright while the empty space (such as that surrounding the brain) still appears dark. However, in both scans, the brain matter appears in different shades of grey. The skull will appear bright in a T1 scan, but light gray in a T2 scan.