The strength of the signal depends on proton density and on two relaxation times, T1 and T2. T1 depends on the time the protons take to return to the axis of the magnetic field and T2 depends on the time the protons take to dephase. A T1-weighted image is one in which the contrast between tissues is mainly due to their T1 relaxation properties while in a T2-weighted image the contrast is due to the T2 relaxation properties. Routinely, T1-weighted images are obtained before and after intravenous contrast administration of a gadolinium chelate (i.e. gadopentatate dimeglumine). T2-weighted images are obtained when further tumour delineation or characterisation is felt necessary: images are acquired in the coronal and sagittal planes.
