Image Contrast in MRI

Basic contrast in MR imaging relies on the differences in the density of water and fat, T1, and T2 of different tissues. The strongest signal is achieved with a long TR (allowing full spin realignment between excitations) and short TE (measuring the signal before significant T2 decay). Because the resulting image contrast is based mostly on the density of protons, such images are appropriately called proton density weighted (Fig. 5A). Beyond this, contrast is added only by making tissues darker. Adding image contrast based on differences in the T1 of tissues is achieved by reducing TR to a value about equal to the average T1 of the tissues of interest. Images with this modification are called T1-weighted images (Fig. 5B). This TR shortening preferentially darkens spins with longer T1 values, which do not recover as quickly as those with shorter T1 values. The addition of contrast based on differences in T2 is best achieved by measuring the MR image signal at a TE about equal to the average T2 of the tissues of interest, resulting in T2-weighted images

Figure 6.  Illustration of slice selection.  (A) Spins at rest are oriented vertically with the magnetic field.  A magnetic field gradient causes the magnetic field strength to be weaker on the left and stronger on the right so that the corresponding Larmor frequency is lower on the left and higher on the right.  Application of a radio frequency (RF) pulse excites a (B) plane of spins whose Larmor frequency falls within the frequency range of that RF pulse.

This increase in TE preferentially darkens spins with shorter T2 values, which decay more rapidly than those with longer T2 values. T1 and T2 contrasts are rarely combined because objects that remain bright on a T1-weighted image tend to be dark on a T2-weighted image and vice versa. Thus, the combination of these weightings would result in an image where all tissues are dark—some from insufficient T1 recovery of spin alignment with B0, others from T2 decay of the precessing magnetization.