Targeted magnetic resonance imaging (tMRI) of small changes in the T₁ and spatial properties of normal or near normal appearing white and gray matter in disease of the brain using divided subtracted inversion recovery (dSIR) and divided reverse subtracted inversion recovery (drSIR) sequences

This review describes targeted magnetic resonance imaging (tMRI) of small changes in the T₁ and the spatial properties of normal or near normal appearing white or gray matter in disease of the brain. It employs divided subtracted inversion recovery (dSIR) and divided reverse subtracted inversion recovery (drSIR) sequences to increase the contrast produced by small changes in T₁ by up to 15 times compared to conventional T₁-weighted inversion recovery (IR) sequences such as magnetization prepared-rapid acquisition gradient echo (MP-RAGE). This increase in contrast can be used to reveal disease with only small changes in T₁ in normal appearing white or gray matter that is not apparent on conventional MP-RAGE, T₂-weighted spin echo (T₂-wSE) and/or fluid attenuated inversion recovery (T₂-FLAIR) images. The small changes in T₁ or T₂ in disease are insufficient to produce useful contrast with conventional sequences. To produce high contrast dSIR and drSIR sequences typically need to be targeted for the nulling TI of normal white or gray matter, as well as for the sign and size of the change in T₁ in these tissues in disease. The dSIR sequence also shows high signal boundaries between white and gray matter. dSIR and drSIR are essentially T₁ maps. There is a nearly linear relationship between signal and T₁ in the middle domain (mD) of the two sequences which includes T₁s between the nulling T₁s of the two acquired IR sequences. The drSIR sequence is also very sensitive to reductions in T₁ produced by Gadolinium based contrast agents (GBCAs), and when used with rigid body registration to align three-dimensional (3D) isotropic pre and post GBCA images may be of considerable value in showing subtle GBCA enhancement. In serial MRI studies performed at different times, the high signal boundaries generated by dSIR and drSIR sequences can be used with rigid body registration of 3D isotropic images to demonstrate contrast arising from small changes in T₁ (without or with GBCA enhancement) as well as small changes in the spatial properties of normal tissues and lesions, such as their site, shape, size and surface. Applications of the sequences in cases of multiple sclerosis (MS) and methamphetamine dependency are illustrated. Using targeted narrow mD dSIR sequences, widespread abnormalities were seen in areas of normal appearing white matter shown with conventional T₂-wSE and T₂-FLAIR sequences. Understanding of the features of dSIR and drSIR images is facilitated by the use of their T₁-bipolar filters; to explain their targeting, signal, contrast, boundaries, T₁ mapping and GBCA enhancement. Targeted MRI (tMRI) using dSIR and drSIR sequences may substantially improve clinical MRI of the brain by providing unequivocal demonstration of abnormalities that are not seen with conventional sequences.