MRI Contrast Agent T1 weighted image (also referred to as T1WI or “spin-lattice” relaxation time) is one of the basic pulse sequences in MRI and demonstrates differences in the T1 relaxation times of tissues.
A T1WI relies upon the longitudinal relaxation of a tissue’s net magnetisation vector (NMV). Basically, spins aligned in an external field (B0) are put into the transverse plane by an RF pulse. They then slide back toward the original equilibirum of B0. Not all tissues get back to equilibirum equally quickly, and a tissue’s T1 reflects the amount of time its protons’ spins realign with the main magnetic field (B0).
T1 weighting tends to have short TE and TR times.
Fat quickly realigns its longitudinal magnetization with B0, and it therefore appears bright on a T1 weighted image. Conversely, water has much slower longitudinal magnetization realignment after an RF pulse, and therefore has less transverse magnetization after a RF pulse. Thus, water has low signal and appears dark.
If T1WIs did not have short TRs, then all the protons would recover their alignment with the main magnetic field and the image would be uniformly intense. Selecting a TR shorter than the tissues’ recovery time allows one to differentiate them (i.e. tissue contrast).
T1-weighted sequences provide the best contrast for paramagnetic contrast agents (e.g. a gadolinium-containing compounds).
T1-weighted sequences include:
T1W spin echo (SE)
T1W gradient echo (GRE)
gadolinium postcontrast sequences (gradient echo sequences)
time of flight 2D or 3D MR angiography sequences
contrast-enhanced MR angiography
dual echo sequence (in-phase and out-of-phase)