T1 weighted image (also referred to as T1WI or the "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 magnetization vector (NMV). Basically, spins aligned in an external field (B₀) are put into the transverse plane by a radiofrequency (RF) pulse. They then slide back toward the original equilibrium of B₀. Not all tissues return back to equilibrium in the same amount of time, and a tissue's T1 reflects the amount of time taken for its protons' spins to 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 an 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. 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)

Practical tips

• signal hyperintensity on T1WI is an important finding and needs to be explained, the potential causes of this appearance are:

  • fat

  • methemoglobin

  • paramagnetic contrast media e.g. gadolinium-based agents

  • melanin

  • slow-flowing blood

  • proteinaceous fluid

  • calcium ³

  • copper ³

  • manganese ³

  • iron ³

• See also Hyperintense on T1-weighted images (mnemonic)

Summary

• TR: short

• TE: short

• fat: bright

• fluid: dark