The simplest way to think about the multitude of sequences available on modern scanners is to divide them according to the dominant influence on the appearance of tissues. This leads to a division of all sequences into proton density (PD) weighted, T1 weighted, T2 weighted, diffusion weighted, flow sensitive and 'miscellaneous'. A number of 'optional add-ons' can also be considered, such as fat or fluid attenuation, or contrast enhancement. This leads to a broad categorisation as follows:
- T1
- gadolinium enhanced
- fat suppressed
T1 weighted sequences
T1 weighted sequences are part of almost all MRI protocols and are best thought of as the most 'anatomical' of images, resulting in images that most closely approximate the appearances of tissues macroscopically, although even this is a gross simplification.
The dominant signal intensities of different tissues are:
- fluid (e.g. urine, CSF): low signal intensity (black)
- muscle: intermediate signal intensity (grey)
- fat: high signal intensity (white)
- brain
- grey matter: intermediate signal intensity (grey)
- white matter: hyperintense compared to grey matter (white-ish)
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 magnetisation 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 its protons' spins realign with the main magnetic field (B0).
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
Summary
- TR: short
- TE: short
- fat: bright
- fluid: dark
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