What are the different types of spin echo of magnetic resonance imaging (MRI)? What are the functional characteristic features of T1,T2 and STIR echo sequences?
The spin echo sequence is a fundamental pulse sequence in MRI A single RF pulse generates a free induction decay (FID) but 2 successive RF pulse produce a spin echo (SE). The time b/w the middle of the first RF pulse and the peak of the spin echo is called echo time (TE)
The different types of spin echo of MRI are
Single echo spin echo
simplest form of SE
consist of 90º – pulse and 180º pulse and an echo
Multi-echo spin echo
As long as T2 relaxation has not completely destroyed the MR signal it is possible to stimulate the system c additional 180º pulse(s) and generate additional echoes.
The amplitude of each echo is progressively smaller due to T2 decay.
echoes are uniformly diminishing.
Fast (Turbo) spin Echo (FSE)
Multiple 180º pulses and echoes also followed each 90º pulse.
echoes are not uniformly diminishing in size c increasing TE due to different phase encoding gradient are being applied c each 180º pulse.
The advantage is
decreased scan time with maintained SNR
Motion artifact are less severe
This technique cope better c poorly aligned magnetic fields than conventional spin echo
Fundamental characteristic features of
Longitudinal relaxation time constant.
Transverse relaxation time constant.
It is the time taken by the body's atom to realign itself after being disturbed by radiofrequency pulse applied in longitudinal axis.
Time taken by the tissue nuclei to realign itself after being disturbed by radiofrequency in a transverse axis.
Non enhanced T2 weight MRI IDENTIFIES
1. Bone marrow
3. Subacute haemorrhage
Fat subacute haemorrhage, melanin, protein rich fluid
Low proton density, calcification
Low proton density, calcification,fibrous
deoxy haemoglobin, inhancellular
methemoglobin, Ferritin, hemosiderin,melanin
Protein rich fluid
Methemoglobin in late sabaute haemorrhage
Factors to determine image contrast and weighting of MRI image
TR (the repetition time)
TE (the echo time)
Short TE <45ms
Long TE>45 ms
Poor constrast Not used
T1 weighted → short TR/Short TE
PD (proton density weighted) → Long TR/short TE
T2 weighted → Long TR/Long TE
STIR (Short TI inversion Recovery)- only fat suppression method available .
STIR is an inversion recovery pulse sequence that uses a TI (Time of inversion) which correspond to the time it takes fat to recover from full inversion to transverse plane so that there is no longitudinal magnetization corresponding to fat.
When 90º RF pulse is applied after the delay time TI the signal from fat is nullified.
STIR is used to achieve suppression of the fat signal in the T1 weighted image. e.g. TI of 150-175 ms achieves fat suppression (variation)
It produces excellent depiction of bone marrow oedema, bone metastases.
Limitation of STIR
It cannot be used as a fat suppression technique post-gadolinium.
Overall signal to noise ration is poor.
The multiple 180º pulses cause deposition of extra energy and result in tissue healing
FLAIR (Fluid Attenuated Inversion Recovery)
Another variation of the inversion recovery sequence.
The signal from fluid . e.g CSF is nullified by selecting a TI corresponding to the time of recovery of CSF from 180° inversion to the transverse plane .
It suppresses the high CSF signal in T2 and proton density weighted images so that pathology adjacent e.g. ∼2000 ms achieves CSF suppression at 1.5 T.