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

1. Single echo spin echo
   • simplest form of SE
   • consist of 90º – pulse and 180º pulse and an echo
2. 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.
3. 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

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Factors to determine image contrast and weighting of MRI image

1. Spin echo
2. TR (the repetition time)
3. TE (the echo time)

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• 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

1. It cannot be used as a fat suppression technique post-gadolinium.
2. Overall signal to noise ration is poor.
3. 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.