# BASIC MRI SEQUENCES

Technologist at Cancer Institute Adyar-Retired. en Cancer Institute Adyar-Retired. Now working as instructor in Shri satya sai medical college
9 de May de 2018

### BASIC MRI SEQUENCES

1.  MRI sequence is order to combination of RF and gradient pulse designed to acquire the data to form the image.  Timing,order,polarity & repetition frequency of RF pulse and applied magnetic field gradients
2. BASIC SEQUENCES
3. Spin echo sequence(SE) Inversion recovery sequence (IR) Gradient echo sequence(GRE) Echo planar imaging(EPI)
4. FREE INDUCTION DECAY
5. Free induction decay (FID) refers a short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse
6. It is induced in the receiver coil by the rotating component of the magnetization vector in the x-y plane which crosses the coil loops perpendicularly
7. This sinusoidal signal decays exponentially with a time constant of T2*.
8. It does not contribute to form MR image.
9. FREE INDUCTION DECAY  Free induction decay (FID) refers a short-lived sinusoidal electromagnetic signal which appears immediately following the 90° pulse.  It is induced in the receiver coil by the rotating component of the magnetization vector in the x-y plane which crosses the coil loops perpendicularly.  This sinusoidal signal decays exponentially with a time constant of T2*.  It does not contribute to form MR image.
10. SPIN ECHO SEQUENCE  It consists of 90 and 180degree RF pulse  The excitatory 90 degree Rf pulse flips net magnetization vector along z axis and due to non homogeneity strat dephasing  By giving 180 degree pulse they strat rephasing to bring protons back into the phase  This rephasing increase magnitude of TM and stronger signal is induces in the receiver coil
11. It consists of 90degree and 180 degree RF pulse
12. Initially 90 degree pulse is given
13. After 90 degree pulse TM vector starts dephasing
14. Next we are going to give 180 degree pulse
15. After giving 180 degree pulse TM vector direction is changed
16. Now TM vector started rephasing
17. Getting good signal to form the image
18. PARAMETERS • Short TR(300-800ms) • Short TE(15-45ms) T1- anatomy • Long TR(1000-2000ms) • Long TE(90-140ms) T2- pathology • Long TR(1000-3000ms) • Short TE(15ms)PD
19. spin echo sequence Dual SE Fast SE Single shot fast SE modification
20. DUAL SPIN ECHO SEQUENCE  Two 180 degree pulse are sent after each 90 degree pulse to obtain dual echoes per TR.  PD+T2 double echo sequence is an example of this modified SE sequence.  It is run with longTR.  After the first 180 degree pulse, since TE is short, image will be proton density weighted (Long TR, shortTE).  Next 180degree pulse, TE will be long giving T2 weighted image (LongTR, LongTE).
21. DUAL SPIN ECHO SEQUENCE
22. FAST (TURBO) SPIN ECHO  In fast SE sequence we are giving multiple 180 degree rephasing pulses are sent after each 90 degree pulse.  In these sequence multiple echos are obtained per TR once echo with each 180 degree pulse.  All echos are used to fill a single k-space.  Since k-space is filled much faster with multiple echos in a single TR the scanning speed is increases considerablly.  Reduce phase encoding and spatial resolution.
23. Generally speaking contrast in fast spin echo is similar to spin echo and used in Muscoloskeletal regions Central nervous system pelvis
24. SINGLE SHOT FAST SPIN ECHO  Scan time is much reduced in SS-FSE than fast spin echo  All lines of k-space is filled in one TR  SS-FSE combines a partial fourier technique  Half of lines acquired in one TR and other half are transposed
25. Uses Cranial nerves Inner ear MR urogram MRCP Spinal imaging
26. INVERSION RECOVERY SEQUENCE  In this sequence we are giving first 180 degree pulse.  Second we are giving 90 degree pulse followed by 180degrees.  After that are we are taking signal.  TI- time of inversion is the time between first 180 degree pulse and 90 degree pulse.
27. INVERSION RECOVERY –MAJOR SEQUENCES FLAIRSTIR
28. STIR FLAIR
29. STIR S • SHORT T • TIME I • INVERSION R • RECOVERY
30. STIR  Fat has a short TI i.e. 80-150ms.  So fat can be suppressed at the STIR sequence.  This sequence can be used with T1 or T2W images.  Mainly used in body imaging.
31. FLAIR FL- fluid A- attenuation I-inversionR-recovery
32. FLAIR  The aim of FLAIR sequence is to suppress liquid signals, mainly in the brain .  Water has a long TI.  Nulling of water signal is seen at TI of 2000-2400ms.  Heavily T2W image is obtained. USES Neuro imaging Post contrast image
33. COMPARISION OF SES AND IRS
34. GRADIENT ECHO SEQUENCE  There is no 180 degree pulse in gradient echo sequence.  Rephasing of TM in GRE is done by gradients; particularly by reversal of the frequency encoding gradient.  Since rephasing by gradient gives good signal.
35. Initially 90 degree pulse is given
36. TM vector started dephasing due to inhomogeneity