BASICS OF MRI

Based on complex interaction between

—— Protons in human body

—— Magnetic field  

—— Radiofrequency energy

 Object to be imaged is placed in a powerful, uniform magnetic field,(B0).

The spins of atomic Nuclei are characterized by

Nuclei align parallel or anti-parallel to B0

As the RF pulse continues, the spins change lower energy to higher energy state.

This leads to “tipping” of the net magnetization toward the transverse plane.

¢¢ Spins phases are coherent (aligned with each other).

¢¢ When Rf is shut off - Spins lose their phase coherence & the signal decays. This process is called transverse relaxation

—— TR (Repetition Time ): Interval between Rf pulses.

—— TE (Echo time): Time between Rf pulse & signal reception.

BASIC IMAGING SEQUENCES

¢¢ T 1 WI

¢¢ T 2 WI

¢¢ T2 * WI

¢¢ GRADIENT ECHO

¢¢ FLAIR

¢¢ STIR  

IMAGING CHARACTERISTICS 

¢¢ T1 WI: TR & TE short

T2 WI: TR & TE long

¢¢ T1 WI: Dark - Water, CSF, edema, Calcium (can be pradoxically bright because of crystalline structure of calcium)  

           Bright - Lipid ,Gadolinium,subacute

blood, protein, Mn,  melanin

¢¢ T2 WI: Dark - Calcium, bone  

       Bright - CSF, water, edema 

¢¢ ECHOPLANAR IMAGING:  

¢¢Single excitation used to collect all multiple images

(40ms)

¢¢Used in applications highly sensitive to even minor proton movement.

USED IN:

vv Diffusion MR vv Perfusion MR vv Functional MR

GRADIENT ECHO

- An excitation pulse with a flip angle lower than

90°  

- No 180° rephasing pulse  -TR is very short and  scan time very less (sec)

-Visualise   hemosiderin and ferritin

-USED IN   ØØMRA  ØØCISS  (Constructive interference in

steady state

T1-WEIGHTED MRI 

¢¢ Use a  (GRE) sequence - short TE and TR.  

¢¢ Due to the short repetition time (TR) this scan can be run

very fast allowing the collection of high resolution 3D datasets.

¢¢ Basic types of MR , contrast used, is a commonly run

clinical scan.  

¢¢ The T1 weighting can be increased (improving contrast)

with the use of an inversion pulse as in an MP-RAGE sequence. ¢¢ Provide good gray matter/white matter contrast.

(ANATOMY)

T2-WEIGHTED MRI 

ØØUse a Spin Echo (SE) -long   T E and  T R.

ØØSE  less susceptible to inhomogeneity in      the magnetic field.

ØØ Well suited to edema as they are sensitive      to water content (edema is characterized by      increased water content):PATHOLOGY T*2-WEIGHTED MRI 

¢ T*2 -  (GRE) sequence- long TE and long TR.

¢ Gradient echo sequence used.  

¢ Does not have the extra refocusing pulse used in        spin echo

¢ So it is subjected to additional losses above the normal T2 decay (referred to as T2′), these taken together are called T*2.  

¢ Increase contrast for certain types of tissue, such as venous blood

FLUID ATTENUATED INVERSION RECOVERY(FLAIR)

 Sequence used to null signal from fluids.

E.g.CSF so as to bring out lesions at fluidparenchyma interface  

 Choosing the inversion time TI (the time between the inversion and excitation pulses), the signal from any particular tissue can be suppressed

CLEAR FLUID  in a CLOSED SPACE will be supressed

FAST FLAIR: Fast spin echo plus flair      

 USES

1. For periventricular & subcortical

abnormalities:  (Cortical & juxtacortical multiple sclerosis lesions,  degenerative diseases).

2. In seizure disorders (e.g MTS):  

—— Sensitive for detecting signal abnormalities demonstrating size asymmetry & abnormal signal within the atrophied hippocampus