Tissue Suppression Techniques

1
Tissue Suppression Techniques

• Inversion Recovery
• Chemical (or spectral) saturation or
  frequency-selective presaturation
• Spatial presaturation in the field of view

2
Inversion Recovery (IR) Techniques

• If TI(ln 2) T1(tissue x)0.693 T1 (tissue x)
  then tissue x is nulled
• STIR (Short TI Inversion Recovery)
• At 1.5T, T1 of fat is approximately 200ms. Then
  TI0.693 x 200 ms140 ms
• FLAIR (Fluid Attenuated Inversion Recovery)
• At 1.5T T1 of CSF is approximately 3600ms. Then
  TI0.693 x 3600 ms2500 ms
• Fast FLAIR
• TR/TI/TEeff10000/2500/112ms, ETL8

3
MR tissue properties
4
Fat Suppression STIRShort Ti IR
Mz
100
MAGNETISATION along Z -AXIS
0
time/ms
-100
100
200
300
400
-Mz
5
Fluid Attenuated IR (FLAIR)
Mz
100
0
MAGNETISATION along Z -AXIS
time/ms
-100
-Mz
2000
6
GRASE vs. Fluid Attenuated IR (FLAIR)
T2 GRASE
Flair
TR 4500 / TF 8 / EF 3 / TE 100
TR 7000 / Ti 2100 / TE 150
7
Advantages and Disadvantages

• Advantages
• No extra RF heating
• No variability caused by magnetic field
  inhomogeneities
• Disadvantages
• Tissues with similar T1 values are all suppressed
  and thus cannot be differentiated
• Long acquisition times caused by long TR
• Low SNR

8
Chemical (spectral) presaturation

• At 1.5T water protons precess 220Hz faster than
  fat protons
• Presaturation pulse with resonance frequency of
  water protons(or fat protons) is applied
  immediately before the 90o pulse of the SE

9

• Advantages
• Resolves tissues with similar T1 values (fat and
  gadolinium-enhanced tumors, or fat and blood
  products)
• No influence on the signal from tissues other
  than the on being suppressed
• Disadvantages
• Suffers from sensitivity to magnetic field
  inhomogeneities
• Requires extra time

10
Spatial Presaturation

• Reduce motion and flow-related artifacts fo
  structures adjacent to-or actually within the FOV
  nest to-the region of interest.
• Spatial presaturation is accomplished by applying
  an extra 90o pulse before the 90o pulse of the
  SE.
• Imaging of the spine a saturation band is placed
  within the FOV anterior to the vertebral bodies
  to suppress artifacts arising from the heart and
  great vessels
• MRA Saturation pulses are placed outside the FOV
  at one end of a vessel to suppress either venous
  flow (for arteriography) or arterial flow (for
  venography)

11
Spatial Presaturation
12
Magnetization Transfer (MT)

• Suppress protein-bound water protons whose
  resonance frequency is approximately 500 to 2500
  Hz away from that of bulk water protons.
• MT saturation pulses are simply off-resonant
  pulses with their center frequency 1000 to 2000
  Hz removed from the Larmor frequency of protons
  and a BW of several hundred to several thousand Hz

13
(No Transcript)
14

• Application
• Time of Flight (TOF) MRA
• To suppress the background brain tissue to
  enhance the visualization of smaller, more
  peripheral vessels
• MT effect in FSE
• In FSE the rapid 180o pulse have a broad BW that
  contains frequencies off the bulk water resonance
  frequency.