This document discusses techniques for fat suppression in MRI. It begins by explaining why fat suppression is useful for tissue characterization and contrast agent visualization. It then covers the molecular and magnetic properties that differ between fat and water, allowing various techniques to exploit these differences. The main techniques discussed are CHEMICAL SHIFT SELECTIVE (CHESS), WATER EXCITATION, DIXON, STIR, SPAIR, and SPIR. Each technique is explained in 1-2 sentences. The document concludes by comparing the techniques and discussing their common clinical applications.

1. FAT SUPPRESSION TECHNIQUES IN MRI
SAKSHAM KUMAR
3rd Year B.Sc RADIOGRAPHY
NIMHANS
BANGALORE

2. OVERVIEW
❖ WHY & HOW FAT SUPPRESSION
❖ FAT vs WATER IN MRI
❖ PHYSICS OF FAT SUPPRESSION
➢ CALCULATION OF WATER - FAT CHEMICAL SHIFT
➢ T1 RECOVERY
➢ T2 DECAY
❖ TECHNIQUES OF FAT SUPPRESSION
■ CHEMICAL SHIFT SELECTIVE (CHESS)
■ WATER EXCITATION
■ DIXON
■ STIR
■ SPAIR
■ SPIR
❖ APPLICATIONS - WHICH ONE & WHEN

3. WHY & HOW FAT SUPPRESSION
WHY ?
❏ Useful in MR Imaging to eliminate strong signals from fatty tissues that interfere
with the signals from adjacent areas and hence improves tissue characterization.
❏ Uptake of paramagnetic contrast agent in the tissue or lesion is visualized after
suppressing the fat.
❏ To improve dynamic range in water-containing structures such as cartilage when
used in conjunction with T1-weighted sequences.
HOW ?
❏ Lipid & water hydrogen proton behaves differently during an MR Imaging
acquisition and fat suppression techniques are based on these differences.

4. FAT vs WATER
★ MOLECULAR
STRUCTURE
H atom arranged
with C & O atom.
Better shielding.
Two H atoms arranged
with O atom. Less
shielding.
★ TUMBLING
RATE
Relatively slow. Relatively fast.
★ RECOVERY
RATE
Hydrogen in fat
recovers more
rapidly along the
longitudinal axis
& loses transverse
magnetization
faster than water.
Hydrogen in water
recovers slower along the
longitudinal axis & loses
transverse magnetization
slower than fat.
Magnitude of transverse magnetization vs amplitude of signal.
PROPERTIES FAT WATER

5. PHYSICS OF FAT SUPPRESSION
CALCULATION OF WATER - FAT CHEMICAL
SHIFT
❖ The strongly electronegative oxygen atom of water molecules
pulls away the protective electron clouds covering the hydrogen
nuclei. Because of this deshielding effect exposes hydrogen proton
to a relatively stronger local magnetic field. Hence they resonate
slightly faster than the more shielded protons in triglyceride (fat)
molecules. Therefore precessional frequency of magnetic moment of
fat nuclei slightly different from that of water results in CHEMICAL
SHIFT
❖ The chemical shift between fat and water has been measured
to be approximately 3.5ppm.
3.5 ppm

6. T1 RECOVERY VALUES
T1 RECOVERY IN FAT
❏ Occurs due to Hydrogen nuclei give up their
energy to surrounding molecular lattice acquired
from RF excitation pulse.
❏ Has low inherent energy & easily absorbs energy
into its lattice from hydrogen nuclei.
❏ Slow molecular tumbling allows T1 recovery
process relatively rapid as well as there is efficient
energy exchange from hydrogen nuclei to the
surrounding molecular lattice.
❏ Magnetic moment of fat hydrogen nuclei quickly
relax & regain their longitudinal magnetization.
T1 RECOVERY IN WATER
❏ Occurs due to Hydrogen nuclei giving up energy to
surrounding molecular lattice acquired from RF
excitation pulse.
❏ Has high inherent energy & does not easily absorb
energy into its lattice from hydrogen nuclei.
❏ High molecular mobility allows less efficient T1
recovery process and does not allow efficient energy
exchange from hydrogen nuclei to the surrounding
molecular lattice.
❏ Magnetic moment of water hydrogen nuclei take
longer to relax & regain their longitudinal
magnetization.

7. T1 RECOVERY IN FAT T1 RECOVERY IN WATER
DIFFERENCE IN T1 RECOVERY BETWEEN FAT AND WATER

8. T2 DECAY VALUE
T2 DECAY IN FAT
❏ The process is efficient
in hydrogen in fat
because of closely
packed molecules,
therefore spin - spin
interactions more likely
to occur.
❏ As a result,magnetic
dephase quickly &
therefore rapid loss of
coherent transverse
magnetization.
❏ So, T2 decay time is
short.
T2 DECAY IN WATER
❏ The process is less efficient
than fat because of
molecules are spaced apart,
therefore spin - spin
interactions less likely to
occur.
❏ As a result, magnetic
moment of hydrogen
nuclei dephase slowly,
therefore gradually loss of
coherent transverse
magnetization.
❏ So, T2 decay time of water
is long.

9. TECHNIQUES OF FAT SUPPRESSION
MAJOR TYPES OF FAT SUPPRESSION USED IN MR IMAGING

10. CHEMICAL SHIFT SELECTIVE (CHESS)
❖ Based on Chemical shift.
❖ A narrow band frequency selective RF pulse (90°)
excites the fat protons.
❖ The transversal magnetization is destroyed afterwards
by spoiler gradients, resulting in no fat magnetization
left for imaging.
❖ Technique is very versatile and can be appended to any
pulse sequence.
❖ Works best at 1.0T and higher and cannot be used
below 0.3T (due to close position of spectral peaks).
❖ Requires homogeneous field to work properly and thus
may fail around metallic hardware, imaging far from
isocenter, or in anatomic regions with susceptibility
distortions (sinuses, head and neck).

11. WATER EXCITATION
❖ Also based on chemical shift.
❖ Nulling of fat is controlled by precise timing of
the interpulse delays that allow water & fat to
go out of phase.
❖ Used primarily in the musculoskeletal system,
especially for evaluation of cartilage.
❖ Less sensitive than most other techniques.

12. DIXON
❖ Based on Chemical shift.
❖ Two series of images are acquired.
➢ In the first series signal from fat (Sf) & water (Sw)
protons are ‘In - phase’ (Sin). by selecting a TE when
the magnetic moment of nuclei in fat & water are in the
phase.
➢ In the second series signal from fat and water protons
are ‘Opposed - phase’ (Sop). by selecting a TE when
the magnetic moment of nuclei in fat & water are out of
phase with each other.
❖ A separate fat & water image can be calculated from equation.
❖ So, DIXON delivers up to four contrasts in one sequence
■ IN-PHASE, OPPOSED PHASE, WATER & FAT
images.
❖ Widely used in imaging of extremities, abdomen & spine.
Sin = Sw + Sf
Sop = Sw - Sf
Sin + Sop = ⦏(Sw + Sf) + (Sw - Sf)⦐⁄2
= (2Sw)⁄2 = Sw
Sin - Sop = ⦏(Sw + Sf) - (Sw - Sf)⦐⁄2
= (2Sw)⁄2 = Sf

13. STIR(Short Tau Inversion Recovery)
❖ Based on different T1 relaxation times of water & fat
protons.
❖ Prior to the excitation pulse of the sequence an
inversion pulse (180°) is applied, which inverts the
spins of all the tissues.
❖ When choosing T1 such that the longitudinal
magnetization of fat at the time when the excitation
pulse is applied is zero, the fat protons will not
contribute to the MR signal.
❖ The strength of desired fat suppression is possible
by modulating T1 values.
❖ T1 values should be in the range of 130-180 ms at
1.5T & in the range of 205-240 ms at 3.0T.
❖ Only technique even works in magnetic field
inhomogeneity.

14. HYBRID TECHNIQUES
SPIR (Spectral Presaturation With Inversion Recovery)
❖ Uses a spectrally selective inversion pulse to flip the fat spins by 110⁰.
❖ After an requisite inversion time (TI), a conventional excitation pulse is applied.
SPAIR (Spectrally Adiabatic Inversion Recovery)
❖ Uses a spectrally selective inversion pulse to flip the fat spins by 180⁰.
❖ Uses an adiabatic pulse to deal with RF spatial nonuniformity (B1 heterogeneity).
❖ After an requisite inversion time (T1), a conventional excitation pulse is applied.

15. APPLICATIONS - WHICH ONE & WHEN
❖ CHESS
➢ Used for fat suppression because of a rapid technique.
➢ Compatible with all pulse sequences (SE & gre)
➢ Fat protons are selectively excited & then dephased with a spoiler gradient.
❖ DIXON
➢ Preferable in case of magnetic field inhomogeneities (e.g..presence of metallic implant, large FOV
etc..)
➢ GOOD AND HOMOGENOUS FAT SUPRESSION.
➢ CAN GIVE FAT FRACTION OR WATER FRACTIONS.
❖ STIR
➢ Appropriate at low magnetic field because of reduced chemical shift between fat & water.
➢ STRONG FAT SUPRESSION WHEN COMPARED TO SPIR
❖ WATER EXCITATION
➢ Mainly used for cartilage imaging.
❖ SPIR/SPAIR
➢ Selectively suppress the fat.

16. CLINICAL APPLICATIONS

17. COMPARISON OF TECHNIQUES

18. CHEMICAL SHIFT ARTIFACTS
❏ Chemical shift is due to the differences between resonance frequencies of fat and water. It
occurs in the frequency-encode direction where a shift in the detected anatomy occurs
because fat resonates at a slightly lower frequency than water.
❏ Commonly found in Fat suppressed MRI sequences, and in MRS.
❏ In MRI, both spin echo sequences (SE) and gradient echo sequences (GRE) may
demonstrate chemical shift misregistration or mismapping. The mismapping occurs in the
frequency-encoding direction, and show up as a bright band on one side and a dark band on
the other side of a fat-soft tissue interface.

19. BIBLIOGRAPHY
★ www.radiopedia.org

20. THANK YOU