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n  Fat saturation is an MRI technique used to suppress the signal from normal adipose tissue..  To suppress the fat signal for a given MR sequence a fat suppression module is typically

Fat suppressi

on imaging

By Roshan Shah

B.Sc MIT 3 rd

year 3 rd

batch

n

 Fat saturation is an MRI technique used

to suppress the signal from normal adipose tissue

 To suppress the fat signal for a given

MR sequence a fat suppression module is typically inserted at the beginning of an otherwise normal MRI sequence

n

 It is used in MRI for mainly two purpose.

To suppress the signal from normal adipose tissue to reduce chemical shift artifact or improve visualization

of uptake of contrast material

Tissue characterization, particularly

in adrenal gland tumors, bone marrow infiltration, fatty tumors, etc

Physics for fat

suppression

 (FAT SAT) FAT SAT) A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequence, so that they produce a negligible signal.

 This technique requires a homogeneous magnetic field and homogeneous volume of tissue.

 To prepare this type of sequence, the following properties should be used.

 Fat and water have different resonant frequencies

 They have different Larmor precession frequencies

 They have different T1 relaxation times.

 This method is reliable for contrastmaterial

enhanced T1 weighted imaging

 It is useful in tissue characterization particularly in area with a large amount of fat

 It also useful for avoiding chemical shift misregistration artifact

 Allows good visualization of small anatomical

details

 Fat suppression can be achieved in a number of different ways

 Short tau inversion recovery (FAT SAT) STIR)

 Chemical shift Selective (FAT SAT) CHESS) Sat.

Fat- Spectral Pre-saturation with Inversion Recovery

(FAT SAT) SPIR)

 Spectral Attenuated Inversion Recovery (FAT SAT) SPAIR)

 Water excitation

 DIXON-based

R

 It is an inversion recovery pulse

sequence with specific timing so as to suppress the signal from fat

Advantages:

1.It suppresses whole of the adipose tissue including water fraction

2.This is only method which can be used even in

magnetic field inhomogeneities.

3.It can be used with low magnetic field

strength.

Disadvantages:

4.Beginning at TI null most of the proton have not

completely relaxation, and are therefore still partially saturated, in this situation will overall produce signal loss and SNR ratio will decrease.

5 It cannot be used post gadolinium to demonstrate contrast enhancement.

3 Long acquisition time

4 Tissue contrast is affected SNR is reduced.

MRI image appearance

 Fluids normally appear bright and fat appear very

dark in a STIR images

 Pathological processes normally increase the water content in tissues Due to the added water

component this results in a signal increase on STIR images

Consequently pathological processes are usually bright

on STIR images

STIR sagittal sequence used in C spine imaging and knee

imaging

S

 Based on the chemical shift between fat and

water.

 In this short-duration RF-pulses tuned to

the resonance frequency of fat.

 RF-pulse tuned to the fat resonance frequency together with a spoiler gradient saturates and dephase fat protons, leaving only water

protons to produce a signal.

 Advantages

 It can be added to any pulse sequence.

 Can be used for post contrast imaging.

 Disadvantages

 It cannot be used at low field strength.

 It also cannot be use in Inhomogeneities of the

static magnetic field.

Can be used in post contrast MR arthrography.

MRI image appearance

MRI image appearance

 Advantages

SPAIR provide better and more homogeneous fat

suppression than SPIR

Tissue contrast is not affected

 Disadvantages

The inversion time is longer

Reduce number of slice for a given TR

MRI image appearance

 Instead of suppressing fat, these

techniques use a short series of

RF pulses(FAT SAT) binomial pulse) to

selectively excite water protons.

 No spoilers are needed.

 used primarily in the musculoskeletal system,

especially for evaluation of cartilage Some 3D applications in the breast and liver have also been reported

 Acquiring 2, 3 or more echoes at different

TE's, "water only" and "fat only" images

can be extracted

water

Out-phase

fat

 Advantages

Insensitive to B0 and B1 Inhomogeneities

4 contrasts delivered

in onemeasurement

 Disadvantages

Increases minimal TR because in- and opposed phase data must be acquired

 The Dixon technique is widely used in

abdominal imaging, imaging of the

extremities, and the spine.

 The opposed phased imaging is useful for

detection of small amounts of fat Ex-

adrenal gland tumors or steatosis.