Fat suppression

imaging

By

Roshan Shah

B.Sc. MIT 3rd year

3rd batch

Introduction

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.

Indication

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) 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.

Advantages

This method is reliable for contrast material 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.

Disadvantage

Inhomogeneities of the static magnetic field.

Inhomogeneities In the radio-frequency field.

Inhomogeneities in volume of tissue.

Methods

Fat suppression can be achieved in a number of different ways. Short tau inversion recovery (STIR)

Chemical shift Selective (CHESS) Fat-Sat.

Spectral Pre-saturation with  Inversion Recovery (SPIR)

Spectral Attenuated Inversion Recovery (SPAIR)

Water excitation

DIXON-based

STIR

It is an inversion recovery pulse sequence with specific timing so as to suppress the signal from fat.

Cont.…

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:

1. 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.

2. 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

CHESS

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.

Cont.…

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

SPIR

 It is the combination of the spectral saturation and STIR routines.

MRI image appearance

SPAIR

It is a hybrid technique combining features of both CHESS and STIR.

 SPAIR uses an adiabatic pulses.

Cont.…

AdvantagesSPAIR provide better and more homogeneous fat

suppression than SPIRTissue contrast is not affected.

DisadvantagesThe inversion time is longer.Reduce number of slice for a given TR.

MRI image appearance

Water excitation

This technique is based on the chemical shift.

Instead of suppressing fat, these techniques use a short series of RF pulses(binomial pulse) to selectively excite water protons.

No spoilers are needed.

Cont.…

Advantages No additional preparation pulse is

necessary.Reduced sensitivity to B1

Inhomogeneities Disadvantages

Increased total measurement timeReduced maximum number of slices.

Application

used primarily in the musculoskeletal system, especially for evaluation of cartilage. Some 3D applications in the breast and liver have also been reported.

DIXON-based

This technique is based on the chemical shift. It uses ‘In phase’ and ‘Out phase’ cycling of

fat and water. Acquiring 2, 3 or more echoes at different TE's,

"water only" and "fat only" images can be extracted

In-phase

water

Out-phase

fat

Cont.…

Advantages Insensitive to B0 and B1

Inhomogeneities.4 contrasts delivered in one

measurement. Disadvantages

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

Application

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.

Thank

you