MRI FAT Suppression
The reason for fat suppression…
1. Objects that you want to determine whether or not fat
2. Artifacts reduction
3. Fat signal suppression through vision can improve visibility
1. It is the differential diagnosis of benign and malignant in the absence of fat.
2. Because it will cause artifacts of high fat, abdominal breathing synchronized to prevent use.
3. By eliminating the high signal to be fat.
This way the dynamic range other than fat spreads and we are easier to observe the objects.
That is why we add fat suppression after contrast-enhanced imaging.
Types of fat suppression
It is based on the difference of T1.
It uses chemical shift.
The difference of T1 is STIR.
After applying the IR pulse, we wait until the longitudinal magnetization of fat becomes 0 and take an image.
The time until fat signal becomes 0 is called TI(inversion time).
At 1.5 T, T 1 of fat is about 250 ms, so TI = 0.693 × 250 = 170 ms.
In 3T, T1 will be extended so it will be longer than this.
STIR is not fat specific and depends on T1.
Even though it is not fat, T1 close to fat are suppressed.
After contrast-enhanced imaging, you must be careful to organizations that T1 shortening.
It is a method independent of the strength of the static magnetic field.
STIR is reversed in all tissues, fat recovery is the fastest, collected signal is negative.
For this reason we will display the absolute value.
Also, other tissues other than fat gather signals during recovery, so SN decreases.
There are several kinds using chemical shift.
First of all, it is CHESS method.
If it is 1.5 T, the difference between the resonance frequency of water and fat is 224 Hz.
This is a method of applying 90 ° pulse to suppress the signal 224 Hz away from water.
If you can suppress the signal in this way it is fat.
The higher the magnetic field, the wider the resonance frequency difference, CHESS method is advantageous.
In 3T, the difference between the resonance frequency of water and fat is 447 Hz.
Next, it is SPAIR method.
This applies a frequency selective 180 ° IR pulse, and then start imaging wait until the longitudinal magnetization of fat is 0.
This becomes stronger to B1 non-uniformity.
The difference with STIR is that it is frequency selective.
SN other than fat does not decrease.
It takes more time than the CHESS method, but the effect of fat suppression increases.
In fat suppression using chemical shift, there is fat (5.3 ppm, 4.2 ppm) with no difference between water and chemical shift.
The fat signal can not be suppressed.
Next, it is Dixon method.
At 1.5 T, the 224 Hz resonance frequency is different, so the phases (in phase) will be the same every 1/224 = 4.5 ms.
It is opposite phase at 2.3 ms (opposed phase) of this half.
When collecting signals at TE 2.3 ms, water and fat signals in the same voxel are canceled out and the signal decreases.
Signals of parts that are only fat in voxels such as subcutaneous fat will not decrease.
Next, it is a method using binomial pulse.
In accordance with the timing of in phase and opposed phase, excitation pulse is split and only water is excited.
For example, if it is a 90 ° pulse, apply 45 ° pulse first and apply 45 ° pulse at the opposed phase timing, the fat magnetization will be 0.
The division method is Pascal’s triangle: 1: 1, 1: 2: 1, 1: 3: 3: 1.
The greater the number of divisions, the greater the effect of fat suppression, but the shortest TE will extend.
Since I am a beginner in English, please correct the mistake grammar.