Illustrations on MRI Techniques Using Matlab - Digital Signal Processing for Medical Imaging Using Matlab - page 58

3.2.2 Momentary Peak Due to Spin Echo

The following experiment demonstrate the momentary peak of the resultant mag-
nitude at every pixel achieved after applying 180 ◦ pulse. This is due to rephasing.
This is called spin echo. Spin echo helps to obtain the significant signal for a while
so that the hardware can capture the required signal during dephasing. The experi-
ment involves obtaining the spatial domain image at three different time instances
as described below.
Time instant: 0.2

1. Initialize k and n .
2. Apply G y gradient for the duration of 0.1.
3. Apply the read-out G x gradient for the duration of 0.1.
4. Sample the value at the end of read-out duration.
5. Sample value corresponds to G x ,

G y .
6. Note that the resultant magnitude at every pixel (at time instant t = 0 . 2) corre-

sponds to the values obtained after dephasing for the duration of 0.2.

Time instant: 0.4
1. Initialize k and n (same as that of the one used in the case of time instant t = 0 . 2).

2. Apply G y gradient for the duration of 0.1.
3. Apply G x gradient for the duration of 0.1.
4. Apply 180 ◦ RF pulse.
5. Apply read-out G x gradient for the duration of 0.2.
6. Sample the value at the end of read-out duration.
7. Sample value corresponds to G x , − G y .

8. Note that the resultant magnitude at every pixel (at time instant t

4) corre-
sponds to the values obtained after rephasing for the duration of 0.2 (obtained
during read-out phase), after the time instant t

= 0 .

2.
9. This is the spin-echo instant at which momentary peak has occurred.

= 0 .

Time instant: 0.5

1. Initialize k and n (same as that of the one used in the case of time instant t = 0 . 2 ) .

2. Apply G y gradient for the duration of 0.1.
3. Apply G x gradient for the duration of 0.1.
4. Apply 180 ◦ RF pulse.
5. Allow relaxation explicitly for the duration of 0.1. This helps to cancel out partially
the phase introduced in steps 1 and 2. This step acts as the rephasing stage.
6. Apply read-out G x gradient for the duration of 0.2. During this phase, the first
0.1 s acts as the rephasing stage to obtain the echo at 0.4. The next 0.1 s acts as
the dephasing stage.
7. Sample the value at the end of read-out duration.
8. Sample value corresponds to G x , − G y .