Illustrations on MRI Techniques Using Matlab - Digital Signal Processing for Medical Imaging Using Matlab - page 52

Fig. 3.2 Illustration of the phase constants of the individual magnetic moments at every pixel of
the image slice of size 11 × 11 (refer Sect. 3.2 )

3. Apply 180 ◦ pulse and allow the magnetic moment to dephase for 0.3. During this
phase, after the time duration of 0.2, there exists the momentary peak occur in the
resultant magnitude of the magnetic moment of every pixel. This is called spin
echo (refer Sect. 3.2.2 ). Usually the transverse components reduces drastically
due to factors other than the spin-spin relaxation. The transverse component
becomes almost zero before the hardware perform the procedure to capture the
MRI signal. But spin echo guarantees the availability of the signal, so that the
hardware can capture the required signal.
4. Apply G x gradient for the duration of 0.2. During this period, there is the cancel-
lation of phase introduced due to the G x component. This is known as Gradient
echo. This helps to synchronize the hardware to sample at the particular sample
time to choose the proper position in the K-space. This is similar to the one used
in proton-density MRI image.
5. Sample the real and imaginary component of the signal s ( t ) to obtain the sample

.
6. Proper scaling factor is used so that the final basis (to perform IFFT2) look like
the standard form e − j 2 π xG x

of the K-space at ( G x , − G y )

e − j 2 π yG y

11 . This is the process of discretization.
7. The above steps are repeated for the complete scan in the K-space. For every
time, we have to wait for the longitudinal component to reach maximum before
applying 90 ◦ RF pulse. (This is illustrated by repeating the steps 1-5 by varying
the values of G x and G y ranging from

11

− 5 to 5 with the interval of 1).

The obtained K-space is viewed as the function of G x and G y

G y .TheK-
space is rearranged and are subjected to IFFT2 to obtain the MRI image f

=−

( x , − y )