Illustrations on MRI Techniques Using Matlab - Digital Signal Processing for Medical Imaging Using Matlab - page 67

Fig. 3.10 a Actual magnitude of the transverse component immediate after applying the second
90 ◦ pulse; b K-space; c reconstructed image, corresponding to the time constant T 1

4. After approximately 3 times T 2 ( , transverse component is almost zero. Dur-

ing this duration, the longitudinal component gradually increases (refer Fig. 3.9 ).
This is described by the factor T 1 (

x , y )

, )

.
5. At this moment, apply another 90 ◦ degree pulse. This is required to obtain the
transverse component. Now the magnitude of the magnetic moment in the indi-
vidual pixel of the image slice are different at various locations (refer Fig. 3.10 a).
6. Apply G y gradient for the duration of 0.0000000009.
7. Apply

x y

G x gradient for the duration of 0.0000000009.
8. Apply Gx gradient for the duration of 0.0000000018. During this phase, after the
time duration of 0.0000000009, there is the cancellation of phase introduced due
to G x . This is known as Gradient echo. This helps to synchronize the hardware
to sample at the particular instant during real time to choose the proper position
in the K-space.
9. Sample the real and imaginary component of the signal s

−

( t ) to obtain the sample

.
10. 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.
11. The above steps (2-10) 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).