Basic Physics of MR Signal and Image Generation - Advance Image Processing in Magnetic Resonance Imaging - page 20

MO 1

Medium T1

Short T1

Long T1

0

− MO − 1

t = TI

Time

FIGURE 1.8 Plot of Mz vs. time in the IR pulse sequence.

to this sequence is shown in Figure 1.9 . The initial 90 ° pulse rotates the equilib-

rium magnetization M 0 by 90 ° about the x ′ axis, creating transverse magnetization

oriented along the y ′ axis (Figure 1.9a and Figure 1.9b). During the following

period, τ = TE/2, this transverse magnetization decays because of spin-spin relax-

ation and the inhomogeneity of the B 0 magnetic field.
As previously said, inhomogeneity of the field causes the nuclei in different
regions of the sample to have different Larmor frequencies; this means that the
isochromats are off-resonance from the B 1 field to different extents and hence
will precess in the xy plane at slightly different frequencies. Therefore, individual
isochromats will be seen to dephase in the xy plane following the 90 pulse.

The position of a few isochromats at a time TE/2 after the pulse is shown in
Figure 1.9c; the isochromats are displayed spread out on both sides of the y

°

′
axis, because the Larmor frequency of some isochromats will be greater than
the nominal value ( ∆ω > 0) whereas that of others will be lower ( ∆ω < 0).

Consequently, isochromats with greater and less than zero will be seen to

precess in opposite directions in the rotating frame. At sufficiently long TE/2
values, isochromats will be completely dephased in the xy plane and M xy will
be reduced to zero. In addition to spin-spin relaxation, inhomogeneity of the
field contributes to the gradual reduction of the net magnetization in the xy
plane. However, the additional decay of the magnetization due to field inhomo-
geneity can be reversed, applying a 180

∆ω

° pulse. Application of a 180 ° pulse at

time TE/2 following the initial 90 ° pulse causes all the isochromats to rotate by

180 axis; this brings the isochromats to their mirror-image position

(Figure 1.9d). Following the 180

° about the x ′

pulse, the frequency and direction of preces-
sion of isochromats in the xy plane remain same as prior to the 180

°

pulse as
the Larmor frequency of each isochromat is unchanged. Hence, immediately

°