Basic Physics of MR Signal and Image Generation - Advance Image Processing in Magnetic Resonance Imaging - page 18

Interrogation of the nuclear spin system with multiple pulses provides infor-
mation that is not accessible via one-pulse experiment. In fact, multipulse experi-
ments, such as multiple gradient-echo, inversion-recovery, and spin-echo sequences,
enable experimental determination of proton density, T1, and T 2, respectively.

1.7.1 G RADIENT E CHO

Gradient-echo pulse (GE) includes an -degree RF pulse (usually a low flip angle

is defined for fast imaging) followed by a time-varying gradient magnetic field
in order to generate (and then acquire) an echo signal instead of a decaying FID.

α

For physical concepts of magnetic field gradients, see Section 1.8 . The key

concept underlying GE formation is that a gradient field can dephase and rephase
a signal in a controlled fashion so that one or multiple echo signals can be created.
After the application of an -degree RF pulse, a negative gradient (for example,

along x axis) is switched on; as a result, spins in different x positions will acquire
different phases, which can be expressed as:

α

t
=− −

∫

ϕ (,) xt γ xGd τ γ xGt (1.25)

x x

0

so that the loss of spin phase coherence becomes progressively worse as time
elapses after the excitation pulse. When the signal decays to zero, a positive
gradient of the same strength is applied; the transverse magnetization components
will gradually rephase, resulting in a regrowth of the signal. The spin phase angle
is now given by:

t

∫

ϕ (,) xt =− γ xGt ′ +−

′

γ xGd τ (1.26)

x x

t

The phase dispersal introduced by the negative gradient is gradually reduced over
time after positive gradient is switched on at t = t ′ . After a time t ′ , the spin phase

ϕ is zero for any x value, which means that all the spins have rephased, and

therefore an echo signal is formed. Time t ′ is called echo time , TE.

1.7.2 I NVERSION R ECOVERY

In the inversion-recovery (IR) pulse, the equilibrium magnetization M 0 is ini-
tially perturbed by a 180 pulse. Following a short time period TI (time of

inversion), a second perturbation is introduced in the form of a 90

°

° pulse. This

can be written as a 180 -FID sequence. In order to examine the effects

of this pulse sequence, assume that the RF pulses are applied along the x

° -TI-90 °

′ axis

of the rotating frame and are on-resonance. Prior to the 180 ° pulse the magne-

tization is at equilibrium ( Figure 1.7a ). The 180 ° pulse causes the magnetization

to rotate by 180 axis, and at the end of the pulse it will be oriented

along the negative z axis (Figure 1.7b). Because, in this case, the 180

° about the x ′

° pulse