Fundamentals - Preventive Biomechanics - page 18

Table 3.1 Approximate values of human tissue relaxation parameters at B 0 ¼ 1 : 5 T and body
temperature
Tissue T 1 (ms) T 2 (ms)

Fat 250 60

Muscle 900 50

the decay of the transverse magnetization. The shape of the exponential recovery
and decay curves of the longitudinal and transverse magnetization, respectively,
are determined by the time constants T 1 and T 2 ; respectively, cf. Fig. 3.2 .
As can be seen in Fig. 3.2 a, after time T 1 ; the longitudinal magnetization has
returned to 63.3 % of its equilibrium value M 0 ; and after time T 2 ; the transverse
magnetization has lost 63.3 % of its original value M ? ð t 0 Þ: Approximate values for
T 1 and T 2 for fat and muscle tissue are given in Table 3.1 .
Both mechanisms, the longitudinal magnetization recovery and the transverse
magnetization decay, describe the return to equilibrium of net magnetization. They
are referred to as Relaxation. The decay of the transverse magnetization, as dis-
cussed previously, assumes homogeneity of the external magnetic field. In prac-
tice, however, the field exhibits inhomogeneities, which accelerate the decay of
transverse magnetization, referred to as dephasing (loss of phase-coherent rota-
tion) and thus lead to a revised decay time T 2 :
Both relaxation processes are driven by different mechanisms and are inde-
pendent. Transverse relaxation decays due to the dephasing effect where direc-
tional changes (of phase coherence) of the single magnetic dipole moment vectors
in the x-y-plane cause the magnetization to cancel out, i.e. to decay. The loss of
phase coherence is due to variation in the local magnetic fields affecting the single
magnetic dipole moments, i.e. the combination of the external field and the local
fields generated from neighboring dipole moments. These field fluctuations lead to
different local precessional frequencies and finally to the dephasing effect.
Approximate values for T 1 and T 2 for fat and muscle tissue are listed in Table 3.1 .

3.1.4 Resonance Condition

Relaxation of longitudinal and transverse magnetization is achieved if the mag-
netic moments that initially precess about the static external field z-axis are
transiently rotated away from this direction into the x-y-plane transverse to the
external magnetic field B 0 : This is accomplished by adding other magnetic fields,
e.g. a (transient) radio frequency (rf) field B 1 ; perpendicular to the external field. In
contrast to B 0 ; the rf-field is not static but oscillates, since it is generated from coils
supplied by an alternating current with the frequency x : It shows that an adequate
(in terms of amplitude) B 1 -field to most effectively rotate the magnetic moment
into the transverse x-y-plane is obtained by adding two linearly polarized fields
with polarization planes orientated at right angles. The polarization planes have the