Esaote O-scan eXP, User Manual

Page: 241 / 242

350004900 Rev. 07 5 / 6
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General Principle
s
of Ma
gnet
ic
Resonance Imaging
T
1
depends on tissue composition, structure and surroundings. Moreover,
given that precessing frequency depends on the intensity of the magnetic
field (Larmor equation) in the presence of a stronger magnetic field
protons precess faster. and it is more difficult for them to transmit energy
to a lattice with slower fluctuating magnetic fields. This is why T
1
is longer
in a stronger magnetic field.
The spin-spin relaxation has two causes: non-homogeneities of the
external magnetic field and the non-homogeneities of the local magnetic
fields within the tissues. If there are no significant differences in magnetic
field strength inside a tissue, the protons move in phase for a long time,
and therefore T
2
is longer. On the contrary, larger differences in local
magnetic fields cause larger differences in precession frequencies, thus
protons shift out of phase faster and T
2
is shorter.
From signal to image
When the proton system in a strong magnetic field is disturbed by an RF
pulse, the protons are in phase and transverse magnetization can be
detected. This can be seen as longitudinal macroscopic magnetization
tilted by 90° on the transverse plane. An RF pulse which tilts the
magnetization by 90° is called a 90 ° pulse. According to the strength and
the duration of the RF pulse it is possible to obtain different tilt angles: for
instance, one may have a 180 ° pulse.
Just after a 90° pulse, the transverse magnetization starts to decrease
because the protons are losing phase coherence, while the longitudinal
magnetization increases. Transverse and longitudinal magnetization
vectors add up to a sum vector. This sum vector performs a spiral motion
when it changes its direction from being in the transverse plane (no
longitudinal magnetization) to its final position (no transverse
magnetization). The resulting signal, which disappears with time, is called
a FID ( free induction decay ) signal.
Different tissues may have different relaxation times. Let us now see what
happens when tissues having different relaxation times are disturbed with
consecutive 90° pulses, i.e. a pulse sequence . If the time TR between two
consecutive pulses is long enough, the different tissues have regained all
of their longitudinal magnetization, thus the transverse magnetization
after the following pulse will be the same for all the tissues involved in the
experiment. Vice versa, if the TR is short, as the T
1
of tissues is different,
after a certain interval a tissue will have regained more of its longitudinal
magnetization than another one. If another 90° pulse is sent at that
moment, the transverse magnetization will be greater in the first tissue
with respect to the second. Thus the antenna will receive a stronger signal
from the first than from the second. The selection of a sequence of pulses
will determine which type of signal will be obtained from a tissue.
Therefore the pulse sequence for a specific study must be selected and
described with care.