Esaote O-scan eXP, User Manual

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4 / 6 Appendix A
Moreover, as soon as the RF pulse is switched off, the whole system,
previously disturbed by the pulse, returns to the original quiet state, so
that the newly established transverse magnetization starts to disappear
(the process is known as transverse relaxation ) while the longitudinal
magnetization grows back to its original size ( longitudinal relaxation ). The
reason for this process depends on the energy level of the protons
system: the protons that were raised to a higher energy level by the RF
pulse return to their lower energy level when the RF pulse is switched off.
This process is not instantaneous but occurs gradually: the protons
become out of phase thereby generating increasingly smaller transverse
magnetization. In the meantime, the energy picked up from the RF pulse
is transmitted to the surrounding protons, the so-called lattice . This
explains why the longitudinal relaxation is also known as spin-lattice
relaxation .
The time it takes for the longitudinal magnetization to return to its original
value is called longitudinal relaxation time or more simply T
1
. It does not
give the exact duration of the relaxation process but is rather a constant
describing how fast this process is.
Another interesting phenomenon is related to the non-homogeneity of the
magnetic field where the patient is placed: its strength is different in
different points within the examination volume, thus causing unwanted
different precession frequencies. Moreover, each proton is influenced by
the small magnetic fields from neighboring nuclei, also not distributed
uniformly, again causing different precession frequencies. Therefore,
when the RF pulse is switched off, the protons are no longer forced to
move in synchrony and, as they have different precession frequencies,
they soon become out of phase. In particular, in a short time interval, the
protons will be 180° out of phase, cancelling out their magnetic moments
in the respective plane. This interval is a time constant known as the
transversal relaxation time or spin-spin relaxation (due to the spin-spin
interaction): the same time constant is also called T
2
.
T
1
is longer than T
2
. In biological tissues T
1
is about 300 to 2000 msec and
T
2
is about 30 to 150 msec, according to the type of tissue examined. As
it is difficult to precisely locate the end of the longitudinal and transverse
relaxation, T
1
and T
2
are not defined as the times when relaxation is
completed, but are defined respectively as the time when approximately
63% of the original longitudinal magnetization is reached (T
1
) and the
time when transverse magnetization decreases to 37% of the original
value (T
2
). These percentages are derived from mathematic equations
describing signal intensity (63% = 1-1/e; 37%=1/e).
By measuring the relaxation times, it is possible to obtain some degree of
tissue characterization. For example, liquids have a long T
1
and a long T
2
while for fatty tissues T
1
and T
2
are both short. Pathological tissues often
have a higher water content than the surrounding normal tissues.