Esaote O-scan eXP, Руководство пользователя

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350004900 Rev. 07 3 / 6
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General Principle
s
of Ma
gnet
ic
Resonance Imaging
The role of RF pulse
To sum up, when a patient is placed inside a strong magnetic field, the
body protons tend to align themselves along the field axis, resulting in a
magnetic moment (the so-called macroscopic magnetization) in the
direction of the external field. It would be ideal to measure this
magnetization, but this is not possible as it is in the same direction,
parallel to the external field. By sending a short burst of electromagnetic
waves, called radiofrequency (RF) pulse , to the patient, it is possible to
disturb the protons in order to change the direction of the magnetization.
Not all RF pulses disturb the alignment of protons. A pulse that can
exchange energy with the protons is required, i.e. one that has the same
frequency, the same angular velocity as the protons: the Larmor
frequency. When the RF pulse and the protons have the same frequency,
protons may pick up some energy from the radio wave and show a
phenomenon known as resonance , analogous to the one observed in
acoustic experiments.
Another phenomena also occurs. Due to the RF pulse, the protons no
longer point in random directions, and become “in phase”, i.e. they move
in synchrony, pointing in the same direction at the same time, so that
their magnetic vectors add up in this direction. The result is a magnetic
vector with a transverse component: this is called transverse
magnetization. This moves in phase with preceding protons, inducing an
electrical current which is the actual MR signal . This can be picked up by
an antenna: the receiving coil.
Relaxation times
One of the problems is identifying the source of the signal in the human
body. To solve this problem, the rule mentioned above can be used as a
reference: Z
0
= J B
0
. This rule means that the precession frequency is
directly proportional to the field intensity. The trick is then to take a
magnetic field, which has different strength at each point of the patient
cross-section, so that protons in different places precess at different
frequencies. As they precess with different frequencies, the resulting MR
signal from different locations also has a different frequency: therefore,
the frequency is the way to assign a signal to a certain location. The
frequency encoding described above is implemented by applying linear
field gradients along the three main orthogonal directions as in a
Cartesian coordinates system. The most common technique is known as
Spin warp technique where different points may precess in a direction
with different frequencies, while in the orthogonal directions a difference
in the precession phase can be detected. This is the reason why the first
gradient is usually called Readout Gradient while the orthogonal versions
are called Phase Encoding Gradients . In the case of data acquired using
this particular technique, the Fast Fourier Transform (2DFFT or 3DFFT) is
used to transform raw data into images.