2. Basics of MRI
• Magnetic resonance imaging is based on
the phenomenon of nuclear magnetic
resonance.
• Resonance is defined as amplified
response to a stimulus that has the same
natural frequency.
3. Basics of MRI
Consider the simplest atom; hydrogen, its nucleus consists of
only one proton.
Protons are positively charged.
Electrons are not involved in MR imaging.
Hydrogen atoms (protons) are the simplest and most abundant
element in the body.
All current MRI techniques are based on receiving and
processing signals from protons.
4.
5. Protons have a rotational axis, and since they are positively
charged, they create a small magnetic field an a magnetic
axis.
In nature, the orientation of these axes is random.
Magnetic moment: The direction of a single hydrogen atom nucleus.
6.
7. In the presence of a magnetic field, protons spin around an axis
that is aligned with the main magnetic field.
The dominant axis of these spinning protons is oriented parallel
to the main magnetic field.
8. Precession is the rotation of protons around the main
Magnetic field.
Net magnetization is the sum of all magnetic moments.
10. The Larmor frequency:
The speed of frequency of precession. It is
proportional to the scanners magnetic field.
The Larmor equation:
w=&.B
w= larmor frequency in MHz
&=The gyromagnetic ratio in MHz/Tesla
B= The magnetic strength in Tesla
11.
12. Resonance only occurs if the RF frequency matches the
larmor frequency of the nuclei.
In order to see the precession of the net magnetization
We should flip the net magnetization from longitudinal
to transverse plane.
13.
14. Recovery of the net magnetization from the transverse
plane is associated with the induction of MR signal.
Free Induction Decay (FID): First MR signal after the
RF pulse.
