Download Principles of Magnetic Resonance

 What is Larmor Frequency?
 What is its purpose in image
acquisition?


It is the frequency at which the hydrogen
precess
When hydrogen is near an externally applied
magnetic field, an increase or decrease
causes the precessional frequency or
resonant frequency also increase or decrease.


ω0 = B0*Ɣ
Example:
 If we have an MRI capable of having a magnetic
strength of 1.5 Tesla. What is the precessional
frequency of hydrogen?
 Answer:
▪ 1.5T x 42.6MHz/T = 63.9MHz
 After the atom is “pushed” by an
electromagnetic energy and the RF
pulse is stopped.
 Two things happens;
 They align with the applied magnetic
field
 Attain the normal rate of precession.
The RF energy that was absorbed by the
atom is released, this energy decays
rapidly and is referred to as Free Induction
Decay.
 A single pulse will not provide enough
information for imaging techniques, so this
pulse is repeated multiple times.
 An interval at which this pulse is repeated
is called repetition time or TR and is
usually in milliseconds.


The repetition time determines how
much longitudinal (T1) occurs.
 TR controls the T1 weighting of the image.

The FID signal needs to be focused so a
large signal appears at certain time.
 Echo, and the time it takes to appear after
the RF pulse is applied is called the echo
time (TE).

Is produced from the nuclei remaining in
the transverse plane.
 Dephasing and precession occurs at
different rates.

A refocusing pulse is given at a given
time, usually 1800, at half the echo time
(TE/2).
 It flips the entire system into its mirror
image.
• Those nuclei precessing at a faster rate,
but are now going in opposite direction
and now are behind the slower
precessing nuclei.
• At the echo time, TE, all nuclei have
reached the same point and the receiver
coil detects a large signal, which refers to
as echo.
A
RF
Pulse
B
Refocusing Pulse
C
T1 and T2 relaxation time
 The term returning to equilibrium is
called relaxation and can be
thought of as two-step process.
 T1 relaxation or Longitudinal
relaxation
 T2 relaxation or Transverse relaxation
It is controlled by a time constant referred
to as T1.
 It is the time it takes about 63% of the
nuclei to realign with the external
magnetic field.
 After the magnetic moment is flipped 900
by the application of a pulse of RF energy,
the pulse is turned off. This is followed by a
gradual return to equilibrium along the z
axis.




It occurs simultaneously with the T1
relaxation.
It involves the rate of precession of different
nuclei.
When RF pulse is applied all atoms are
precessing together, in phase, at the exact
same rate. Once it stops, each atom finds
itself in a slightly different magnetic
environment.
 The time constant controlling how
fast this process occurs is called the T2
relaxation.
 As with T1 values, the T2 value is the
time it takes for 63% of the nuclei to
be out of phase with each other.
 The T2 happens more rapidly than the
T1.
T2 value is much shorter than the T1
value.
 Typical T1 for biological tissues range
from 200 to 1,000ms, whereas T2 50 to
150ms.
 At higher applied magnetic fields, T1 is
increased while T2 are caused by local
effects and are unaffected by field
strengths.

There are Inhomogeneities in the local
magnetic field around each nuclei which
make the actual dephasing go faster.
 Inhomogeneities;

 Variation in the applied magnetic field
 Changes in magnetic susceptibilities
 Chemical shifts
 Changes caused by spins of nuclei moving
in vascular tissues.
 During T2 relaxation, all of the
nuclide phase their spins by
transferring
energy
to
neighboring
atoms,
this
process is referred as spinspin relaxation.
During relaxation process, the
atoms emit RF energy of exactly the
same energy that they had absorbed;
these atomic magnets return to the
equilibrium state that they were in
before. This energy that is emitted by
the atoms is responsible for
generating the MR image.
T1 is primarily affected by the
magnetic field of the overall
environment, while the T2 relaxation
is predominantly affected by the
magnetic field in the local vicinity.
The RF signal that is given off by
the atoms returning to equilibrium is
detected by an antenna and analyzed
by computer for MR imaging.