2. WHAT IS MRI?
Magnetic resonance imaging (MRI) is a noninvasive medical imaging
technique that helps diagnose medical conditions.
MRI uses a powerful magnetic field, radio frequency pulses and a
computer to produce detailed pictures of organs, soft tissues, bone and
virtually all other internal body structures.
An X-ray is very effective for showing doctors a broken bone, but if they
want a look at a patient's soft tissue, including organs, ligaments and
the circulatory system, then they'll likely want an MRI. And, another
major advantage of MRI is its ability to image in any plane.
4. THE FIVE SIMPLE STEPS
Patient is placed in a magnet.
Radio wave is sent in
Radio wave is turned off
The patient emits a signal
The emitted signal from the patient is used for reconstruction of the
image
5. THE PHYSICS OF MRI
Human body is made of cells, cells are made of various atoms, and
all atoms contain protons in their nucleus.
Protons carry positive charge and possess spin.
We know, moving charge is electric current, and electric current
induces magnetic field.
Thus, proton has its own magnetic field, and act as
tiny bar magnets.
6. FIRST, THE PATIENT IS PLACED INSIDE
THE MAGNET
When the protons are placed in an external magnetic field, they
align themselves according to the external magnetic field like a
compass needle aligns itself along the magnetic field of earth.
The protons may however align in two different ways: parallel or
antiparallel to the external magnetic field.
These different types of alignments have different energy levels.
Naturally the preferred state of alignment is the
one which requires less energy.
7. LONGITUDINAL MAGNETIZATION
Proton pointing in opposite direction cancels each
others magnetic effect in respective direction.
As there are more protons aligned parallel to the
external magnetic filed, there is a net magnetic
movement aligned with or longitudinal to the
external magnetic field
In a strong external magnetic field a new magnetic
vector is induced in the patient, who becomes a
magnet himself.
This new magnetic vector is aligned with the
external magnetic field, and thus called longitudinal
magnetization.
8. PROTON PRECESSION
In the presence of an external magnetic field protons
show a certain type of movement called precession,
similar to the ‘wobbling’ movement of a spinning top just
before it is about to stop.
It is important to know how fast proton precess. The
number of times proton precess per second is called
precession frequency. It Depends upon the strength of
the magnetic field in which protons are placed.
The precession frequency is calculated using Larmor
Equation:
9. NEXT, A RADIO WAVE IS SENT TO THE
PATIENT
The purpose of this RF pulse is to disturb the protons which are
peacefully precessing in alignment with the external magnetic field.
For this we need a RF pulse that can exchange energy with the
protons, a RF pulse with same frequency as the precessing protons.
Only when the RF pulse and the protons have the same frequency,
can protons pick up some energy from the radio wave, a
phenomenon called resonance (this is where the "resonance" in
magnetic resonance come s from). We can calculate the
frequency of the necessary RF pulse from Larmor Equation.
10. EFFECT OF RF PULSE
Some of the protons pick up the energy
and move from lower energy level to
higher energy level, that is, some of the
protons that were previously pointing
along the magnetic field align them
against the magnetic field
(antiparallel). This causes the
longitudinal magnetization to decrease,
as the number of excess protons
aligned along the external magnetic
field decreases
11. EFFECT OF RF PULSE
The RF pulse causes the protons to
precess in sync. They now point in
the same direction at the same
time, thus their magnetic forces add
up in the direction they are pointing.
This results in a magnetic vector
pointing to the side to which the
protons precess, that is in the
transverse direction (in the X-Y
plane). This is called the transversal
magnetization.
12. NEXT, THE RADIO WAVE IS TURNED
OFF
Before RF pulse there was only longitudinal magnetization.
After the 90dgr RF pulse there is only transversal magnetization and this is spinning
around
With time after the removal of RF pulse the transversal magnetization decreases
and longitudinal magnetization increases in spiral motion
13. OBTAINING MRI SIGNAL FROM THE
PATIENT
When an antenna is placed near the
tissue, an electric current is set up in the
antenna due to the spiralling movement of
the magnetic vector from the transversal to
longitudinal direction. Due to the spiralling
movement, the magnetic vector gradually
moves away from the antenna and thus
the amplitude of the current induced
reduces gradually. This is called FID signal,
free induction decay.
14. COMPUTING AND DISPLAY
The received signal is then fed into a computer and, amazingly, a quarter of a second
later an image appears on the screen.
15. REFERENCES
MRI Made Easy by Prof Dr Hans H. Schild; Published by Schering, AG.
Basic MRI Physics by Evert J. Blink.
http://science.howstuffworks.com/mri.htm
https://www.cis.rit.edu/htbooks/mri/inside.htm
http://en.wikipedia.org/wiki/Physics_of_magnetic_resonance_imagi
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