2. CONTENTS
INTRODUCTION
HISTORY
MRI VS CT SCAN
HOW MRI WORKS
COMPONENTS OF MRI
COMPUTER SYSTEM
COOLING OF MAGNETS
ADVANTAGES
DISADVANTAGES
SHAPE OF MRI MACHINE
2
3. Introduction
MRI is a type of scan that uses strong magnetic fields
and radio waves to produce detailed images of inside of
the body.
An MRI scanner is a large tube that contains powerful
magnets.You lie inside the tube during scan.
MRI perhaps the best application of superconductivity
which directly affected the humanity across the globe.
3
4. Introduction
Prof Peter Mansfield was awarded Nobel
prize in 2003 for his discovery in MRI with
Prof Paul C Lauterbur of USA.
The concept of NMR imaging used in
present day MRI system was proposed
by Paul Lauterbur as early as 1973.
4
5. Notes
MRI is perhaps the best application of superconductivity which directly affected the
humanity across the globe. It is a common tool with the radiologist in diagnostic
hospitals for imaging various soft tissue parts of the human body and for detecting
tumors. The concept of NMR imaging used in present day MRI systems was proposed
by Paul Lauterbur as early as 1973. MRI exploits the presence of vast amount of
hydrogen (protons) in a human body as the water content in a human body is said to
be about 80 %. When protons in the tissues of the body, aligned in a static magnetic
field (B0), are subjected to resonant RF excitation, they absorb energy. Proton relaxes
back and emits resonant signal which is a characteristic of the tissue. The signal is
picked-up by a receiver located inside the magnet bore and is used to construct the
image using Fourier transform. Since the NMR signal frequency is proportional to the
magnetic field the whole tissue can be mapped by assigning different values of the
proton frequency to different proton locations in the sample using well computed
field gradient. All MRIs use proton NMR for mapping proton density which is
different in different types of tissues. The images show contrast which helps in
identifying these tissues and the changes occurring in a sample tissue. MRI turns out
to be an ideal technique for soft tissue regions of the body such as brain, eyes and soft
tissue part of the head. Since bones have low density of protons they appear as dark
regions.
5
7. MRI VS CT SCAN
CT SCAN
• Uses X-rays for imaging.
• Exposure to ionizing radiation.
• Resolution problem.
• Injection of a contrast medium
(dye) can cause kidney.
• problems or result in allergic or
injection-site reactions in some
people.
• Less cost than MRI.
• Quick process and easily available.
ct-scan-vs-mri-scan-4-638.jpg
MRI
• Uses large external field, RF pulse
and 3 different gradient fields.
• MRI machines do not emit
ionizing radiation.
• Good resolution & 3-D
reconstruction.
• Gadolinium contrast is relatively
nontoxic.
• More cost.
• Lengthy process and non
availability.
ct-scan-vs-mri-scan-6-638.jpg
7
8. Notes
Why we are using MRI instead of CT scan , here some
comparison between this two.
CT scan uses x ray technology to produce image but MRI
uses large magnetic field to elicit image.
Certain advantages of CT scan over MRI as it is less
expensive, easily available , quick process .
But still we are going for MRI technology because it has no
ionization radiation so no harm to body, produce good
resolution 3D image and each n every inner injury can be
detected. So every one now preferring MRI although it has
high cost.
8
9. HOW MRI WORKS
MRI exploits the presence of vast amount of hydrogen in a
human body as the water content in human body is said to
be about 80%.
At the centre of each hydrogen atom is an even smaller
particle , called proton. Protons are like tiny magnets and
are very sensitive to magnetic fields and has magnetic spin.
MRI utilizes this magnetic spin properties of protons of
hydrogen to elicit images.
Then why our body can’t like magnets?
9
10. HOW MRI WORKS
10
•The protons i.e. hydrogen ions in a body
•are spinning in a haphazard fashion and
•cancel all the magnetism.
•That is our natural state.
•When there is large magnetic field acts
•on our body, protons in our body line up in
•same direction.
•In same way that magnet can pull the
needle of a compass.
11. Notes
Human body is largely made of water molecules, which
consists of smaller particles i.e hydrogen and oxygen atoms.
Protons lies at the centre of each atom, which is sensitive to
any magnetic fields and hence this proton serves as a
magnet. Normally water molecules in our body are
randomly arranged, but upon entering on the MRI scanner
first magnet causes body’s water molecules to align in one
direction and second magnet was then turned on and off in
a series of quick pulses, causing each hydrogen atom to
alter their alignment and quickly , switches back to their
original relaxed state, when switched off.
11
13. COMPONETS OF MRI
13
1) Main magnet
(superconducting
magnet)
2)Gradient coils
3)RF coils(radiofrequency)
Schematic diagram of MRI scanner
14. Notes
A superconducting magnet is the heart and most expensive part of an MRI
scanner.MRI magnets need high homogeneity and high temporal stability
similar to NMR spectrometers. However, the magnetic field requirement in
the present day MRI scanners for clinical use is limited to 3T only. Another
major difference with NMR magnet is that sample size is much larger.
The main magnet is superconducting, cooled to LHe temperature and
mounted in an efficient cryostat with a horizontal bore to accommodate the
patient. Inside the main magnet is a set of gradient coils for changing the
field along the X, Y and Z directions required for imaging. Inside the gradient
coils are the RF coils producing the field B1 for rotating the spin by an angle
dictated by the pulse sequence. These coils also detect the signal emitted by
the spins inside the body. At the centre is a patient table which is computer
controlled.
The magnet, the RF body coil and the gradient coil assembly represent the
three major subsystems that comprise the resonance module of the MR
scanner.
14
17. COMPONETS OF MRI
1.Superconducting magnet
A superconducting magnet is the heart and most
expensive part of an MRI scanner.
The magnetic field requirement in the present day MRI
scanners for clinical use is limited to 3T only.
The main magnet is superconducting, cooled to LHe
temperature and mounted in an efficient cryostat with
a horizontal bore to accommodate the patient.
5195771_orig.gif
17
18. COMPONETS OF MRI
2.Gradient coils
Gradient coils are used to produce deliberate variations
in the main magnetic field.
There are usually three sets of gradient coils, one for
each direction.
The variation in the magnetic field permits localization
of image slices as well as phase encoding and frequency
encoding.
The set of gradient coils for the z axis are Helmholtz
pairs, and for the x and y axis paired saddle coils.
18
19. Notes
It generates secondary magnetic field with in primary
magnetic field, they are located in bore of primary magnet.
They are arranged in opposition to each other to +ve and
–ve pulse.
Gradient coils are set of magnetization coils, which cause of
variation in magnetic field. They must be able to cause
spatial variation along the direction of man magnetic field.
They are along with RF pulse are responsible for slice and
voxel formation.
Gradient is extra magnetic field which is added to the
magnetic field.
19
20. COMPONENTS OF MRI
2.Gradient coils
X coil – create a varying
magnetic field from left to
right.
Y coil- create a varying
magnetic field from top to
bottom.
Z coil- create a varying
Magnetic field from head to toe.
20
22. COMPONETS OF MRI
3. RF Coils
Same as Radio waves – high wavelength, low energy
electromagnetic waves.
RF coils are the "antenna" of the MRI system
That transmit the RF signal and receives the return signal.
They are simply a loop of wire either circular or
rectangular.
Inside the gradient coils are the RF coils producing the
field B for rotating the spin by an angle dictated by the
pulse sequence. These coils also detect the signal emitted
by the spins inside the body. At the centre is a patient table
which is computer controlled.
22
24. COMPONENTS OF MRI
3.RF Coils
Start RF pulses (Excitation- Protons jump to higher energy
state by absorbing radiation).
24
25. COMPONENTS OF MRI
3.RF coils
Stop RF pulses (Relaxation- Protons return to
their original state emitting radiation)
25
26. Notes
RF used to transmit RF pulses receiving signals in MRI produce
best possible images. It can make magnetization of hydrogen
nuclei , turn it 90 degree away from magnetic field.
Some low energy (parallel protons) flip to a high energy (anti
parallel) state decreasing longitudinal magnetization.
Protons process in phase, at a result net magnetization vector
turns towards the transverse plane, i.e. right angles to the
primary magnetic field = transverse magnetization.
Each proton is rotating around its axis 63,000,000 rotation per
second. The 63MHz rotation is in the frequency range called
Radio frequency.
Rotation speed α magnetic field strength
26
28. Notes
Receives RF signal and performs analog to digital
conversion.
Digital signal representing image of body part is stored in
temporary image space or case space. It store digital signal
during data acquisition, digital signal then sent to an image
processor were a mathematical formula called Fourier
transformation is applied to image of MRI scan is displayed
on a monitor.
28
29. COOLING OF MAGNETS
MRI (magnetic resonant imaging) machines work by generating a very
large magnetic field using a super conducting magnet and many coils
of wires through which a current is passed. Maintaining a large
magnetic field needs a lot of energy, and this is accomplished using
superconductivity, which involves trying to reduce the resistance in
the wires to almost zero. This is done by bathing the wires in a
continuous supply of liquid helium at -269.1C.
A typical MRI scanner uses 1,700 liters of liquid helium, which needs
to be topped up periodically.
Recently small special purpose refrigerators have been proposed for
recondensation of evaporated helium, which together with a
cryocooler for the radiation shields give a complete closed refrigeration
system.
29
31. Notes
In this figure the cryostat has an outer vacuum case (OVC) made of metal, one thermal shield (usually at a
temperature of 40–50 K) and the helium vessel, housing the magnet assembly. Top left shows a typical
cryocooler in its vertical orientation, ready to fit into the cryocooler sleeve, as indicated.
The liquid helium fill level to keep the magnet superconducting at 4 K is also shown. For a complete fill,
typically 1500–2000 l is used. Depending on the temperature gradient that may develop inside the magnet
(from bottom to top) and on the superconducting coil design, which defines coil stability, lower fill volumes
may be tolerable. The minimum allowable volume may also differ between the ramping process and the
subsequent persistent operation of the ramped magnet.
Any advanced/alternative cryogenic concept for MRI applications needs to address all the following operating
modes:
Energy saving pre-cooling of the magnet down to the
operating temperature (usually done with liquid nitrogen or a recoverable liquid helium facility).
Magnet ramp up to full field, preferably with captured boil-off helium gas during ramp.
Normal operating condition (NOC) with extra heat loads (due to gradient heating) that reduce the cryogenic
margin, and ensure no helium loss (zero boil-off/recovery).
Ramp down.
Shipping ‘ride-through’ (from factory to MRI site), optimizing losses to minimize the cost.
Cooldown to operating temperature or refill at the customer site, with high-efficiency transfer.
Safe ramp up at the customer site.
Cryocooler technology is constantly progressing. Currently, the dual-stage cryocooler cools the thermal shield
thermally linked to its first stage. The second stage is connected to the recondenser which re-liquefies
escaping helium gas from the helium vessel.
31
32. COOLING OF MAGNETS
32
LASER COOLING SYSTEM(LCS)
• LCS is one of the recent technologies used to cool magnet in
MRI. The temperature of a laser system can determine its
lifetime, performance and safety.
• In laser cooling, atomic and molecular samples are cooled down
to nearly absolute zero through the interaction with one or more
laser fields.
• The basic principle of laser cooling is Doppler effect .
• The Doppler effect, or Doppler shift, is the change in wavelength
and frequency caused by the movement of an observer relative
to the source.
33. LCS
In Doppler effect the frequency of light is tuned slightly below an
electronic transition in the atom. Because the light is detuned to lower
frequency, the atom will absorb more photons if they move towards
the light source. If light is applied from two opposite directions, the
atom will scatter more photons. If this process continuous, the speed
of the atom reduces and hence the kinetic energy also reduces. Which
reduces the temperature of the atom, and hence cooling of the atom is
achieved.
As per Doppler cooling, if a stationary atom sees the laser neither red
shifted nor blue shifted, it does not absorb the photon. An atom
moving away from the laser sees that the laser is red shifted, then also
it does not absorb photon. If an atom is moving towards the laser and
sees that it is blue-shifted, the it absorbs the photon and thus the
speed of the atom will get reduced.
33
35. Notes
In this proposed system four temperature sensors are fixed
on the four sides of the superconducting magnet. It can
predict the temperature level at the superconducting
magnet, and transmit it to the controller. So the controller
has to be designed for making the cooling effective. And we
have to place our model in controller so that it can provide
the corresponding wavelength of laser for the predicted
temperature .
35
36. ADVANTAGES OF MRI
No ionizing radiation & no short/long-term effects
demonstrated.
Variable thickness, any plane
Better contrast resolution & tissue discrimination
Various sequences to play with to characterize the
abnormal tissue.
Many details without I.V contrast.
36
37. DISADVANTAGES
Very expensive
Dangerous for patients with metallic devices placed within
the body.
Difficult to be performed on claustrophobic patients.( fear
of closed space)
Movement during scanning may cause blurry images.
RF transmitters can cause severe burns if mishandled.
Not easily available
37
