1. MRI machines use a magnetic field, radio
waves, and a computer to detect the
properties of living tissue.
1946, phenomenon called "Nuclear Magnetic
Resonance explained by Felix Bloch and Edward
M. Purcell were awarded the Nobel Prize in
Physics in 1952.
2. Damadian - 1977
First ever MRI image of human body detect cancer
tissue
Homogeneous part of field very limited so patient table
was moved to collect each voxel
Took 4hrs to collect single slice
3.
4. Magnet – for nuclear alignment
Radio frequency source – for RF excitation
Magneticfield gradient system – for spati al
encoding
Computer system – for the image formation
process and the user interface
Image processor – to convert ‘ signals ’ into
images.
5.
6. Magnetic Field Strength
• Magnetic field strength can be measured in Tesla (T)
or Gauss (g).
• 10,000 g =1T
7. Magnets Earths magnetic field is 0.3-0.7G
Magnets used for imaging mostly between .5 to 7 Tesla
1] permanent magnets
2] Electromagnets
(solenoid)
3] Resistive magnets
4] Superconducting
magnets
5] hybrid magnets.
8. Ferromagnetic materials have
half - filled electron shells.
ferromagnetic materials
retain their
magnetization even when
the external magnetic field
has been removed
10. Advantage of permanent magnets is
Main advantage of permanent magnets is that they require no
power supply or cryogenic cooling -relatively low in operating
costs.
The magnetic field created by a permanent magnet has lines of
flux running vertically from the south to the north pole
(bottom to the top) of the magnet, keeping the magnetic field
virtually confined within the boundaries of the system
11. Problems associated with the weight of these systems.
The weight of a permanent magnet can be on the order
of 15 000 kg,
as compared with some superconducting
electromagnets that weigh 5000 kg.
12. Permanent magnet scanners are
temperature sensitive, and to maintain
homogeneity of magnet and image quality
a constant temperature must be maintained
Not fluctuate by more
than 1 kelvin
13. Electromagnets
Michael Faraday ’ s law of electromagnetic induction
B0 = kI
where
I is the current flowing through the wire
k is the proportionality constant (quantity of
charge on each body)
B 0 is the strength of the magnetic field.
14. Solenoid electromagnets
Resistance is a property of the wire that can pose as an
obstacle.
Resistance will convert the current into heat. In order to
maintain the magnetic field, there must be a constant current.
This type of magent is called a resistive magnet.
15. Resistive magnets
• Have field strengths up to 0.3T
• Every conductor shows resistance flow of current
So needs more current to be applied to create
a static magnetic field.
16. Superconductor
At very low temperatures[0k/ 273.5 c, the resistivity
of some materials becomes zero. Materials in such
state are called
superconductors.
Current in a ring shaped super conducting material has
been observed
to flow for years in the absence of a potential
difference
17. Superconducting Magnets[niobium-titanium]
• Resistance is reduced by cooling the coils
[0k /-273c].
liquid helium is used to cool the wires and
remove resistance [liquid form only at the
extremely low temperature of −270 °C (about 4 K
]
• Without resistance, the electrical current can
flow within a closed circuit. There is no need for
any external power to be applied. The flowing of
electrical current without resistance is known as
superconductivity.
18. Superconducting Magnets
• Most superconducting magnets are solenoid by
design and exhibit a horizontal magnetic field.
• Superconducting magnets can achieve very
high field strengths, clinical scanners range
between 0.5T and 1.5T.
19. Gradient coils
Gradient magnetic fields[G] are superimposed over the
main magnetic field[Bo].
These fields are produced by applying a current in the
gradient coils.
There are three sets of gradient coils in MR systems.
20.
21.
22. The coil that is used to vary the intensity of the
magnetic field in the left to right direction is the X
gradient coil-saddle coil
23. The gradient coil that is used to vary the intensity of
the magnetic field in the anterior to posterior direction
is the Y gradient coil.-saddle coil
24. The gradient coil that is used to vary the
magnetic field in the head to foot direction is
the Z gradient coil.-helm holtz coil
25. gradient coil produces variation in the main magnetic
field
Used to
1] selection of slice -selectively excite nuclei in one
slice of tissue
2] phase encoding
3]Frequency encoding
45. Receive coils detect the
weak signal emitted by the
spins as they precess in
the B0 field.
46. Surface coils are a type of receiver RF coil used in MRI
to receive the radiofrequency signal
Surface coils are small and are shaped so that they can be
placed near the part of anatomy being imaged.
surface coils have good signal to noise ratio for the tissue
adjacent to the coil. They also allow for smaller voxel size
which in turn allows for improved image resolution .,
surface coils have a smaller field of view..
Surface coils have a relatively simple design which consists of
a loop of wire.
47.
48. Volume coils which are used to both transmit and receive the
radiofrequency signal in MRI.
body coil – which is a volume coil built into the bore of the
magnet which transmits the radiofrequency for most
examinations.
smaller volume coils –
such as in head and occasionally in extremity imaging (e.g. the
knees).
Typical head imaging makes used of head coils, or birdcage
coils
Volume coils generally have a homogenous RF excitation
across a large volume.,
downside of receiving a lot of noise when the target region of
interest is small.
49.
50. Phased array coils are an example of a receive-
only RF system.
Collection of multiple surface coil into a larger array
whose individual signals are combined to create one
image.
As signal coils detect signal based on proximity, they
have a high sensitivity but limited anatomical field of view.
The combination of multiple surface coils into an array
allows for a good signal-to-noise ration over a larger field
of view.
51. example of this is in spine imaging, where
single surface coils might provide quite
good signal for the adjacent small section
of vertebra , but are too small to image
the entire longitudinal length of the spine.
By aligning multiple surface coils together,
the entire spine can be imaged in good
quality
