Diagnostic MRI

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2. CONTENTS
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 INTRODUCTION
 HISTORY
 MRI VS CT SCAN
 HOW MRI WORKS
 COMPONENTS OF MRI
 COMPUTER SYSTEM
 COOLING OF MAGNETS
 ADVANTAGES
 DISADVANTAGES
 SHAPE OF MRI MACHINE

3. Introduction
3
 MRI is a type of scan that uses strong magnetic fields
and radio waves to produce detailed images of insideof
the body.
 An MRI scanner is a large tube that contains powerful
magnets.You lie inside the tube duringscan.
 MRI perhaps the best application ofsuperconductivity
which directlyaffected the humanity across the globe.

4. Definition
Magnetic Resonance Imaging
Magnet
Radio Frequency = Resonance
Imaging
It is a non-invasive method for mapping internal
structure within the body which uses non-ionizing
electromagnetic radiation and employes radio
frequency radiation in the presence of carefully
controlled magentic fields to produce high quality
cross-sectional images of the body in any plane

5. Introduction
 Prof Peter Mansfield was awarded Nobel
prize in 2003 for his discovery in MRIwith
Prof Paul C Lauterbur of USA.
The concept of NMR imaging used in
present day MRI system was proposed
by Paul Lauterbur as early as1973.
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6. Notes
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 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. Theconceptof NMR imaging used in presentday MRI systemswas 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 bean ideal technique forsoft tissue regionsof the bodysuch as brain, eyesand soft
tissue part of the head. Since bones have low density of protons they appear as dark
regions.

7. History
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8. MRI VS CT SCAN
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CT SCAN
• Uses X-rays forimaging.
• Exposure to ionizingradiation.
• Resolutionproblem.
• Injection of a contrast medium
(dye) can causekidney.
• problems or result in allergic or
injection-site reactions in some
people.
• Less cost thanMRI.
• Quick process and easilyavailable.
MRI
• Uses large external field, RFpulse
and 3 different gradientfields.
• MRI machines do notemit
ionizing radiation.
• Good resolution & 3-D
reconstruction.
• Gadolinium contrast isrelatively
nontoxic.
• Morecost.
• Lengthy process andnon
availability.

9. Notes
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 Whyweare using MRI instead of CT scan , here some
comparison between thistwo.
 CT scan usesx ray technology toproduce image but MRI
uses large magnetic field to elicitimage.
 Certain advantages of CT scan over MRI as it is less
expensive, easily available ,quick process .
 Butstill weare 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. Soeveryone now preferring MRI although it has
high cost.

10. HOW MRI WORKS
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 MRI exploits the presence of vast amount of hydrogen in a
human body as the water content in human body is said to
be about80%.
 At the centre of each hydrogen atom is an even smaller
particle , called proton. Protons are like tiny magnets and
areverysensitive to magnetic fields and has magneticspin.
 MRI utilizes this magneticspin propertiesof protonsof
hydrogen to elicitimages.
 Then whyour bodycan’t like magnets?

11. HOW MRI WORKS
•The protons i.e. hydrogen ions in abody
•are spinning in a haphazard fashionand
•cancel all the magnetism.
•That is our naturalstate.
•When there is large magnetic fieldacts
•on our body, protons in our body line up in
•same direction.
•In same way that magnetcan pull the
needle of acompass.
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12. Notes
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 Human body is largely made of water molecules, which
consistsof smallerparticles i.e hydrogen and oxygen atoms.
 Protons lies at thecentreof 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
directionand second magnetwas 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 switchedoff.

13. HOW MRI WORKS
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14. COMPONETS OF MRI
1) Main magnet
(superconducting
magnet)
2)Gradient coils
3)RF coils(radiofrequency)
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Schematic diagram of MRIscanner

15. Notes
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 A superconducting magnet is the heart and most expensive part of an MRI
scanner.MRI magnets need high homogeneity and high temporal stability
similarto NMR spectrometers. However, the magnetic field requirement in
the present day MRI scanners for clinical use is limited to 3T only. Another
majordifferencewith NMR magnet is thatsample 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 directionsrequired for imaging. Inside thegradient
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 bodycoil and thegradientcoil assemblyrepresentthe
three major subsystems that comprise the resonance module of the MR
scanner.

16. COMPONETS OF MRI
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18. Different Types of MRI
Coils in MR Systems
• Gradient coils
• RF coil
1. Transmit Receive Coil
2. Receive Only Coil
3. Transmit Only Coil
4. Multiply Tuned Coil

19. COMPONETS OF MRI
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1.Superconducting magnet
 A superconducting magnet is the heart andmost
expensive part of an MRIscanner.
 The magnetic field requirement in the present day MRI
scanners for clinical use islimited to 3T only.
 The main magnet is superconducting, cooled to LHe
temperatureand mounted in an efficientcryostat with
a horizontal bore to accommodate thepatient.

20. COMPONETS OF MRI
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2.Gradientcoils
 Gradient coils are used to producedeliberatevariations
in the main magneticfield.
 Thereare 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.

21. Notes
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 It generates secondary magnetic field with in primary
magnetic field, theyare located in boreof primary magnet.
They are arranged in opposition toeach otherto +veand
–ve pulse.
 Gradientcoils are setof magnetization coils, which causeof
variation in magnetic field. They must be able to cause
spatial variation along thedirectionof man magnetic field.
 They are along with RF pulseare responsible forsliceand
voxel formation.
 Gradient is extra magnetic field which is added to the
magnetic field.

22. COMPONENTS OF MRI
2.Gradientcoils
X coil – create a varying
magnetic field from leftto
right.
Y coil- create a varying
magnetic field from topto
bottom.
Z coil- create a varying
Magnetic field from head totoe.
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24. COMPONETS OF MRI
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3. RF Coils
 Same as Radio waves – high wavelength, low energy
electromagnetic waves.
 RF coils are the "antenna" of the MRIsystem
 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 computercontrolled.

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26. COMPONENTS OF MRI
3.RF Coils
Start RF pulses (Excitation- Protons jump to higher energy
state by absorbing radiation).
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27. COMPONENTS OF MRI
3.RF coils
Stop RF pulses (Relaxation- Protons returnto
their original state emittingradiation)
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28. Notes
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 RF used to transmit RF pulses receiving signalsin MRI produce
best possible images. It can make magnetization of hydrogen
nuclei , turn it 90 degree away from magneticfield.
 Some low energy (parallel protons) flip toa high energy (anti
parallel) state decreasing longitudinal magnetization.
 Protons process in phase, at a result net magnetizationvector
turns towards the transverse plane, i.e. right angles to the
primary magnetic field = transversemagnetization.
 Each proton is rotating around itsaxis 63,000,000 rotation per
second. The 63MHz rotation is in the frequency range called
Radio frequency.
 Rotation speed α magnetic field strength

29. Comprehensive Receiving coils
 standard configuration：
QD head coil QD Neck Coil QD Body Coil
QD Extremity Coil Flat Spine Coil Breast Coil

30. Making
Images of the
NMR Signal
• Uniform magnetic field to set the stage (Main Magnet)
• Gradient coils for positional information
• RF transceiver (excite and receive)
• Digitizer (convert received analog to digital)
• Pulse sequencer (controls timing of gradients, RF, and
digitizer)
• Computer (FFT to form images, store pulse sequences,
display results, archive, etc.)

31. COMPUTER SYSTEM
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32. Notes
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 Receives RF signal and performsanalog todigital
conversion.
 Digital signal representing image of body part is stored in
temporary image space or case space. It store digital signal
during dataacquisition, digital signal then sent toan image
processor were a mathematical formula called Fourier
transformation is applied to imageof MRI scan is displayed
on a monitor.

33. COOLING OF MAGNETS
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 MRI (magneticresonant imaging) machineswork bygenerating avery
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, whichneeds
to be topped upperiodically.
 Recently small special purpose refrigerators have been proposed for
recondensation of evaporated helium, which together with a
cryocooler forthe radiation shieldsgiveacompleteclosed refrigeration
system.

34. COOLING OF MAGNETS
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35. Notes
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 In this figure the cryostat has an outervacuumcase (OVC) made of metal, one thermal shield (usuallyat 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 cryocoolersleeve, 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, lowerfill volumes
may be tolerable. The minimum allowable volume may also differ between the ramping process and the
subsequent persistent operation of the rampedmagnet.
 Any advanced/alternative cryogenic concept for MRI applications needs to address all the following operating
modes:
 Energy saving pre-cooling of the magnet down tothe
 operating temperature (usuallydone with liquid nitrogen ora recoverable liquid helium facility).
 Magnet ramp up to full field, preferably with captured boil-off helium gasduring ramp.
 Normal operating condition (NOC) with extra heat loads (due to gradient heating) that reduce the cryogenic
margin, and ensure no helium loss (zeroboil-off/recovery).
 Rampdown.
 Shipping ‘ride-through’ (from factory to MRI site), optimizing losses to minimize thecost.
 Cooldown to operating temperature or refill at the customersite, with high-efficiency transfer.
 Safe ramp upat the customersite.
Cryocoolertechnology is constantlyprogressing. Currently, the dual-stage cryocoolercools 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 heliumvessel.

36. COOLING OF MAGNETS
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LASER COOLING SYSTEM(LCS)
• LCS is one of the recent technologies used tocool magnet in
MRI. The temperature of a laser system can determine its
lifetime, performance andsafety.
• 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 principleof lasercooling is Dopplereffect .
• The Dopplereffect, or Dopplershift, is thechange in wavelength
and frequency caused by the movement of an observer relative
to thesource.

37. LCS
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 In Doppler effect the frequency of light is tuned slightly below an
electronic transition in theatom. 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 theatom reducesand hence the kineticenergyalsoreduces. Which
reduces the temperatureof theatom, and hencecooling of theatom is
achieved.
 As per Doppler cooling, if a stationary atom sees the laserneither red
shifted nor blue shifted, it does not absorb the photon. An atom
moving away from the lasersees that the laser is red shifted, then also
itdoes notabsorb photon. If an atom is moving towards the laserand
sees that it is blue-shifted, the it absorbs the photon and thus the
speed of the atom will getreduced.

38. LCS
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39. Notes
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 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 thecooling effective. And we
have to place our model in controller so that it can provide
the corresponding wavelength of laser for the predicted
temperature .

40. MRI Artifacts
Motion
related
artifacts
Para-
magnetic
artifacts
Phase Wrap
artifacts
Frequency
artifacts
Susceptibility
artifacts
Clipping
artifact
Chemical
Shift Artifact Spike artifact
“Zebra”
artifact

41. MotionArtifacts  Motion artifacts are caused by
phase mis-mapping of the protons.
Para-MagneticArtifacts  Para-magnetic artifacts
are caused by metal (~ iron
Phase WrapArtifacts  Phase wrap artifacts are
caused by mis-mapping of phase.

42. Frequency Artifacts  Frequency artifacts are
caused by „dirty‟ frequencies. Faulty electronics,
external transmitters, RF-cage leak, non-shielded
equipment in the scanner room, metal in the patient,
Susceptibility Artifacts  Susceptibility is theability
of substances to be magnetized, for example iron in
blood.
ClippingArtifact  Signal clipping or „over flow‟
occurs when the receiver gain is set to high during the
pre-scan.

43. Chemical Shift Artifact  Chemical shift artifacts are
caused by different resonance frequencies of hydrogen
in lipids and hydrogen in water
SpikeArtifact Aspike artifact is caused by one
„bad‟ data point in k-space
“Zebra” Artifact  The “Zebra” artifact may occur
when the patient touches the coil, or as a result of
phase wrap.

44. Indications
• Diagnosing: strokes; infections of the
brain/spine/CNS; tendonitis
• Visualising: Injuries; torn ligaments – especially in
areas difficult to see like the wrist, ankle or knee
• Evaluating: Masses in soft tissue; cysts; bone
tumours or disc problems.

45. Contraindications
• The strength of the magnet is 5000 times stronger
than the earth so all metals must be removed.
• People with pacemakers or metal fragments in the
eye cannot have a scan
• There has not been enough research done on babies
and magnetism, so pregnant women shouldn’t have
one done before the 4th month of pregnancy – unless
it is highly necessary.

46. ADVANTAGES OF MRI
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 No ionizing radiation & no short/long-termeffects
demonstrated.
 Variable thickness, anyplane
 Bettercontrastresolution & tissuediscrimination
 Various sequences toplaywith tocharacterize the
abnormal tissue.
 Many details without I.Vcontrast.

47. Advantages
The MRI does not use ionizing
radiation, which is a comfort to
patients
• Also the contrast dye has a very
low chance of side effects
• ‘Slice’ images can be takenon
many planes

48. DISADVANTAGES
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 Veryexpensive
 Dangerous forpatientswith metallicdevices placed within
the body.
 Difficult to be performed on claustrophobicpatients.( fear
of closed space)
 Movementduring scanning maycause blurry images.
 RF transmitters can cause severe burnsif mishandled.
 Not easilyavailable

49. Disadvantages
1. Claustrophobia-Patients are in a very enclosed
space.
2. Weight and size - There are limitations to how big
a patient can be.
3. Noise - The scanner is very noisy.
4. Keeping still - Patients have to keep very still for
extended periods of time.
5. Cost - A scanner is very, very expensive, therefore
scanning is also costly.
6. Medical Contraindications - Pacemakers, metal
objects in body etc.

50. SHAPE OF MRI MACHINE
CLOSED MRI
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OPEN MRI

51. COMPARISON
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CLOSED MRI
 High field typically 1.5T –3T.
 High image quality
 Fast imaging
 Advanced application
 Increased patient anxiety.
 Claustrophobic patients
problems.
 High acoustic noise levels.
OPEN MRI
 Low field typically 0.2T–
0.4T
 Low image quality
 Slow imaging
 Limited application
 Less patient anxiety.
 Claustrophobic patients
handling.
 Lower acoustic noiselevels.

52. SOME MRI IMAGES
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