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Magnetic resonance imaging (mri) asit meher ppt

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MRI technology

Magnetic resonance imaging (mri) asit meher ppt

  1. 1. PRESENTED BY:- ASIT KUMAR MEHER 1
  2. 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. 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. 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. 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
  6. 6. History 6
  7. 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. 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. 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. 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. 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
  12. 12. HOW MRI WORKS 12
  13. 13. COMPONETS OF MRI 13 1) Main magnet (superconducting magnet) 2)Gradient coils 3)RF coils(radiofrequency) Schematic diagram of MRI scanner
  14. 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
  15. 15. COMPONETS OF MRI 15
  16. 16. 16
  17. 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. 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. 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. 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
  21. 21. 21
  22. 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
  23. 23. 23
  24. 24. COMPONENTS OF MRI 3.RF Coils Start RF pulses (Excitation- Protons jump to higher energy state by absorbing radiation). 24
  25. 25. COMPONENTS OF MRI 3.RF coils Stop RF pulses (Relaxation- Protons return to their original state emitting radiation) 25
  26. 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
  27. 27. COMPUTER SYSTEM 27
  28. 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. 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
  30. 30. COOLING OF MAGNETS 309652071_orig.gif
  31. 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. 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. 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
  34. 34. LCS 34
  35. 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. 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. 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
  38. 38. SHAPE OF MRI MACHINE 38 CLOSED MRI OPEN MRI
  39. 39. COMPARISON CLOSED MRI OPEN MRI  High field typically 1.5T – 3T.  High image quality  Fast imaging  Advanced application  Increased patient anxiety.  Claustrophobic patients problems.  High acoustic noise levels.  Low field typically 0.2T – 0.4T  Low image quality  Slow imaging  Limited application  Less patient anxiety.  Claustrophobic patients handling.  Lower acoustic noise levels. 39
  40. 40. SOME MRI IMAGES 40
  41. 41. 41
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