MRI Cyrogenics


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This is the seminar on MRI Cryogenics, which i did in second year of my MTech.

Published in: Technology, Health & Medicine

MRI Cyrogenics

  1. 1. 1.5 T MRI Scanner during installation at Ontario Veterinary College, University of Guelph, Canada.
  2. 2. MRI: The biggest and most important component in an MRI system is the magnet. The magnets in use today in MRI are in the 0.5- tesla to 2.0-tesla range, or 5,000 to 20,000 gauss. Magnetic fields greater than 2 tesla have not been approved for use in medical imaging, though much more powerful magnets -- up to 60 tesla -- are used in research.
  3. 3. The MRI magnet surrounding you, on the other hand, is a superconducting magnet; it conducts electricity, thereby creating a magnetic field. The superconducting magnet you’re inside could be up to 3 tesla – 60,000 times the force of the Earth’s magnetic field 0.5 Gauss.
  4. 4. Limits of variables given by Food . and Drug Administration Maximum field to 8 Tesla. Maximum noise to 99 dB.
  5. 5. Block Diagram of MRI:
  6. 6. RF coil Arrangement:
  7. 7. Introduction MRI is a true three-dimensional imaging technique, where the object itself is active, responding in various ways to the radiation, which is sent into it– not only onto it. In this way it has some resemblance to computed tomography (CT, CAT) and positron emission tomography (PET), where one also can get information from small specific volumes deep inside a human body. MR imaging is based on nuclear magnetic resonance (NMR) principles and discerns tissues by the characteristics of the signal emitted by their nuclei in a strong magnetic field when subjected to resonant RF excitation.
  8. 8. Major step was the utilization of Super conducting magnets, which can create magnetic fields that are a magnitude higher than ordinary resistive coils. A magnetic resonance imaging magnet includes a cryocooler penetration assembly and a superconducting coil assembly.. An essential part of the Superconducting main magnet system is the cryocooler, which is used to cool the radiation shields of the main magnet system and to recondense the helium present in the main magnet system.
  9. 9. Cold end(4 K) 2nd stage 1st stage
  10. 10. ADVANTAGES High reliability Utilizing reliable compressors Moderate cost Good service Over 20,000/yr made DISADVANTAGES Large and heavy Intrinsic vibration from displacer Low efficiency (valve loss)
  11. 11. The cryocooler has moving internal parts, which constitute a source of distortions of the magnetic field in the examination volume. The reason is that the moving parts have conducting and/or magnetic materials, and field distortions arise as a result of the eddy currents induced in the conducting materials and as a result of the moving magnetic field of the magnetic materials.
  12. 12. Furthermore, the moving parts lead to vibrations which are transmitted to other portions of the MRI system, and field distortions arise as a result of the eddy currents induced in these other portions when they vibrate in the magnetic field. The cryocooler penetration assembly has a thermal station within a thermal box within a vacuum vessel. The superconducting coil assembly has a magnet cartridge within a thermal shield within a vacuum enclosure .
  13. 13. A flexible bellows hermetically connects the vacuum vessel and the vacuum enclosure. The weight of the cryocooler penetration assembly is supported independent of the superconducting coil assembly which, together with the flexible connections, isolates the vibrations of the cryocooler coldhead (which is attached to the cryocooler penetration assembly) from the superconducting coil assembly thereby improving imaging quality.
  14. 14. Overview of MRI : Magnetic Resonance Imaging or MRI is a modern diagnostic technique for acquiring information from the interior of a body. Usually this is a human body or an animal, but MRI is also used in the industry for more technical purposes. The greatest advantage of MRI is that it can create three-dimensional images of the object under study without hurting the object in any way and without using any ionizing radiation.
  15. 15. The body must be placed in a strong magnetic field, more than ten thousand times the magnetic field of Earth. A radio signal is sent into the body, where it is absorbed by hydrogen atoms. The hydrogen atoms in the body respond by sending back a signal to a detector. The strength of this signal mirrors the amount of hydrogen in various parts of the body.
  16. 16. When creating an image of an organ, the signal must be acquired from every part of the organ point by point by a scanning procedure. To accomplish this, the magnetic field is rapidly varied with gradients in three dimensions.
  17. 17. The gradients divide the object studied into a great number of volume elements, called voxels, each of which has a volume of about one cubic millimeter or less. Every voxel gives rise to one signal with a unique amplitude. Calculating the amplitudes of all these signals and their space coordinates in the studied body is more or less what MRI is about.
  18. 18. Scanner construction and operation The three systems form the major components of an MRI scanner: A static magnetic field. An RF transmitter and receiver. Three orthogonal, controllable magnetic gradients.
  19. 19. Cross-section of cylindrical MRI Scanner
  20. 20. An outline of a cylindrical Whole body system, which includes the main magnet creating the background field ,gradient system and RF body coil, is shown in Fig. These three subsystems compete for the same annular space that also provides maximum accommodation and comfort for the patient.
  21. 21. Need of Superconducting Magnet: The option of creating a magnet by running electrical current through wire coils – an electromagnet. The problem is the electrons making up that current are forever bumping into the fidgety atomic particles of the material through which they are traveling, slowing them down considerably. Given the resistance the current encounters, providing the vast amount of power required to overcome it and generate a magnetic field sufficient to operate an MRI would be prohibitively expensive.
  22. 22. Take special coils and surround them with something really, really cold – liquid helium, at 452.4 degrees below zero on the Fahrenheit scale, does quite nicely. The result?
  23. 23. Those over-caffeinated atoms in the conducting wire are frozen into submission. Slowed to a virtual halt, they allow the current to sail right through the miles of wires snaking through an MRI scanner. This technology allows for the construction of hugely powerful magnets like the one surrounding you right now.
  24. 24. Would you think that being in the middle of such a powerful force would make you feel different – tingly or something?
  25. 25. It doesn’t. However, on an atomic level, it’s quite a different story, which takes us from the “M” of MRI to the “R” – Resonance.
  26. 26. You’re made up mostly of water, which means a large number of the atoms inside your body are hydrogen atoms. This turns out to be quite fortuitous, because hydrogen atoms happen to be built in such a way that they react dependably to the forces they will be subjected to inside this scanner.
  27. 27. It is of special interest that the signal from hydrogen in lesions and tumors often decay slower than surrounding tissues and therefore can be detected in the image. The contrast of the image is created by the experimental procedure and is not inherent in the imaged body.
  28. 28. Forces subjected to patient inside the scanner : Main magnetic field Pulses of radio waves.
  29. 29. In the nucleus of every hydrogen atom is a positively-charged proton that spins (or precesses, scientifically speaking) around an axis, much in the same way as a child’s top. This spinning generates its own tiny magnetic field, giving the proton its own north and south poles.
  30. 30. Now, the nuclei of other atoms spin, too, but for a number of reasons (including, as we’ve mentioned, their sheer quantity), MRI is generally interested only in hydrogen atoms. Under normal circumstances, these hydrogen protons spin about willy-nilly, on randomly oriented axes.
  31. 31. However, when these atoms are placed in a more powerful magnetic field, it’s as if a drill sergeant blew a whistle: the protons line up at attention. in the direction of the field. Now that the magnet has gotten the hydrogen protons lined up at attention, the scanner is ready to subject them to the next step, the one that will result in an actual signal.
  32. 32. A radio transceiver, also called an RF coil, which can communicate with your hydrogen atoms via radio frequency (RF) waves. These waves are close in frequency to those of your favorite FM station. In fact, the room in which the MRI scanner is located is probably shielded so that the local easy listening station doesn't interfere with your images. Your technologist is using that coil to send RF pulses at your spine.
  33. 33. When the RF pulse stops, the protons release that absorbed energy, return to their previous alignments and, in so doing, emit a signal back to the coil . The signal gets turned into an electric current, which the scanner digitizes. The lower the water content in an area, the fewer hydrogen protons there will be emitting signals back to the RF coils.
  34. 34. Different types of MRIs display this data differently, but in any case you get a variety of shades of grey that reflect the different densities. In some scans, the weaker the signal, the darker that part of the image will be. So bone will be fairly dark, while fat will be light.
  35. 35. Gradient Magnets: The gradient magnets. There are three of them in the machine (called x, y and z), each oriented along a different plane of your body, all of them far less powerful than the main magnet. They modify the magnetic field at very particular points and work in conjunction with the RF pulses to produce the scanner’s picture by encoding the spatial distribution of the water protons in your body.
  36. 36. Using medical terminology, the transverse (or axial, or x-y) planes slice you from top to bottom; the coronal (x-z) plane slice you lengthwise from front to back; and the sagittal (y-z) planes slice you lengthwise from side to side. However, the x, y and z gradients can be used in combination to generate image slices that are in any direction, which is one of the great strengths of MRI as a diagnostic tool.
  37. 37. The technologist and radiologist have the ability to alter imaging parameters (like the timings of the RF pulse and gradients) to emphasize areas of injury or disease or to acquire higher image resolutions.
  38. 38. Shades of gray: Here’s a picture (sagittal view) of your spine! Now it’s clear what the trouble is. See the dark disc that, unlike the others, protrudes into the spinal canal? That’s a herniated disc compressing the nerves of the spinal cord. MRI scans can display more than 250 distinct shades of grey, each reflecting slight variations in tissue density or water content. It is in those subtle shades that radiologists unlock the secrets of the tissues.
  39. 39. Herniated disc:
  40. 40. Why Cryogenics in MRI? The spatial resolution of MRI Tomography improves as the strength of available magnetic field increases. The magnetic field strength is a function of current conducted in superconducting loops. Conventional MRI equipment useful in diagnostic medical imaging requires high DC magnetic fields, such as 5000 gauss or greater.
  41. 41. The basic physics principle says that as you cool electrical components and circuits, two things happen: The impedance and resistance of those circuits go down. The noise, which is a result of thermal activity in the materials, also comes down. So, you wind up with lower resistance and lower noise.”
  42. 42. The stronger the magnetic field the higher the frequency of the RF-pulse. The proportionality factor is called the gyromagnetic ratio, which is γ = 42.58 MHz/T for protons of Hydrogen atoms. For human imaging however, magnetic fields in the order of 0.1–4 T are commonly used. This means that the RF-pulses will have frequencies up to about 170 MHz –not far from commercial TV and FM radio stations, which can interfere with the imaging and vice versa.
  43. 43. The gyromagnetic ratio γ, is the radio frequency at which the nucleus absorbs energy in a magnetic field of 1 Tesla. In MRI still almost only hydrogen is utilized. In special cases one can make use of Phosphorous- 3114 or of Fluorine-1915, Helium-3 and Xenon- 189 , which do not occur naturally in humans.
  44. 44. Simple MRI magnet design with horizontal bore and magnetic field
  45. 45. The diameter of the bore is usually about 60–80 cm, which limits the size of objects (patients) to be imaged. The effective length of the bore can be almost any size but it is usually limited to less than one meter. A magnet of this size may be used for imaging any part of a human being, as long as he/she is not too large, or an animal .
  46. 46. Sketch of two MRI magnets in series for imaging larger objects
  47. 47. Risks of MRI 1) External Projectile Effects 2) Internal Projectile Effects 3) Other magnetic Effects 4) Radiofrequency Energy 5) Gradient field changes 6) Acoustic Noise 7) Quenching of the Magnetic Field 8) Claustrophobia
  48. 48. Objects categories : MR-Safe: The device or implant is completely non-magnetic, non-electrically conductive, and non-RF reactive, eliminating all of the primary potential threats during an MRI procedure. MR-Conditional: A device or implant that may contain magnetic, electrically conductive or RF-reactive components that is safe for operations in proximity to the MRI, provided the conditions for safe operation are defined and observed (such as 'tested safe to 1.5 teslas' or 'safe in magnetic fields below 500 gauss in strength'). MR-Unsafe: Nearly self-explanatory, this category is reserved for objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room.
  49. 49. The largest object I know of being pulled into a magnet is a fully loaded pallet jack.
  50. 50. Few Psychological effects observed during MRI Examination. A slight increase of body temperature, a few tenths of degrees, caused by the RF pulses. Nerve stimulation in limbs, when the magnetic fields are switched rapidly.
  51. 51. Safety in MRI Imaging MRI is in most cases assumed to be a very safe diagnostic modality. It is not invasive, contrast agents can often be avoided and it does not use ionizing radiation. The strong magnetic field has probably no impact on the human body, but many metallic implants are not compatible with MRI and deaths have occurred.
  52. 52. Limitations of MRI : High-quality images are assured only if you are able to remain perfectly still while the images are being recorded. If you are anxious, confused or in severe pain, you may find it difficult to lie still during imaging. A person who is very obese may not fit into the opening of a conventional MRI machine. The presence of an implant or other metallic object often makes it difficult to obtain clear images and patient movement can have the same effect. Breathing may cause artifacts, or image distortions, during MRIs of the chest, abdomen and pelvis. Bowel motion is another source of motion artifacts in abdomen and pelvic MRI studies. Assessment of the lungs is limited.
  53. 53. MRI may not always distinguish between tumor tissue and edema fluid. It cannot detect calcium present in a tumor. Detection of calcium (in tumors or other issues) is limited with MRI. MRI typically costs more and may take more time to perform than other imaging modalities.
  54. 54. Breakthrough: The next breakthrough could be the construction of “high temperature” super conducting magnets, which would need only liquid nitrogen at 77 K and not expensive liquid helium at 4 K in order to be cooled to super conducting condition, i.e. to conduct an electric current without any resistance at all.
  55. 55. Gifford-McMahon cryocoolers with ever-growing capacity have contributed significantly toward achieving this goal—this first allowed the elimination of liquid nitrogen as a thermal shield coolant and more recently the implementation of zero boil-off designs (0BO) using a helium recondenser dramatically increased helium refill intervals. Pulse tube coolers with lower vibration and fewer moving parts hold potential for the next step in reliability and patient comfort.
  56. 56. He (L) Recondensing system for MRI superconducting magnet (Utilizing 4 K cryocooler as well as cooler for radiation shield)
  57. 57. MRI technology is still in its infancy. Manufacturers are constantly improving machine designs, and scientists are discovering new applications, from monitoring wine quality to detecting lies; one MRI study revealed that people used twice as many regions of the brain to tell lies as they did to tell the truth.
  58. 58. WIPs in Toshiba America Medical Systems, Tustin, Calif. The Excelart Vantage 3T The Excelart Vantage Plus powered by Atlas technology—a 1.5T large-bore system. It provides a field of view up to 55 cm. Other WIPs include an 8-channel knee coil, a 6-channel wrist coil, an elliptical-shaped bore to better accommodate obese patients, and JET motion-correction software.
  59. 59. The Excelart Vantage at both 3T and 1.5T is powered by Atlas technology, and the Vantage 1.5T with Atlas received FDA clearance during Radiological Society of North America (RSNA). The Excelart Vantage 3T features a new magnet design and a short-bore combination as well as Pianissimo Technology, the last of which reduces acoustic noise. Goals of the system are to enable whole-body imaging and spectroscopy that more aggressively captures information.