Equipment

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• Equipment

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• Magnetic resonance imaging (MRI) scan requires
  the use of a very strong magnetic field.
• Unlike other devices used in radiology, MR
  imaging uses no radiation.
• The magnet is contained in the housing of the
  scanner and this creates a magnetic field
  oriented down the center of the magnet.

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• The patient is placed within the magnetic field
  by lying on a table which is placed through the
  center of the opening of the magnet, similar to
  lying on a road running through a tunnel.

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• The strength of the magnetic field is measured in
  units called gauss or Tesla
• 10,000 gauss equals 1 Tesla.
• The earth's magnetic field is approximately 0.6
  gauss.
• The strongest magnetic field permitted in MRI
  scanning of humans is 1.5 Tesla (1.5T).

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• Three types of magnets are available for use in
  MRI.
• Most MRI scanners in use today are superconductng
  magnets.
• Resistive magnets are electromagnets, similar to
  superconducting magnets, but they are air cooled
  therefore have greater resistance to current and
  create weaker magnetic fields.

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• Permanent magnets are made of solid magnetic
  material, similar to bar magnets, and create the
  weakest magnetic fields.
• However, they can be arranged in a configuration
  that doesn't require the patient to be surrounded
  by the magnet and are used in Open MR scanners.

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• The strongest is a superconducting magnet.
• This is a type of electromagnet in which current
  flowing in a circular direction in a coil of wire
  creates a magnetic field oriented down the core
  of the coil.
• In superconducting magnets, the wire conducts the
  current without significant resistance because it
  is cooled to a temperature close to absolute zero
  by being bathed in a jacket of liquid helium
  and/or liquid nitrogen.

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• The picture shows the actual magnet (the outer
  container resembles a thermos and contains the
  superconducting wire surrounded by liquid
  helium).

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• Creating an Image

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• The physics of MRI are extremely complex.
• When a patient is placed within and MR scanner,
  the protons in the patients tissues (primarily
  protons contained in water molecules) align
  themselves along the direction of the magnetic
  field.

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• A radiofrequency electromagnetic pulse is then
  applied, which deflects the protons off their
  axis along the magnetic field.
• As the protons realign themselves with the
  magnetic field, a signal is produced.
• This signal is detected by an antenna, and with
  the help of computer analysis, is converted into
  an image.

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• The process by which the protons realign
  themselves with the magnetic field is referred to
  as relaxation.
• The protons undergo 2 types of relaxtion
• T1 (or longitudinal) relaxation and
• T2 (or transverse relaxation) relaxation.

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• Different tissues undergo different rates of
  relaxation, and these differences create the
  contrast between different structures, and the
  contrast between normal and abnormal tissue, seen
  on MRI scans.

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• T1 weighted images emphasize the difference in T1
  relaxation times between different tissues.
• In these images, water containing structures are
  dark.
• Since most pathologic processes (such as tumors,
  injuries, CVA's, etc.) involve edema (or water),
  T1 weighted images do not show good contrast
  between normal and abnormal tissues.
• However, pathologic processes do demonstrate
  excellent anatomic detail.

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• T2 weighted images emphasize the difference in T2
  relaxation times between different tissues.
• Since water is bright on these images, T2
  weighted images provide excellent contrast
  between normal and abnormal tissues, although the
  anatomic detail is less then that of T1 weighted
  images.

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• Proton density images emphasize neither T1 or T2
  relaxation times, and therefore produce contrast
  based primarily on the amount of protons present
  in the tissue.

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• Intravenous contrast is often used to improve the
  sensitivity of MR imaging,
• especially in the brain and spine.

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• MR contrast agents contain gadolinium, which
  increases T1 relaxation and causes certain
  abnormalities to "light up" on T1 weighted
  images.
• These agents contain no iodine, and allergic
  reactions are extremely rare.

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• Image Orientation

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• MRI images can be obtained in any imaging plane
  without moving the patient.
• However, three standard views are usually used

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• Transverse (axial) Imagine the patient is lying
  on their back and is sliced across from right to
  left.
• You are viewing from the patient's feet.

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• Coronal Imagine the patient is standing in front
  of you and is sliced across from right to left.
• You are viewing from the front of the patient.

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• Sagittal Imagine the patient is standing
  sideways and is sliced across from front to back.
  
• You are viewing from the side of the patient.