MRI

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MRI

• Phillip W Patton, Ph.D.

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RAPHEX General Question 2001

• D43 In MRI, the RF frequency is dependent on
  the
• A. Diameter of the body part being imaged.
• B. Magnetic field strength.
• C. Pulse Sequence.
• D. RF coil.
• E. Relaxation time.

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RAPHEX Answer to 2001 Question D43

• B. The frequency used in MRI depends on the
  product of the magnetic field strength and the
  gyromagnetic ratio of the element being imaged.
  Since most MRI is of hydrogen atoms, the
  frequency depends only on the magnetic field
  strength.

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RAPHEX General Question 2001

• D46 Gradient fields in MRI are principally
  used to
• A. Maintain a uniform magnetic field in the field
  of view.
• B. Eliminate perturbations in the magnetic field
  due to site location.
• C. Shorten T1 to reduce scan time.
• D. Provide spatial localization.
• E. Measure the spin coupling.

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RAPHEX Answer to 2001 Question D46

• D. Gradient fields are used to modify the
  magnetic field so that resonance will be
  slightly different at different points in the
  volume of interest. It is then possible to
  localize these points.

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RAPHEX General Question 2002

• D51A higher intensity MRI signal is produced
  by
• A. Long T1, long T2.
• B. Long T1, short T2.
• C. Short T1, long T2.
• D. Short T1, short T2.

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RAPHEX Answer to 2002 Question D51

• C. The MRI signal strength depends on the value
  of the magnetization vector along the constant
  field (z) axis, which is then tipped into the
  x-y plane to produce the MRI signal. A short
  T1 implies a rapid return of the magnetization
  to the z axis following excitation by the RF
  field. A long T2 implies a long dephasing time
  for the individual nuclei involved in producing
  the magnetization. Both these effects will lead
  to a large value of magnetization and a large
  MRI signal.

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RAPHEX General Question 2003

• D53For hydrogen imaging in a 1.0 T MRI unit,
  the frequency of the RF signal is about
  ________ .
• A. 40 Hz
• B. 40 kHz
• C. 40 MHz
• D. 400 MHz
• E. 4 GHz

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RAPHEX Answer to 2003 Question D53

• C. The frequency for hydrogen is 42 MHz x
  magnetic field strength in tesla.

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Raphex 2001 Diagnostic Questions

• D43. In MRI, the RF frequency is dependent on
  the
• A. Diameter of the body part being imaged
• B. Magnetic field strength
• C. Pulse sequence
• D. Relaxation time
• E. RF coil

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RAPHEX Answer to 2001 Question D43

• B. The frequency used in MRI depends on the
  product of the magnetic field strength and the
  gyromagnetic ratio of the element being imaged.
  Since MRI is mainly of hydrogen atoms, the
  frequency depends only on the magnetic field
  strength.

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Raphex 2001 Diagnostic Questions

• D46. Gradient fields in MRI are principally used
  to
• A. Eliminate perturbations in the magnetic field
  due to site location
• B. Maintain a uniform magnetic field in the field
  of view
• C. Measure the spin coupling
• D. Provide spatial localization
• E. Shorten T1 to reduce scan time

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RAPHEX Answer to 2001 Question D46

• D. Gradient fields are used to modify the
  magnetic field so that resonance will be slightly
  different at different points in the volume of
  interest.

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Raphex 2000 Diagnostic Questions

• D48. In MRI images, motion during the scans
  results in ghost images which appear in the
  ______ direction.
• A. Amplitude
• B. Frequency encoding
• C. Phase encoding
• D. Relaxation
• E. Slice thickness

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RAPHEX Answer to 2000 Question D48

• B. Motion usually appears as a series of ghost
  images of reduced intensity displaced in phase
  encoded direction.

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Raphex 2001 Diagnostic Questions

• D50. Which of the following does NOT generally
  affect the total exam time of an MRI study?
• A. of acquisitions
• B. of frequency encoding steps
• C. of phase encoding steps
• D. of pulse sequences in the study
• E. TR

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RAPHEX Answer to 2001 Question D50

• A. frequency encoding steps are determined only
  by the resolution setting of the ADC.

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Raphex 2003 Diagnostic Questions

• D54. In MRI the signal-to-noise ratio can be
  increased by all of the following except
• A. Switching from a volume to a surface coil
• B. Increasing the number of acquisitions
• C. Increasing the static magnetic field strength
• D. Decreasing the slice thickness
• E. Increasing TR

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RAPHEX Answer to 2003 Question D54

• D.

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Raphex 2001 Diagnostic Questions

• D45. Superconducting magnets, compared to
  resistive magnets
• A. Are less expensive
• B. Are more easily turned off
• C. Do not require liquid helium
• D. Have higher field strength

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RAPHEX Answer to 2001 Question D45

• D. Superconducting magnets have field strengths
  of 0.3 10 T, while resistive magnets have
  strengths below 0.2 T.

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Raphex 2003 Diagnostic Questions

• D57-D59. Match the following MRI terms. (Answers
  may be used more than once.)
• A. Gradient fields
• B. RF
• C. Shim coils
• D. T1
• E. T2
• D57. Used to adjust magnetic field uniformity
• D58. Used to localize MR signal
• D59. Used to tip the net magnetization of spins

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Raphex 2003 Diagnostic Answers

• D57-D59. Match the following MRI terms. (Answers
  may be used more than once.)
• A. Gradient fields
• B. RF
• C. Shim coils
• D. T1
• E. T2
• D57. Used to adjust magnetic field uniformity C
• D58. Used to localize MR signal A
• D59. Used to tip the net magnetization of spins B

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Raphex 2000 Diagnostic Questions

• D49. Patients who have MRI scans should be
  screened to eliminate those who have
• A. Internal steel fragments
• B. Metallic prostheses
• C. Pacemakers
• D. Surgical clips
• E. All of the above

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RAPHEX Answer to 2000 Question D49

• E. Pacemakers can be affected an cause the pulse
  rate to change dramatically. Some surgical clips
  are magnetic and can move causing bleeding.
  Metallic prostheses can heat due to the RF pulses
  distorting the images. Steel workers with small
  fragments in their eyes have lost vision during
  an MRI scan.

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Raphex 2002 Diagnostic Questions

• D57. All of the following are MRI artifacts
  except
• A. Chemical shift
• B. Ring
• C. Susceptibility
• D. Wrap-around
• E. Zipper

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RAPHEX Answer to 2002 Question D57

• B. Bounce point artifact refers to a zero signal
  that can occur with certain inversion recovery
  pulse sequences. Chemical shift is a displacement
  difference between lipids and water. Zipper is a
  series of dropouts due to RF problems.
  Wrap-around represents aliasing due to the FOV
  being smaller than the object size.