Flashcards in this deck (18)

• What effect does the static magnetic field of an MRI scanner have on hydrogen protons?

  The static magnetic field (B₀) causes hydrogen proton spins to align either parallel or anti-parallel to the field, creating a net magnetization vector along the field direction.
  mri magnetic_field
• What effect does a gradient magnetic field have on hydrogen protons?

  It causes the precession frequency (Larmor frequency) to vary with spatial location, leading to spins accumulating different phases based on position.
  mri gradient_field
• What happens with local field inhomogeneity in MRI?

  It leads to local variations in precession frequency, causing faster dephasing of spins and signal loss in that region.
  mri field_inhomogeneity
• Which TE and TR values best emphasize differences in proton densities?

  ii. TE = 10 ms ; TR = 5000 ms. Long TR removes T1 weighting, and short TE minimizes T2 effects, emphasizing proton density differences.
  mri proton_density
• Which choice distinguishes sample A from sample D?

  Choice: iv. TE = 20 ms ; TR = 1000 ms. Sample D has a longer T1, and A will appear brighter due to less recovery of D before each pulse.
  mri sample_distinction
• What are the four methods to optimize penetration in ultrasound imaging?

  • Frequency Selection: Lower frequencies increase penetration.
  • Gain Adjustment: Amplifies received echoes.
  • Dynamic Range: Adjusts grey scale to enhance weak echoes.
  • Time-Gain Compensation (TGC): Compensates for depth-dependent attenuation.
  ultrasound penetration
• Why does improving image penetration worsen resolution?

  Lower frequency improves penetration but reduces resolution due to longer wavelengths producing poorer axial and lateral resolution.
  ultrasound resolution
• What is a pulse sequence in MRI?

  A pulse sequence is a series of RF pulses and gradients used to control signal acquisition and contrast.
  mri pulse_sequence
• What are the differences in excitation pulse flip angles between spin-echo and gradient echo?

  • Spin echo: 90° flip angle
  • Gradient echo: <90° flip angle
  mri flip_angle
• How are echoes formed in spin-echo and gradient echo sequences?

  • Spin echo: 180° refocusing
  • Gradient echo: Gradient reversal
  mri echo_formation
• What are the projectile risks associated with MRI?

  • Magnetism
  • RF burns
  • Heating (SAR)
  • Hearing damage (gradient noise)
  • Implanted device malfunction
  mri risks
• How can an MR Operator reduce acquisition time?

  • Decrease TR
  • Reduce phase encodes
  • Parallel imaging
  mri techniques
• Order the following from highest to lowest CT number: Copper, Liver, Water, Lung, Air.

  • Copper
  • Liver
  • Water
  • Lung
  • Air
  ct tissues
• What is the CT number for Copper?

  Thousands of HU (very high)
  ct copper
• What is the CT number for Liver?

  +40 to +60 HU
  ct liver
• What is the CT number for Water?

  0 HU (reference point)
  ct water
• What is the CT number for Lung?

  Roughly -700 to -900 HU
  ct lung
• What is the CT number for Air?

  About -1000 HU (lowest density)
  ct air