Imagining

• T2 relaxation


  Transverse decay or spin-spin relaxation, where protons are dephasing and the net transverse magnetization vector is being lost
• T1 relaxation


  Longitudinal recovery or spin-lattice relaxation, where the longitudinal magnetization vector is being recovered
• T2 relaxation happens much quicker than T1 relaxation
• Time of Echo (TE)


  The amount of time that is waited after the RF pulse to sample the transverse magnetization signal
• Time of Repetition (TR)

  The time between the first RF pulse and the next RF pulse
• Short TE


  Highlights T2 differences in tissues, but with high signal and little contrast
• Longer TE


  Highlights the differences in T2 relaxation rates between tissues
• Short TR


  Highlights T1 differences in tissues, as tissues haven't fully regained longitudinal magnetization
• Longer TR


  Negates T1 contrast, as tissues have fully regained longitudinal magnetization
• Creating a T1-weighted image


  Use short TE to avoid T2 contrast, and reduce TR to highlight T1 differences
• A short TE and long TR pulse sequence creates high signal with no T1 or T2 contrast
• Keeping T2 contrast out of the image


  1. Keep TE time short

  2. Don't allow time for T2 differences to occur

  3. Reduce transverse relaxation
• Highlighting T1 differences between tissues

  1. Reduce TR time

  2. Measure transverse magnetization vector after 90 degree RF pulse
• Reducing TR time


  Affects the transverse magnetization vector
• T1 weighted image


  • Contrast is predominantly due to T1 differences in tissues
  • CSF is dark
  • Fat is bright
  • Muscle is intermediate
• Creating a T2 weighted image

  1. Keep TR long

  2. Increase TE time

  3. Highlight differences in T2 relaxation
• T2 weighted image


  • CSF is bright
  • Fat has intermediate signal
  • Muscle has low signal
• CSF is brighter in T2 weighted image than T1 weighted image
• Proton density weighted image

  • Contrast is based on the number of protons available for nuclear magnetic resonance
  • Fat and fluid have highest signal
  • Muscle has intermediate signal
• Muscle signal is intermediate in proton density weighted image because its longitudinal magnetization vector was smaller compared to fat and fluid
• Proton density weighted image shows low signal from menisci, ligaments, and subchondral bone plates
• Proton density weighted image allows identification of tears in ligaments or meniscus by contrast between bright fluid and surrounding low signal structures
• Long TR


  Highlights T1 differences
• Short TR


  Highlights T1 differences
• Long TE


  Highlights T2 differences
• Short TE


  Highlights T2 differences
• Long TR and short TE creates a proton density weighted image
• Remembering specific TR and TE values is not required, just need to know the order of magnitude (hundreds, thousands)
• Next topic is spatial localization in MRI images
• Any material that exhibits measurable radiation related changes can be used as detector for ionising radiation

  
• Changes that can be used to detect ionizing radiation

  • Change of colours
  • Chemical changes
  • Emission of visible light
  • Electric charge
• Active detectors


  Immediate measurement of the change
• Passive detectors


  Processing before reading
• Requirements for detecting ionizing radiation


  • Medium for interaction
  • Measurable signal from the interaction
  • Electronics to detect the signal

  
• Detector


  Produces an observable signal when interacting with radiation

  
• Sensor


  Monitors the detector and converts detector signal to an electrical signal
• Electronics assembly


  Supplies operating voltage, processes signal from sensor then sends to readout unit for display
• Readout unit


  Displays instrument reading in rate mode (cps, dpm, mrem/h, etc..) and/or scaler mode (counts, mrem, uSv, etc ...)
• Radiation Measurement Principles


  1. Radiation enters a medium, deposits energy

  2. Produces ionizations, scintillations (signal)

  3. Signal converted to an electrical pulse

  4. Pulse is amplified (original pulse is small)

  5. Amplified pulses are counted / sorted by their energies, and recorded

  6. Digital display of counts, spectrum or images

  
• Types of detectors


  • Counters
  • Spectrometers
  • Dosimeters