Gamma Camera - Uniformity and Other Related Items

    1. Causes of Nonuniformity
      1. Define uniformity - ability to reproduce a uniform distribution of radiation being detected. Consider this when assess a flood source that has uniform distribution and what causes it not be uniform
      2. Response to where the photon interacts with the crystal (above) will cause a lack of response as it approaches the edge of the PMT (photocathode)
        1. Where there are two PMT edges interact the positioning of the events are directed at each other further causing misposition of photon location
            Poor PMT response causes nonlinearity/uniformity
        2. Z-pulse variation from each PMT is another source of nonuniformity. Results in (Both a. and b.):
          1. Causes cold and hot PMTs
          2. Causes wavy lines
    2. Uniformity Corrections - nonuniformity can have as much as +/- 15% variation resulting is visual hot/cold spots
      1. Manually using a point source all PMTs can be tuned and this reduce nonuniform to about +/10%
      2. Count Skimming/Adding
        1. Determines the average counts per pixel from a uniform source
        2. Deviations per pixel are then stored
        3. As an image is collected counts are either subtracted (skimmed off) or added "on the fly"
        4. Every isotope used with this camera requires its own correction matrix
        5. Nonuniformity is reduced to about +/-5%
      3. Energy and linearity corrections
        1. Linearity corrections are stored at the pixel level usually at the factory or by service dude

          2. Peak Drift based at pixel level

          1. Energy peaks vary slightly at the PMT and pixel level
          2. Energy correction of the photopeak is applied at the pixel level with )Z
          3. When acquiring data these corrections factors are applied at the pixel level
          4. End results - improves energy resolution and reduces scatter
        2. Linearity correction
          1. Application of an appropriate linear phantom data is acquired in one projection to collect X coordinates. The system then creates )X to correct the positions which generates a straight line in the X direction
          2. Phantom is rotated 90 degrees to determine )Y positions applied
          3. Corrections are then applied to all pixels in the matrix

          3. Correcting the Flood

        3. End results
          1. Energy and linear corrections will generate a 3-5% non uniformity
          2. Adding count skimming, isotope specific acquisition with appropriate collimator will generate 1-3% nonuniformity
          3. Up to a 120 million count correction flood (sensitivity map) must be acquired to have a correction map
    3. Autotuning
      1. PMTs HV drift over time and autotuning is applied by making small adjustments as needed to assure that the variation of the pulse does not change
      2. Methods
        1. Photopeak monitoring - HV is monitored at PMT level and adjusted with a reference value
        2. Split-Photopeak monitoring - Each PMT has two high peak windows. When drifting occurs HV is adjusted by maintaining a ratio between the two peaks
        3. LED monitoring - LED flashes and generates photoelectron with the PMTs. The LED sets the standard and the PMTs adjust the HV when there is a drift in photoelectron collection
      3. If a PMT goes out of tune more than 3 keV then service dude takes over because the PMT can no longer correct itself
    4. Other points of interest
      1. PMTs are effected by magnetic fields and may be shields with mu-metal to reduce any low magnetic field. Would PMTs be able to function near an MRI unit?
      2. Keep your camera's room temperature constant with a suggested range of 68 to 70 degrees F.
      3. If temperature increases by 9 degrees or more within an hour the crystal may crack
      4. Beware of power surges and always use a batter backup
      5. Consider gamma ray photon interference from adjacent rooms and/or PET
      6. An old camera may show crystal hydration or optical gel deterioration

        7. CsI Camera Head

      7. Pixelated detector
        1. Has many small crystals, usually made of semiconductor material

          2. CsI Crstyal and Parts

        2. Each crystal is attached to a single position-sensitive photomultiplier (PSPMT)
        3. Collimator holes are lined up to individual crystals
        4. It's like creating a matrix on the detector head
        5. Resolution my not be as good, however, sensitivity is outstanding, and it allows for first pass procedures of the myocardium

        8. CZT Camera from GE

      8. CZT Camera (Cardiac)
        1. Pixeled system with CsI(Tl) crystals with silicon photodiodes
        2. There are 9 detectors in the camera head which rotate back and forth
        3. Has great counting statistics and shorten's imaging time
    5. Crystal materials
      1. NaI(Tl), CsI(Tl), CsI(Na), LaBr3 each have different properties (density, deadtime, light output, etc.)
      2. CZT is a semiconductor - better energy resolution and less thickness needed. It also has to operate below room temperature

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    Chapter 5 - Nuclear Medicine Instrumentation by J. Perkeges