JPNM Physics

Some Fundamental Physics Considerations

Document created May 30, 1996; updated May 31, 1996 Instrumentation Consderations

All of the physics relating to colllimated F-18 imaging is found in any GOOD book on atomic physics. For example, The Atomic Nucleus by Evans is one of the most complete. Chapter 25 "Attenuation and Absorption of Electromagnetic Radiation" has excellent quantative data from which the tables below were compiled.

At 500 keV:

Mass Attenuation Coefficients
cm2/g
MaterialDensity PhotoCompt scatCompt absTot absTot atten
Water1.0<<0.0010.0630.0300.0300.093
NaI3.670.0160.0500.0250.0470.091
Lead11.30.0800.0470.0240.100.15


Linear Attenuation Coefficients
cm-1
MaterialDensity PhotoCompt scatCompt absTot absTot atten
Water1.0<<0.0010.0630.0300.0300.093
NaI3.670.0590.180.0920.170.33
Lead11.30.900.530.271.11.7

Some important points to note are the following:

  1. In water, at this energy (511 kev), there is no photoelectric effect but there is significant Compton scattering.
  2. Lead has strong photoelectric effect AND very signifcant Compton scattering. This implies Compton scattering in the collimator will be significant.
  3. NaI has signifigant everything, i.e. photelectric effect and Compton scattering both happen to an appreciable degree. This demonstrates that NaI is a relatively poor detector at this energy.
  4. First order "shielding" calculations can be based on the half vaue layer:

    At 500 keV: At 150 keV:

    HVLH2O = 7.5 cm HVLH2O = 4.6 cm

    HVLNaI = 2.1 cm HVLNaI = 0.32 cm

    HVLPb = 0.41 cm HVLPb = 0.034 cm

  5. With F-18, for the same attenuation effect (first approximation), we need 1.6 times as much water, 6.6 times as much NaI and 12 times as much lead compared to Tc-99m.

  6. Robert E. Zimmerman, zimmer@bwh.harvard.edu