1X-ray Interactions

Photon Energy and Momentum E = hf P = h/ h = 4.136 x 10-15 eVS (Plancks Constant) Probability of interactions depends on E

Coherent scattering

AKA: Rayleigh or Classical

2Coherent scattersummary

No energy is transferred

No ionization occurs

In diagnostic radiology < 5% of interactions are Coherent processes

Photoelectric (inner shell) and Compton (outer shell) interactions

PE and Compton effects are important in Radiology

Photoelectric effectPhoton energy > K-shell binding energy

3Photoelectric Effect summary

Z3

1/E3(above K-shell binding energy)

No scatter photons(Any patient characteristic x-rays are

low energy & get absorbed locally)

4Compton scatter is in ALL directions, and is approximately equal intensity in every direction at diagnostic x-ray energies.

Compton scatter summary

Occurs with outer shell electrons

Probability electron density (electrons/cc)

Probability falls with increasing photon energy ( 1/E)

Produces scatter that reaches image receptor

Photoelectron/Compton scatter electron fate

~34 eV to produce one electron-ion pair10 keV electron results in ~300 electron-ion pairs

5Attenuation of Radiation

Start with No - number N at depth t isN = No x e-t

= linear attenuation coefficient (cm-1)t = absorber thickness (cm)

[ e = base of natural logarithm (~2.7) ]

Fraction that is transmitted is e-tFraction that interacts is (1 - e-t)

6Attenuation is the sum of Coherent, Compton and Photoelectric processes

Half Value Layer

HVL reflects Eaverage (i.e. Penetration)

7Beam hardening

Adding filters reduces number of photons, Increases their average energy Eaverage

Most State Regulations Require

HVL > 2.5 mm Al (@ 80 kVp)

If HVL is < 2.5 mm AlBeam has low energy photons that irradiate

patient, but dont contribute to image!

HVL can be increasedby adding filters (Al, Cu, or Sn)

8Eaverage will Contrast (and vice versa)

/ is numerically the same as for water, because the density is 1 g/cm3

http://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/water.html