Resident Physics Lectures

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Resident Physics Lectures

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1. Resident Physics Lectures Christensen, Chapter 5 Attenuation

2. Beam Characteristics Quantity number of photons in beam

3. Beam Characteristics Quality energy distribution of photons in beam

4. Beam Characteristics Intensity weighted product of number and energy of photons depends on quantity quality

5. Beam Intensity Can be measured in terms of # of ions created in air by beam Valid for monochromatic or for polychromatic beam

6. Monochromatic Radiation (Mono-energetic) Radioisotope Not x-ray beam all photons in beam have same energy attenuation results in Change in beam quantity no change in beam quality # of photons & total energy of beam changes by same fraction

7. Attenuation Coefficient Parameter indicating fraction of radiation attenuated by a given absorber thickness Attenuation Coefficient is function of absorber photon energy

8. Linear Attenuation Coef. Why called linear? distance expressed in linear dimension “x” Formula N = No e -mx where No = number of incident photons N = number of transmitted photons e = base of natural logarithm (2.718…) m = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm)

9. Linear Attenuation Coef. Units: 1 / cm ( or 1 / distance) Properties reciprocal of absorber thickness that reduces beam intensity by e (~2.718…) ~63% reduction 37% of original intensity remaining as photon beam energy increases penetration increases / attenuation decreases attenuating distance increases linear attenuation coefficient decreases Note: Same equation as used for radioactive decay

10. Linear & Mass Attenuation Coefficient coefficient (m) Linear atten. Coef. 1 / cm absorber thickness(x) linear cm

11. Mass Attenuation Coef. Mass attenuation coefficient = linear attenuation coefficient divided by density normalizes for density expresses attenuation of a material independent of physical state Notes references often give mass attenuation coef. linear may be more useful in radiology

12. Monochromatic Radiation Let’s graph the attenuation of a monochromatic x-ray beam vs. attenuator thickness

13. Monochromatic Radiation Yields straight line on semi-log graph

14. Polychromatic Radiation (Poly-energetic) X-Ray beam contains spectrum of photon energies highest energy = peak kilovoltage applied to tube mean energy 1/3 - 1/2 of peak depends on filtration

15. X-Ray Beam Attenuation reduction in beam intensity by absorption (photoelectric) deflection (scattering) Attenuation alters beam quantity quality higher fraction of low energy photons removed Beam Hardening

16. Half Value Layer (HVL) absorber thickness that reduces beam intensity by exactly half Units of thickness value of “x” which makes N equal to No / 2

17. Half Value Layer (HVL) Indication of beam quality Valid concept for all beam types Mono-energetic Poly-energetic Higher HVL means more penetrating beam lower attenuation coefficient

18. Factors Affecting Attenuation Energy of radiation / beam quality higher energy more penetration less attenuation Matter density atomic number electrons per gram higher density, atomic number, or electrons per gram increases attenuation

19. Polychromatic Attenuation Yields curved line on semi-log graph line straightens with increasing attenuation slope approaches that of monochromatic beam at the peak energy mean energy increases with attenuation beam hardening

20. Photoelectric vs. Compton Fractional contribution of each determined by photon energy atomic number of absorber Equation m = mcoherent + mPE + mCompton

21. Attenuation & Density Attenuation proportional to density difference in tissue densities accounts for much of optical density difference seen radiographs # of Compton interactions depends on electrons / unit path which depends on electrons per gram density

22. Photoelectric Effect Interaction much more likely for low energy photons high atomic number elements

23. Photoelectric vs. Compton As photon energy increases Both PE & Compton decrease PE decreases faster Fraction of m that is Compton increases Fraction of m that is PE decreases

24. Photoelectric vs. Compton As atomic # increases Fraction of m that is PE increases Fraction of m that is Compton decreases

25. Interaction Probability

26. Interaction Probability

27. Interaction Probability

28. Relationships Density generally increases with atomic # different states = different density ice, water, steam no relationship between density and electrons per gram atomic # vs. electrons / gram hydrogen ~ 2X electrons / gram as most other substances as atomic # increases, electrons / gram decreases slightly

29. Applications As photon energy increases subject (and image) contrast decreases differential absorption decreases at 20 keV bone’s linear attenuation coefficient 6 X water’s at 100 keV bone’s linear attenuation coefficient 1.4 X water’s

30. Applications At low x-ray energies attenuation differences between bone & soft tissue primarily caused by photoelectric effect related to atomic number & density

31. Applications At high x-ray energies attenuation differences between bone & soft tissue primarily because of Compton scatter related entirely to density

32. Applications Difference between water & fat only visible at low energies effective atomic # of water slightly higher yields photoelectric difference electrons / cm almost equal No Compton difference Photoelectric dominates at low energy

33. Photoelectric Effect Exiting electron kinetic energy incident energy - electron’s binding energy electrons in higher energy shells cascade down to fill energy void of inner shell characteristic radiation

34. K-Edge Each electron shell has threshold for PE effect Photon energy must be >= binding energy of shell For photon energy > K-shell binding energy, k-shell electrons become candidates for PE PE probability falls off drastically with energy SO PE interactions generally decrease but increase as photon energy exceeds shell binding energies

35. K-Edge step increase in attenuation at k-edge energy K-shell electrons become available for interaction exception to rule of decreasing attenuation with increasing energy

36. K-Edge Significance K-edge energy insignificantly low for low Z materials k-edge energy in diagnostic range for high Z materials higher attenuation above k-edge useful in contrast agents rare earth screens Mammography beam filters

37. Scatter Radiation NO Socially Redeeming Qualities no useful information on image detracts from film quality exposes personnel, public represents 50-90% of photons exiting patient

38. Abdominal Photons ~1% of incident photons on adult abdomen reach film fate of the other 99% mostly scatter most do not reach film absorption

39. Scatter Factors Factors affecting scatter field size thickness of body part kVp

40. Scatter & Field Size Reducing field size causes significant reduction in scatter radiation One of the most effective ways of minimizing operator exposure is to reduce field size through collimation. Even a relatively small reduction in field size can often result in a substantial reduction in operator exposure. This occurs for two reasons. The first is that a smaller beam irradiates a less volume of tissue so that there is less tissue to act as a scatter radiation source. Secondly reducing beam size means that scatter radiation must travel further through the patient before exiting. The increased travel distance means a less intense scatter field for the operator. A fluoroscopist should always collimate the x-ray beam to a size no larger than is required clinically.One of the most effective ways of minimizing operator exposure is to reduce field size through collimation. Even a relatively small reduction in field size can often result in a substantial reduction in operator exposure. This occurs for two reasons. The first is that a smaller beam irradiates a less volume of tissue so that there is less tissue to act as a scatter radiation source. Secondly reducing beam size means that scatter radiation must travel further through the patient before exiting. The increased travel distance means a less intense scatter field for the operator. A fluoroscopist should always collimate the x-ray beam to a size no larger than is required clinically.

41. Field Size & Scatter Field Size & thickness determine volume of irradiated tissue Scatter increase with increasing field size initially large increase in scatter with increasing field size saturation reached (at ~ 12 X 12 inch field) further field size increase does not increase scatter reaching film scatter shielded within patient

42. Thickness & Scatter Increasing patient thickness leads to increased scatter but saturation point reached scatter photons produced far from film shielded within body

43. kVp & Scatter kVp has less effect on scatter than than field size thickness Increasing kVp increases scatter more photons scatter in forward direction

44. Scatter Management Reduce scatter by minimizing field size within limits of exam thickness mammography compression kVp but low kVp increases patient dose in practice we maximize kVp

45. Scatter Control Techniques: Grid directional filter for photons Increases patient dose

46. Angle of Escape angle over which scattered radiation misses primary field escape angle larger for small fields larger distances from film

47. Scatter Control Techniques: Air Gap Gap intentionally left between patient & image receptor Natural result of magnification radiography Grid not used (covered in detail in chapter 8)

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