Resident Physics Lectures (Year 1)

1. Resident Physics Lectures (Year 1) • Christensen, Chapter 5 Attenuation George David Associate Professor Medical College of Georgia Department of Radiology

2. Diagnostic X-Ray Beam Characteristics • Characteristic Radiation • Discrete energies • Characteristic of target material • Bremsstrahlung • Energy range 0 – kVp selected

3. Beam Characteristics • Quantity • # photons in beam 1, 2, 3, ... ~ ~ ~ ~ ~

4. ~ ~ ~ ~ ~ ~ ~ Beam Characteristics • Quality • energy distribution of photons in beam 1 @ 27 keV, 2 @ 32 keV, 2 at 39 keV, ... ~

5. 324 mR Beam Characteristics ~ • Intensity • weighted combination of photon # & energy • depends on beam • Quantity • Quality • Beam intensity can be quantified by measuring ionizations • Meter reading in milliRoentgens (mR) ~ ~ ~ ~ ~ ~ ~

6. Roentgen? • Unit of measurement for amount of ionizing radiation that produces 2.58 x 10-4 Coulomb/kg of air @ STP • 1 C ~ 6.241509324×1018 electrons 1901

7. Beam Intensity • Can be measured in terms of # of ions created in air by beam • Valid for both mono-energetic & poly-energetic beams 324 mR

8. Mono-energetic Radiation • 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

9. Attenuation Formula N = No e -mx where No = # incident photons OR # photons penetrating absorber of 0 thickness N = # transmitted photons e = base of natural logarithm (2.718…) m = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm) • No • N x Mono-energetic radiation beam

10. If x=0 (no absorber) 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) • No • N X=0 Mon-energetic radiation beam

11. Mon-energeticRadiation • Let’s graph the attenuation of a monochromatic x-ray beam vs. attenuator thickness 60% removed 40% remain Mono-energetic radiation beam

12. 1 .1 .01 .001 Mono-energetic Radiation • Yields straight line on semi-log graph Fraction (also fraction of energy) Remaining or Transmitted • N / No = e -mx 1 2 3 4 5 Attenuator Thickness Mono-energetic radiation beam

13. Poly-energetic Radiation • 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

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

15. Half Value Layer (HVL) • Absorber thickness that reduces beam intensity by exactly 1/2 • Units of thickness • value of x which makes N equal to No / 2 N = No e -mx • X1/2 = HVL = .693 / m N/No=0.5 = e -mx Mono-energetic radiation beam

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

17. 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

18. Poly-energetic Attenuation • Curved line on semi-log graph • line straightens with increasing attenuation • slope approaches that of monochromatic beam at peak energy • Mean energy increases with attenuation • beam hardening 1 .1 Poly-energetic Fraction Transmitted .01 Mono-energetic .001 Attenuator Thickness

19. 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

20. Scatter Radiation • NO benefit • no useful information on image • detracts from film quality • exposes personnel, public • 50-90% of photons exiting patient

21. Scatter Factors • Factors affecting scatter • field size • thickness of body part • kVp An increase in any of above increases scatter.

22. II Tube II Tube X-Ray Tube X-Ray Tube Scatter & Field Size • Reducing field size causes significant reduction in scatter radiation

23. 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

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

25. Scatter Management • Reduce scatter by minimizing • field size • within limits of exam • thickness • mammography compression

26. Scatter Control Techniques:Grid • Directional filter for photons • Increases patient dose

27. Grid Structure

28. Purpose Focal Spot • Directional filter for photons • Ideal grid • passes all primary photons • photons coming from focal spot • blocks all secondary photons • photons not coming from focal spot “Good” photon Patient “Bad” photon X Grid Film

29. Scatter Control Techniques:Air Gap Grid Air Gap • Gap intentionally left between patient & image receptor • Grid not used Patient AirGap Patient Grid ImageReceptor