Projection Radiography (X-Ray)

1. Projection Radiography (X-Ray) Instructors: Brian Fleming and Ioana Fleming flembri@pha.jhu.edu, ioana@cs.jhu.edu January 7th, 2010

2. Today • X-Ray production • Interaction with matter / tissue • Instrumentation • Applications

3. 1. Atomic Structure Balanced == Neutral -- No Charge! Missing Electron == ? Extra Electron == ?

4. Electrons • Orbiting in shells

5. Electron Binding Energy • Atom’s ground state – lowest energy configuration • Basic principle: bound energy < unbound energy + electron energy • Binding energy is difference • Binding energy of hydrogen electron: 13.6 eV 1 eV is the kinetic energy gained by an electron that is accelerated across a one (1) volt potential

6. Ionization and Excitation • Ionization is “knocking" an electron out of the atom creates 1 electron and 1 ion (what charge?) • Excitation is “knocking" an electron to a higher orbit

7. Characteristic Radiation What happens to ionized or excited atom? • Return to ground state by rearrangement of electrons • Causes atom to give of energy Energy given off as radiation • infrared • light • x-rays

8. Ionizing Radiation Radiation with energy > 13.6 eV ionizes H • Energy required to ionize: • Air: 34 eV • Lead: 1 keV • Tungsten: 4 keV (average binding energies) • Radiation energies in medical imaging • 30 keV - 511 keV • can ionize 10 - 40,000 atoms

9. Particulate Radiation • Any subatomic particle (proton, neutron, electron) can be considered to be ionizing radiation (nuclear, beta) if it possesses enough kinetic energy to ionize an atom • An electron accelerated across 100 kV potential difference yields a 100 keV electron

10. Ion Drives

11. Ion Engines

12. Medical Particle Beams

13. What are X-Rays ? Electromagnetic EM Radiation • radio, microwaves, • infrared, visible light, ultraviolet • x-rays, gamma rays • Particle / photon: E = h * ν • Planck's constant h = 4.14 * 10-15 eV-sec • f is frequency • Electromagnetic wave: λ = c / ν • C = 3 * 108 meters/sec; speed of light X-Rays vs. light vs. radio waves

14. 2. How are X-Rays produced? X-rays are produced when accelerated electrons interact with a target, usually a metal absorber, or with a crystalline structure. Electron radiative interactions: • Characteristic x-rays: • Electron ejects an inner-shell electron • Reorganization generates x-ray • Bremsstrahlung x-rays • Electron “grazes" nucleus, slows down • Energy loss generates x-ray (primary source of x-rays from an x-ray tube)

15. X-Ray Spectrum

16. EM Radiation Interactions w/ matter Completely different than particulate radiation (electron) interactions: • Photoelectric effect • Compton scattering

17. Photoelectric effect • Photon with energy 40keV enters • Photoelectron from K-shell with energy (40-33.2)=6.8keV exits • Electron from M- to K-shell • Characteristic radiationat (33.2-0.6)= 31.6KeVin a random direction. • The Atom now has positive charge • What if the energy is higher/lower? • Atom completely absorbs incident photon • All energy is transferred • Atom produces • - characteristic radiation, and/or • - energetic electron(s) • Characteristic radiation might be • - x-ray • - Other light (very important) Iodine Energy levels K -33.2keV L -4.3keV M -0.6keV K L M Example

18. Compton Scattering • Photon collides with outer-shell electron • Photon is not absorbed, but it loses energy and it changes direction (angle θ) • E - Energy of incident photon • E’ - Energy of scattered photon • m0 is rest mass of electron • m0c2 = 511 keV

19. Medical Imaging • Photoelectric effect • Responsible for contrast between tissues • Compton effect • Undesirable • How can we control the angle? • Important concepts • Attenuation • Dose

20. Attenuation Beam Strength • Photon count = number of photons in the burst • Energy flow = how much energy the bust is carrying Intensity of an x-ray beam = energy fluence rate (per unit area per unit time) • The process describing the loss of strength of a beam of electromagnetic radiation. • Tissue-dependent attenuation is the primary mechanism behind contrast in radiology.

21. Linear Attenuation Coefficient • Assuming “narrow beam” geometry = same width as the beam detector • Homogeneous slab of thickness Δx • Fundamental photon attenuation law • N = N0 e -μ Δ x • μ = linear attenuation coefficient • In terms of intensity: • I = I0 e -μ Δ x • This is known as Beer’s Law

22. Attenuation Coefficient • The linear attenuation coefficient μ of all materials depends on the photon energy of the beam and the atomic numbers of the elements in the material. • Since the mass of the material itself provides the attenuation, attenuation coefficients are usually characterized by μ/ρ, where ρ is the material density.

23. Attenuation Coefficient Human Density ~ 1 g/cm3 Δx = 20cm N0 = 1,000,000,000,000 Exercise 1: Eγ = 20 KeV Exercise 2: Eγ = 100 KeV N = 2,000 ΔE = 999,999,998,000 * 20 keV = 2e13 keV N = 33,000,000,000 ΔE = 967,000,000,000 * 100 keV = 9.6e13 keV

24. EM Radiation Dose • How many photons? → fluence • How much energy? → energy fluence • What does radiation do to matter? →dose

25. Exposure = the creation of ions • How many ions are created? • ExposureX, the number of ion pairs produced in a specific volume of air by EM radiation • SI Units: C/kg (charge per mass) • Common Units: Roentgen, R 1 C/kg = 3876 R

26. Dose As EM radiation passes through a material, it deposits energy into it by the photoelectric effect and Compton scattering. • How much energy is deposited into material? • DoseD, the energy deposited per unit volume • SI unit: Gray (Gy) 1 Gy = 1 J/kg (energy per mass) • Common unit: rad 1 Gy = 100 rads 1 R of exposure yields 1 rad of absorbed dose in soft tissue.

27. So did we kill our test subject? • 2 x 1013 keV = 3.2 x 10-3 J • Mass = 80 kg • 3.2e-3 J / 80 kg = 0.00004 rads = 0.04 mRad • 9.6 x 1013 keV = 1.54 x 10-2 J • 1.54e-2 J / 80 kg = 0.00019 rads = 0.19 mRad • Typical chest x-ray dose ~ 0.1 mRad • 1000 Rad =

28. Dose Equivalent Different types of radiation, when delivering the same dose, can have different effects on the body. • Dose equivalentH H = D * Q • Q = quality factor, • Q ≈ 1 for x-rays, gamma rays, electrons, beta, • Q ≈ 10 for neutrons and protons, • Q ≈ 20 for alpha particles. • Since Q ≈ 1, H = D • SI unit, Sievert (Sv). More common, rems

29. Effective Dose = The sum of dose equivalents to different organs or body tissues, weighted to produce a value proportional to risk (the body is not irradiated uniformly) • Annual effective dose (average) = 100 mrems • Chest x-ray = 0.1 mrems • Fluoroscopic study = several rems

30. Biological Effects of X-Rays • Injury to living tissue results from the transfer of energy to atoms and molecules in the cellular structure. • Atoms and molecules become ionized or excited. • These excitations and ionizations can: • Produce free radicals • Break chemical bonds • Damage molecules that regulate vital cell processes

31. Prompt and Delayed Effects • Radiation effects can be categorized by when they appear • Prompt, acute effects – skin reddening, hair loss and radiation burns which develop soon after large doses of radiation are delivered over short periods of time • Delayed effects – cataract formation and cancer induction that may occur months or years after a radiation exposure.

32. Prompt Effects • Will develop within hours, days or weeks depending on the size of the dose. The larger the dose the sooner the effect will occur • Limited to the site of the exposure.

33. Prompt Effects • The skin does not have receptors that sense radiation exposure. No matter how large a radiation dose a person receives, there is no sensation at the time the dose is delivered. • Some people who have received large doses claim to feel a tingling at the skin, however it is believed that the tingling is due to static charge at the skin surface rather than the direct sensation of radiation exposure.

34. Delayed Effects • Cataracts – induced when a dose exceeding 500 rems is delivered to the lens of the eye. Radiation induced cataracts may take months or years to appear. • Extremely unlikely to receive a substantial dose to the eye working with todays units.

35. Delayed Effects • Cancer studies of people exposed to high doses of radiation have shown there is a risk of cancer induction associated with high doses. • Studies demonstrate that cancer risk is linearly proportional to the dose • Radiation induced cancers may take 10-15 years to appear.

36. Cancer Risk EstimatesPutting Risk into Perspective • 1 in a Million chance of death from activities common in society • Smoking 1.4 cigarettes in a lifetime (lung cancer) • Eating 40 tablespoons of peanut butter (aflatoxin) • Spending two days in Los Angeles (air pollution) • Driving 40 miles in a car (accident) • Flying 2500 miles in a jet (accident) • Canoeing for 6 minutes (drowning) • Receiving a dose of 10 mrem of radiation (cancer)

37. Personnel Exposure Limits • Annual Dose Exposure limits have been established based on the recommendations of national and international commissions. • Exposures at or below these limits should result in no exposure effects

38. Exposure Effects • 1000 rad – second degree burns • 2000 rad – intense swelling within a few hours • 3000 rad – completely destroys tissue • 400 rad – acute whole body exposure is LD 50/30* *LD 50/30 – lethal to 50% of population within 30 days if not treated

39. Projection Images: The creation of a two-dimensional image “shadow” of the three dimensional body. X-rays are transmitted through a patient, creating a radiograph. • chest x-rays • mammography • dental x-rays • fluoroscopy • angiography • computed tomography

40. The three standard orientations of projection (slice, tomographic) images Axial, Transaxial, Transverse Coronal Frontal Sagittal Oblique Slice: an orientation not corresponding to one of the standard slice orientation.

41. 3. Radiographic System

42. X-Ray tube

43. Anode Angle Anode angles in diagnostic x-ray tubes range from 7 to 20 degrees, with 12- to 15-degree angles most common. The smaller the angle, the better the resolution.

44. X-Ray Tube Components • Filament controls tube current (mA) • Tungsten - preferred because of its high melting point (3370°C) • Cathode and focusing cup • Anode is switched to high potential • 30 -150 kVp • Made of tungsten • Bremsstrahlung is 1% • Heat is 99% • Spins at 3,200-3,600 rpm • Glass housing; vaccum

45. Exposure Control • kVp applied for short duration • Older machines have a fixed “shutter speed” • Newer machines allow for variable exposure times • Tube current (mA) controlled by filament current and anode voltage mA * exposure time = mAs • Max energy -controlled by anode voltage V (keV) • Radiation Dose -controlled by current and time (mAs)

46. X-Ray Spectrum

47. Filtration • Low energy x-ray will be absorbed by the body (ouch!), without providing diagnostic information • Filtration: Process of absorbing low-energy x-ray photons before they enter the patient • Inherent Filtration • Within anode • Glass housing • Added Filtration • Aluminum • Copper/Aluminum • Measured in mm Al/Eq

48. Restriction • Goal: To direct beam toward desired anatomy

49. Compensation Filters • Goal: to even out film exposure

50. Colimators • Grids