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
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
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
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
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
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 * 10 8 meters/sec; speed of light X-Rays vs. light vs. radio waves
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)
EM Radiation Interactions w/ matter Completely different than particulate radiation (electron) interactions: Photoelectric effect Compton scattering
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 radiation at (33.2-0.6)= 31.6KeV in a random direction. The Atom now has positive charge What if the energy is higher/lower? K L M Iodine Energy levels K -33.2keV L -4.3keV M -0.6keV Example 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)
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 m 0 is rest mass of electron m 0 c2 = 511 keV
Medical Imaging Photoelectric effect –Responsible for contrast between tissues Compton effect –Undesirable –How can we control the angle? Important concepts –Attenuation –Dose
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.
Linear Attenuation Coefficient Assuming “narrow beam” geometry = same width as the beam detector Homogeneous slab of thickness Δx Fundamental photon attenuation law N = N 0 e -μ Δ x μ = linear attenuation coefficient In terms of intensity: I = I 0 e -μ Δ x This is known as Beer’s Law
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.
Attenuation Coefficient Human Density ~ 1 g/cm 3 Δx = 20cm N 0 = 1,000,000,000,000 Exercise 1: E γ = 20 KeV Exercise 2: E γ = 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
EM Radiation Dose How many photons? → fluence How much energy? → energy fluence What does radiation do to matter? → dose
Exposure = the creation of ions How many ions are created? Exposure X, 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
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? Dose D, 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.
So did we kill our test subject? 2 x 10 13 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 10 13 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 =
Dose Equivalent Different types of radiation, when delivering the same dose, can have different effects on the body. Dose equivalent H 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
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
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
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.
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.
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.
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.
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.
Cancer Risk Estimates Putting 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)
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 Whole Body – Radiation Workers5 rem/year (5000 mrem/yr) Extremities – Radiation Workers50 rem/year 50,000 mrem/yr General Public0.1 rem/year (100 mrem/yr)
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
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
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.
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.
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
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)
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
Restriction Goal: To direct beam toward desired anatomy
Compensation Filters Goal: to even out film exposure
Effectiveness in scatter reduction? grid ratio = h / b 6:1 to 16:1 (radiography) or 2:1 (mammography)
Grids Radiation is absorbed by grid –grid conversion factor –Typical range 3 < GCF < 8 Grid visible on x-ray film –move grid during exposure –linear or circular motion
Intensifying Screen Film stops only 1-2% of x-rays Film stops light really well Phosphor = calcium tungstate Flash of light lasts 1 x 10 -10 second ~1,000 light photons per 50 keV x-ray photon Reflective layer prevents light from going backwards
Radiographic Cassette Cassette holds two screens; makes “sandwich" One side is leaded. Why?
Contrast Agents Contrast compounds containing barium or iodine, which are radio opaque, can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels. The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen. Air –Does not absorb x-ray –“opposite” type of contrast –By Inflating the lungs, air provides contrast for lung tissues
X-ray Shoe Fitting Device In the late 1940's and early 1950's, the shoe-fitting x-ray unit was a common shoe store sales promotion device and nearly all stores had one. It was estimated that there were 10,000 of these devices in use. This particular shoe-fitting x-ray unit was produced by the dominant company in the field, the Adrian X-Ray Company of Milwaukee WI, now defunct. Brooks Stevens, a noted industrial designer whose works included the Milwaukee Road Olympian train and an Oscar Meyer Wienermobile, designed this machine.
The primary component of a shoe-fitting x-ray unit was the fluoroscope which consisted essentially of an x-ray tube mounted near the floor and wholly or partially enclosed in a shielded box and a fluorescent screen. The x-rays penetrated the shoes and feet and then struck the fluorescent screen. This resulted in an image of the feet within the shoes. The fluorescent image was reflected to three viewing ports at the top of the cabinet, where the customer, the salesperson, and a third person (mom) could view the image at the same time. The radiation hazards associated with shoe fitting x-ray units were recognized as early as 1950. The machines were often out of adjustment and were constructed so radiation leaked into the surrounding area.