Physics of Radiology PDF

Physics of Radiology  Overview Radiation X-ray production – X-ray machine – Atomic interactions Film processing Radiation safety  Wilhelm Conrad Röentgen Professor of Experimental Physics – Würzburg University Discovered x-rays – 8 November, 1895 Awarded fi...

Physics of Radiology  Overview Radiation X-ray production – X-ray machine – Atomic interactions Film processing Radiation safety  Wilhelm Conrad Röentgen Professor of Experimental Physics – Würzburg University Discovered x-rays – 8 November, 1895 Awarded first Nobel Prize for Physics – 1901  Electromagnetic Radiation Electric and magnetic fields traveling together Large spectrum X-rays – Electron energy transition outside nucleus -rays – Emitted from unstable atomic nuclei M o r g a n , 5 t h  Electromagnetic Radiation All travel at speed of light – 3.0 x 108 m/s Dual Nature Velocity= frequency x wavelength c=ʋλ Photon - a discrete bundle of energy h Energy = h ʋ =hc / λ = Planck’s constant c = speed of light  eV Basic unit of photon energy Energy an e- achieves accelerated across 1 volt 15 eV causes ionization in living tissue Diagnostic radiographs – 60 - 100 keV e- = electron  X-ray production X-rays produced when highly energetic electrons interact with matter and convert their kinetic energy into electromagnetic radiation A device that accomplishes this task consists of: – An electron source (filament, or cathode) – An evacuated path (vacuum) for electron acceleration – An external energy source to accelerate the electrons (electric field ) – A target or anode  X-rays Production 2 types of interaction of e- with target Collision interaction Characteristic radiation Irradiative interaction Bremsstrahlung (braking radiation)  Characteristic radiation – an accelerated e - removes a bound e- from the target. As another e – replaces it, energy given off as radiation. Characteristic e- radiation  Bremsstrahlung (braking radiation) – e- passes close to the nucleus of the target material – electrostatic interaction causes the e- to bend form its path – energy given off as x-rays radiation Bremsstrahlung e-  X-ray tubes Main components: – Cathode – Anode – Glass (or metal) envelope – Tube housing  Radiation Tube Head Design Electron s Anode Cathode X-ray photons Copyright U. of Wash. Environmental Health and Safety. Used with permission.  Cathode The cathode consists of a filament of tungsten wire surrounded by a focusing cup The filament circuit provides a voltage up to about 10 V to the filament, producing a current of up to about 7A Electrical resistance heats the filaments and releases electrons (Thermionic effect) Electrons liberated from the filament flow through the vacuum of the tube to the anode when a positive voltage is applied to the anode relative to the cathode  Anode The anode is a metal target electrode that is maintained at a positive potential difference relative to the cathode Tungsten is the most widely used anode material because of its high melting point (3422°C) and high atomic number (Z=74) – Tungsten anode can handle substantial heat deposition without cracking or pitting of its surface  Anode configurations Simplest type of x-ray tube has a stationary (fixed) anode – Consists of tungsten insert imbedded in a copper block – Copper supports the tungsten target, and it removes heat efficiently from the target – Small target area limits heat dissipation rate, limiting the maximum tube current and thus the x-ray flux – Used in dental x-ray units, portable x-ray machines, portable fluoroscopy systems  Rotating anode – Rotating anodes used for most diagnostic x-ray applications – Greater heat loading and consequent higher x-ray output capabilities – Electrons impart energy to a continuously rotating target, spreading thermal energy over a large area and mass Morgan , 5th Edition, 1993 Line focus principle  X-ray Machine Components timer kV p Step-up Trans. oil filte Step- r m collimato down A Trans. r exposure switch The low-voltage circuit (green in diagram above) controls the heating of the filament in the x-ray tube. The high-voltage circuit (red in diagram) controls the voltage across the x- ray tube.  Anode / e- Interaction 90% of energy is lost as heat – Need high melting point target Tungsten – Rotating anode Helps dissipate heat Prevents pitting  Spectrum of X-ray Energies Most e- undergo multiple radiative interactions Wide range of e- energy striking anode – Maximum energy = kVp e- = electron Thrall & Widmer, 1998  8 1.4 10 8 1.4.10 8 1.2 10 8 1 10 kVp_100 7 kVp_90 8 10 kVp_80 kVp_70 7 6 10 kVp_60 7 4 10 7 2 10 0 0 0 10 20 30 40 50 60 70 80 90 100 0 keV1 , keV2 , keV3 , keV4 , keV5 100 Bremsstrahlung and Characteristic x-rays at various maximum tube potential (kVp)  Factors affecting x-ray emission Output of an x-ray tube described by the terms quality, quantity and exposure – Quality: Average energy – Quantity: number of x rays – Exposure is nearly proportional to the energy fluence of the x-ray beam and therefore has quality and quantity associated characteristics  Factors X-ray production efficiency, exposure, quality and quantity are determined by: – X-ray tube target material – Voltage – Current – Exposure time – Beam filtration – Generator waveform  Target (anode) material Affects efficiency of bremsstrahlung radiation production Output exposure roughly proportional to atomic number Energies of characteristic x-rays depend on target material Target material affects quantity of bremsstrahlung radiation and the quality of characteristic radiation  Tube voltage (kVp) Determines the maximum energy in the bremsstrahlung spectrum and affects the quality of the output spectrum Efficiency of x-ray production is directly related to tube voltage Changes in kVp must be compensated by corresponding changes in mAs to maintain the same exposure Effect of tube voltage 1.0 200 kV Relative 0.5 Intensity 100 kV 50 kV 0 0 50 100 150 200 X Ray Energy keV  Tube current (mA) Tube current is equal to the number of electrons flowing from the cathode to the anode per unit time Exposure of the beam for a given kVp and filtration is proportional to the tube current Effect of tube current 1.0 40 mA 20 mA Relative 0.5 Intensity 0 X Ray Energy Emax  Exposure time Exposure time is the duration of x-ray production Quantity of x-rays is directly proportional to the product of the tube current and exposure time (mAs)  Beam filtration Beam filtration modifies the quantity and quality of the x-ray beam by selectively removing low-energy photons in the spectrum This reduces the photon number (quantity) and shifts the average energy to higher values, increasing the quality Filtration of x-rays No filter X Ray Intensity With filter Emin X Ray energy Emax  Total Filtration Aluminum filter (s) Adde d 2.5 mm Glass window Total 70 Oil/Metal barrier kVp of x-ray tube 1.5 mm Inheren t  Generator waveform Generator waveform affects the quality of the emitted x-ray spectrum For the same kVp, a single-phase generator provides a lower average potential difference than a three-phase or high-frequency generator Both the quantity of x-rays produced and the quality of the x-ray spectrum are affected  Summary X-ray quantity is approximately proportional to: Z target × kVp 2 × mAs X-ray quality depends on kVp, generator waveform, and tube filtration Exposure depends on both quality and quantity Half-Value Layer Indicates the quality (energy) of the x-ray beam  Interaction with Matter Coherent scattering Photoelectric effect Compton scattering Pair production Photodisintegration  Coherent Scattering Photon interacts with object and changes its direction – No photon absorption – No change in photon energy ~5% of x-rays that strike patient Disadvantageous to radiograph production – Film fog  Photoelectric Effect Important in diagnostic radiology X-ray striking patient is totally absorbed – No scattering Probability of occurrence – Direct (atomic number)3 Magnifies absorption differences between tissues – Inverse (energy)3 Decreased contrast for higher kVp settings Thrall & Widmer, 1998  Compton Scattering Results in almost all of scatter reaching film Probability of occurrence – Increases directly with photon energy – Independent of atomic number Thrall & Widmer, 1998  Making a Radiograph Object between x-ray tube and x-ray film Low energy photons absorbed by patient – Not useful for image production – Filters reduce patient exposure Only possible because of differential absorption of x-rays – Photoelectric effect T h r al l & W i d  Collimators - Localizers Adjust the size and shape of the x-ray field emerging from the tube port Cones ( Fixed shape & size ) Diaphragm (Adjustable parallel-opposed lead shutters define the x-ray field) Cone Cone Diaphragm T h r al l & W i d Heel Effect  Scatter Radiation Decreases image detail Produces generalized film fog Grid reduces scatter reaching film  Grids Alternating lead and aluminum strips X-rays aligned with grid reach film X-rays not aligned are absorbed – Scatter radiation Some primary beam absorbed – Must increase mAs Grid  Grids Use when part thickness > 10 cm – Amount of scatter a function of thickness Types – Parallel – Focused – Stationery – Moving T h r al l & W i d Grid Types stationary – image of grid superimposed on radiograph Moving – Bucky or Potter-Bucky – grid moves back/forth during exposure – blurs image of grid – requires more radiation to use  Photographic film Photographic film has low sensitivity for x-rays directly; a fluorescent screen (phosphor) is used to convert x-ray to light, which exposes film Film Composition: – Transparent plastic substrate (acetate, polyester) – Both sides coated with light-sensitive emulsion (gelatin, silver halide crystals 0.1-1 mm). Exposure to light splits ions atomic silver appears black (negative film) – Blackening depending on deposited energy (E = I t) – Optical density (measure of film blackness) for visible light: D = log (Iincident/Itransmitted) – D >> 2 = "black", D = 0.25 … 0.3 = "transparent (white)" with standard light box (diagnostic useful range ~ 0.5 - 2.5)  Film characteristic curve (H and D curve) I Relationship between film exposure and optical density D Film characteristics: – Fog: D for zero exposure – Sensitivity (speed S): Reciprocal of exposure XD1 [R] that produces D of one: XD 1 Linear region S = 1/XD1  Film characteristic curve II Film characteristics continued: – Film gamma g (maximum slope): – Contrast C = DD/Dlog X D2 − D1 γ = log X 2 − log X 1 max Film Contrast, gamma latitude  Intensifying Screens More efficient to turn x-rays to light to expose film Screens composed of layers of phosphorescent crystals Thickness and size of crystals can be varied – Determines speed of cassette – Decrease detail with increase size/thickness  Screen / Film Combinations Sandwiching phosphor and film in light tight cassette. Lateral light spread through optical diffusion limits resolution, can be minimized by absorbing dyes Screen thickness is tradeoff between sensitivity and resolution X-ray Film emulsion X photons ray Crystal Light- s tight Phosphor screen cassette Foam Fil m Light spread  Characteristics of Fluorescent Screen Fluorescence wavelengths are chosen to match spectral sensitivity of film: CaWO2: 350nm-580nm, peak 430 nm (blue) Rare earths: Gd: green cassette photoreflective La: blue layer fluorescent screen photosensitive layer film substrate Dual-coated film, two screen layers Optically reflective layers  CASSETTES light tight holders for film sized for film FRONT vs. BACK ID window Types – cardboard vs. screen Morgan , 5th Edition, 1993  Factors Affecting Image Detail Motion Film Speed Focal spot size Focal spot - film distance Object - film distance Grid use Distortion  Factors Affecting Contrast Contrast = differences in film blackness between areas in radiograph Subject contrast Film contrast Fog and scatter  Subject Contrast Thickness differences Physical density differences Atomic number differences X-ray beam energy (kVp)  Film Processing Most common errors are in these steps Developing Fixing Final wash  Film Developing Reduce exposed silver halide crystals to metallic silver – Supply e- to Ag+ ions Rate depends on time and temperature e- = electron  Film Fixing Convert undeveloped silver halide to soluble compound Clears undeveloped silver from film  Final Wash Remove excess chemicals and residual silver Inadequate wash retained fixer AgSO4- brown film  Fluoroscopy Purpose To visualize, in real time: organ motion ingested or injected contrast agents insert stents cautherize small blood vessels  over-couch x-ray tube, II below table under-couch x-ray tube, II above table  Radiation Quantities and Units Exposure Absorbed Dose Equivalent Dose Effective Dose  Exposure Total electrical charge per unit mass that x and Gamma ray photons generate in air – SI unit : Coulomb per kilogram – Traditional unit: Roentgen 1 C/kg = 3.88 *103 R  Measurement of Radiation Exposure in medical radiography Free air ionization chamber: Determines radiation exposure by measuring the amount of ionization an x ray beam produces within its air collection volume.  Absorbed Dose (D) The amount of energy per unit mass absorbed by the irradiated objects. The amount of energy absorbed by a structure depends on the atomic number of the tissue composing the structure and the energy of the incident photon. ( Effective atomic number) – Bone (13.8) , – Soft tissue (7.4)  Units of Measurements SI unit : Gray (Gy) = J/Kg Traditional unit: RAD (Radiation Absorbed Dose) Deposition of 100 ergs of ionizing energy per gram of target material. 1 Gy = 100 Rads  Typical Values of D Radiotherapy dose = 40 Gy to tumour (over several weeks) Annual background dose = 2.5 mGy whole body Chest PA = 160 uGy entrance surface dose  Equivalent Dose (HT) HT = D. WR Average absorbed dose in a Radiation weighting factor tissue or organ in the human chosen for the type and body energy of the radiation HT is used for radiation Radiation type & energy W range R protection purposes when a X & gamma ray, electrons 1 person receives exposure ( every energy) from various types of ionizing Neutrons < 10 keV 5 radiation 10 keV – 100 keV 10 Protons 2 HT = (D.WR)1 + (D.WR)2 + (D.WR)3 Alpha particles 20  Units of Measurements SI unit : Sievert (Sv) Traditional Unit: REM (Roentgen Equivalent Man) 1 Sv = 100 rem for x-rays: WR=1 , HT = D 1 Sv = 1 Gy  Effective Dose (Sv , rem) Tissue or organ WT E = D. WR. WT Gonads 0.20 Red bone marrow 0.12 some organs are more Colon 0.12 Lung 0.12 radiosensitive than Stomach 0.12 others Bladder 0.05 WT is a concept for Breast 0.05 the relative risk Liver 0.05 Oesophagus 0.05 associated with Thyroid 0.05 radiation of different Skin 0.01 body tissue Bone surfaces 0.01 for x-rays: WR=1 Remainder 0.05 E = D. WT E = ∑ DT ×WT  Typical Values of E Barium enema = 7 mSv CT abdomen = 10 mSv Conventional abdomen = 1 mSv Chest PA = 20 uSv Annual dose limit for radiation workers = 20 mSv Annual background dose = 2.5 mSv. Radiobiology The study of the action of ionizing radiations on living things.  Biological Effects Absorption of energy in biological material leads to excitation or ionization. – excitation: raises electron to higher energy state without ejecting it from the atom. Radiations can be emitted from this process – ionization: ejects electron from the atom. Radiation is also released in the process  Overview Stochastic vs. Non-stochastic Effects Dose Response Curves Mechanisms for Biological Damage – Direct vs. Indirect Specific Biological Effects Risk Estimates  Stochastic (Random) Effects Occur by chance Occur in both exposed and unexposed individuals Probability of occurrence increases as dose increases  Stochastic Effects Cancer – Leukemia – Bone Cancer – Lung Cancer Mental Retardation Genetic Effects No radiation induced genetic effects have been observed in humans Could occur if DNA is damaged  Non-stochastic (Deterministic) Effects A certain minimum dose must be exceeded before occurrence The magnitude of the effect increases as dose increases There is a clear causal relationship between exposure and occurrence  Non-stochastic Effects Cataracts Skin Erythema Early effects (Acute Effects) – Hematopoietic Syndrome – Gastrointestinal (GI) Syndrome – Central Nervous System Syndrome  Linear No Theshold Response 1 Applies to P stochastic effects ro b Assumes that any a 0. bi 5 amount of lit radiation has y of some detrimental E 0 ff 0 50 10 effect e Dos 0 ct e  Nonlinear Theshold Response 1 Applies to P ro b T h non-stochastic a 0. bi 5 es h effects lit ol y d of D E 0 o ff 0 se 50 10 e Dos 0 ct e  Acute Radiation Syndrome # A Large Gamma Radiation Dose in a Short Duration – LD50/30 = 450 rad Lethal dose to 50% of population in 30 days without medical attention  Hematopoietic Syndrome Blood changes may be seen at dose as low as 14 rad Blood changes almost certain at doses above 50 rad Hematopoietic Syndrome appears at about 200 rad Blood Count (Lethal Dose) RBC C el Neutrophil l s R Lymphocytes e Platelet d s u ct io 24- 1 2 3 hr week weeks weeks n  Gastrointestinal Syndrome Threshold of 1000-10000 rads Damages intestinal lining Nausea and vomiting within the first 2 - 4 hours May develop diarrhea Associated with sepsis and opportunistic infections At 10 days could develop bloody diarrhea resulting in death  Central Nervous System Seen with radiation dose >10000 rads Micro vascular leaks edema Elevated intracranial pressure Death within hours  Survival Time Survival Time Hematopoietic Gastrointestinal CNS/ CVS 200 Rads 1000 Rads 100,000 Rads  Direct and Indirect Action Biological effects of radiation result principally from damage to DNA – DNA the “critical target” – Target can be directly damaged by radiation – Target can be indirectly damaged  Direct and Indirect Action of Radiation in Biological Systems H OH O · H Indirect Action Direct Action 2 nm 4 nm  Indirect Action H2O H2O++e- H2O+ H++OH H2O+e- H2O- H2O- H+OH- OH+OH H2O2 Chain of events in X-ray absorption Incident X-ray Photon Fast Electron (e- ) Ion Radical Free Radical Chemical Changes for Breakage of Bonds Biological Effects  The Cell Cycle Cell cycle components G – M, G1, S, G2 2M Cycles in culture – crypt cells, 9 - 10 hours – stem cells (mouse skin) 200 hr S – due to G1 Cells most radiosensitive in M, G2 Resistant in late S G G 0 1  Cell Radiosensitivity – Sensitivity Directly Proportional to Reproductive Capacity – Sensitivity Inversely Proportional to Cell Differentiation White Blood Cells (Lymphocytes>Granulocytes) Basal and Endothelial Cells Sperm Producing Cells Red Blood Cell Producing Cells Tissue Sensitivity Highest to Lowest Bone Marrow Kidney Liver Esophagus Bladder  Concepts of Radiation Protection limiting ionizing radiation exposure Uses ALARA concept – As Low As Reasonably Achievable views use of radiation in terms of RISK vs. BENEFIT Provides Standards and Guidelines (S&G) for appropriate use of radiation  ALARA GOAL: decrease exposure whenever possible does not eliminate radiation exposure provides guidelines to limit the risk of bodily injury  Standards & Guidelines (S&G) set by the scientific community federal & state legislation based on S&G Organizations ICRP (International Commission on Radiological Protection) NCRP (National Council on Radiation Protection & Measurements) NRC (Nuclear Regulatory Commission)  Radiation Protection Basics There are three methods that can be used to reduce your exposure. Time Distance Shielding  Time The less time that you spend in a radiation field will lower your exposure. Dose = Dose rate x time  The farther away people are from a radiation Distancesource, the less their exposure. RADIOACTIVE SOURCE  Distance vs. Exposure 100 cm 10 Sv per hour 50 cm 40 Sv per hour 10 cm 1000 Sv per hour Inverse Square Law  Shielding Any dense material placed between you and the source you will lower your exposure.  Shielding X, Neutron Paper sheet Aluminum Lead Paraffin  Shielding Personal shielding – lead aprons - at least 3 - 5mm Pb equivalent – thyroid / eye shielding during fluoroscopy – lead glove (5mm + Pb eq.) if hands likely to be in beam Protective barriers – lead glass / acrylic for windows, lead sheets in doors or plaster board walls, brick walls – minimise directing primary beam at widows / doors – best position to be located during x-ray exposure  Risk Comparisons Fatal risk of 1:1000000 (10-6) – 40 Tablespoons of Peanut Butter – 100 charcoal broiled steaks – 2 days in New York City – 1.5 Cigarettes – 10 mrem of radiation – 300 miles in a car – 1000 miles in a jet  Relative contribution factors to death smoking 10 cigarettes / day 1 in 200 all natural cause - aged 40 1 in 850 flu (all ages) 1 in 5,000 accidents on the road 1 in 8,000 radiation: effective dose - 10mSv 1 in 10,000 leukaemia 1 in 12,500 accident at work 1 in 43,000 radiation: (worker in nuclear industry) 1 in 57,000 being hit by lightning 1 in 10,000,000 radioactivity release (nuclear power) 1 in 10,000,000 UK figures (Plaut, 1993)  DNA Ionization Increased rate of – Mutations – Abortions and fetal abnormalities – Susceptibility to disease Decreased life span – Risk of cancer – Risk of cataracts  Radiation Protection Obtain maximum diagnostic information with minimum exposure to personnel and general public Blind adherence to rules cannot substitute for the exercise of good judgement!  Radiation Safety Radiation worker – Not exceed age (yr) x 10 mSv – Not exceed 50 mSv/year General public – Not exceed 1 mSv Medical/dental exposure not included – Pregnant - not exceed 0.5 mSv 10 mSv = 1 rem  Fate of DNA Damage Repair Mutations Cell death  Personnel Monitoring Check adequacy of radiation safety program Disclose improper radiation protection practices Detect potentially serious radiation exposure situations Do not wear detection devices for medical/dental radiographs  Basic Radiation Safety Rules for Diagnostic Radiology Thrall DE. Textbook of Diagnostic Veterinary Radiology, 3rd Edition. Page 5.