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CT Physics Usman Mahmood, MS, DABR Lead Diagnostic Medical Physicist Department of Medical Physics Memorial Sloan-Kettering Cancer Center e-mail: [email protected] 1 Text and Reference Books REFERENCE TEXTS: Medical Imaging Physics, W.R. Hendee and E.R. Ritenour, Wiley-Liss Publisher, fourth edition, 2002. The Essential Physics of Medical Imaging, 2nd Edition. J.T. Bushberg, J.A. Seibert, E.M. Leidholdt, and J.M. Boone, Lippincott Williams and Wilkins Publisher, 2002. Advances in Medical Physics. A. B. Wolbarst, K. L. Mossman, and W. R. Hendee, Medical Physics Publishing, 2008. Radiology Review: Radiologic Physics. E.L. Nickoloff, Elsevier/Saunders Publisher, 2005. Review of Radiologic Physics, 3rd Edition. W. Huda, Wolters KluwerLippincott Williams & Wilkins Publisher, 2010. Computed Tomography, 2nd Edition. E. Seeram, Saunders Publishers, 2001. Mosby – Exam Review for CT 2 Some Online Resources Sprawls Educational Foundation – The Physical Principles of Medical Imaging – http://www.sprawls.org/resources/ International Atomic Energy Agency (IAEA) – Radiation Protection of Patients website – http://rpop.iaea.org – CT Tutor.com (have fee associated with them) **All images are from Stewart Bushong “Radiologic Science for Technologists” 3 X-ray Fundamentals Review Usman Mahmood, MS, DABR Overview Electromagnetic Radiation X-Ray Tubes and X-ray Production X-Ray Generator Interaction of Radiation with Matter Fundamental Principle CT systems are basically density measuring devices An image of an object (i.e. person, animal, ancient mummy etc.) may be reconstructed on the basis of the attenuation that occurs as x-radiation is transmitted through it. (Quoted from Mosby review book) As shown above, a x-ray beam striking a patient variably interacts with some tissues of the body. This then produces a “shadow” of the internal anatomy. Electromagnetic Radiation Photons have no mass and no charge Have magnetic and electric fields changing in sinusoidal fashion Characteristics: Travel in straight line Wavelike Invisible Capable of penetrating through a solid object – – – – • • At higher energies, they have properties that are similar to physical particles Meaning that even though they have no mass or charge, they are capable of knocking out electrons (also known as the ionization process) – What is an x-ray and how is it different than a gamma ray or other EM radiation? • One of the most energetic forms of light! • Both gamma and X-rays are part of the EM spectrum and are indistinguishable. • However, the primary or only real difference is that Gamma-rays originate from the nucleus of an atom. • X-rays originate from outside the nucleus of an atom. X-Ray Ionizing Radiation: – Is radiation that is able to produce a change (ionization) in matter on the atomic level. Ionization of an atom refers to removal or addition of electrons from the atom. There are two types of ionizing radiation: particulate and electromagnetic. X-Ray is a type of electromagnetic radiation Described as a wave like fluctuation of electric and magnetic fields – Photons are energy disturbances moving through space at the speed of light X-Ray Generators The x-ray generator provides the operator control of the radiographic techniques: – – – – Tube voltage (kVp), Tube current (mA), Exposure duration and delivers power to the x-ray tube. High Voltage Generators Modify incoming voltage and current in order to provide the x-ray tube with the power to provide an xray beam – CT Scanners now use High Frequency Generators (ripple is < 1%). – Typically located inside the CT gantry X-ray Generator Autotransformer – Is designed to supply a precise voltage to the filament circuit kVp = kilo-voltage peak (controls the energy or quality of x-rays that are prodcued – overall penetrating ability) mA – milli-amperage or X-ray tube current (quantity of radiation or “photon fluence”). High Voltage Transformer – A “step-up” transformer. Increases the output voltage from the auto-transformer to the kVp necessary for x-ray production. Bridge Rectifier – Current from a wall plug is 60 Hz AC (alternating current). Converts AC to a direct current (DC) (means electrons flow in one direction). Necessary for efficient and safe operation of x-ray tube. X-ray Production X-radiation is created by taking energy from electrons and converting it into photons with appropriate energies. This energy conversion takes place within the x-ray tube. X-ray Tube Glass envelope: maintains a vacuum inside the tube. Filament: The part of the cathode that emits electrons resulting in a tube current Focusing cup: metal shroud surrounding the filament Target: Region of the anode struck by electrons emitted by the filament Rotor: Rotating part of the electromagnetic induction motor located inside the glass envelope Window: thin section of the glass envelope through which the useful beam emerges Tube Glass Envelope The primary functions of the envelope are – 1. Ensures a vacuum, which allows for more efficient X-ray production. i. 2. If gas/air is present, then electrons flowing from cathod to anode may interact with the gas/air… hence causing fewer x-rays to be produced, and generally more heat is generated. Provide structural support and electrical insulation for the anode and cathode assemblies Cathode Basic Function of Cathode: The basic function of the cathode is to expel the electrons from the electrical circuit and focus them into a well-defined beam aimed at the anode. (kept at negative potential) Consists of 2 components 2 Filaments – Where the electrons come from. – – Focusing Cup – helps direct (“focus”) the x-rays to a specific spot on the anode. Cathode Filament – Material Tungsten High atomic number, High melting point, Thermal conductivity 1 – 2 cm coil of wire – Purpose: Focusing Cup: Material Nickel Purpose: Emit a low level negative charge wherein emitted electrons from the filament cannot repel from one another, they are held together in a cloud Electron emission when heated to 2200° C – Aka Thermionic Emission Dual Focus = Two filaments housed within one focusing cup (creating large and small focal spots (FS)). Cathode Assembly Small quantities of tungsten from the filament will vaporize and be deposited on the floor of the envelope – Generally at the window Over time when sufficiently built up: – Deposit acts as a filter which in turn reduces the efficiency and intensity of the useful beam – Compromises the vacuum within the envelope – Creates a conducting surface which could conduct a current Arcing Anode The anode is the component in which the xradiation is produced. It is a relatively large piece of metal that connects to the positive side of the electrical circuit. Has 2 functions: – to convert electronic energy into x-radiation – to dissipate the heat created in the process Disk: – Beveled edge (better heat dissipation and smaller effective focal spot size) – “Area” of x-ray production – Focal (Target) Track Area on the surface of the anode disk in which incoming electrons from the filament interact – High speed rotation evenly distributes heat over the entire track (+) Target (-) Decelerates the electrons – Anode: Positive electrode Properties – High melting point – High “Z” material Tungsten (W): “Z”=74, melts @ 3,370°C e- always flow from negative to positive Lead Housing Attenuates x-rays emitted in directions other than through the tube window Housing leakage - <1 mGy/hr @ 1 meter Anode Assembly Induction Motor 2 Parts – Rotor: Located within the envelope Armature on which the disk sits – Stator: Located outside the envelope Series of electromagnets whose currents and electrical fields function to “spin” the rotor within X-ray Production An x-ray tube is an energy converter. It receives electrical energy and converts it into two other forms: x-radiation and heat. Main thing to remember: – Approximately 0.2 % of energy during interactions produces x-ray Main production is HEAT – Heat is problematic in any x-ray based system Solutions: – Tungsten utilized for its thermal conductivity – Oil is utilized surrounding the tube which helps it cool faster – Rotation of the anode allows for a greater dissipation of heat over a larger surface X-ray Tube Heating 99.5% of all interactions in the x-ray tube produce heat 0.5 % produce x-rays – Tungsten Target 90 % - Bremsstrahlung 10 % - Characteristic Heat units (HU)= kVp x mA x time Tube Ratings – Heat Curves Tube Cooling – Rate of Cooling X-ray Production Entire area on the focal track where the electron stream impacts and x-ray photons are produced: – Actual Focal Spot Area on the focal track where the x-ray photons are produced which are only directed out towards the image receptor: – Effective Focal Spot Otherwise known as the Line Focus Principle: - Focal spot is area of target where x-rays are emitted - By angling target (bevel) the “EFFECTIVE” area of target is smaller than the actual area of the electron interaction. Affected by the angle of the focal spot, generally between 5 – 20 ° X-Ray Production Focal Spot is related to detail – Large Effective Focal Spot = Less detail – Small Effective Focal Spot = Better detail Usage of the small focal spot concentrates heat into a smaller area – Extended usage could lead to quicker anode pitting Anode Heel Effect Radiation intensity is greater at the cathode end of the tube then at the anode end due to the absorbing properties of the anode… More pronounced at certain distances and used in conventional x-ray The smaller the anode angle, the larger the heel effect The difference in intensity across the useful beam of an x-ray field can vary by as much as 45% X-Ray Tube Care Potential causes for failure: – Vaporized Tungsten (may lead to arcing) – Pitted Anode High exposures eventually lead to heat creating small areas of melting aka pits on the focal track Leads to vaporized tungsten and arcing – Cracked Anode Large exposure to “cold” anode can potentially crack the anode when great heat load causes fast expansion of a cold surface WARM UP – Gassy Tube Caused by compromise of the vacuum within the envelope Reduces amount of x-ray produced and could cause oxidation and burnout of the filament X-ray Production Summary.. Primary function of X-ray tube – 1. Generate free electrons or an electron cloud that accumulates at the cathode (free electron accumulation is aks space charge) . 2. Apply high voltage (50 kV to 150 kV supplied by generator) to accelerate electrons from cathode (negative potential) to anode (positive potential). 3. Allow for high energy electrons to interact with anode (tungsten based target) so that x-rays can be produced. X-ray Production 3. Allow for high energy electrons to interact with anode (tungsten based target) so that x-rays can be produced. - Distance between cathode and anode is about 1 cm. - When electrons from the cathode (aka projectile electrons) strike the atoms of the anode, energy is transferred from the electron to the atom. - The projectile electrons interact with - 1. Orbital electrons or inner shell electrons 2. Nucleus of atom 99% of the projectile electron energy is converted to heat. 1% is used for production of X-rays. - Characteristic X-ray Incident electron knocks out an inner shell electron ~ ionization – Incident e- must have energy (speed ½ mv2) the binding energy of the e- being knocked out. As e- from outer shells fill the inner shell vacancy – a characteristic x-ray is produced at an energy equivalent to the difference between the binding energies of the shells of the vacated e- and the e- that takes its place The x-rays produced are at specific energies characteristic of the binding energies of the target atom Bremsstrahlung (braking radiation) Electron interacts with the nucleus of an atom. The electron decelerates and changes direction. Energy of the x-ray depends on the amount of energy the electron looses during deceleration KE=½ mv2 Eout hυ E in E out h Broad range of energies produced – known as spectrum! the greater the Z of the target atom the greater the chance of Bremsstrahlung interaction X-ray Spectrum Spectrum refers to range of types and quantity of x-rays Here, the relative number of x-rays emitted is plotted as a function of energy of each individual x-ray (known as polychromatic spectrum) Bremsstrahlung x-rays have a range of energies and form a continuous emission spectrum If possible to measure the energy in each emitted x-ray, would find that energies range from peak “electron” energy all the way down to zero. Ex. If x-ray tube operated at 100 kVp, can have xrays up to only 100 keV X-ray Spectrum Key Points – Increase in tube current (mA) from 200 mA to 400 mA means 2 x as many electrons will flow from the cathode to the anode (i.e. mAs will be doubled). Mean 2x as many x-rays are produced. Figure below. – As kVp is raised, the x-ray quantity increases with the square of the kVp and the spectrum shifts to the right towards higher energy (more penetrating) x-rays. Higher beam quality. Filtration Photon output varies in energy or wavelength – Lower energy x-rays do not penetrate, therefor do not contribute to final image. Why filter? 1. Remove long wavelength (low penetrating) photons which will not contribute to the quality of the image and only contribute to acquired dose; Harden the beam 2. Shape energy distribution across beam in order to produce a more uniform beam Filtration Inherent Filtration – Materials which are a permanent part of the tube and its housing Envelope - Window Dilectic oils which surround the tube – As tube ages, inherent filtration increases as the filament evaporates Deposits down onto the window which in turn places it in the path of the beam Added Filtration – Anything added to sufficiently harden the beam Al / eq = Aluminum equivalent X-ray Tube Heating 99.5% of all interactions in the x-ray tube produce heat 0.5 % produce x-rays – Tungsten Target 90 % - Bremsstrahlung 10 % - Characteristic Heat units (HU)= kVp x mA x time Tube Ratings – Heat Curves Tube Cooling – Rate of Cooling Technical Factors mA – Milliamperage s Amount of current applied to the filament responsible for the burning off of electrons Number of electrons crossing tube from cathode to anode – Seconds Length of time that the current is applied to the filament mAs – Directly proportional to the intensity of the photons produced Exposure rate or Number of photons – Directly proportional to the dose that the patient receives Technical Factors kV – Kilovoltage Controls quality (and quantity) of the x-ray beam Selection controls speed and energy levels of the electrons applied across the x-ray tube Increasing or Decreasing e- energy results in photons with either greater or lesser penetrability Does affect quantity – 15 % rule… Ie. 80 kV beam will be more penetrating than 70 kV beam…it will also affect photon quantity as 80 kV beam will have double the photons created… QUIZ! Which factors affect the x-ray spectrum? – kVp – mA – Exposure time – Filtration – Target material QUIZ! Which factors affect the x-ray spectrum? – kVp YES! – mA NO! – Exposure time NO! – Filtration YES! – Target material YES! X-ray Beam - Primary vs. Remnant Primary vs. Remnant (exit) The radiation which exits the x-ray tube makes up the primary beam. This radiation has not yet interacted with matter. •Remnant radiation is radiation which exits the patient after it has interacted with the anatomy under investigation. 3 = primary beam. 4 = secondary beam. 5 = Scatter radiation. 6 = remnant radiation Interaction of Radiation and Matter Photon Interactions • 3 major interactions between x-ray and matter 1. Photoelectric Effect 2. Compton Scattering 3. Coherent Scattering* * Happens at kV levels below diagnostic therefore does not contribute to imaging in CT Classical (Rayleigh or elastic) Scattering • • • • • Excitation of the total complement of atomic electrons occurs as a result of interaction with the incident photon No ionization takes place No loss of E The photon is scattered (reemitted) in a range of different directions, but close to that of the incident photon Relatively infrequent probability - 5% c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 37. Compton Scattering • • • Dominant interaction of x-rays with soft tissue in the diagnostic range and beyond (approx. 30 keV 30MeV) Occurs between the photon and a “free” e- (outer shell e- considered free when Eg >> binding energy, Eb of the e- ) Encounter results in ionization of the atom and probabilistic distribution of the incident photon E to that of the scattered photon and the ejected e A probabilistic distribution determines the angle of deflection c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 38. Compton interaction probability dependent on electron density Compton Scattering • • • Compton interaction probability is dependent on the total no. of e- in the absorber vol. (e-/cm3 = e-/gm · density) With the exception of 1H, e-/gm is fairly constant for organic materials (Z/A = 0.5), thus the probability of Compton interaction proportional to material density () Conservation of energy and momentum yield the following equations: • Eo = Esc + Ee- • E0 Esc = 1+ E0 1- cosθ 2 m ec , where mec2 = 511 keV Compton Scattering Esc as a function of E0 and angle (q) – Excel spreadsheet Compton Scattering • • • • • As incident E0 both photon and e- scattered in more forward direction At a given fraction of E transferred to the scattered photon decreases with E0 For high energy photons most of the energy is transferred to the electron At diagnostic energies most energy to the scattered photon Max E to e- at of 180o; max E scattered photon is 511 keV at of 90o c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 39. Photoelectric Effect • • • Interaction of incident photon with inner shell eAll E transferred to e- (ejected photoelectron) as kinetic energy (Ee) less the binding energy: Ee = E0 – Eb Empty shell immediately filled with e- from outer orbitals resulting in the emission of characteristic x-rays (Eg = differences in Eb of orbitals), for example, Iodine: EK = 34 keV, EL = 5 keV, EM = 0.6 keV c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 41. Photoelectric Effect 1. Photoelectric: Incident photon with an energy level the same as or slightly more than the binding energy of an inner shell electron interacts with that electron Photon gets completely absorbed Ejects this electron out of its orbit Electron with a higher energy in an outer shells migrate towards the nucleus emitting the energy difference between shells to fill the vacancy Atom is ionized Photoelectric Effect • Edges become significant factors for higher Z materials as the Eb are in the diagnostic energy range: • • • • • Contrast agents – barium (Ba, Z=56) and iodine (I, Z=53) Rare earth materials used for intensifying screens – lanthanum (La, Z=57) and gadolinium (Gd, Z=64) Computed radiography (CR) and digital radiography (DR) acquisition – europium (Eu, Z=63) and cesium (Cs, Z=55) Increased absorption probabilities improve subject contrast and quantum detective efficiency At photon E << 50 keV, the photoelectric effect plays an important role in imaging soft tissue, amplifying small differences in tissues of slightly different Z, thus improving subject contrast (e.g., in mammography) Dependence on Z and E • Photoelectric component – Z3/E3 • Compton component – Z0 logE/E Mass Attenuation Coefficient c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 1 st ed., p. 26. Attenuation scattered transmitted absorbed • Attenuation – reduced intensity – is the linear attenuation coefficient; probability of interaction per unit path length. – Total attenuation is a due to combination of scattered(Compton effect) and absorbed photons(Photoelectric effect) I = I0 - I