Harrington Intro Imaging Midterm PDF

RADIOLOGIC PHYSICS AND SAFETY - Operator must know o How to: ▪ Properly operate x-ray machine and features specific to that unit ▪ Properly position patient ▪ Control image quality (kV...

RADIOLOGIC PHYSICS AND SAFETY - Operator must know o How to: ▪ Properly operate x-ray machine and features specific to that unit ▪ Properly position patient ▪ Control image quality (kVp, mA, grid, etc.) ▪ Minimize radiation levels (colimitation, special dose rate controls) ▪ position personnel for minimum radiation exposure ▪ Properly use shielding devices and personnel monitoring devices o How radiation is distributed in the room o Applicable laws and institution policies - Atom o Smallest particle of matter which still maintains all the properties of that element o Consists of nucleus with surrounding electron shells ▪ Atomic no. (Z) = # of protons ▪ Atomic mass (A) = # of protons + neutrons o Quantum Theory ▪ Atoms and molecules can only exist in certain energy state ▪ When an atom or molecule changes its state it must emit or absorb an amount of energy sufficient to bring it to another state o Electron Orbital Shells ▪ Energy of atoms is quantized → e- revolve around the nucleus at discrete energy levels Binding energy – amount of energy the e- has in each shell o Closer to the nucleus has an increase BE o Bound electrons = neg. total energy ▪ Calculating e- in a shell = 2(n2) Generally no more than 8 e- in a shell - Electromagnetic Waves o EM radiation is emitted anytime electric charges are made to accelerate ▪ Once underway, a changing electric field creates changing magnetic field a short distance away, which creates an electric field a little farther away, etc. etc. ▪ Sinusoidal electric and magnetic fields are directed perpendicular to each other ▪ Constant, straight line s = c (3x108m/s) o Number of peaks passing per second is the frequency (f) ▪ SI units = Hz ▪ 1 Hz = 1 cycle per second ▪ Distance between 2 adjacent peaks is constant and equals wavelength (λ) ▪ Wavelength x frequency = velocity (c) o EM radiation has wavelike properties ▪ It is absorbed and emitted in discrete localized energy bundles (photons) Each photon carries so little energy, can only experience their collective effect E = h x f (h = Planck’s constant) o High energy EM can ionize matter ▪ Energy is always transferred to any material with which it interacts o Ionization ▪ An atom gaining or losing an e- Results in a net charge Positive = cation Negative = anion ▪ X-rays ionize atoms ▪ Biological effects (short and long term) May disrupt atomic structure Can cause temporary or permanent cellular damage ▪ Can penetrate matter ▪ Causes some materials to fluoresce ▪ Reacts with silver halide of film - 2 Types of Electricity o Static – electric charge at rest (unit in coulombs) o Current – moving electric charge (unit in amperes) Direct Current Alternating Current - Flow of e- in one direction - e- flow in alternating opposite directions (+ → - or vice versa) - Delivers more power than AC source of - amplitude of sinusoidal AC voltage must be square root of 2 times greater than steady same voltage value of DC current to perform same amount of useful work (RMS) - each change from - to + = 1 pulse; 2 pulses =1 cycle - 3 basic X-ray machine components o Generator o X-ray tube o Control panel - In an X ray machine o Absolutely can not have pulsating X-ray tube from AC current ▪ Not safe ▪ Poor images ▪ Hard on tube - X ray generator o Supplies electric power to x ray tube ▪ Begins with source of electrical energy Wall plug, 115 or 230 V, 60 Hz Ac is standard Modified to meet tube standards ▪ Filament heating requires approx. 10 V ▪ E- acceleration requires between 40k and 150k V (40-150 kVp) o Produces smooth, high voltage direct current from low voltage AC current ▪ Transforms low voltage AC to high voltage AC ▪ Rectifies high voltage AC current to DC current with diodes ▪ Smooths the voltage dips with capacitive filter, offset circuitry or increase frequency - Transformers o Current in a wire can generate a magnetic field (B) o Current in a coil of wire creates weak mag field = solenoid o If iron core is placed in the center of the solenoid, the mag field becomes much more intense (electromagnet) ▪ Solenoids follow the right-hand rule → o A wire carrying a current experiences a force in a magnetic field ▪ Cannot be parallel to field B ▪ F=BxIxL B is in Teslas (T) or Weber’s/m2 I = current in Amperes L is length of wire F in Newtons o A changing magnetic field produces a transient electric field and a changing electric field produces a transient magnetic field ▪ Moving a bar magnet near an electromagnet changes the electric field and magnetic field o Current flow thru one coil can cause mutual inductance in a 2nd coil wrapped around the same iron core or rod ▪ Basis of transformer o Induced EMF’s exist only in coil if flux through coil is changing o EMF = -N x charge in flux per unit time ▪ N = # of wire coils ▪ AC current flows thru primary coil → creates changing magnetic field in core → this field induces current in the secondary coil o No current flows when B is stable ▪ So you can’t use DC current ▪ AC current has continuously changing voltage that works well for transformers Current flow in one direction while voltage is positive Flows in opposite direction while voltage is negative o Laws of Transformers 𝑵𝒑 𝑽𝒑 ▪ 𝑵𝒔 = 𝑽𝒔 Np = # of turns in primary coil Vp = voltage in primary circuit Ns = # of turns in secondary coil Vs = voltage in secondary circuit ▪ 𝑽𝒑 ∗ 𝑰𝒑 = 𝑽𝒔 ∗ 𝑰𝒔 Vp = voltage in primary coil Vs = voltage in secondary coil Ip = current in primary coil Is = current in secondary coil ▪ Step up transformer – has more coils in secondary circuit (↑V) ▪ Step down transformer – has more coils in primary circuit (↓V) o Use a step up for the x ray tube and a step down for the filament ▪ So generator transforms up and then down - Current Rectification o Uses a diode ▪ Basically takes AC current and alters it through the rectifier so that it all enters with current going in the same direction o Better rectification = better image ▪ Full-wave rectification creates a better image than half wave rectification - Smoothing Out o Principle Disadvantages of full-wave rectification (unfiltered) ▪ Tube pulsates, and anode receives rapidly varying amounts of energy ▪ Intensity of x ray beam varies over each half cycle ▪ Quality (effective energy) of beam varies over each half cycle Increased patient dose via “soft” radiation o Capacitor stores energy and releases it when current stops flowing ▪ Creates slower decrease in current in full-wave rectified DC current o Ripple Factor ▪ Variation in the voltage across the x ray tube expressed as a % of its max value ▪ Ripple Factors: Half-wave, unfiltered = 100% o Voltage goes from 0-100 each cycle Half-wave, filtered = 20% Full-wave, unfiltered = 100% Full-wave, filtered = 9% Six pulse = 13.5% (from 86.5 -100) Twelve pule = 3.5% (from 96.5 -100) High frequency = 1% o Best image quality because of lowest ripple factor o Three phase generator creates the same voltage output as a single-phase generator, but more efficiently - X Ray Tube Head o Evacuated glass envelope and CRT (cathode regulated tube) o Negatively charged tungsten cathode ▪ Coil filaments ▪ Electron stream comes from cathode filament o Positively charged copper anode ▪ Embedded tungsten target o Window o Added filtration o Beam limiting device - X Ray Generation o Electrons thermionically “boil-off” the filament embedded in the cathode (-) o Electrons “shot” at (+) anode target by applying strong potential difference (kVp) o High KE e-s interact with target atoms to produce photons in x-ray wavelength ▪ Electrons all acquire the same high terminal KE (monochromatic keV) ▪ About ½ the speed of light o Electrons not absorbed are conducted away → complete the circuit (mA) o X ray beam is polyenergetic, photons are comprised of varied energies (spectrum) ▪ The energy of the photon depends on what e- shell it interacts with in the tungsten target ▪ Electrons closer to the nucleus have higher BE and give off higher energy photons o X rays directed toward window and filter o Cathodes ▪ Focusing cup (-) surrounds coil “aims” e- at anode target o Otherwise they could scatter from the filament ▪ Current applied to tungsten filament Attains approximately +2000 degrees C ▪ Electrons boil off coil and form negatively charged electron cloud (thermionic emission) o “Filament” circuit ▪ Voltage across filament is 10V ▪ Current thru filament is 4 A Power dissipated (IxV) = 40 W ▪ High resistance in filament causes temp to rise so high and “boil off” the e- (>2200 C) ▪ Step Down transformer at filament circuit to decrease voltage so current can be increased (↑ mA)  o X ray tube current ▪ Electrons emitted from filament form a negative cloud (aka space charge) Prevents further e- emission ▪ Electrons attracted to anode when increased (+) potential applied to anode ▪ Tube current (mA) = flow of e- across vacuum gap (from filament) to anode completes circuit ▪ “Space Charge” effect Space charge is an e- cloud and highly electronegative Via electrostatic repulsion, space charge makes it difficult for subsequent e- to be emitted from cathode filament Residual Space charge o Acts to limit the # of e- available and therefore limits current flow in the x ray tube o X ray tube output and Current ▪ Tube output is proportional to tube current At low kVp the tube is space charge limited (so it is kVp controlled) At saturation voltage, all e- are pulled away from the filament and tube current is maximized o Tube current is now largely controlled by filament heating o i.e. once saturation is attained, the current is determined by # of e- available by filament heating o So at a given kVp, you can only pull so many e- out of the filament ▪ kVp affects tube current for a given filament current At low peak voltages (5%) Interactions (typically low energy) in which radiation undergoes a change in direction without a change in wavelength o Only interaction between x ray and matter that does not cause ionization Absorption of low energy photon→ vibration of atom→ emission of radiation→ atom returns to undisturbed state o Photoelectric Reaction ▪ Interaction with matter where x ray is absorbed and not scattered ▪ An e- escapes with KE = difference between the energy of incident x ray and e- binding energy ▪ Yields weak characteristic radiation in biological systems Photon enter “biological” atoms that are low atomic number and so have low energy e- These low energy e- that are emitted only travel a few mm before being absorbed elsewhere in the body ▪ Has 3 products 1. Photoelectron 2. Characteristic radiation 3. Positive ion  ▪ Photoelectric Probabilities Incident photon must have sufficient energy to overcome BE of e- PE rxn most likely to occur when photon energy and electron BE are nearly the same The tighter electron bound in orbit, PE more likely o Higher atomic number increases PE probabilities o Higher atomic number atoms will require higher energy photons to have a photoelectric reaction Dropping the kVp makes fat, bone and muscle clearer on an image by increasing the number of PE interactions, but also increases radiation dose PE interactions create bright spots on an image and areas where just radiation passes through is just black o Compton Scatter ▪ Photon interacts with outer orbital e-, imparting some of its energy → ejecting the e- from orbit ▪ Photon continues on in altered path, scattered, with less energy (longer wavelength) The greater the change in direction, the more energy lost by the photon ▪ Compton interactions create gray areas → higher kV increases # of Compton interactions and produce an overall darker image Basics of Attenuation - Attenuation is defined as: o The partial absorption of the x ray beam, the reduction in intensity that occurs as it traverses matter body part - 4 factors affecting attenuation o Kilovoltage o Density o Atomic number (Z) ▪ Higher atomic number atoms absorb more o Electrons per gram of tissue - Polyenergetic Attenuation o Each filter the beam passes through increases the HVL ▪ Subsequent filters must be thicker in order to reach the HVL of the new beam with a higher average kVp - Functionally monoenergetic after 3-4 HVLs Units of Radiation Deposition and/ or Safety - Radiation Detection and Measurement o Absorbed Dose (D) ▪ Measures the amount of radiation energy (E) absorbed per unit mass (M) of the absorbing medium (must be specified) ▪ Absorbed dose D = E/M Units: o Gray (Gy) – SI system o Rads – non-SI system Don’t quantify biological damage (requires conversion factor QF) ▪ Not source related One gray = 1 joule of energy deposited per kilogram One rad – 100 ergs of energy deposited per gram o 1 Gy = 100 rads o 1 rad = 10 mGy ▪ Linear Energy Transfer (LET) Energy absorbed by the medium per unit length of travel (keV per micrometer) Proportional to (particle charge)2 Inversely related to particle KE o Photons, electrons, gamma and x rays – Low LET (a lot of penetration but only some energy deposited) o Neutrons, protons, and alpha particles – High LET (don’t penetrate very far but deposit a lot of energy) ▪ Dose Equivalent (H) Attempts to quantify biologic damage from deposition of radiation in tissues H = absorbed dose x quality factor H = D x QF Quality factor depends on the LET value QF = 1.0 for beta particles, electrons, gamma rays and x rays o 1 rad = 1 rem in diagnostic radiology (because QF = 1.0 for x rays) QF may be as high as 20 for alpha particles/ heavy nuclei Units o Sievert (Sv) – SI system (1 Sv = 100 rem) o Rem (rad. Equiv. man) – non-SI system (1 rem = 10 mSv) IMAGING - Based on the difference in attenuation between body parts - Bones that appear bright absorb a lot of radiation so that the image receptor beneath them did not receive a lot of radiation - Interactions with film o A “latent” image is formed ▪ Film needs to be developed to bring out the image ▪ Heavily irradiated areas turn black Ionic Ag+ reduced to metallic Ag0 Image must then be developed ▪ Complete attenuation of x rays leaves film relatively “clear” ▪ Partial attenuation/ transmission = gray o Silver halide crystal ▪ Atomic arrangement inside hexagonal film crystal is cubic Sensitivity speck is AgS surface defect Somewhat nonrigid crystal with negative surface charge o Atoms and electrons may migrate Absorbed photons liberate halide e- within the grain ▪ Photon interactions within silver-halide Photoelectrons and Compton electrons are produced that migrate thru-out the crystal dislodging other electrons Some are trapped by sensitivity speck Ag+ Electron that silver picks up comes from Bromine ▪ In each crystal, 60%) produces fog ▪ Decreased humidity (< 40%) = increased static risk Light & Radiation o Avoid low level, diffuse light ▪ Safelight must match film type Still only safe at a distance o Lead-line storage bin o Keep loaded cassettes away from x ray unit ▪ Unprocessed film fogs at about 0.2 mR Shelf life Transmittance Optical Film - Optic Film Density (OD) (T%) Density (OD) o Used to measure film blackening 100 0 OD = log10(I0/It) 50 0.3 I0 is light intensity incident on film 25 0.6 It is light transmitted through film 10 1 o Light Transmittance Formula 5 1.3 %T = (It / I0) 1 2 ▪ For every 0.3 change in optical density, the %T inversely changes by a factor of 2.0 0.1 3 o OD is proportional to the # of photons reaching film ▪ Can be measured using densitometer ▪ As OD increases, % transmittance decreases ▪ Useful range of ODs from 0.3 to 2 0.3 = 50% transmittance 2.0 = 1% transmittance Above 2.2 needs “hot” light - Characteristic Film Curve o Plot relation between exposure and OD ▪ Every 0.3 increase doubles the exposure Steeper curve = higher contrast (more black and white) ▪ Base + fog = OD without exposure Typical range 0.1-0.2 OD units ▪ Film speed inversely related to exposure (short E = increased speed) - Image Receptors: Film Screens o Intensifying screens ▪ The ability of crystals of certain inorganic salts (phosphors) to emit light when excited by x rays = fluorescence ▪ Intensifying screens absorb x ray photons and emit many more visible light photons which then expose film ▪ Reduces patient dosage 20-50x (Primary Function) Shorter exposure times Facilitate radiography of thick body parts which otherwise requires high exposures Decreases risk of motion blurring ▪ But exposure without screens is sharpest ▪ 4 basic layers Base made of plastic or cardboard Reflective (titanium dioxide) or absorptive layer Phosphor layer – varying thickness Protective plastic cover layer ▪ One x ray photon can recruit a thousand light photons Reduces sharpness of image somewhat - Direct vs Screen-Film Exposure Stage of Process Direct Screen-Film Incident X-ray photons 1000 20 X rays absorbed by film 10