Physics Test 1.pdf

Physics Test 1 Main Technical Factors kV or kVp: - Controls max energy of photon in beam - Impacts contrast resolution - Lower kVp provides consistent photons = higher exposure mA: - Controls number of electrons flowing from cathode to anode - Impacts exposure (more photons= more exposure) mAs: - C...

Physics Test 1 Main Technical Factors kV or kVp: - Controls max energy of photon in beam - Impacts contrast resolution - Lower kVp provides consistent photons = higher exposure mA: - Controls number of electrons flowing from cathode to anode - Impacts exposure (more photons= more exposure) mAs: - Controls quantity when tube is energized - Main controlling point for total quantity or radiation produced during an exposure AEC - Used instead of manual settings- compensates for patient thickness/ density Located inside bucky or detector Terminates exposure as soon as required quantity of remnant beam reaches image receptor Provides CONSISTENT quality of image Only provides imaging for selected cells Electrons def into trigger device, activates once predetermined amount of electrons are received Tube Housing Tube: - Protects tube from environment, and user from tube (burnt, shocked, radiated) - Contains insulating oil (helps dissipate heat) - Made of metal (cast steel, lead lined- great at absorbing x-ray photons) - Metal provides sturdy support, and lead absorbs off focus radiation Envelope (outside of X-ray tube itself) - Maintains vacuum, provides optimal environment (no air or gas, clear path) - Made of pyrex glass (withstands heat) or metal (can prolong, and high melting point - Pyrex glass- may turn yellow/ brown from tungsten which may give electrons alternating path - Does not happen with metal Filament: - Part of cathode where electrons come from (Thermionic emission) - Made of wired thoriated tungsten as it has high melting point - More electrons at lower filament currents due to thoriums efficiency- prolong life Focusing cup: - Part of cathode, surrounds filament, makes filament and electron beam stay together - Pushed charges together so it reaches anode effectively - Made of metal, and cup has negative charge to prevent electrons from repelling each other - Tube Housing Target: - Part of anode (+) - Where we want electrons to collide - Made of tungsten (high melting point) & rhenium (adds strength) Stationary anode: - Where electrons crash into - Good for low volume Rotating anode disc: - Heat dissipates - Made of tungsten and graphite (where electrons hit is tungsten, rest is graphite) Anode stem: - Connects rotating disc to rotator assembly (makes it spin) - Made of molybdenum (poor conductor, stops heat from spreading to motor) Rotor: - Inside of x-ray tube - Rotates anode disc - Made of copper iron (copper good conductor, iron magnetic property) Stator: - Outside x-ray tube (surrounding rotor) - Does not move, but initiates spinning of rotor - Made of electromagnets, powers the motor Induction Motor 1. 2. Current flowing through inductor creates magnetic field Moving magnetic field induces an electric current Outside the tube: - Alternating current passed through stator, creating magnetic field AC power moves back and forth, so does magnetic field Focal Spot - Area of target bombarded by electronswhere x-rays are produced Optimal image-smallest area possible Optimal heat loading- largest area possible Must compromise! Line Focus Principle: - Angling the target creates the illusion of smaller focal spot (in length only) Steeper angle, smaller it appears Allows us to keep focal spot fairly large Actual focus spot: - Target SA that is struck by electrons Effective focal spot: - Size that focal spot appears to be Width is same for actual and effective (size of cup) Benefits and Limitations of AEC Benefits: - Do not need to guess attenuation of structures being imaged Consistency in contrast is produced in image Limitations: - Only cell selected is imaged, so knowing location is critical What Cell to Utilize Chest: Left & right cells to image lungs Spine: Abdomen: Second cell to image lumbar spine All cells to image everything needed X-Ray Production - Electrons boiled off cathode to hit anode Voltage applied, electrons travel through difference Projectile electrons gain KE as they leave cathode Hit anode at focal spot, and release energy in form of x-rays or heat (95% heat, 5% x-rays) X-ray tube provides ideal environment for x-ray production Characteristic Radiation - Projectile electrons interact with inner shell K electron If energy overcomes K shell BE, then it can be released, is then replaced with L shell or other shell electron This produced characteristic x-rays Results in either: 1. 2. Electron cascade (replacement of K shell) Auger electron (energy released is transferred to orbital electron, none left for diagnostic imaging as very low left) Bremsstrahlung Production - Projectile electrons have interactions with nucleus positive charge The nucleus positive charge causing projectile electrons to slow down and interferes in trajectory Path slightly altered: low energy Brem Path extremely altered: high energy Brem X-ray Emission Spectrum - Representation of intensity of x-rays emitted Can be calculated by total # of x-rays emitted = area under curve of emission spectrum General shape: parts can be shifted, the spike can differ, mA changes amplitude (height changes), KvP changes max energy (width change) The larger the area under curve, the higher the intensity or quantity Characteristic x-rays: electrons from cathode and K shell Effect of mAs Doubling mAs will result in: - Twice as many electrons and photons travelling from cathode to anode Twice as many x-rays Amplitude will double on the graph The start, and the end of the graph will remain identical Increasing kVp - - The efficacy of Brems and Characteristic production increases; resulting in greater proportion of incident electrons striking anode and producing x-rays Thus, more x-ray photons produced I.e 50 kVp set-1-2 electrons is max amount given off I.e 120 kVp set, more interactions possible, more photons, thus more x-rays As kVp increases, area under curve increases ( double kVp= quadruple area) Everything on graph shifts to right Filtration - Inherent: present by nature (oil, glass envelope) Added: material added to remove low energy, i.e aluminum or copper which absorbs low energy photons) Brems is further reduced on left Beam intensity decreases (amplitude will drop) Beam energy increases (shifts to right) Results in reduced x-ray intensity but increased effective energy More added, more low energy removed Effect of Target Material - As atomic number increases, Brems increases (larger nucleus & charge) As atomic number increases, higher energy x-rays increases Characteristic x-rays have differed spikes due to different energies Square Law and Inverse Square Law Inverse: - Describes the decrease in intensity as distance from source is increases Photons diverse as they move away from source Square law: - Then rate of mAs increase needed to maintain intensity is inversely proportional to inverse square law Contrast - High contrast- lots of black and white - Low contrast- lots of grey Contrast resolution: - Ability of system to distinguish portions with similar densities thus similar greys - Impact of scatter radiation: photons that get redirected (produces noise) - Noise decreases contrast resolution and decreases image contrast and ability to distinguish elements Scatter Radiation - Photon changes trajectory, interacts with outer shell electron, ionizes it, and changes path - Degrades image quality- does not deposit anatomical information Factors that impact: - kVp- as it increases, absorption goes down and increases scatter, however incidence of scatter also goes down - Tissue thickness- more possibility of scatter - Collimation- when anatomy that does not need to be seen is imaged, will have scatter SR Ratio - Scatter: primary ratio - How many scatter for photon travelling - Abdomen: 3:1 ratio (75% is noise) - Beam Restricting Devices Aperture Diaphragm - Peace of lead with hole cut out in middle - Instead of collimate, you slide the lead peace - Good for nasal or skulls Collimator - Hooks to bottom of tube housing - Has light bulb and mirror and dials Positive Beam Limitations: - Senses how big detector is and automatically adjusts collimation - Collimation should never be larger than image receptor Grids - Remove scattered radiation from remnant beam that will reach IR Located between patient and IR Photons travel in predictable course, can determine the expected path of divergence (dependent on SID) Using when imaging thicker body parts (thicker than 10cm) kVp should be greater than 60 Grid contains radiopaque (x rays absorb) strips and radiolucent (x-rays pass through) strips alternating Lead (high Z) for opaque, aluminum (low Z) for lucent Grid Construction Parameters Grid ratio: - Height to width interspace Higher ratio= greater stopping power for angled scatter photons Grid ratio= h/D h= lead strip height D= interspace height Grid frequency: - Number of radiopaque strips per unit length Affects visibility of grid on image The combination of frequency and ratio, determines quantity of lead in grid Grid Surface X-ray Absorption - As photons reach grid, percentage will interact with lead on surface %= width of grid septa/ (width of grid septa + interspace width) x 100 Bucky Factor: Air Gap Technique - - When this technique is used, IR positioned 10-15 cm from patient, and large portion of scatter do not interact with image receptor This decreases scatter reaching image receptor Reduce scatter reaching IR by allowing distance for angled scatter photons to travel so they miss the IR No grid required, however unsharpness occurs due to magnification Attenuation - - The reduction of x-ray photons and intensity of beam due to energy being lost from additional interactions Due to absorption & scatter Coherent Scattering - Interaction between very low energy x-rays and matter (mainly in mammography) - X-ray photons interact with entire atom (Rayleigh) or single electron (Thompson) causing electrons to momentarily shift, however it does not result in ionization Two types: - Thompson- 10 keV, interacts with SINGLE ELECTRON (not ionized) resulting in vibration, but overall initial photon remains same - Rayleigh- 15-20 keV (mammo), interacts with WHOLE ATOM causing excitation, however no overall lost energy Compton Scatter - Main interaction of x-rays of soft tissue Incident x-ray photon interacts with outer shell electron, and ionizes, electron is ejected from atom - Scattered photon has reduced energy and has changed direction - Wavelength of scatter is greater than incident - 26 keV to 30 MeV Probability of Compton - If incident photon energy increases, probability increases, only if all other interactions are at higher energies - Overall, number of compton scatter decreases as the energy increases as higher energy photons are more penetrating, harder to stop Photoelectric Effect - Incident photon interacts with an inner shell electron BE of incident photon is greater than inner shell (K or L) Atom is ionized with inner shell electron vacancy Electron is filled by outer shell Probability of PE: - Proportional to: Z^3/E^3 - Thus, P.E of is 22 times greater in Ba than Ca for photon in particular energy Probability of Compton and PE Pair Production - Only occur at energies over 1.02 MeV Interacts with electric field of nucleus Forms positron pair- two photons given off in 180 degrees (0.511 MeV each) No importance in diagnostic imaging Photodisintegration - Above 10 MeV Photon is entirely absorbed and excited the nucleus Nucleus instantly emits a nucleon or other nuclear fragments No relevance to imaging General Radiography - - Balance between compton optimizes image quality Lower-energy X-rays are preferred to maximize the photoelectric effect for better contrast With higher-energy sources, Compton scattering becomes more dominant