Physics Crap

sihirlifil's version from 2017-09-03 00:44


Question Answer
X-rays = what kind of radiation? (Which means?)Electromagnetic, ionizing (= removes electron)
How do x-rays travel?Constant velocity, in a straight line (though direction can be changed)
X-ray energy is proportional to ____ and inversely proportional to ____Frequency / wavelength
What electric charge do x-rays have?None. They are unaffected by magnetic or electric fields
(X-ray wavelength)(10^-10)
How are x-rays and matter related?Interact and are either absorbed or scattered. Certain substances fluoresce
What happens when x-rays penetrate tissue?Some of the energy is absorbed
Can cause biological changes when atoms are ionized
X-rays are generated where?X-ray tube
2 interactions that produce x-raysCollisional
What is the cathode made of? Charge?Tungsten filament in molybdenum (poor conductor) focusing cup; negatively charged
What is the anode made of? Charge?Rotating tungsten disc on molybdenum rod (stationary = copper rod); positively charged
Electrons are produced where? How?Cathode filament. Electric current is run through to heat it
Higher filament temperature = more electrons produced (controlled by mA)
What happens when electrons leave the cathode?Travel through vacuum inside x-ray tube to the anode
The mA controls...Amount of current, i.e. number of electrons produced at the cathode (via cathode filament temperature)
What allows electrons to accelerate?Voltage difference between negative filament (cathode) and positive target (anode)
Collisional interaction is a result of?Characteristic radiation
Electron from cathode kicks out electron from the inner shell of the tungsten anode, outer shell electron replaces it and releases energy as a characteristic x-ray which is a useful part of the beam (electron that was kicked out are low-energy and don't escape the target)
Radiative interaction is a result of?Braking radiation a.k.a. Bremsstrahlung
Electron from cathode bends around tungsten nucleus at anode and slows down because of the difference in electrical charge, releasing energy as an x-ray that contributes to the beam (this electron produces more useful x-rays through additional interactions)
Energy of the x-rays produced is a function ofkVp(kVp = max achievable x-ray energy. Most is lower energy)
Quantity of the x-rays produced is a function ofmA
Most of the energy spectrum produced isBremsstrahlung; lower energy
Components of an x-ray tubeAnode
Glass tubing with vacuum, surrounded by oil and lead-lined (shields x-rays)metal housing
Light beam diaphragm
Types of anodeStationary
What is the focal spot?Location on target (anode) where x-rays are produced
____% of energy created by electrons hitting the target is heat90%
Why is tungsten a good material for the target?High melting point (6192F) and high atomic number (74)
This means it can get really hot and withstand the heat energy from electrons hitting the target (90% of total energy = heat), and higher positive charge = more effective braking = more Bremsstrahlung = more x-rays
How is heat dissipated at the target?Rotates to prevent melting
Effective focal spot depends onFilament size and focal spot angle
Focal spot: Larger angle = & smaller anlge = Larger & smaller effective focal spot
Focal spot size affectsImage sharpness. Small focal spot = more detailed; large blurs edges (penumbra)
Function of electrical supplyHeats tungsten filament to produce electrons via mA (current)
Accelerates electrons from cathode to anode via kV
Components of electrical supplyPower supply (mains) to tungsten filament
High voltage transformer
Rectifier circuit
Step down transformer fxnReduces incoming current heating the cathode (otherwise mA current too high for the filament, will burn it)
Step up transformer fxnIncreases voltage to order of Kilovolts (otherwise voltage too low, x-rays don't have enough energy)
Autotransformer fxnEnables mains electricity voltage compensation
Provides constant voltage to filament circuit & 1ry voltage to step up transformer (since level of incoming current fluctuates, need to even it out)
Normal current is AC / DC?AC
Why is a generator needed?Increase voltage and rectify the current (make it flow in one direction since it's alternating current)
Radiographs need DC at higher voltage
What does rectifying do?Keeps target positive (keeps electrons flowing + to -), preventing damage to cathode
Why is the anode angle tilted?Concentrates heat over a larger area (incr actual focal spot) even though effective focal spot is still small (keeps image sharp)
Increasing kVp...Increases potential difference between filament and target
Electrons are accelerated to higher velocities, have more energy when striking the target
X-ray machine controls arekV selection
mAs selection
Exposure button (prep & take)
Light beam diaphragm
kVp = ?Maximum energy of x-rays
mAs = ?Number of x-rays (mA x time)
Can be made up of any combination of mA and time but want to keep exposure time low (movement artifact, radiation safety)
X-ray exposure determined bykVp and mAs
Collimation isRestricting beam to area of interest via lead shutters (reduce volume of tissue exposed)
Why is collimation beneficial?Less scattered radiation created in the patient
Improved image quality
Radiation safety
What is the light beam diaphragm?Collimator. Delineates area of patient that is irradiated
Types of x-ray machinesPortable
High frequency portable
Secondary radiation is produced when...X-rays penetrate the patient
Types of secondary radiationPhotoelectric effect (PE)
Compton scatter
(Coherent scatter)
Photoelectric effect =Type of scatter from characteristic radiation (i.e. inner electron kicked out, outer moves in, ejected e- continues interacting)
Compton scatter =Outer shell electron ejected from tissue atom nucleus, ejected e- is eventually absorbed by patient. Can be high energy --> Compton / PE interaction --> exit patient & fog film
How does patient size affect Compton scatter?Incr size of patient (& exposed area) = incr scatter
When can you use a grid?Any structure >10cm
A grid isPanel of parallel lead & aluminum lamella. Lead absorbs x-rays that aren't perpendicular with film
Grid ratioHeight of lead strips:width of spaces between them
Grid factorincrease in mAs needed to compensate for radiation absorbed by the grid
You place the grid...Between patient & film screen / detector
The x-ray image is a result ofDifferential absorption of x-rays in the patient's tissues
Film blackening is directly related toAmount of x-rays striking the film (=mAs)
absorption in penetrated tissue (=thickness & matter)
Energy of x-rays (=kVp) (higher energy more likely to penetrate patient)
Inverse square lawExposure is inversely proportional to the distance between x-ray source and imaging system / film (FFD) (further away from source = lower exposure)
Decrease in x-ray beam intensity is proportional to ^2 of distance from source
At 2x the distance, there is ____ of x-rays available1/4
Increasing the FFD ___ the intensity of x-ray beamReduces
Reducing the FFD ___ the intensity of x-ray beamIncreases
An overexposed rad looksDark (burned toast)
An underexposed rad looksLight
Radiographic film isPhotographic film with an emulsion containing silver halide crystals --> atomic silver when exposed to x-rays & processed
How do we get clear areas on the film?Undexposed silver halide crystals washed off during fixation
Photographic film more sensitive to x-rays or light?Light
Fxn of intensifying screensPhosphor crystals transform x-rays to light, which amplifies image and reduces # x-rays needed
How many screens in the cassette? Where?2, in close contact with the film
Fast screen = what kind of crystal layer? Resolution?Thick layer of Large crystals, low resolution, need lower exposures
Slow / detail screen = what kind of crystal layer? Resolution?Thin layer of small crystals, high resolution, need higher exposures
___ is needed to differentiate tissues on a radiographContrast
High contrast = ___ shades of grayFew
Low contrast = ___ shades of graymany (wide latitude)
Settings for high contrast imageHIGH mAs/low kVp (PE predominates) e.g. for abdomen, msk (low inherent contrast)
Settings for low contrast imagelow mAs/HIGH kVp (Compton predominates) e.g. for thorax (high inherent contrast)

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