Radiography Flashcards
what is the bohr model
the idea that an atom consists of neutrons, protons and electrons. neutrons and protons give the mass number, protons give the atomic number and electrons give the chemical properties. Electrons circulate in orbits/shells around the nucleus, closer to the nucleus - higher electrostatic force, and higher binding energy is required for removal.
what is the use of a transformer
to alter the voltage, step up - increase potential difference thus increase voltage, step down - decrease potential difference thus decrease voltage. Both are required in an x-ray tube
what is rectification
changing the current from alternating to direct
what is divergence
the x-ray beam is not parallel, when it leaves tube, it diverges out to so it is not as intense, as the distance doubles, the dose quarters
how are x-rays produced
when fast moving electrons rapidly decelerate and diverge. happens when electrons (produced by spreading voltage through tungsten, then through transformer to positive tungsten) collide with the nucleus of an atom - electromagnetic energy produced. May also collide with electrons in surrounding orbits - displace them and re-organisation of atom causes release of energy
what metal surrounds the target to absorb heat
copper
what metal can shield the x-ray tube
lead
what is collimination
control of the size and shape of the x-ray beam using lead. ensures the patient is only radiated at the area required, the rest of the beam is absorbed by the lead
apart from lead, copper and tungsten what other metal is involved in x-ray production
aluminium - filters low energy photons
what is the use of a spacer cone
to reduce magnification and increase focus to skin distance
what are the possibilities for the x-ray beam in regards to interacting with tissue
pass straight through unaffected - patient receives no dose
absorbed - completely absorbed, doesnt leave the patient
scattered - hits tissue and changes direction, either with or without the loss of energy
what is attenuation
change in intensity of the x-ray beam, due to absorption and scatter. Alters the amount of photons hitting the receptor, white image, required for contrast
what is photoelectric absorption
when the x-ray photon interacts with electrons in the k-shell of an atom. Photon has energy just higher than binding energy, causes displacement of electron, any energy left in the photon is used for kinetic energy for electron. Photon disappears and is fully absorbed. Atom is unstable - causes electron to fill shell and recruit another electron - energy released in form of heat
what is the compton effect
when the x-ray photon interacts with an electron in the outer shell. Binding energy is lower and photon causes displacement of electron. Photon is then scattered and loses some energy. Atom is then unstable - electron is recruited. action of photon is then dependant on level of energy left, can go on to interact with another atom
what influences the chances of photoelectric absorption
higher atomic number - more likely
lower photon energy - more likely
density of material
what influences the chances of compton effect
density of material - proportional
independant of photon energy and atomic number
what affect can scattered photons have on image quality
if photon hits object passed the receptor and scatters backwards - interacts with image receptor and causes fogging which degrades the image
what reduces the scatter effect
collimination - less area irradiated, less likely to get scatter
film surround by lead pocket to prevent fogging
what impact does photon energy have on image quality
if photon energy is low - more photoelectric absorption, thus more attenuation and contrast - but higher patient dose
if photon energy is high - less absorption, less contrast and worse image but lower patient dose
need to find a balance
what is the difference between a true and an oblique lateral cephalogram
true - image receptor and MSP parallel, x-ray beam perpendicular
oblique - image receptor and MSP at an angle, beam at an oblique angle to both
what are some things that should be done when taking lateral ceph
should be at a reproducible distance - should also be long enough to reduce magnification
should wear thyroid collar
should colliminate
what is parallax
an apparent change of a position of an object caused by a real change in the position of the observer
how can you carry out parallax
change in direction, know what direction it has moved
have a stable reference point
know what structure you are checking
observe movement
if the x-ray beam moves to the right, and so does the structure of interest, where is the structure positioned
lingually or palatally
what is the x-ray shadow
the x-ray beam that has been attenuated, picked up by image receptor
what is the relationship between pixels and resolution
more pixels - more contrast - better quality image - better resolution
explain how a phosphor plate works
phosphor plate contains crystals, crystals become excited by the x-ray beam, produces a latent image. phosphor plate is then scanned by a laser, laser causes the excited crystals to emit light. the light then produces the image in the scanner which is connected to a computer
explain how a solid state sensor works
sensory is part of the receptor and connected to a computer via a wire or wireless. the x-ray beam excites crystals, produces latent image, image detected by sensor, sent to computer immediately.
what are the two types of digital receptors
solid state and phosphor plate
compare phosphor plate and solid state
phosphor plate is thinner, less bulky and more flexible - more comfortable. however involves scanning, which is light sensitive. solid state is bulky and uncomfortable, no scanning required, thought to be more durable but more expensive.
in a film receptor, what is responsible for production of latent image
the emulsion, contains silver hallide crystals, can be sensitised by x-ray photons. those crystals not sensitised are removed, creates latent image
what is the relationship between film speed and dose
faster film speed - lower dose. but also larger crystals thus lower resolution
what steps are involved in chemical processing
developing - sensitising crystals
washer - removing residual developer solution
fixing - removing non-sensitised crystals and fixing emulsion
washer - removing residual fixing solution
drying - removing excess water and allowing handling
what alters developing
time, concentration of solution and temperature
compare film and digital receptors
digital receptors allow for storage of images online, reduces physical storage space. also allows for sharing and backing up of images, as well as manipulation of image - brightness etc. however, digital has a lower resolution than film, may be lost if technical issue and requires diagnostic level computer monitor
in quality assurance what must be checked
image receptor
image uniformity
image qulity
what faults can be seen on an image receptor
scratching, cracking and delamination
how is image uniformity checked
pass x ray beam through nothing, unattenuated, should all be the one colour
how is image quality checked
x-ray beam passed through wedge step, compared against baseline
what are the image quality ratings
diagnostically acceptable and diagnostically unacceptable
what should be seen on a bitewing radiograph
full crown of all posterior teeth, mesial of most anterior to distal of most posterior, all approximal surfaces with minimal overlap
what should be seen on a peri-apical radiograph
full root, full crown and periapical bone
give examples of some issues with fault analysis
cone cutting - collimation issue
poor exposure
poor resolution
