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
