Diagnostic imaging Flashcards
CR
- computed radiography
- used to show moving images
CT
- computed x-ray tomography
- makes images of sections through body
scintigraphy
uses gamma rays which are injected to show metabolism
MRI
- magnetic resonance imaging
- uses radiowaves to image things like the brain or spinal cord
ultrasound
shows echogenicity of tissues
2 types of x-ray image
positive and negative
bone colour of positive x-ray images
dark
bone colour of negative x-ray images
white
how x-rays are made
by electrons colliding into a heavy metal (e.g. tungsten)
where are x-rays produced
x-ray tube
components of an x-ray tube
glass or metal envelope, -ve cup shape behind the filament, +ve tungsten target with copper behind it.
how are x-rays produced in x-ray tube
electorns are fired of by filament and aimed by -ve cup shape at the tungsten target. x-rays are angled down towards patient by angle of tungsten target
function of copper behind tungsten target in x-ray tube
conducts heat away from tungsten target
colimator (radiography)
found under the x-ray tube it shapes the x-ray beam
x-ray absorption depends on
density and thickness of tissue and the atomic number of element sin the tissue
5 types of tissue seen radiographically in order of absorption
air, fat, soft tissue, bone and metal
what colour does air show up on radiograph
lucent/black
colour of bone on radiograph
white/opaque
2 types of contrast medium in radiograph
positive and negative
positive contrast medium colour on radiograph
radiopaque (white) as it absorbs lots of x-rays
example of positive contrast mediums radiograph
iodine, or barium sulphate (GIT)
negative contrast medium colour radiograph
radiolucent (black) as absords no x-rays
example positive contrast medium radiograph
gasses (air)
radiograph mAs
- milliamperes per second
- number of electrons passing accross the x-ray tube
increasing mAs
increases number of x-rays produced so more will get through and you can decrease exposure time and so reduce motion blur
kVp radiograph
- kilovoltage peak
- peak voltage applied accross the x-ray tube
increaseing kVp radiograph
increases the energy of the beams so more get through
values low kVp
45-60
value middle kVp
65-75
values high kVp
over 80
x-ray machine button
- has 2 stages
- 1st stage prepares machine
- 2nd stage sends the beam
x-ray sunburn
caused by low energy x-rays
preventing x-ray sunburn
aluminium sheet filters out low energy x-rays
effects of scattered radiation on the image
makes image grey, fuzzy and indistinct
4 things that reduce radiation scattering
- use colimator to reduce volume of tissue being irradiated
- put a layer of lead between table and cassette
- increase kVp to make beams go through patient better
- use an x-ray grid to reduce scattered radiation getting to cassette
what needs to be done to compensate for use of x-ray grid
increase mAs by the grid factor
appearance of x-ray grid
thin lead strips parallell to 1* beam. about 24 per cm
roentgen
- ability of radiation to ionise the air
- measured in couloms per kg of air (C/kg)
becquerel
- Bq
- rate of radioactive decay
- measured as disintegration per second
gray
- Gy
- absorbed dose of radiation
- measured in joules per kg of tissue (joule/kg)
sievert
- Sv
- dose equivalent radiation
- measures the effect of radiation
- = gray x quality factor
max dose of radiation for people at work
20 mSv per year
Sv measured by
film badge or thermoluminescent detector (TLD)
ALARA (radiation)
- as low as reasonably possible
- radiation wise it should be ALARA
you can achive ALARA radiation in 5 ways
- wear lead aprons and gloves
- keep as far away from the source as possible
- use lead mobile barriers or walls
- do things at arms length
- have a long line for the exposure button
controlled area of radiation
- is the area immediately around the 1* beam in which the radiation dose exceeds permissible limit
- usually 2 metre radius around 1* beam
- needs warning signs around it
radiation protection adviser
- person who inspects radiography facilities in an area and writes reports on improvements
- also sets local practice rules
radiation protection supervisor
- person in practice
- responsible for notifying health and safety executive theat radiography is performed at the practice
- responsible for following local rules
- looks after dosimeter readings
- reports radiography problems
