mobile x-ray Flashcards
mobile x-ray equipment components
generator the drive unit weight brakes and locks tube support exposure beep warning collimator
mobile x-ray drive unit
- either motor driven or motor assisted
- electric motor with power supplied by batteries
mobile x-ray weight
- have consideration for lifts and manoeuvrability, counterweight in tube column to aid movement
mobile x-ray units
- mostly for hospital use and too heavy to be carried
- motor driven
- exposure output is less than room based units
portable x-ray units
- for home visits, military and veterinary work
- can be carried and set up
- low exposure output
limitations for mobile units
- low power input - thus has a shorter and narrower exposure range compared to units in x-ray rooms
- takes longer to take images that require a higher exposure
ideal characteristics of a mobile unit
- has a DAP meter
- has a focal point distance of 55-205 cm from the floor
- single-phase charging - Power flows through a single conductor.
ideal characteristics of mobile machine (generator and x-ray tube)
- mAs range - 0.1-500
- exposure times 0.001-1.25
- focal spot 0.3/1.0 mm
- power 20 kW
ideal characteristics of a mobile machine (wireless portable detector)
detector sizes - 35x43cm and 24x30cm
image resolution - up to 3.4Lp/mm
SNR
Signal to noise ratio
weakness of a digital system
- images can be viewed by clinicians immediately after x-ray has been taken
- image may have low resolution at this point and may be used as a diagnostic image - could lead to misdiagnosis
disadvantages of portable units
- restricted exposure factors - cannot use high kVp
- limitation on the length of exposure time
- Cannot use units for a long time to prevent damage
high frequency generator
- mains AC
- Rectifier
- Inverter
- high voltage transformer
- x-ray tube
what does the rectifier do
- converts AC to DC
issues with capacitor discharge mobile units
- with long exposures - the kV might fall during the exposure - causing under penetration
problems with restricted x-ray tube movement
- lack of tube height SID<180cm for CXR
- most patients are normally supine/semi-erect, which degrades the image
- infection control is difficult to observe
- can damage furniture
advantages of digital mobile x-ray
- can acquire images quickly
- however there is the temptation to ‘ just repeat’ for a better/slightly better image
- efficient
advantages of digital mobile x-ray
- can acquire images quickly
- however there is the temptation to ‘ just repeat’ for a better/slightly better image
- efficient
advantages of analogue x-ray machine
- fully developed technology
- relatively inexpensive
advantages of Digital x-ray machine
- simple to store data
- low radiation exposure of patients
disadvantages of Analogue x-ray machine
- has poorer image quality compared to new digital systems
- complicated reproduction and storage of images
disadvantages of Digtal x-ray machine
- high acquisition costs
- different imaging formats makes it difficult to share
mobile x-ray equipment components (generator)
- constant potential - either mains independent, using rechargeable batteries for all functions or capacitor discharge generator
image acquisition CR
- uses a photostimulable phosphor as an image receptor
- the phosphor absorbs the radiation, trapping electrons at energy levels vis the process of photostimulable luminescence
- the imaging plate is then read by a laser which releases the trapped electrons returning them to their original state ready for the next exposure
DDR image acquisition can be either….
Direct or Indirect
DDR image acquisition (direct)
- directly converts the absorbed x-ray into a proportionally sized electrical charge with no immediate scintillating step
- uses amorphous selenium
DDR image acquisition (indirect)
- indirect conversion - uses a scintillator to convert x-rays to light
- x-ray photons encounter a caesium iodide scintillator and are converted into light
- the needle-like caesium iodide structure acts to minimise scatter
- the light then reaches a low-noise photodiode array and is converted into an electrical charge that is read out digitally before finally sent to the image processor
what factors affect the resultant image quality?
- contrast
- dynamic range- wider range = increased range on contrast
- spatial resolution
- noise
- artefacts - leads/tubes on patient, pacemaker, movement (voluntary & non- voluntary)
- kVp levels - too low = not enough penetration
- focal spot size
Signal to noise ratio
a measure of true signal to noise
- a lower SNR generally results in a grainy appearance to images
what improves SNR
increasing mAs - however patient dose also increases
using high-quality sensors - transfers x-ray photons into light photons
