X-ray Imaging Flashcards
What three specifications affect the quality of an X-ray image?
Exposure factors
Focal spot
Filtration
Why is it important to turn off an X-ray machine when not in use?
When connected to a power supply, the area around it is considered a ‘controlled area’ with access restrictions.
How does kV affect X-rays produced?
The higher the kV, the greater the potential difference across X-ray tube, and the faster the electrons will travel between cathode and anode.
Therefore resultant X-rays will have a higher energy.
What is the typical kV range, and how does increasing this affect the exposure?
Typical range 40-120kV
kV affects penetrating power of X-rays produced, so need to increase kV to X-ray thicker parts of body
An increase of 10kV approximately doubles the exposure
How does mA affect the X-rays produced?
The greater the mA, the hotter the cathode filament gets, and the more electrons are available to be accelerated across the tube.
If more electrons hit the anode, then more X-rays are produced (energy of X-rays is not changed)
How does increasing exposure time affects X-rays produced?
Increasing time of exposure results in an increase in the number of X-rays produced
What are typical mA values, and how does increasing this affect the exposure?
Typical 20-60mA (portable) 1000mA (fixed)
mAs govern quantity of X-rays produced
Increase in mAs needed to radiograph thicker parts of the body
Doubling the mAs doubles the exposure
What do we need to decide on for each region when setting up an exposure chart?
kV mA s Distance Use of a grid
What is the relationship in increase in kV and mAs?
An increase in 10kV is approximately equivalent to doubling the mAs
What is the focal spot? Describe the ideal size.
Focal spot = the area of the anode hit by electrons
Ideally as small as possible - ‘point source’ of X-rays
When would we use fine focus on an X-ray machine?
Used for smaller/thinner areas of anatomy - smaller focal spot.
What is the difference between the actual and effective focal spot?
Angle of anode allows electrons to hit a larger area (actual focal spot), while source of X-ray beam is effectively smaller (effective focal spot)
What is the penumbra of an image?
Slight margin of blurring around edge of a structure - increases with larger focal spot.
What is filtration of X-rays?
Low-energy X-rays insufficient to penetrate patient, so filtered out by thin sheet of aluminium over window of X-ray tube.
Name the three different types of X-ray machine.
Portable
Mobile
Fixed
Describe portable machines.
Small, compact, easily moved
Can be operated from normal electrical socket#
Relatively low output
Often ‘linked’ exposures
Describe mobile machines.
Larger and heavier than portable (wheeled)
Can still usually be operated from normal electrical socket
Higher output than portable machines
Describe fixed machines.
Permanent installations - tube on a gantry
Wired to specialised 3-phase electrical supply
Much higher potential output
How does collimation help in X-raying imaging?
Reduces unnecessary radiation of patient
Reduces production of scattered radiation (improves image quality, less personnel exposure)
What is the Inverse Square Law and why is it important?
Intensity of beam at given point is inversely proportional to square of distance from X-ray tube
Important in radiation safety and exposure for the X-ray image formed
Define film focal distance (FFD) and explain what this means for the exposure factors.
FFD = distance from X-ray tube to X-ray image receptor
If FFD is altered, mAs will need to be altered to produce a comparable radiograph (old mAs x new distance squared / old distance squared = new mAs)
Define object film distance (OFD).
OFD = distance between the object being radiographed and the X-ray detector
How does Object Film Distance affect magnification and how can we reduce this?
If OFD increases, image will be magnified
Keep patient area of interest as close to cassette as possible
Can use marker of known size to determine degree of magnification of image
How does Object Film Distance (OFD) affect image sharpness and how can we reduce this distortion?
Large OFD increases size of penumbra around edges
Keep area of interest parallel to cassette and perpendicular to X-ray beam
In what 3 ways might X-rays interact with matter?
X-ray photons pass through unchanged, in a straight line and do not lose energy (useful image)
X-ray photons are absorbed, proportion depends on nature of material
X-ray photons are scattered, lose some energy, degrade image quality and pose radiation hazard
What factors affect the absorption of X-rays?
Atomic number (high Z = more absorption) Physical density (higher density = more absorption) Thickness of tissues (thicker tissues = more absorption)
Describe how bone, gas and soft tissue would appear on a radiograph.
Bone = high Z, good absorber, white on image
Gas = low density, poor absorber, black on image
Soft tissue = intermediate Z and density, intermediate absorber, grey on image
How are grids used and what do they do?
Used when radiographing thicker areas (approx. >10cm thick)
Placed between patient and cassette
Reduce amount of scattered radiation reaching the film - improve image quality
How do grids affect exposure factors and why does this happen?
Some scattered radiation will pass through spacing material
Some of primary X-ray beam will be absorbed by lead
So higher mAs needed if using a grid
What is a grid ratio and give some typical examples?
Height of lead strips divided by width of spacing material
Typical ratios 6:1 - 12:1
How does grid ratio affect exposure factors?
High grid ratios most effective at removing scatter BUT remove more of primary beam
So higher exposure is needed
