Diagnostic Imaging Flashcards
State properties of x-rays
Part of electromagnetic spectrum (travels at same speed in straight line but varied wavelengths/frequencies)
High energy wave due to short wavelength and high frequency
How is velocity of an x-ray calculated?
Wavelength x frequency
Define wavelength
Distance between two consecutive peaks
Define frequency
Number of times peak passes a fixed point per second
How are wavelength, energy of the wave and frequency related?
Frequency is proportional to energy of the wave and inversely proportional to wavelength
What are gamma rays used for?
Scintigraphy
What are the different types of x-ray machines available and their features?
Portable- small and compact, easy to move, linked exposure, low output as use mains voltage
Mobile- uncommon, larger, higher output than portable
Fixed- permanently installed, tube on gantry, higher output due to specialised electrical supply
What are features on x-ray control pannels?
On off button Mains voltage compressor (usually automatic unless old machine, adjusts incoming voltage, varies 215-240V to keep kV constant) kV control mA control Timer
What is the main legislation for use of x-rays and what is its main priciples?
Ionising radiations regulations 2017
Only radiograph for clinical reasons, exposure to personnel to minimum, dont exceed dose limits
Why is radiation dangerous and what effects can it have on the body?
Invisible, can’t feel, can penetrate, cumulative effects
Carcinogenic, somatic (changes to tissues shortly after exposure particularly rapidly dividing cells), genetic mutations
What are sources of radiation hazard?
Tube head
Primary beam
Scatter
What are ways of reducing effect of scatter?
Collimation
Keep area of interest close to plate
Lead protection worn
Grid used
What safety measures need to be in place for all practices using x-rays?
General- HSE notified of using x-rays, practice has RPS and RPA (radiation protection supervisor and advisor)
Local rules- no manual restraint, protection used in controlled areas, guidelines if pregnant, warning signs for exposure
System of work- legal requirement displayed, details RPA/RPS, description and access to restricted areas, how to respond to incidences, working practices
What is meant by a controlled area and what are features of it?
Where x-rays are taken and dose exceeds 7.5mSv/h
Specific room with lead lined or thick walls to prevent penetration
Large enough for 2 people to be 2m from primary beam
Warning signs and indicators of exposure
Restricted to classified personnel
How are staff protected from x-rays?
Minimal exposure- rotate staff, minimal time taking x-rays
Distance- at least 2m from primary beam, leave room where possible, never manually restrain
Barriers- PPE, lead glass, lead-plywood doors
Dosimetry- follow dose limits, wear dosimeter
What are types of dosimeters available in practice?
Film badges- film blackens in proportion to exposure
Thermoiluminescent detectors- crystal absorbs radiation, heating causes light emission proportional to radiation dose
What are the protocols in place for dosimeter monitoring?
Worn for 4, 8 or 13 weeks then returned to NRPB for reading, reports kept for 2 years
Need individual badges worn at waist level, cant wear outside practice
State the two methods of x ray formation
General emission- major
Characteristic emission- minor
Describe how general emission x-ray formation works
Electrons rapidly decelerate when hit tungsten target as the pass through tungsten atoms and interact with electrons in atoms, energy lost from deceleration released as x-rays
Describe how characteristic x-ray formation works
Incoming electrons knock out electrons from tungsten orbits and electron drops from outer to inner shell releasing energy as x-rays
Describe the x-ray tube
Cathode and tungsten anode in vacuum, immersed in oil to help heat conduction and insulation, surrounded by lead except window for x-ray beam
Describe how x-ray tube generates x-rays
Small current passed through cathode filament heating it up producing cloud of free electrons known as thermionic emission
Focusing cup keeps cloud of electrons together preventing cathode repelling
Potential difference applied across tube causes electrons to hit tungsten target and x-rays are produced
Why is heat a problem in x-ray generation?
Interaction between electrons and tungsten is 99% heat and 1% x-ray generation
Electrons get focused on focal spot so heat dissipation is issue
What are the two types of anodes used to aid heat dissipation of x-ray generation?
Stationary- tungsten in block of copper so conducts heat from target, portable machines
Rotating- anode on disk with angled edge, tungsten track around edge to heat gets evenly distributed and is lost by radiation and convection from disk, on larger machines
What is meant by kV control?
Controls potential difference across tube between anode and cathode
What does mA control do?
Varies small current heating the cathode
How does the timer on x-ray control panel work?
Closing activates kV/high tension and mA/filament circuits
Describe how kV control works
Step up transformer supplies high voltage across tube with higher kV the higher potential difference across the tube, faster electrons will travel so have more kinetic energy and produce higher energy x-rays
Why is higher kV needed, what is typical kV ranges and how does increasing by 10kV effect exposure?
To produce higher quality images and to penetrate thicker tissues
40-120kV
10kV increase doubles exposure
How does mA control effect the amount of x-rays produced
Allows electrons to be emitted from the cathode, higher the mA the hotter the cathode gets and more electrons are accelerated across the tube, producing more x-rays
What are the typical ranges for mA?
Portable- 20-60mA
Fixed- 1000mA
How is time of exposure linked with x-rays production?
Increasing time increases number of x-rays produced
What is known as mAs?
mA and time
What effect does doubling mAs have on exposure?
Doubles exposure
How is mAs affected for thicker tissues?
Needs to be higher
What parameters are considered for exposure charts?
kV mA s Distance from x-ray tube, animal and plate Use of grid Body part
Describe how an exposure chart is created
Note exposure factors used on good images to build up a bank of them for different situations
Take exposure and increase kV when tissues are thicker
Repeat bad images until you get a good image to record
What must be the same to use exposure charts?
Machine
Film focal distance
Digital detector
Use of grid
What is seen for over and under exposure?
Overexposed- areas of blackness
Underexposed- grainy and lacking contrast
What is the relationship between intensity of x-ray and distance from x-ray tube?
Intensity is inversely proportional to square of distance from x-ray tube
Doubling distance from tube quarters intensity of radiation
Define film focal distance/FFD
Distance from x-ray tube to image receptor
What need changing if FFD is changed and how is it calculated?
mAs
New mAs = old mAs x (new distance squared/old distance squared)
Define object film distance
Distance between object radiographed and x ray detector
What is the effect of increasing object film distance on the image produced?
Gets magnified
Reduces sharpness due to penumbra
How is distortion of images prevented?
Keep area parallel to cassette and perpendicular to x-ray beam
What is the focal spot in x-ray machines?
Area of anode hit by electrons
What are advantages and disadvantages of keeping focal spot small as possible?
Advantages- image produced best if there’s a point source of x-rays, limits penumbra
Disadvantages- causes issues for heat dissipation
When is fine focus used in x-ray machines?
Smaller and thinner areas of anatomy
Define penumbra
Margin of blurring around edge of a structure due to beam diverging
What is the difference between actual and effective focal spots?
Target on anode is angled so electrons hit actual focal spot then beam is produced from smaller effective focal spot
Why is filtration of x-rays needed?
X-ray beam contains spectrum of energies
Low energy x-rays have insufficient energy to penetrate to produce an image so thin sheet of aluminium filters them out
What is meant by collimation?
Using light beam diaphragm to reduce aperture therefore primary beam size produced on the patient
What are the benefits of collimation?
Reduce unnecessary exposure to the patient
Reduce scatter produced
Improves image quality
Reduce exposure into controlled area
State the ways x-rays can interact with matter
X-ray photons pass through unchanged
X-ray photons are absorbed
X-ray photons are scattered
How can x-ray photons pass through matter unchanged and what effect does this have on the image produced?
Travel in straight line without losing energy
Forms useful x-ray image
Define radiolucent
Permeable to x-rays
Define radiopaque
Blocks x-rays
Describe how x-rays can be absorbed as they pass through matter
Depending on if material is radiolucent or radiopaque depends on proportion absorbed, depends on atomic number of tissues atoms
How does absorption of x-rays differ between tissues? (general, bone, gas, soft tissue)
General- higher atomic number, higher density and thicker tissue increase absorption due to more interactions
Bone- high atomic number, good absorption
Gas- low density, poor absorber
Soft tissue- intermediate atomic number and density, varied absorption
What colours are seen on x-rays when passing radiolucent and radiopaque tissues?
Radiolucent- black
Radiopaque- white
Between- grey
Describe what causes x-ray photons to scatter when interacting with matter and the impact this has
Deflected in random directions, some losing energy
Reduces image quality and not useful as dont reflect anatomy, increase exposure risk
When and why are grids used?
At high exposures to reduce the large amounts of scatter than will be produced, improving image quality
What is an x-ray grid?
Flat plates with series of thin lead strips to absorb scatter alternating thin radiolucent strips to allow primary beam through cassette
Why does use of a grid increase exposure factors needed?
Some of primary beam gets absorbed by the lead and some scatter passes through spacing material
What is a grid ratio?
Height of strips/width of spacing
What are typical grid ratios and what affect does a higher ratio have?
6:1-12:1
More efficient at removing scatter but removes more primary beam so need higher exposure
What effect does increasing lines per cm on a grid have?
Removes more scatter but absorbs more primary beam so need higher exposure
Define grid factor
Number mAs is multiplied by when using grid
What is a parallel grid and what are its advantages and disadvantages?
Parallel equal height lead strips
Advantages- cheap, easy to use, can use either way up, at any FFD or centring point
Disadvantages- beam divergence means increased primary beam absorption at edges
What is a focused grid and what are its advantages and disadvantages?
Lead strips slope progressively towards grid periphery
Advantages- no image cut off as mimic divergence
Disadvantages- needs correct orientation and centring, correct FFD needed to match divergence, expensive and complex
What is a pseudo-focused grid and what are its advantages and disadvantages?
Parallel lead strips with height reducing to periphery
Advantages- reduction of grid ratio to periphery compensates cut off in parallel grids
Disadvantages- needs correct orientation, more expensive
What is a cross-hatched grid and what are its advantages and disadvantages?
Lead strips at right angles to each other
Advantages- efficient at removing scatter
Disadvantages- need high exposures, accurate centring, expensive
What is a moving grid and what are its advantages and disadvantages?
Parallel grid permanently under radiolucent x-ray table, oscillates during exposure
Advantages- fine lines as in stationary grids blurred so not seen
Disadvantages- expensive, always used
What is meant by digital imaging in radiography?
X-ray machine and table are unchanged and images are produced on various image receptors
What are the advantages of digital imaging?
Low running costs
Higher quality images
Adjustments can be made reducing time and exposure
Easy to store, communicate and retrieve images
What are the disadvantages of digital imaging?
Viewing limited to availability of computer
May have artefacts
Need backing up
Describe how computed radiography works
Uses storage phosphor cassette
Energy from interactions with x-ray beam is stored in phosphor, later released as light when excited by laser beam in plate reader
Light is captured and quantified by photomultiplier tubes which converts to electrical signal as image on screen
How is phosphor plate for CR erased for re use?
Bright light removes residual energy, takes 1-2 minutes
Describe how direct radiography works
Patient information is entered into system
Flat panel detector produces instant image on connected monitor
Processing algorithms and formula reconstructs image depending on anatomical region being viewed
Can manipulate filters, contrast, size, orientation, measurements
How are digital images stored?
Computer files in ditcom format
Regularly backed up, can print at extra cost
Long term storage on PACS for identification, manipulation, back up and retrieval of images
What is meant by PACS and ditcom?
PACS- picture archiving and communication system
Ditcom- digital imaging and communications in medicine
What is meant by a satisfactory radiograph?
A radiograph that can be diagnostic
List factors assessed when looking at image quality
Opacity Contrast Sharpness Technique faults Artefacts
How does opacity affect image quality and what effects opacity?
How black or white overall
Depends on tissue type, thickness, exposure factors, processing of image
How does contrast affect image quality and what effects it?
Difference in tone between areas either long or short scale contrast
Depends on part being x-rayed, atomic number, density, algorithm applied, scatter
How does sharpness effect image quality and when affects it?
How clearly defined the image is
Depends on movement, scatter, object film distance
State some technique faults that effect image quality of radiographs
Poor positioning Not enough projections taken Not using L/R markers No date or patient information Movement blur Incorrect grid use Poor centring and collimation
