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
How do lines per cm on a grid affect exposure factors?
More strips of lead per cm = more scatter removed BUT more primary beam absorbed too
So higher exposure needed
What is a grid factor and what does it depend on?
Grid factor = number by which the mAs must be multiplied if a grid is used
Depends on grid ration, lines/cm
Name the 5 types of grid.
Parallel Focused Pseudo-focused Cross-hatched Moving/Potter Bucky
Describe a parallel grid.
Lead strips parallel and of equal height
Some increase in amount of primary beam absorbed towards edges, since beam is diverging
Cheap, can be used either way up, at any FFD or centring point
Describe a focused grid.
Lead strips slope more towards periphery of grid, no cut-off of image
Must be used right way up
Centre of X-ray beam must be at centre of grid, appropriate FFD used
More expensive than parallel grids
Describe pseudo-focused grids.
Lead strips parallel but height reduces towards periphery of grid
Reduction in grid ratio at periphery compensates for grid cut-off
Describe cross-hatched grids.
Tow sets of lead strips at right angles to each other
Very efficient at removing scattered radiation
Require high exposures, accurate centring, are expensive
Describe moving/Potter Bucky grids.
Parallel grids mounted permanently beneath radiolucent tabletop
Oscillate rapidly during exposure
Means that fine lines (from stationary grid) are blurred out and not seen
X-ray cassette/detector stays stationary beneath grid so image remains sharp
Describe Computed Radiography (CR) and how it is carried out.
Uses cassettes containing a ‘storage phosphor’ plate
Plate is erased by bright light and can be re-used
Process takes around 1-2 mins
Describe Digital Direct Radiography (DR).
Electronic detector directly captures X-ray image
Image is displayed immediately on monitor
What information do we need to set before taking an X-ray image?
Patient information
Anatomical region of interest
Size of animal
What do we need to know about our machine’s ‘exposure index’ (EI)?
Manufacturer’s name for value
Whether it is a proportional or inversely proportional measure of exposure
How can we manipulate a digital image?
Filters / algorithms Contrast / brightness Size (zoom in/out) Orientation (flip/rotate) Annotation / measurement
How can digital X-ray images be stored?
As computer files - dicom format (digital imaging and communications in medicines)
On PACS software (picture archiving and communication system)
Need regular backup!
What are the advantages of digital radiography?
Decreased running costs Time saving Improved images in some cases Easy image retrieval and storage Easy communication of images Portable - instant images in the field
What image quality factors should be assessed?
Opacity Contrast Sharpness Patient positioning Centring of X-ray beam Collimation of X-ray beam
Define radiographic opacity.
Degree of blackening of the radiograph.
What is radiographic opacity influenced by?
Patient (tissue type and thickness)
Exposure factors
Digital algorithm/processing
Define radiographic contrast.
Difference in radiograph opacity between adjacent areas on an image.
What influences radiographic contrast?
The inherent contrast of part being radiographed (Z number, density)
Digital algorithm selected
Amount of scatter reaching film
Define radiographic sharpness.
The ability to distinguish fine detail on an image.
What decreases radiographic sharpness?
Movement (animal, table, X-ray tube)
Scattered radiation
Object film distance
List the miscellaneous digital image artefacts that occur.
Double exposure Digital exposure errors 'Uberschwinger' or rebound artefact Ghost artefact Moire artefact Dirt on light guide (CR only)
What is an ‘uberschwinger’ or rebound artefact?
Occurs when there is a large density difference between adjacent objects (excessive ‘edge enhancement’ by computer algorithm
Most commonly seen around orthopaedic implants
Important to recognise as can mimic implant loosening
What is a ghost artefact?
Incomplete erasure of a CR plate before use
May also occur if CR plate exposed to light
What is a Moire artefact?
Bands across the image
Can arise with CR systems due to interference between frequency of laser reader and number of lines/cm of grid
What happens if there is a dirt on the light guide (CR only)?
Emitted light blocked by dirt from reaching photomultiplier tube
Seen as a line along the image
Line is orientated in direction imaging plate moves during the reading process
CR plate reader cleaning and maintenance should be performed routinely to prevent this occurring
What happens if there is a dirt on the light guide (CR only)?
Emitted light blocked by dirt from reaching photomultiplier tube
Seen as a line along the image
Line is orientated in direction imaging plate moves during the reading process
CR plate reader cleaning and maintenance should be performed routinely to prevent this occurring
Define contrast media.
Contrast media = agents that are more or less opaque than surrounding tissue.
What is the purpose of contrast media?
To see structures not normally/poorly visible, e.g. ureters, urethra, spinal cord
To gain more information about soft tissue structures, e.g. bladder, kidneys, GI tract
What are the two types of contrast media?
Positive contrast, e.g. barium/iodine - white on radiograph
Negative contrast, e.g. air/CO2 - black on radiograph
Describe barium as a contrast agent.
Used in GI tract Various formulations, powder/suspension Inert, non-toxic Reasonably palatable Cheap Good mucosal detail (liquid barium) No osmotic effect
Describe iodine as a contrast agent.
Water-soluble organic iodine-containing preparations
2 main groups - ionic / non-ionic
IV use
Renal excretion
What are the disadvantages of ionic iodinated contrast media?
Irritant extra-vascularly
Toxic in large doses - do not repeat studies multiple times
Viscous
Contraindicated IV in CV or renal insufficiency
Contraindicated for myelography
What are the disadvantages of non-ionic iodinated contrast media?
Slightly more expensive
Viscous
Side-effects may still occur, although rare (anaphylaxis, urticaria, vomiting, pyrexia)
What are the advantages of negative contrast media?
Cheap
Simple to use
Relatively safe
Can combine with positive - double contrast study
What are the disadvantages of negative contrast media?
Very small risk of air embolism
Poor mucosal detail
Produce less contrast than positive contrast agents
Why do we carry out plain radiographs before contrast radiographs?
Check radiographic quality
Check patient preparation
Look for radiological diagnosis
Look for radiopaque lesions that may be obscured by contrast
What alternative methods to contrast radiography have become more popular?
Endoscopy has replaced many GI studies
Ultrasound +/- colour Doppler - portosystemic shunts, UT investigations
CT and MRI have replaced myelography to a degree
Describe a barium swallow.
Indications = dysphagia, regurgitation, suspected rupture
Shows pharynx and oesophagus
Liquid barium/barium and food OR iodinated contrast if suspected rupture
Care if swallowing problems, risk of aspiration
Describe a barium ‘follow-through’ study.
Evaluates stomach and small intestine (e.g. if vomiting cat/dog)
Liquid barium by mouth/stomach tube
Radiographs taken immediately then at intervals
Final images at 24hrs / when all barium in colon
Largely superseded by ultrasound and endoscopy
Describe a barium enema.
Evaluates large intestine Liquid barium infused into rectum post-enema Can follow with air (double contrast) Messy and difficult to interpret Technique superseded by endoscopy
What contrast media do we use for the urinary tract?
Water-soluble iodinated contrast medium
NEVER barium - cannot use IV and irritant in bladder, can cause granulomatous cystitis
Describe excretory urography.
Identify/assess kidneys and ureters
Bolus of contrast given into peripheral vein
Radiographs taken immediately (nephrogram), 5 mins (pyelogram), 10 mins (ureterogram), 15 mins (ureterovesicular junction)
Describe cystography.
Delineates bladder
Contrast introduced via urinary catheter
Pneumocystogram (air only) = bladder location, shows large masses or marked thickening
Positive contrast cystogram (iodinated contrast) = leakage
Negative contrast cystogram (both agents) = delineation of wall and content
Describe (vagino-)urethrography.
Delineates urethra (and vagina)
Water-soluble iodinated contrast medium, introduced via Foley catheter into distal urethra/vestibule
Radiograph taken at end of injection
Indications as for cystography
Describe myelography.
Delineates sub-arachnoid space
Localises lesions of spinal cord
NON-ionic water-soluble contrast medium (ionic causes seizures/arachnoiditis)
CT and MRI have largely replaced myelography
What nursing considerations should we have for myelography?
Keep head elevated to prevent media entering the brain, as this could cause the patient to seizure
A patent IV catheter should be available
IV diazepam should be available
