Diagnostic imaging Flashcards
How are xrays produced?
Interaction with fast moving electrons with a metal target
What makes up the cathode?
Tungsten wire filament
How are free electrons produced at the cathode?
Electrical current is run through the tungsten wire filament causing a cloud of free electrons which are focussed into a beam by a negatively charged focussing cup
What is the relationship between current and no of electrons?
More current = higher no of electrons
What is the anode made of?
Solid tungsten
What are the two mechanisms of xray formation?
General emission - ‘braking’ speed
Characteristic emission - electron knocked out of atom
What is the area of the anode that is hit by electrons and produces xrays called?
Focal spot
What is produced when electrons interact with the anode?
Heat and x-rays
How does a stationary anode remove heat?
Conduction - copper
How does a rotating anode remove heat?
Convection - molybdenum
How does a high voltage (kV) affect xrays?
Electrons travel faster, have more kinetic energy so xrays have a higher ENERGY
When do you use higher voltage (kV)?
When radiographing thicker parts of the body - more penetration
What affect the quantity of xrays produced?
mA and time
What is the inverse square law?
The exposure is inversely proportional to the square of the distance from the xray tube
What is used to filter out low energy xrays?
Thin sheet of aluminium
What are the three ways xrays interact with matter?
Pass through unchanged
Absorbed
Scattered
What factors affect absorption of xrays?
Atomic number
Density
Thickness
What does xray image formation depend on?
Differential absorption
What does computed radiography use to capture the image?
A storage phosphor plate
How is the storage phosphor plate read?
Energy is stored in plate and later excited by a laser beam in a plate reader and converted to electrical signal
What does direct digital radiography use to capture the image?
Electronic detector
What is contrast?
The difference in density between two adjacent areas on a radiograph
How does voltage affect contrast?
Higher kV increases penetration so lowers the contrast
How does scattered radiation affect contast?
Causes overall blackening of the film so reduces contast
How do you reduce scattering of radiation?
Use a grid
Collimation
What is collimation?
Reducing the size of the primary beam to the area of anatomy that we want to see
When are grids used?
When radiographing thicker objects
What are grids made up of?
Thin lead strips alternating with thin radiolucent strips
What is a side effect of using a grid?
Need to increase the exposure
What is the grid factor?
Multiple of mAs needed when using grid compared to same exposure without grid
What does the grid factor tend to be?
2-6x
What are the different grid types?
Parallel
Focussed
What is a side effect of using a parallel grid?
Get cut off on the sides
What do you have to make sure when using a focussed grid?
Its the right way up otherwise will get massive cut off
How do you alter the collimation?
Using a light beam diaphragm to alter aperture size
What affects the sharpness of radiographs?
Movement blur
Focal spot size
Distance
What is the focal spot?
Point of xray source
How does focal spot affect the sharpness of the image?
Smaller focal spot = sharper image
What are the two distances that affect sharpness?
Film/focal spot distance
Object/film distance
What is the focal film distance?
The distance from the x-ray tube to the film plate
How does focal film distance affect sharpness?
The closer the x ray machine is, the more divergence there is so its less sharp
What does penumbra mean?
Spread of xrays after a point when they start to diverge
What is the object film distance?
Distance between the object (animal) and the film plate
How does the object film distance affect the sharpness?
Larger OFD leads to larger penumbra and so less sharpness - want to be as close as possible
How is magnification calculated using the two distances?
FFD/(FFD-OFD)
How do you reduce magnification?
Longer FFD
Shorter OFD
When does distortion occur?
When an object is not parallel to the film
What is a film fault only found on cassette based systems?
Double exposure - two images on one plate
What is a sign of underexposure in digital photography?
Very grainy and pixelated image
How do you correct underexposure?
Increase exposure factors - kV and/or mA
What causes the Uberschwinger artefact in radiographs?
When there is a large density difference between adjacent objects
What does the Uberschwinger artefact look like in radiographs?
Excessive edge enhancement causing radiolucent black ring around artefact
What causes a ghost artefact in radiographs?
Incomplete erasure of a plate
What are moire artefacts in radiographs?
Bands across image
What causes moire artefacts in radiographs?
Interference between the frequency of the laser reader and the grid
What can cause one line across the radiograph image?
Dirt in the cassette reader eg. dust particle
What is the ALARA principle?
As Low As Reasonaby Achievable - radiation
What methods of protection are used for radiation?
Space/distance
Barriers
Protective clothing
Time
What legislation surrounds radiation?
Ionising radiation regulations 2017
What personnel are required for radiation legislation?
Radiation protection advisor - external
Radiation protection supervisor - internal
How big should the controlled area for radiation use be?
2m - when it is connected to the mains supply
What is the device called that monitor radiation levels on individuals?
Dosimeters
What is the legal dose limit for workers over 18?
6mSv
What is an MRI?
Cross sectional imaging technique - slices
What does MRI use to generate images?
Radiowaves and a strong magnet - pulses and echoes
What are some advantages of MRI?
Good for soft tissue
Doesnt use ionising radiation
Good for imaging areas which arent very accessible eg. brain and spinal cord
What are some disadvantages of MRI?
Expensive
Need to be perfectly still
No metal
What is an artefact in MRIs?
Metal distorts magnetic field eg. microchip
What does CT stand for?
Computed tomography
How does CT generate and image?
Ionising radiation - ring of rotating xray detectors
How is the CT information displayed?
Different planes and windows
What is CT used for?
Anatomical reconstructions of bony structures eg. complex joints
Also lymph nodes, lungs
How is CT better than radiographs?
Avoids superimposition of structures - can get 3D structures
What are some disadvantages of CT?
Not widely available
Ionising radiation - higher dose than radiography
What does scintigraphy use to generate an image?
Radioisotope - technetium 99m gives out gamma rays
Bound to a substance that binds to a specific area of the body eg. bone
How does scintigraphy work?
Bound isotope is injected IV
Pattern of radiation emitted is analysed
What are the main uses of scintigraphy?
Skeletal injury in horses
Ectopic thyroid tissue in hyperthyroid cats
What are some advantage of scintigraphy?
Can localise problems that cant be seen otherwise
Assesses function, not just structure as depends on metabolism
What are some disadvantages of scintigraphy?
Uses ionising radiation
Remains radioactive for a while
Poor anatomical detail
What does an ultrasound use to generate an image?
High frequency sound waves - 2-18 MHz
How are ultrasound waves produced?
Disc in a transducer has voltage applied and expands and contracts proportional to the voltage
This movement causes sound waves
What is the piezoelectric effect?
Conversion of electrical energy to kinetic energy and therefore a sound wave
How are ultrasounds read?
Sound returns to the transducer
Pressure of the sound wave distorts the disc generating voltage proportional to the pressure
Voltage processed by machine
What is acoustic impedance?
Density of tissue x speed of sound in tissue
What does a change in acoustic impedance cause?
Reflection of ultrasound waves
What is non-specular reflection?
When beam hits small structures and is re-radiated in all directions - causes texture to organs
What are the two display modes of ultrasound?
B mode - brightness
M mode - motion
How does the B mode of ultrasound work?
Images a slice - image built up of lots of lines of ultrasound beam
See movement in real time
What does brightness depend on in B mode of ultrasound?
Amplitude of signal
What does position depend on in B mode of ultrasound?
Time the signal takes to return
When is M mode of ultrasound used?
In cardiac work
How does the M mode of ultrasound work?
Movement of points along a single line - position vs time
What do you need to avoid when doing an ultrasound exam?
Bone or gas
Hair
What do you need to use with the ultrasound transducer?
Acoustic gel
What are the three different types of ultrasound transducers?
Phased
Linear
Convex
What is a phased transducer?
Beam is steered electronically
Diverges at depth - end up with a fan shaped image
What is a linear transducer?
Beam is steered manually
Multiple elements - triggered in groups
What is a convex transducer?
Elements are arranged in a curve
Which transducer is better for superficial structures?
Linear
What are the advantages of a phased array?
Easy to manipulate
Small contact area
Wide field at depth
What does a high frequency of ultrasound give?
Good image resolution
But cant penetrate as far
What are high frequency ultrasounds used for?
Superficial structures in larger animals
All structures in smaller animals
What do low frequency ultrasounds give?
Poorer resolution
Can image deeper structures
What are low frequency ultrasounds used for?
Deeper structures
Larger animals
What does hyperechoic mean?
Appears white on ultrasound image
What does hypoechoic mean?
Appears grey on ultrasound image
Often used as a relative term
What does anechoic mean?
Appears black on ultrasound image
What does echogenic mean?
Appears white on ultrasound image - same as hyperechoic
What does echolucent mean?
Appears black on ultrasound image - same as enechoic
What are 4 common ultrasound artefacts?
Acoustic enhancement
Acoustic shadowing
Reverberation
Mirror image
What is acoustic enhancement?
Brighter area distal to fluid filled structures
Why does acoustic enhancement occur?
Low attenuating structure so no echoes are caused
Then structures deeper to it cause echoes that it thinks are denser so is brighter
What is acoustic shadowing?
A black area below a structure that is highly attenuating/causes lots of reflection
What transition causes the most acoustic shadowing?
Soft tissue to gas
Soft tissue to bone is slightly less
What causes reverberation artefacts on ultrasound scan?
When sound bounces between interfaces giving multiple echoes
Mainly gas interfaces - closely spaced reflective surfaces
What do reverberation artefacts look like?
Comet tail
When do mirror image artefacts occur?
At highly reflective interfaces - typically diaphragm-lung interface
What does a mirror image artefact look like?
Bright white line with mirror image of the liver on either side
What are the advantages of ultrasound?
Quick
Safe
Non-invasive
Real time
What are the disadvantages of ultrasound
Need to clip hair
Gas and fat hinder
Needs experience
Non-specific findings eg. dont know what the lesion is
What is the doppler shift in ultrasound?
When the sound waves hit small moving targets eg. RBC
Changes the frequency of the sound
What can you use doppler ultrasound for?
Direction of blood flow
Velocity of blood flow
Turbulent blood flow
What are the Roentgen signs for in radiology?
Features that help you to filly describe a lesion
What are the 6 Roentgen signs in radiology?
Size Shape Position Opacity Margination Number
How is size used as a Roentgen sign?
Measured relative to other organs - increase or decrease
May become apparent or displace another organ
How can you tell if shape and position of a structure has changed in a radiograph?
Know what normal is
What can cause an increase in opacity on a radiograph?
Excess fluid or soft tissue
Deposition of bone/calcium
Foreign body
What can cause a decrease in opacity on a radiograph?
Abnormal gas accumulation
Loss of normal tissue eg. demineralisation
What can cause poorly defined marginations on a radiograph?
Abdominal fluid
Inflammatory change
Malignant bone lesions
What is different in the Roentgen signs between radiologic and ultrasonographic?
Echogenicity in ultrasounds rather than opacity
What can make structures more hyperechoic/brighter on ultrasound?
Fat
Glycogen
Collagen
Crystalline material
What can make structures more hypoechoic/darker on ultrasound?
Oedema
Out of the spleen, kidney and liver, which is the most echogenic and which is the least ecogenic?
Spleen is most echogenic/bright
Then liver
Then kidney is the least echogenic
How does the gall bladder present on ultrasound?
Anechoic
What does the stomach look like on an ultrasound?
5 layered appearance
Has rugal folds
How does the renal medulla present on ultrasound compared to the cortex?
Hypoechoic to anechoic - much blacker
What does the bladder look like on ultrasound?
Echogenic double line with thin muscle layer between - hypoechoic
What are radiographs better at imaging than ultrasound?
Fat, air
What are radiographs worse at imaging than ultrasounds?
Fluid, soft tissue
