imaging Flashcards
properties of X-rays
high energy:
- short wavelength
- high frequency
how are x-rays produced?
the interaction of fast moving electrons with a metal target
production of x-rays
- contains anode and cathode (tungsten)
- cathode heats up, cloud of e- forms
- thermionic emission
- p.d is applied sending the e- to a target/focal spot on the anode producing x-rays
general x-ray emission
- e- interact with tungsten atoms e-
- energy lost through x-radiation
characteristic/minor emission
- one of incoming e- knocks an e- from tungston atom
- an e- from outer shell drops to inner shell to fill gap
- energy is released as x-ray
x-ray tube
- anode and cathode are in pyrex tube
- tube is immersed in oil to aid conduction of heat
- tube surrounded by lead except small window
exposure factors of x-ray
- kV control (voltage)
- higher energy x-ray for thicker parts of body
- mA control (current)
- heats the cathode, higher frequency of x-rays
setting up an exposure chart
when you get a good x-ray, record exposure factors used and measure thickness of the anatomy x-rayed
control panel of x-ray machine
- kV control
- mA control
- timer
collimation definition and prupose
reducing the size of the x-ray beam using diaphragm
- reduces unnecessary radiation of the patient
- reduces production of scattered radiation
- improved image quality
- less exposure to staff
effect of distance on x-ray intensity
intensity is inversely proportional to the square of distance from the x-ray tube
- important to keep distance the same to get same result with the same exposure factors
3 possibilities of interaction of x-rays with matter
- photons pass through unchanged (black/darker on image)
- photons are absorbed (white on image)
- photons are scattered (degrades image quality)
purpose of a grid
- useful when x-raying thick areas
- reduces scattered radiation reaching the film
grid structure
- thin strips of lead
- allows primary beam travelling in straight line pass through
- needs high exposure factors
grid ratio
- height of the lead strips divided by the width of the spacing material
- higher ratio removes more scatter but also removes some of primary beam
grid factor
number by which the mAs must be multiplied if a grid is used
types of grid
- parallel
- focused
- must be used right way up, strips slope more towards edge - pseudo-focused
- compensates for grid cut-off which occurs in parallel grid - cross hatched
- require very high exposures - moving/potter-bucky
- parallel grid mounted beneath table top
- removes fine lines that appear when using stationary grid
effects of ionising radiation
carcinogenic- tumor development of cells
somatic- direct changes in body tissues that occur not long after exposure has occured
genetic/mutation- chromosomal mutation
sources of radiation hazard
- tube head
- primary beam
- scatter radiation
safety measures to protect staff from exposure
- lead apron, gloves and thyroid shield
- tight collimation
- chemical restraint so no need for staff to be present
- dosimeters to measure exposure
how should dosimeters be worn?
- on neck or chest area
- area that faces radiation source
digital radiography
direct capture of x-ray image displayed on computer
digital radiography advantages
- decreased running costs
- reduced repeat rate
- time saving
- decreased radiation dose
- improved images due to manipulation
- portable (field x-rays)
digital radiography disadvantages
- set up costs
- limited by availability of computers
- ensuring adequate back-up of files
radiographic opacity definition
how black or white overall?
radiographic contrast definition
differences in black/white/grey
radiographic sharpness definition
clarity of image
what are contrast media?
agents that are more or less opaque than surrounding tissue
what do contrast media do?
delineate organs/cavities within body to see structures that are usually poorly visable
types of contrast media
- positive contrast
- barium, white on radiograph
- negative contrast
- gases, black on radiograph
barium as contrast agent
- used in GI tract
- reasonably palatable
- cheap
- non toxic
ionic iodinated contrast
- irritant extra-vascularly
- toxic in large doses
- viscous
non-ionic iodinated contrast
- more expensive
- viscous
- side effects can still occur (anaphylaxis, nephrotoxicity)
negative contrast media advantages
air/CO2
- free
- simple, relatively safe
- can combine with positive contrast agents
(double contrast)
negative contrast media disadvantages
- risk of air embolism
- poor mucosal detail
- less contrast than positive contrast agents
barium swallow technique
indications= dysphagia, suspected rupture
- shows pharynx and oesophagus
- care if cant swallow (aspiration risk)
barium ‘follow-through’ technique
- shows stomach and small intestine
- liquid barium (stomach tube or mouth)
- take x-ray immediately then at intervals
barium enema technique
- evaluates large intestine
- liquid barium infused into rectum post-enema
- messy/difficult to interpret
- superseded by endoscopy
contrast in urinary tract
- never barium (bladder irritant)
- use water-solule iodinated contrast
cystography techniques
- pneumocystogram
- positive contrast cystogram
- double contrast cystogram
myelography
- delineates subarachnoid space
- localised lesions of spinal cord
- non-ionic water-soluble contrast
production of ultrasound
- high frequency sound wave
piezoelectric effect
1. voltage applied to disc within transducer
2. disc expands or contracts
3. sound wave produced
recieving the ultrasound signal
- sound returns from tissues to transducer
- pressure of sound wave distorts disc which creates voltage
- voltage processed by machine and displayed
acoustic impedance
density of tissue x speed of sound in tissue
ultrasound specular reflection
- soundwaves hit large smooth surface giving a bright/mirror reflection line
ultrasound non-specular reflection
- beam hits small structures with re-radiates in all directions giving texture to organs
ultrasound B-mode
- images a slice through the patient
ultrasound M-mode
- used in cardiac work
- movement of points along line followed
ultransound abdomen clipping site
xiphisternum to pubis
- follow costal arch up to lumbar muscles
ultrasound heart clipping site
- right side
- 4th-6th intercostal space
- costochondral junction to sternum
ultrasound left kidney clipping site
- behind last rib below lumbar muscles
ultrasound right kidney clipping site
- last 2-3 intercostal spaces below lumbar muscles
advantages of ultrasound
- good soft tissue detail
- internal structure of organs
- functional information (movement)
disadvantages of ultrasound
- expensive equipment
- need to clip hair
- difficult to interpret
- gas/fat/bone hinders examination
types of ultrasound transducer
- phased array
- convex
- linear
advantages of phased array transducer
- easy to manipulate
- small contact area
- wide field at depth
advantages of linear transducer
- large contact area
- large field of view near skin
- good for superficial structures
positioning aids
- radiolucent wedges and plastic troughs
- sandbags (will be seen on radiograph)
- tape
naming of projections
describe the path of the x-ray beam from the tube to the image receptor
- ventrodorsal
- plantarodorsal
lateral projections named after side animal is lying on
CT/MRI
- slices through the animal
scintigraphy
functional remodelling activity of bone
principles of CT
- ionising radiation
- similar to radiography
- rotating x-ray machine to take 360 degree view
uses of CT
- gives multiple detailed reconstructions from one scan
- used to make custom made surgial implants
- 3D models for surgical planning
disadvantages of CT
- limited availability/expensive
- uses higher doses of ionising radiation than x-ray
scintigraphy principles
- uses radioisotope
- bound to another substance which determines where in the body it will localise
e.g. HDP binds to bone - bound isotope is injected into patient
uses of scintigraphy
- dectection of skeletal injury in horses in areas difficult to radiograph
- cases where its difficult to localise source of lameness
- detection of thyroid nodules in hyperthyroid cats
disadvantages of scinitgraphy
- uses ionising radiation
- patient remains radioactive after scan
- poor anatomical detail (difficult interpretation)
how does MRI work?
- nuclei with odd no. of protons and/or odd no. neutrons it will spin creating its own magnetic field
- when the patient enters the feld, the neutrons line up
- patient bombarded with radiowaves and nuclei become disorientated
- this emits a radiosignal
advantages of MRI
- good contrast resolution
- excellent anatomical detail of soft tissues
- doesn’t use ionising radiation
MRI uses
- neurology cases
- soft tissue lesions
disadvantages of MRI
- not widely available/expensive
- requires GA
- powerful magnetic field
physical safety in aquisition of radiographs in horses
- be ready to move x-ray equipment out of way
- avoid touching horse with equipment
radiation safety in aquisition of radiographs in horses
- minimise number of people around horse
- PPE
