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1.22.4.09 - Radiography and Ultrasonographic Image Formation - Coggle…
1.22.4.09 - Radiography and Ultrasonographic Image Formation
restraint and movement
restraint
general anesthesia
contrast studies (NOT oral barium)
positioning aids
sedation
oxygen supplementation and good monitoring are important
suitable for most thoracic and abdominal radiography
may be used for some musculoskeletal studies where G.A. is undesirable
no small animal should be manually restrained except in exceptional clinical cases
managing pain and sick patients
provide analgesia in painful cases
other procedures may needed before radiography in critically ill patients
sedation may be the best restraint
anaesthesia for manipulation of painful areas
critically ill patients may resent lateral recumbency
movement blur - affects image quality - e.g like breathing, limb movement etc, This is minimised by adequate restraint and and correct machine settiings, with short exposures (increase mA and decrease the time to keep the same mAs)
large animal consideration
most are radiographed with standing sedation
rare to use general anaesthesia
manual restraint is usually required - must follow safe practice
principles of controlling movement blur are the same as for small animals
planning a radiographic study
need two orthogonal views to fully evaluate an area
specific body areas have further requirements
equine consideration
common joints
hock
carpus
stifle
fetlock
standard views of equine limbs
lateromedial
dorsopalmar
2 x 45º oblique views
patient preparation
species and body area specific
starving cats and dogs for 24 hours before abdominal studies
allow urination and defecation
clean limbs of mud/dirt and brush feet etc.
radiographic grids
x-rays are scattered
scattered radiaton
is a safety issue
leads to a poorer quality image
more scattered radiation produced for thicker animals
grids filter out photons that arent travelling forwards
grids reduce scatter radiation
considerations
only the centre beams are travelling vertically
width and height determine how much scattered radiation is filtered
grid factor
needs a higher amount of exposure
grids
may be stationary
may be moving
when to use grids
when the part being imaged is more than 10cm thick
Avoiding pitfalls of digital radiography
Set-up order:
Pink = Positioning
Camels = Centring
Collect = Collimation
Extra = Exposure
Large = Labelling
Apples = Artefacts
Correct equipment set up:
Use a gird for body areas over 10cm thick
When using a cassette on a table top:
Start with cassette roughly in the middle of the table
Centre the primary beam in the middle of the cassette
Place middle of area of interest roughly at centre of primary beam
THEN just can make fine adjustments
Digital image formation: processing
Image info from the plate or detector is processed by the system software
The software contains different algorithms for different bodt areas
MUST select the correct body area before processing
Advantages
Wide latitude (dynamic range) – so many structures are clearly visible
Compensate for moderate degree of over- or under-exposure, so fewer repeat radiographs needed
Fast examination times
Can manipulate image post-processing
Easy storage and distribution of images
How to avoid faults:
High exposure factors
Cassettes, plates and processors need to be kept clean and well maintained for good results
Post-processing manipulation
Some transient manipulation can be done after processing, eg, brightness and contrast can be adjusted. If the image has detail missing, you cannot adjust it to find it. If some images are enhanced, some details may be supressed - so dont over do it
File types
Files are produced in DICOM format - these contain lots of diagnostic detail and have embedded patient info and image acquisition detail - important for quality control and legal reasons
DICOM should always be used for dianosis
Can export images to smaller file types like jpeg, but these contain less info and cannot be manipulated. These are useful for showing to clients or for publication - don't use for diagnosis.
Viewing types
Need to use good quality, high brightness/high resolution LCD screens
This applies to all monitors used for diagnosis, not just the acquisition unit
Some laptop screens, for example, are not suitable
Whatever screen is used, reducing background light will significantly improve viewing performance
Artefacts
Misleading appearances
Due to:
Incorrect exposure used
Errors in collimation
Poor maintenance of equipment
Must recognise and correct
After imaging:
Networking/file transfer
View images away from x-ray room
Must be able to transfer files to other practices
Storage and distribution of images
Must be stored securely and accessibly
PACS server often used
Physical or cloud based- and off-site back ups must be an option
Radiographic contrast media
What are they?
Substances admimistered to patient that are either more or less radiopaque than the surrounding tissue
Provide detail of organ size, shape, position, internal detail and sometimes function
Useful for hollow organs
Contrast media
Negative contrast media
Low physical density
Low radiographic opacity
Radiolucent appearance
Air, gases
Mostly used in bladder and GI tract
Can be used with positive contrast agents to give double contrast study
Advantages:
Cheap
Quick
Convenient
Relatively safe
Disadvantages
Poor mucosal detail if used alone
Air slowly eliminated from body
Theoretical risk of air embolus in blood stream, so CO2 can be used which is safer but it isn't used often
Positive contrast media
High atomic number
High radiographic opacity
Radiopaque appearance
Barium, meglumine diatrozoate, iohexol
Barium sulphate
Used for GI contrast studies
Administer as suspension, paste or mixed with food
Advantages
Low toxicity
Inert
Excellent mucosal detail
Possibly therapeutic
Relatively cheap
Disadvantages
Care with aspiration - NOT under GA
Irritant if enters body cavities - take care if suspected perforation
Water soluble iodine perparations
Imaging of cardiovascular system, urinary tract, joints, salivary glands, tear ducts, fistulas/sinuses, gastrointestinal tract, myelography
Types:
Ionic: suitable for IV or direct administration (but NOT myleography) e.g. meglumine diatrozoate
Non-ionic: suitable for myelography and any other use – e.g. iohexol. Recommended for all applications as fewer side effects.
Gastro-intestinal preparations – specifically for GI studies
Advantages
Versatile – can be injected IV or directly administered
Rapidly absorbed if leak into body cavities
Disadvantages
Hyperosmolar (ionic) unpleasant side effects if conscious (nausea, vomiting, etc.)
Irritant if injected perivascularly (ionic)
Large doses of iodine are toxic
Contra-indicated IV in hypovolaemia, hypotension and cardiac or severe renal failure (stabilise first)
Rarely may cause iodine-induced acute kidney injury
Ideal properties for contrast agents
Have different radiopacity from the tissue under examination
Accurately delineate the body part being examined
Be neither toxic nor irritant
Persist for the duration of the study
Be totally eliminated after the study
Be easily administered
Be cost effective
Can have adverse affects, e.g if injected into the wrong place in the body, it could cause problems.
Principles
Always take plain (survey) radiographs first:
Assess adequate technique
Contrast media contra-indicated?
May give a diagnosis
Assess patient preparation
Decide on suitable technique
Compare with subsequent films
Make enough images
Ensure adequate study
Contrast studies
Definitions:
Myelography (contrast radiography of spine – subarachnoid space)
Intravenous urography (IVU) or excretion urography (intravenous injection of contrast excreted by kidneys)
Cystography (contrast radiography of bladder)
Urethrography (urethra)
[Cardio-]Angiography (contrast radiography of [heart and]blood vessels)
Arthrography (joints)
Dacryocystography - lacrimal duct obstructions
Contrast medias used
Positive contrast cystography (infusion of positive contrast into bladder)
Pneumocystography (infusion of air into bladder)
Double contrast cystography (infusion of positive contrast followed by air into bladder)
Many contrast studies are superseded by other imaging techniques e.g MRI for spinal conditions or ultrasounds for cardiac disease
Reasonings for:
Alternative imaging techniques are not always available (location or cost)
The usefulness of these techniques is directly related to the skill of either the acquirer, the interpreter or both (e.g. ultrasonography)
MRI and CT are not feasible for most of large animal spines.
Sometimes contrast radiography gives better results
As a result, contrast radiography may be more appropriate
Ultrasonography
Reflection of sound at boundaries in the body
2D B-mode is most commonly used in practice
Modes
M mode
Motion mode
One line of the B mode image is plotted against time
Doppler ultrasound
Blood flow towards the transducer results in the reflected frequency being higher than the transmitted frequency
Blood flow away from the transducer results in the reflected frequency being lower than the transmitted frequency
This difference is called Doppler shift and its magnitude is related to the velocity of the blood cells by the doppler equation
2 dimensional colour flow CF
Superimposes colour over 2D B-mode or M mode
Colour in the sample volume represents: mean velocity and direction
Direction of flow: BART (Blue Away Red Towards
Continuous wave CW
Pulse wave PW
Displayed with velocity (Y axis) and time (x-axis) of the graph. Flow towards transducer is recorded above baseline, flow away from transducer is displayed below the baseline
CW doppler
All velocities in the path of the beam are measured
Cannot discriminate depth
Can measure high velocities
PW doppler
Allows evaluation of velocity within a specific area
Can determine depth from which signal originates
Physical limits to velocity that can be measured
Use in diagnosis:
Measurement of abnormal flow
Measurement of pressure gradients
Measurement of volume of flow
Ultrasound artefacts
Some artefacts are useful to indicate the nature of structures and cannot be avoided:
Examples:
Electrical noise artefacts
Small to medium sized non-uniform echoes on images
Usually in hypo-anechoic areas
Caused by electrical equipment
Enhanced by gain being too high
To resolve or prevent: turn of other electrical equipment, and reduce TGC
Acoustic shadowing
Anechoic shadowing deep to structure being imaged
Occurs in deep tissue interfaces with marked impedance differences
Caused by total reflection of sound waves at tissue interfaces with impedance differences
Acoustic enhancement
Structures displayed more clearly deep to fluid-filled organs, such as bladder
Caused by low attenuation of sound beam through anechoic structures
May cause area of apparent increased echogenicity
Can be useful to improve imaging
Reverberation artefacts
Two common appearances
Numerous parallel lies present of decreasing intensity as go deeper - caused by poor transducer contact (air between probe and skin- use more gel)
'Comet tail' or /Ring down' artefact - caused by reverberation between two highly reflective interfaces e.g between gas bubbles in GIT
To resolve: poor contact- use lots of coupling gel, ensure good contact with patient by clipping hair or use a smaller transducer
Mirror artefacts
Faint reversed image of structure/organ adjacent to the original, usually separated by a curved echogenetic structure
Occurs commonly at the diaphragm and may be mistaken as a rupture
To resolve: alter transducer angle and change the gain and TGC settings