Imaging modalities =
MRI = magnetic resonance imaging
CT = computed tomography
Scinitagraphy
MRI - Magnetic resonance imaging
- A cross sectional imaging technique
- uses a strong magnet and radio waves
images are slices through the body at different planes
MRI continued
Relys on magnets and radio waves - will see different information depending on the pulse sequence used
- Detection and analysis of the radio signals emitted by the patient gives information about the chemical composition of the patient
- Appearance of tissues will vary depending on timing of ‘pulses’ (inputs of radio waves) and ‘echoes’ (signals from tissue)
MRI continued
computer will reconstruct the informations received into slices
images can be acquired in any orientation in the body
Different ‘sequences’ provide information on tissue type
Advantages of MRI
- Very good anatomical detail, particularly of soft tissues
- Does not use ionising radiation
- will be safe to patient and operator of equiptment
- good for imaging areas that are not very accessible to other techniques
- Particularly useful for brain and spinal cord
- Useful for accurate assessments of extent of speed of a lesion e.g. retrobulbar lesions, nasal lesions, pelvic masses
Disadvantages of MRI
- Not widely available in practice
- Expensive (cost f equipments and installation, maintenance, time to do)
- Animals need to be perfectly still so usually under GA, adds to risks and costs
- Due to powerful metallic field there must be no metallic objects around
MRI safety
- Is a large permeant magnet so metal objects should not be taken into MRI
- will need to use MRI compatible tablets, oxygen cylinders, anaesthetic machinery and monotoring equipment
- need to take care with implants
- pace makers are a NO
what will happen to a microchip in a MRI?
No damage to the animal
The location of the chip may just cause some distortion in the image
Standing MRI
Usually for horses
Used to image distal limbs- only practical for this
A ‘C’ shaped magnet is positioned around the leg whilst images are acquired
Gives good anatomical detail
Will need software to correct for motion during image acquisition - as sedated horse will likely be wobbly
CT - Computed tomography
A cross sectional imaging technique
Uses ionising radiation
Patient is placed inside a ring containing x-ray detectors
A x-ray source is rotated round the patient and the ‘image’ is picked up on the detectors on the other side
CT continued
A computer analyses information for all detectors and forms an ‘image’ cross sectional slices
Patient only needs to be scanned once
The representation of tissues is the same as radiography e.g. bone = white, air = black
CT ‘windowing’
= Reconstructing the image with different grey scales to highlight different bits
Displays information to optimise certain tissues
- is computer manipulation
Uses of CT
- Gives detailed anatomical structures - good for bone e.g. elbows
- Can still be used for soft tissue but will be less detailed in these areas as MRI
- Scanning time is shorted that MRI
- Availability is increasing
CT of elbows
Can take a transverse plane image which will avoid superimposition that would otherwise happen on a radiograph
will be able to see coronoid process
CT of nasal/ aural lesions
CT is more sensitive for detections of changes to the nasal turbinates and bullae
CT of lymph nodes
Lyphnodes are routinely seen on CT
- On radiographs they are only visible is they are enlarged
CT of lung masses and metastases
CT is better and more sensitive for detection of lug metastases - will detected them earlier than a radiograph
3D CT reconstruction
reconstruction of many CT places to 3D model - can be rotated and viewed in all aspects
possible to 3D print bones from this data
Facilitates surgical planning
Disadvantages of CT
- Not widely available in practice
- Expensive compared to radiography
- Scan time is several seconds - quicker than MRI but slower than x-ray
- Uses ionising radiation - has higher patient doses than radiography
- Animal would need deep sedation or GA
Scintigraphy
Uses a radioisotope - commonly technetium 99^m
is a gamma ray isotope
This will be bound to another substance that will determine where in the body it will localise
e.g. hydroxymethylene diphoshonate (HDP) which binds to bone
increased binding with increased metabolism - so increased metabolism of bone will mean that more will bind there
How does scintigraphy work?
The bound isotope is injected into the patient, usually by IV
Time is allowed for the isotope to reach the tissue of interest
A gamma camera is used to detect the pattern of gamma radiation emitted from the patient
Main veterinary uses of scintigraphy
- Skeletal injury in horses, especially ones difficult to radiography e.g. pelvis, spine
- LA and SA where it is difficult to localise a source of lameness
- For ectopic thyroid tissue (thyroid tissue further down neck or in thorax) in hyperthyroid cats
Advantages of scintigraphy
- Increasing in availability in large equine centres
- Has an element of functional assessment as the uptake of the bound isotope depends on metabolism so can assess the function and not just the structure
Disadvantages of scintigraphy
- Uses ionising radiation- risks to patient and operator
- patient will remain radioactive for sometime afterwards (48hrs)
- Need to minimise contact time with patient
- need to consider disposal of bedding and feces as will be radioactive
- Poor anatomical detail so can be difficult to interpret images - but can see different density
- Additional legislation apply: environmental agency due to disposal of isotopes
