Advanced Equine Imaging Flashcards
What are the Equine imaging options?
- Radiography
- US - MRI
- CT
- Scintigraphy
What different MRI options ?
- ‘Low field’ MRI ->
- Distal limb under standing sedation
- Open bore magnet
‘High field’ MRI
- Heads (& limbs) under GA
- Closed bore magnet
Describe how MRI works?
- Cross sectioning imaging (slice by slice)-> Frontal, Sagittal, Transverse
- Grayscale images
- Uses strong magnetic fields and radio waves to produce images
- Pathological processes change magnetic properties of tissues
When do we use MSK MRI?
- Lameness is localised to a specific region, but the source of lamaness is inconclusive on radiography or us
- Penetrating foot injuries
- Standing MRI where GA is inadvisable
- Racehorses in trainign
- Rehab guidance (ready to return to work)
Why do we want to localise the lesion with lameness BEFORE MRI ?
Because MRI expensive and takes longer compared to Xray/US so streamline process to know exactly WHAT to scan
Lameness that resolves with abaXial sesamoid block - where is the lesion here
Tx for above case?
- Box rest – months
- NSAIDs
- Gradual return to exercise, starting with in-hand walking
- Remedial farriery
- Weight management
- Intralesional therapies may be possible - see chronic lameness lecture
=> rescan in 6months
How does CT work?
- CT uses a rotating x-ray to produce stack of slice images
- no superimposition
- using a contrast medium can be helpful in horses with neck problems to detect poss sites of compression of the spinal cord
What indications for CT?
- MSK dx
- Dental & sinus issues
- headshaking - brain lesions
Detail use of CT for MSK specifically?
- Joint and cartilage disease - very sensitive modality for cartilage disease
- More sensitive to bony changes compared to radiography
- Evaluation of lesion relevance (vs. incidental findings)
- Surgery planning (especially fracture repair)
How to interpret CT?
- hyper(white) or hypo(black) attenuation
- Subjective or objective interpretation of tissue attenuation (quantified with HOusefield units)
- Iodinated contrast can be ised
Why do we use contrast on CT?
- This accumulates in blood vessels and within tissues with increased vascularity
- Improves soft tissue contrast
- Neoplastic and inflammatory lesions are hyperattenuating
- Necrotic tissue and abscesses are hypoattenuation
how to image for equine cervical spine dx?
- Laterolateral radiographs have limited diagnostic use
because the APJs are superimposed on each other - Oblique radiographs can be obtained, but they are
challenging in practice! - Compression on the spine? – need CT +/- myelography
Describe Myelography CT?
- Myelography is required to definitively diagnose focal spinal cord compression
- Iohexol contrast medium is injected into the subarachnoid space
- Dynamic and static compression can be diagnosed
How does scintigraphy work?
- Injection of a radiotracer drug intravenously
-Diphosphonate (DP) compounded with radioactive technetium (Tc)- Diphosphonate (DP) binds to exposed hydroxyapatite crystals in bone
- Technetium emits gamma rays, which are detected by a gamma camera
- Image acquisition 2-2.5h after injection, under standing sedation
What should be done before injecting for scitigraphy?
Preventing “cold limb syndrome”: before the injection the horse is lunged
(unless contraindicated) and their legs are bandaged to improve blood flow
and distribution of Tc around the body
How should we manage radioactive horse after this?
The horse is radioactive (particularly its blood and urine) – they require
isolation for some period of time. You can’t turn the radiation off!
What factors influence uptake of DP/Tc ?
How should we assess inc/dec radiopharmaceutical uptake (IRU)
- Mild/moderate/marked
- Focal/diffuse/linear
- Cortical/subchondral
- Location – name of bone (diaphyseal, metaphyseal, epiphyseal)
how to interpret scintigrpahy?
- Compare to contralateral side (but remember could be a bilateral pathology)
- Orthogonal views are required to fully interpret
- IRU is not necessarily synonymous with pain causing lameness
- Need cross-reference with diagnostic analgesia and imaging
what sites would have IRU that are not clinically significant
Any other IRU can be seen ?
When do we use scintigraphy?
- Clinical signs suggestive of stress related bone injury without specific localising clinical signs
- Inability to localise pain using diagnostic analgesia e.g., intermittent lameness, or multi limb pain
- Dangerous behaviour of horse – can’t perform diagnostic analgesia safely
- Unexplained poor performance
- Suspected thoracolumbar or pelvic region pain
who are not good candidates for scintigraphy?
– in sports horses (non-racehorses) – scintigraphy has poor sensitivity. So, it is not a good
indiscriminate screening test (Quiney et al, 2018).
* Ideally scintigraphy should be combined with:
* Diagnostic analgesia – are the areas of IRU clinically significant for the horse?
* Further imaging – what pathology does the IRU represent?
List KEY points about scintigrapy?
When do we use scintigraphy?
- Scintigraphy is the most sensitive
diagnostic modality for detecting
STRESS FRACTURES in racehorses. - Typically, in locations that are
difficult/impossible to block with
local analgesia. - Early detection reduces the risk of
catastrophic fractures
What is a PET scan (Positron emission tomography)?
- Another nuclear medicine
imaging modality - Again, functional rather than
anatomical information - 3D imaging
What do we use PET scan for?
Distal limb, especially the racehorse fetlock – detecting pre-clinical changes that might
predispose racehorses to catastrophic breakdown injury
“Bone scan plus” PET?
- Better spatial resolution [characterise lesions better]
- Cross-sectional information [no superimposition]
PET with CT or MRI?
- Early lesion detection e.g., look at the subchondral bone (before apparent on
plain CT or MRI) - Clinical significance of findings e.g., enthesis (e.g., proximal suspensory
enthesis – determine active vs inactive)
