23 Diagnostic Imaging for Equine Lameness

• Diagnostic imaging is important in decision-making for equine lameness and surgery
• This includes relating imaging findings to the clinical picture and using advanced imaging in severe cases

Equine Athlete Expectations

• Equine athletes are highly specialized and varied

• They are highly competitive, requiring soundness

• Subtle lameness can have a significant impact on performance

• Regardless of the method, a diagnosis is always the goal

• Signalment, History, and Conformation are important

Diagnostic Imaging Modalities

• Radiography and Ultrasonography are modalities
• Nuclear Scintigraphy and Computed Tomography (CT) are modalities
• Magnetic Resonance Imaging (MRI), Thermography, and Arthroscopy are modalities

Limitations of Imaging

• Horses bear weight differently than how they are positioned for X-rays
• This can lead to clinically silent findings such as false positives, which may or may not be relevant
• False negatives may occur due to limitations with the modality, errors in acquisition, or errors in interpretation

Radiography

• Radiography is often the first line of imaging used in equine practice
• High-quality digital portable machines are available
• Powerful, track-mounted overhead machines can be used
• Computed Radiography (CR) and film radiography are still useful
• Distal limb imaging for lameness diagnosis often involves standard views
• Axial skeleton radiography includes cervical and thoracolumbar regions
• Head radiography includes dental and airway assessments

Radiograph Technologies

• Film is becoming outdated due to processing and supply constraints
  • Pro: Slow start-up costs
  • Con: Requires appropriate kVp and mAs settings, physical film storage and is limited to one shot per film
• CR is a viable digital option for low-volume mixed animal practices
  • Pro: Plates are less expensive to replace with most processing advantages of DR
  • Con: One shot per plate is allowed until erased, which is slow
• DR is the current industry standard and continually improving
  • Pro: Allows for post-acquisition image processing with fast unlimited viewing and retake potential
  • Con: Systems can be expensive and have potential technology malfunctions

Radiography: The Good, the Bad, and the Ugly

• Considerations include Film vs CR vs DR
• Radiography is best for calcified tissue, sensitive to changes in bone, greater than 2mm
• It is less sensitive for changes smaller than 1mm, and poor for evaluating soft tissue
• Radiography involves ionizing radiation, provides a 2D image of a 3D object and may be delayed, depending on disease

Ultrasound

• Ultrasound is common modality for equine practice
• The modality is user-dependent-imaging capabilities may be limited
• Often thought of as a second line imaging modality, used following radiographs
• It can also be the first line if indicated or when used in combination
• It is highly portable with high-quality machines
• Some units are cell phone/tablet-linked and cloud-based
• Other units are laptop-sized or large cart-based
• It can be used to diagnosis any structure with sound transmission
• Can also be used for US-guided injections and surgical planning

Physics of Ultrasound

• Ultrasound beams are perpendicular to the probe for best pickup
• If it is not perpendicular, tissues will appear darker than normal, termed "off-incidence"
• "Off-incidence" is common when imaging tendons/ligaments, can also be harnessed to be a benefit
• This problem is addressed by slowly rocking the probe at different angles while scanning

Image Acquisition

• Patient Preparation & positioning is crucial
• Clip hair with a #40 blade
• Clean skin using alcohol and/or scrub
• Apply acoustic gel
• Use a stand-off pad
  • Gel pad fits over the probe to increase distance from the skin
  • It can reduce artefacts and create a contact shadowing artefact
  • It may improve contact for tendon imaging and compressability

Metacarpal/Metatarsal Ultrasound

• Most common region evaluated
• Includes transverse and longitudinal views
• Evaluation covers the SDFT, DDFT, AL-DDFT and the Suspensory ligament
• Suspensory ligament evaluation includes: Origin, Body, Branches
• Imaging zones include measurements from landmarks such as, Accessory carpal bone or head of 4th metatarsal
• Zones average 4cm per zone, labeled 1A, 1B, 2A, 2B, 3A, 3B, 3C

Pastern Ultrasound

• Scanned abaxially on the margins of the SDFT from ZnP1A-C

Stifle Ultrasound

• Collateral ligament is identified

Ultrasound: The Good, the Bad, and the Ugly

• Good: Inexpensive, portable, non-invasive, cross-sectional soft tissue evaluation
• Bad: Underestimates injury site, especially in mild injuries
• Ugly: Easy to introduce artefacts and is highly operator dependent

Nuclear Medicine

• Injected intravenously is a radiopharmaceutical (Technetium-99m)
• It emits gamma rays as it decays
• Tc-99 is bound to a carrier, often localizing to specific tissues
• For horses, this is most commonly hydroxyapatite in bone or metabolically active bone
• Gamma rays are detected by a gamma camera
• The scan is based on exposure time (expressed in counts)
• Images are displayed showing intensity on a “map” of the skeleton
• Most useful for non-displaced fractures and bone remodeling, AKA the "Bone Scan"
• High-level, high-volume equine practices perform nuclear medicine, as radiation regulations and cost make it impractical

PET Scans

• Positron Emission Tomography scans include:
  • Sodium Fluoride: Bone tracer
  • FDG (flurodeoxyglucose): Soft tissue tracer
• The principle is the same as T-99 based nuclear medicine
• PET scans have lower radiation exposure and faster tracer elimination
• They often involve 3D imaging of limbs, as opposed to the 2D of T-99
• Results can be overlaid with MRI or CT
• PET scans can be used for early detection before things are visible
• Can differentiate clinically irrelevant lesions and monitor healing/rehab of active lesions
• They are experimental, therefore, use carries potential for false positives

Nuclear Medicine: The Good, the Bad, and the Ugly

• Nuclear medicine is very sensitive and functional for metabolically active bone conditions
• It is expensive, personnel-intensive, associated with radiation exposure, and requires isolation for 1-2 days
• It is insensitive for “cold” conditions and has less resolution

Computed Tomography (CT)

• Uses x-rays to create cross-sectional slices of anatomy
• Improved contrast resolution vs radiographs
• CT is the modality of choice for assessing bone. MRI is preffered for soft tissues
• Very useful for surgical planning or evaluating complex anatomy
• Limited availability
• Need for general anesthesia
• Standing CT machines for horses do exist with cone beam imaging

CT Indications

• Has surgical planning indications:
  • Fractures
  • Osteomyelitis
  • Sinus Surgery
• Can be used to evaluate complex anatomy
  • Equine skull
  • Sinuses
  • Teeth
  • Cervical Spine
  • Appendicular skeleton

CT Contrast Studies

• IV contrast given to evaluate tissue perfusion
• Inflammation or neoplasia will typically contrast enhance
• A time delay is often needed, which allows for perfusion
• A jugular or other peripheral vessel is often used, with the median artery used in limbs
• Angiography & fistulography can also be used

CT: The Good, the Bad, and the Ugly

• CT provides excellent bone detail, cross-sectional imaging, and limited expense
  • Can fit foals inside
  • One scan takes 2 minutes, with total procedure taking 20-45 minutes
• It is generally limited to the head and distal limbs in adults and typically needs general anesthesia
  • Standing CT machines are currently manufactured and accessible, and are changing CT use in horses
  • CT has poor soft tissue detail, limited availability, and a cost that some clients may not be able to afford

Arthroscopy

• Useful to directly visualize some lesions not easily seen on imaging modalities
• Helpful in scenarios with: Cartilage damage, Stifle injuries (although still limited), Degree of joint contamination
• Good: can directly image & address all in one procedure
• Bad: access to some joint regions is limited

Thermography

• Uses infrared thermal detectors to assess surface temperatures
• Very sensitive to differences in temperature
• It is rapid, portable, and safe
• Also affected by environment and air movement
• Costly equipment for professional level cameras
• Its role has not yet been found in mainstream equine imaging
• It is useful for certain conditions: Laminitis/Cellulitis, Back pain/saddle fit, Acute soft tissue & bone injury

Magnetic Resonance Imaging (MRI)

• Gold standard for musculoskeletal imaging
• Best likelihood to get a diagnosis, often when conventional imaging fails
• Some indication for use as a primary imaging modality (ie. foot lameness)
  • Must have the lesion localized
• Scan quality varies with magnet strength
  • 0.3 tesla (T) vs 1 T vs 3T vs 7T
• Excellent for soft tissues, multi-planar imaging, early damage, and inflammation

MRI Field Strength

• High field >1.0 T (1, 1.5, 3 T common)
  • General anesthesia is required
  • Better resolution = better images = improved diagnostic accuracy
• Low field (typically 0.3 T)
  • Lower cost and more accessible
  • May use standing sedation for patient compliance, though leads to a lower resolution

MRI Slice Planes

• Standard MRI slice planes
  • Transverse (Axial)
  • Sagittal
  • Dorsal (Coronal or Frontal)
• Any imaging plane is possible

MRI Sequences

• Each tissue has different amounts of protons & their own uniques T1, T2, & T2* relaxation properties
• Imaging sequences harness these differences to get planned, specific tissue appearances

T1 Weighted Spin Echo

• Good anatomic detail, esp. bone trabecula, with good resolution
• High Signal shows as (bright/white) for medullary cavity or fat & other tissues
• Low Signal shows as (dark/grey or black) for Synovial fluid, Tendons, Ligaments, Cortical Bone
• Allows the use of IV contrast imaging using Gadolinium

Proton Density

• Best at providing Anatomic detail
• High Signal shows as (bright/white) for Synovial fluid, Medullary cavity or Fat & other tissues
• Low Signal shows as (dark/grey or black) for Tendons, Ligaments, Cortical Bone

T2 Weighted Spin Echo

• Part of duel echo
  • Part one: Proton density
  • Part two: T2-weighted spin echo
• Low magnetic susceptibility
  • No magic angle artifacts
  • Lower signal = lower anatomic detail
• High Signal (bright/white)
  • Synovial fluid
  • Medullary cavity
• Low Signal (dark/grey or black
  • Tendons
  • Ligaments
  • Cortical Bone

Inversion Recovery (STIR)

• Suppresses fat signal, which is Important in marrow cavity
• High Signal (bright/white): Synovial fluid, Inflammatory fluid, Vasculature
• Low Signal (dark/grey or black0: Tendons, Ligaments, Cortical Bone, Medullary cavity

3D Gradient Echo

• Volume acquisition for this modality
  • Thinner slices
  • 3D reconstruct possible
• Clinical Application includes:
  • Fast imaging
  • Higher resolution is common for some structures
• The Drawbacks are:
  • Poor soft tissue resolution with High magnetic susceptibility
• Can be T1 or T2* weighted

Ultra Short Time Echo MRI

• Class of special sequence to image dark structures
  • Tendon
  • Cortical bone
  • Cartilage (calcified cartilage zone)

MRI Safety

• No ionizing radiation
• Ferromagnetic materials: pose a ballistic danger to everyone.
  • Pure Al, Ti, & Cu generally are safe All other metals: Need to tested & labeled to be MRI safe Credit cards: Can be erased
• Burns
  • Conductive materials in the magnet bore can heat & cause burns
  • ECG wires are the biggest area of concern

Body Parts Imaged with MRI

• Foot
• Pastern
• Fetlock
• Metacarpus/Metatarsus
• Carpus
• Tarsus
• Distal Radius/Tibia
• Head & cranial cervical vertebrae

MRI: The Good, the Bad, the Ugly

• It is good to use highly detailed multi-planar imaging of bone, fluid, & soft tissues
• It is limited by: horse body size & magnet availability, time consuming (90 min/scan), costly, and variable quality dependent on field
• The ugly: safety lapses can be deadly and equipment is costly

Choosing Imaging Modalities for Equine Lameness

• Client relationship, expectations, & financial constraints must be navigated
  • Including: Their values & history, Performance expectations, Value of horse vs their budget
• Thorough history guides decision making
• Clinical exam and clinical intuition guides modality selection
• No significant findings is not a bad outcome
  • More information is never bad, and it shortens the differential list