Photo: Rood & Riddle Equine Hospital

Magnetic resonance imaging (MRI) might offer evidence of laminitic changes in a horse's hoof before the disease is otherwise identified. Equine radiologist and consultant Alexia McKnight, DVM, Dipl. ACVR, of McKnight Insight, in Chadds Ford, Pa., shared her anecdotal experience identifying laminitic changes via MRI during her presentation "Equine MRI Basics--What a Horse Owner Needs to Know" at the 2012 International Equine Conference on Laminitis and Diseases of the Equine Foot, held Nov. 2-3 in Monterey, Calif.

"Always expect the unexpected when you're doing MRI," McKnight said, adding that most of what we know about laminitis and MRI is through investigative research on a few clinical cases and from lameness cases not initially associated with the disease.

MRI has proven itself as the "gold standard" for diagnosing neurologic and musculoskeletal issues in people, McKnight told the group of horse owners and practitioners. In equine medicine, she explained, MRI has also gained popularity and become more accessible over the past 15 years. With greater availability, dedicated equine MRI systems, and improved MRI unit design (i.e., more open designs and standing MRI options for horses), veterinarians now use this technology frequently to investigate lower-limb lamenesses as well as the head (including the brain and sinuses).

An MRI study results in highly detailed multiplane and multislice cross-sectional images. The body, both human and horse, is abundantly made of hydrogen atoms in water and fat that are constantly spinning within and between cells. Within the presence of the magnetic field, MRI works to polarize those hydrogen atoms during the imaging process, allowing anatomic information to be decoded, McKnight explained. Complex algorithms crunch the captured data into two-dimensional images that show structures and defects often indiscernible with other diagnostic imaging modalities such as radiography (X ray), ultrasound, and computed tomography (CT) scans.

Radiography and CT scans, for example, best show bone, while ultrasound can produce exquisite resolution of accessible soft-tissue structures, such as many tendons and ligaments. MRI evaluates both bone and soft tissues with superior contrast and anatomic detail, giving practitioners and radiologists an unprecedented view of what's going on inside the leg and foot and making irregularities easier to identify.

Practitioners turn to MRI because it has proven itself in pinpointing lameness causes when other diagnostic imaging options have not, said McKnight.

While MRI is especially useful in identifying bony, soft tissue, and articular changes in the lower leg, it's not typically used to diagnose laminitis. However, while reviewing MRI images for other lameness issues and comparing those findings to horses with known laminitis, McKnight started seeing structural changes indicative of early or mild laminitis. In her experience, McKnight said, "MRI can help identify laminitis and risk factors before the horse knows he has it."

She said those clues can include various combinations of:

  • Laminar stretching;
  • Loss of corium distinction (deep, inner layer of the sensitive hoof wall);
  • Coffin bone reactions;
  • Signal and architectural changes of the dorsal hoof wall;
  • Signal and architectural changes of the sensitive sole;
  • Hoof wall defects;
  • Fluid pockets and tracts within the hoof wall ("The foot is oozing exudate," McKnight said.);
  • Abscesses; and
  • Vascular abnormalities (predominantly using an additional MR venography technique).

Based on past experience, McKnight said she can look back and appreciate the subtleties of some of these laminitis indicators in cases she's reviewed during the past seven years. "In addition, in some horses that present with known laminitis for MRI assessment, the changes found seem to be helpful in prognosticating the clinical outcome," McKnight concluded.