choice of pulse sequences of which the nomenclature varies between
manufacturers, a lack of uniformity or consensus exists regarding the most
appropriate sequences to use. Image quality can be influenced by many different
parameters, including time, signal-to-noise ratio, size of the object, slice thickness,
field-of-view and other imaging specs. In addition the technique gives rise to a
number of previously unknown imaging artefacts that may mimic the presence of a
lesion or render a scan non diagnostic. It is important to know how signal
characteristics are influenced by these factors, so that the operator can assure
image quality, especially in view of the high expense of MRI. As MRI is unlike any
other imaging modality, can produce intricate artefacts and imaging acquisition
involves some complicated physics, interpretation of images does not come naturally
but requires dedicated training with a fairly steep learning curve. As part of this
learning curve a thorough familiarity must be developed with normal MRI anatomy
and the limits of normal variation. The next step of the learning process is to validate
signal abnormalities with histopathological information. High and low field systems
are both in clinical use in equine practice, but image resolution and detail depend
directly on the strength of the magnetic field, which can result in a large difference in
image quality between the different systems in use. Patients have to be kept under
general anaesthesia for examinations in a high field magnet. Appropriate sedation
and dedicated software is required for low field MR examinations in order to
eliminate the interference of patient movement.
The foot
In view of the complicated anatomy of the foot, including the deep location of all
tendoligamentous and osteo-articular structures inside the inaccessible hoof capsule,
it is not surprising that the main surge in new information from MRI has occurred
here. MRI has identified previously undocumented causes of foot lameness and new
discoveries continue to be made. MRI has radically changed the list of differential
diagnoses of foot lameness, especially with regard to injuries of the DDFT, the impar
ligament, the collateral ligaments of the distal interphalangeal joint, the spongiosa of
the navicular bone, hyaline and fibrocartilage of the distal interphalangeal joint and
navicular bone, and subchondral bone of the middle and distal phalanges. The
current relative incidence of these injuries can be learned from recent MR reports.
With the identification of new injury types and improved diagnosis of the correct
cause of lameness, new steps are being taken in the validation of efficacy of existing
treatment modalities and the development of new approaches
Collateral Ligament Injury
Lesions may be identified ultrasonographically, but only a limited length of the
ligament can be assessed. Ultrasonographic assessment revealed lesions consistent
with desmitis characterized by enlargement of the ligament, and areas of reduced
echogenicity were identified in 85 of 313 horses (27%), with lesions confirmed with
the use of MRI (Dakin et al. 2009).
Radiographically osseous cyst-like lesions can sometimes be appreciated at the
insertion of the ligament to the distal phalanx, but this is rare. In addition, various
degrees of mineralization of the ungular cartilages have been associated with the
presence of CL injury. It has previously been suggested that forces mediated by
ligamentous attachments to the cartilages may be transmitted differently through a
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
225