FOOT IMAGING - WHAT HAS MRI TAUGHT US?
Michael Schramme
VetAgro Sup, Campus Vétérinaire de Lyon, Marcy L’Etoile, Rhône-Alpes, France
Introduction
New imaging modalities and advancements in older, commonly used modalities are
providing a more accurate diagnosis of lameness in equine patients. These new
techniques are increasing understanding of disease and leading to earlier
identification and intervention. Advanced imaging leads to a more specific diagnosis
and specific diagnoses should lead to new methods of treatment. Given the plethora
of techniques available however, the right choice of technique can be daunting. It
should consider the strengths and limitations of each one and the economic
circumstances of the patient.
Advantages of MRI over conventional imaging
Radiography has limitations when evaluating complex regions. Radiographs are twodimensional representations of three-dimensional objects. Multiple soft tissue and
bone structures are superimposed on each other. Radiography provides little
information about internal or recessed anatomy such as intra-articular ligaments.
Radiography relies on recognition of disrupted or altered contours, size or shape
changes and abnormal radiopacity. Some of these signs are best seen when the Xray beam is tangential or parallel to the margins of a structure, producing a profile
with minimum superimposition. Radiopacity changes may not be visible until mineral
loss is advanced (>30%).
Diagnostic ultrasonography also has many limitations. Deep structures situated
beneath bone, gas or large vascular structures are poorly visualized. Sonography is
a dynamic evaluation process. It is difficult to impossible to infer the context and
relation of the structure of interest to the surrounding anatomy from still images.
Sonography can only evaluate the surface of bone structures.
MRI does not use ionizing radiation to produce medical images. It produces a greyscale image of hydrogen protons in tissues, based on the measurable energy
release when protons alter their orientation in a large magnetic field. The higher the
amount of free water and the higher the rotational freedom of hydrogen protons in a
tissue, the higher the signal strength generated by that structure. Therefore MRI has
high intrinsic contrast and resolution, resulting in good anatomic separation between
different tissues (tendon, ligament, cartilage, bone, joint fluid). In particular the
superior soft tissue contrast and anatomical detail have high diagnostic value. Next
to anatomic information, MRI also displays information that is primarily physiologic or
pathophysiologic. As a cross-sectional imaging modality, MRI is able to scan an
object in any plane.
The main disadvantages of MRI are its cost (installation and running costs), its still
limited availability and the need for specialist training. As there is a bewildering
Proceedings
of
the
South
African
Equine
Veterinary
Association
Congress
2016
224