46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
157
magnetic field variation). T2 weighted sequences are good for demonstrating fluid in
tissue and hence give good information about pathology within the region being
studied. STIR sequences suppress the signal coming from fat, which allows the
differentiation of fluid (such as oedema) from fat. This is particularly important when
evaluating bone, and STIRs help to differentiate between normal fat within the bone
and pathologic fluid/oedema. The time to complete a STIR sequence is prolonged
relative to the T1 and T2 weighted sequences, and this can result in motion artifact
in the standing animal. STIRS are not very good at looking at anatomic detail for this
reason.
The strength of a magnetic field is measured in Teslas; the higher the Tesla, the
faster the acquisition of an image, the better the image quality, and the more
expensive the equipment. Most MR units used in veterinary medicine have a Tesla
range between 0.2 and 1.5T. There are two main types of magnets. Closed magnets
generally have a relatively large magnet (1T or more), use a gantry, and require
general anaesthesia to acquire the images. The size of the gantry is a limiting factor,
and in adult horses imaging is usually confined to the carpus/hock and distal, and the
head and cranial neck. In many foals, imaging can also be performed on the caudal
vertebrae, thorax and abdomen. There are also open units available, such as the
Hallmarq system. These magnets generally have a lower strength, in the magnitude
of 0.3T.1, 2
The MRI unit I have experience within Australia (located at the University Veterinary
Teaching Hospital Camden (UVTHC) is a Hallmarq system. One of the principle
advantages of this system is it allows imaging of the standing horse under sedation,
negating the need for general anaesthesia. Another advantage is a significant lower
purchase price and lower maintenance costs. There are some disadvantages to this
system, however. Image quality is not as good if compared to a closed, high Tesla
magnet. Motion artifact can be a problem, especially with an uncooperative patient
or imaging above the pastern region. Recent upgrades to motion correction
software have made this less of an issue. Motion artifact is intensified as you move up
the leg, and in general it is more difficult to obtain good quality images of the carpus
or hock than the foot, for example. The coil size available for the Hallmarq system
limits the MR examination in the adult horse to the carpus/hock and distal. In
addition, precise localization of the lesion to a specific region is important to limit
acquisition of MR images to a specific area, thus keeping the time required for
imaging reasonable.
Interpretation of MR images is much more complicated than interpreting radiographs
or ultrasound.
Different imaging sequences are used to help evaluate normal
structure and to document pathology. Further techniques can be used to help
determine whether the pathology is acute or chronic. This can complicate
interpretation, as structures such as tendons, synovial fluid and bone can appear
different depending on the sequence (contrast technique) run. In addition, there are
a complex array of “artifacts” that can be produced on MR images, including those
produced by recent diagnostic anaesthesia.3 In my opinion, having an imaging
specialist review these images is essential to get the most information from the
study.
There are many indications for MR imaging in horses. MRI should be considered in
157