Page 431 - Adams and Stashak's Lameness in Horses, 7th Edition

Diagnostic Imaging 397

Since cartilage has limited ability to repair itself, it is capsule in lameness of the foot. Ultrasonographic
useful to determine its biochemical composition early findings may be equivocal in lameness associated with
VetBooks.ir occurring osteoarthritis significantly affects cartilage Radiography is incapable of detecting subtle or early
the palmar/plantar soft tissues of the distal limb.
on in clinical disease. One study showed that naturally
relaxation times as determined by dGEMRIC in the DIP
cartilage or subchondral bone abnormalities in joint
joints of horses. 15 lameness. Radiographic abnormalities are also absent in
many forms of osseous trauma (e.g. bone contusions,
bone edema, occult subchondral bone loss). Although
HOW TO READ AN EQUINE MRI STUDY MRI is the only imaging modality that can assess all tis­
sues in a single examination, the availability of MRI
A standard MRI protocol produces hundreds of should not result in the omission of radiographic and
high‐detail images, each of which may contain impor­ ultrasonographic examinations. MRI should not replace
tant information. In order to turn this information into but rather complement radiographic and ultrasono­
a useful diagnosis, a methodical and consistent graphic findings. Information obtained from radio­
approach should be used to reading the MRI study. graphic, ultrasonographic, and scintigraphic examinations
Any suspect abnormality should be defined by signal helps in the interpretation of MRI findings and a full set
intensity, size, shape, and contour in comparison with of diagnostic images always provides a better basis for
normal. Throughout the evaluation process the clini­ an accurate diagnosis. Radiography in particular has
cian should cross reference any suspected signal abnor­ better bone versus soft tissue contrast when compared
mality in two ways, i.e. by anatomical cross referencing with MRI and is therefore more sensitive to subtle bone
and by contrast cross referencing. Anatomical referenc­ contour changes like osteophytes, enthesophytes, and
ing is performed by comparing the appearance of a sus­ small osteochondral fragments. Ultrasonography gives a
pected lesion with its appearance in other image planes better representation of fiber pattern in tendons and
of the same contrast weighting. Contrast cross refer­ ligaments and is not plagued by magic angle and flow
encing refers to the process of comparing the signal artifacts that occur in structures like the suspensory
intensity of a suspected lesion in a PD or T1‐weighted ligament branches or distal sesamoidean ligaments,
image with its signal intensity in the corresponding T2‐ especially during standing MRI.
weighted and fat‐suppressed images. As a general rule, There are numerous advantages of MRI over other
an abnormality should be identifiable in at least two imaging modalities. MRI does not use ionizing radiation.
different imaging planes and two different contrast It has high intrinsic contrast and resolution, particularly
weightings for it to be considered a “true” lesion. If an for soft tissues, resulting in good anatomic separation
abnormality can only be seen in one pulse sequence in between different tissues. Next to anatomic information,
one orientation, then there is a high likelihood that the MRI also displays information that is pathophysiologic.
finding is an artifact. As a 3D cross‐sectional imaging modality, MRI is able to
Frequently more than one “true” lesion is identified scan an object in any image plane.
during an MRI examination, causing a diagnostic The main disadvantages of MRI are its cost (installa­
dilemma, as not all apparent abnormalities are equally tion and running costs), its still limited availability, its
associated with pain and lameness and “normal” signal limited accessibility mostly restricted to comfortable
variation may occur. The clinician needs to decide on the examination of the distal limbs for standing magnets, its
likely hierarchy of clinical significance of lesions encoun­ poor suitability as a screening technique (unlike CT), the
tered, in light of the clinical exam findings and current need for general anesthesia with high‐field magnets, the
knowledge of which MRI lesions are most common. relatively lower tissue signal and interference of patient
movement with low‐field magnets, and the need for ded­
icated specialist training. Image quality can be influ­
INDICATIONS, CASE SELECTION, ADVANTAGES, enced by many different parameters, including time,
AND DISADVANTAGES OF MRI signal‐to‐noise ratio, size of the object of interest, slice
thickness, field of view, and other imaging specifica­
MRI is indicated when a lameness problem has been tions. In addition, MRI gives rise to a number of unfa­
localized to an anatomical area and other imaging miliar imaging artifacts that may mimic the presence of
modalities have failed to provide an unequivocal diag­ lesions or render a scan nondiagnostic. It is important to
nosis. Lameness should first be localized to an anatomi­ know how signal characteristics are influenced by all of
cal area because unlike nuclear scintigraphy, MRI is not the abovementioned parameters, so that the clinician
a screening technique. Accurate knowledge of the locali­ can assure high image quality. The large number of
zation of the cause of lameness, as well as the pitfalls images generated with each study also makes interpretation
encountered with diagnostic analgesia is indispensable time consuming.
when interpreting MR images. These rules apply, even in
the face of newer, more powerful 3 T magnets with faster
scanning times now allowing routine screening of the MAGNETIC RESONANCE IMAGING OF THE FOOT
foot, pastern, and fetlock regions in horses with distal AND PASTERN
limb lameness with only short general anesthesia times.
MRI is particularly useful in anatomical areas where Introduction
conventional imaging modalities have limitations, like MRI has become the gold standard of diagnosis in
the foot, the palmar/plantar soft tissues, and the joints of horses with foot lameness, because of its higher sensitiv­
the distal limbs. Ultrasonography is limited by the hoof ity and specificity than radiography, ultrasonography,