156 Clinical Features
low-field, high-field), various manufacturer
names for sequences, individual preferences,
minimal evidence of efficacy of one protocol
versus another, various breed conformations,
and coils and algorithms that are optimized for
humans.Asabasicprotocol,thejointshould
be examined using a local coil, preferably one
that surrounds the joint, and the joint should be
located in the magnetic isocenter. The degree
of joint flexion may be dictated by coil config-
urations, but 90◦flexion may improve imaging
of the CrCL (Podadera et al. 2014). Proton
density (PD) weighted fast spin–echo (SE)
sequences should be obtained in the transverse,
sagittal, and dorsal planes using a small field-
of-view centered on the area of interest and,
importantly, a large matrix size. Additionally,
a fat-suppressed PD-weighted sequence (e.g.,
short-tau inversion recovery or STIR) should
be obtained in one, two, or three planes. For
larger joints, a slice thickness of 3–4 mm should
be used, but for smaller joints use 1–2 mm
slices. Very thin slices (1 mm) may require a
T2-weighted 3D gradient recall echo sequence.
Additional sequences or planes may be of use
in certain conditions. Intravenous or intra-
articular contrast medium is not used routinely.
Image orientation is accomplished by obtaining
scout images in three planes. The dorsal plane
is aligned parallel to the patellar tendon on the
sagittal scout and parallel to the caudal aspect
of the femoral condyles on the transverse scout.
The transverse plane is made parallel to the
distal aspect of the femoral condyles on the
dorsal scout and perpendicular to the patellar
tendon on the sagittal scout. The sagittal plane
is made such that slices bisect the patellar
tendon and intercondylar fossa of the femur on
the transverse scout and parallel to the patellar
tendon on the dorsal scout (Winegardneret al.
2007). High-quality images depend on the
patient being still, which often requires anes-
thesia. Implants or metallic objects may reduce
image quality (Banfield & Morrison 2000). This
point may be moot when surgical implants
or metal debris are far enough away from
structures of interest, such as the cruciate liga-
ments or medial meniscus, to not interfere with
interpretation (Davidet al. 2012a). Additionally,
alterations to image acquisition parameters can
reduce the severity of the susceptibility artifact
to produce a diagnostic image (David et al.
2012b; Simpleret al. 2014). High-field scanners
provide superior signal-to-noise ratio, but
are not always available.Synovial structures
Evaluation of synovial joints is one of the most
important reasons for hospital visits relating to
gait abnormality or lameness. Synovial joints
consist of a fibrous joint capsule, synovial mem-
brane, fluid-filled joint space, and articular car-
tilage (Dyceet al. 2002). The combination of the
joint capsule and synovial membrane is seen as
a low SI structure on MR images (Rubin 2005).
The synovial membrane usually is too small to
differentiate as a separate structure: it also lines
joint recesses, bursas, and tendon sheaths. The
normal synovial membrane does not enhance
(or only minimally enhances) after intravenous
contrast-medium administration (Rubin 2005).
An inflamed synovial membrane is thick and
may be nodular or mass-like, especially when
chronic (Rubin 2005). When there is active
inflammation the synovial membrane rapidly
enhances with intravenous contrast-medium
administration (Rubin 2005). Therefore, intra-
venous contrast-medium administration might
be helpful when an inflammatory arthropa-
thy (e.g., immune-mediated, infectious) is sus-
pected. Joint effusion (Figure 21.1) that is due
to increased synovial fluid production has sim-
ilar signal characteristics as normal joint fluid.
Joint fluid that contains proteinaceous debris or
blood products has a decreased (darker) and
variable SI (Rubin 2005). This different appear-
ance may be referred to as ‘dirty’ versus ‘clean’
fluid. The appearance of normal articular carti-
lage reflects its chemical composition and three-
dimensional histological organization (Rubin
2005). For example, articular cartilage has a low-
PD SI that progressively increases from deep-to-
superficial. Cartilage abnormalities appear as
defects or SI changes in the smooth articular
surface, which may extend to the subchondral
bone plate. Traumatic articular lesions are often
sharply demarcated (Rubin 2005).Meniscus
Within the stifle joint, there are lateral and
medial menisci, which are crescent-shaped