Ankle and Foot 47 - Department of Radiology - University of ...

2248 VII Imaging of the Musculoskeletal System
Figure 47-53. Comparison of T2-weighted, fatsuppressed
(A) and inversion recovery (B) MRIs in a
35-year-old with plantar fasciitis. Both of these sagittal
images of the calcaneus delineate the plantar
aponeurosis (open arrowheads), as well as the edema
in the adjacent heel fat pad (white arrowheads) and in
the bone marrow at its origin (white arrow).
A
B
Protocols
Our ankle/foot MRI protocols call for acquiring images in
three of the standard planes and typically take 45 to 60
minutes. We start with straight sagittal images to survey the
bones. For an ankle MRI, the FOV needs to include the
distal tibia and fibula, all of the tarsal bones, and the bases
of the metatarsals. For a foot MRI, the FOV needs to include
all of the phalanges, all of the metatarsals, and the midfoot
tarsal bones back to the Chopart joint. In cases in which
the area of clinical concern is vague, the sagittal FOV can
be enlarged to include the ankle and forefoot. When they
are available, the radiologist should check these first sets
of sagittal survey images, particularly the edema-sensitive
sequence. If the radiologist observes edema in the talar
dome, images can then be acquired in the mortise coronal
and mortise sagittal planes (our OLT protocol). If there is
edema elsewhere in the tarsal bones, oblique axial and
oblique coronal images are acquired (our tarsal stress fracture
protocol). If the marrow edema is in a metatarsal, this
is usually best demonstrated with short- and long-axis
images (our metatarsal stress fracture protocol). If no bone
marrow edema is found, we generally proceed with our
tendon protocol (straight axial and oblique coronal) unless
some other site is clinically requested.
Sequences
Because detecting abnormally edematous signal in the
bones, tendons, and surrounding soft tissues is the key to
all musculoskeletal MRI, we run edema-sensitive sequences
in all the planes we image. This can be either a fatsuppressed
fast-spin echo T2-weighted sequence or an
inversion recovery sequence. Although for the most part
these two sequences are equivalent (Fig. 47-53), we find
that we get the best images when we use inversion recovery
for the straight sagittal survey plane and T2-weighted
sequences in all other planes.
T1-weighted images, being inherently fat sensitive,
well demonstrate the normal fat in yellow bone marrow
as well as the subcutaneus fat and the deeper fat between
muscles and tendons. We use T1 weighting in all imaging
planes whenever the tendons are not the primary site of
interest.
When the tendons are the site of clinical concern,
we use proton-density–weighted images, along with T2-
weighted sequences, in the straight axial and oblique
coronal planes. The straight axial plane well images all 10
of the ankle tendons in cross section at and above the level
of the ankle joint. The oblique coronal plane well images
the medial and lateral ankle tendons in cross section as
they curve under the malleoli. Tears in the substance of the
ankle tendons are usually best seen with proton-density–
weighted images (Fig. 47-54). However, because protondensity–weighted
images are relatively insensitive for fluid,
they should always be read side-by-side with edemasensitive
images to look for abnormal amounts of fluid in
the tendon sheaths, indicative of active tenosynovitis.
• Use of Contrast
Although the intravenous administration of a gadoliniumbased
contrast agent (IVGd) is not needed for most
musculoskeletal MRI, there are particular circumstances in
which its use is invaluable.*
Whenever possible, we use IVGd when there is a clinical
concern for an inflammatory arthropathy or synovitis,
such as in rheumatoid arthritis (Fig. 47-55). The IVGd
causes T1 signal enhancement of the hypervascularized
inflammatory synovium (pannus) but not of the adjacent
synovial fluid, a distinction that may not otherwise be seen
on noncontrast T1-weighted or T2-weighted images.
Likewise, we prefer to use IVGd whenever possible in
cases of infection. IVGd can help distinguish an enhancing
phlegmon from a nonenhancing pus pocket. Also, IVGd
can distinguish cellulitis, which demonstrates contrast
enhancement of the edematous skin, from “nonspecific
edema from other causes” that does not enhance.
*During the year leading up to the publication of this chapter, the U.S. Food
and Drug Administration (FDA) has issued warnings describing the risk for nephrogenic
systemic fibrosis (NSF) after exposure to gadolinium-containing contrast
agents in patients with acute or chronic severe renal insufficiency. Information
can be found at http://www.fda.gov/cder/drug/infopage/gcca. Before any contrast
agent is administered to any patient, that patient should be screened in
accordance to your institution’s policies.
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