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

47 Ankle and Foot 2237 47
Figure 47-39. The os calcaneus secondarius (OCS)
is an occasionally seen normal variant that resides
between the anterior process of the calcaneus (APC)
and the lateral pole of the navicular (N).
Radiographs
Figure 47-40.
Flow chart for imaging ankle and foot (see text).
CT
MRI US NM
Assess cortex
• Fractures
-Calcaneus
-Distal tibia
-Lateral
process of talus
• Arthritis
• Fusions
• Coalitions
-Osseous
-Nonosseous
Everything else
• Tendons
-Tears
-Tenosynovitis
• Masses
-Soft tissue
-Osseous
• Bone pathology
-Occult fracture
-Osteochondral
lesions
-Infection
• Tendons
-Achilles
• Toes
-Morton’s
-Plantar plate
• Masses
-Vascularity
-Cyst vs solid
• Foreign bodies
-Wood
• Screening
-Sesamoid
• Charcot
Figure 47-40 is a flow chart outlining which modalities we
use to image various pathologic processes.
CT is used when we specifically need to assess
bone cortex. The ability to reformat CT data into twodimensional
cross-sectional images in any plane desired
makes it the ideal modality to assess the intra-articular
extent of fractures, especially complex fractures involving
the distal tibia or calcaneus. This is particularly helpful to
orthopedic surgeons as part of their presurgical planning.
CT data can also be volume rendered into threedimensional
projections to show the alignment of comminuted
fractures. CT is also useful for showing fractures
that are difficult to see radiographically, such as the lateral
process of the talus or the anterior process of the calcaneus.
CT can be used to show arthritic narrowing of joints that
are difficult to visualize radiographically, such as the subtalar
joint. In patients who have undergone a surgical
arthrodesis in an attempt to fuse a painful arthritic joint
who remain symptomatic, CT can show the degree of bone
fusion at the cortical level. CT is also the best modality to
show the abnormal bone cortex in cases of tarsal coalition,
both solid osseous and nonosseous coalitions.
MRI is essentially used for everything else. MRI is the
best way to evaluate all of the ankle tendons at once for
tears or tenosynovitis. It is the best way to evaluate masses
arising from either the soft tissues or bones of the extremities.
MRI is extremely sensitive for the detection of bone
marrow and soft tissue edema/inflammation, and as such
is it useful for the detection of conditions that may be
radiographically occult, including stress fractures, osteochondral
lesions, and infection.
Ultrasonography (US) can be used to perform a
focused examination of the soft tissues of the ankle or foot.
Unlike MRI, which images all of the ankle bones and
tendons at once, US is used when we wish to examine one
specific tendon. US is particularly useful when we wish to
examine a torn Achilles tendon in a dynamic fashion to
see how much the tendinous gap opens between plantar
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