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

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47 Ankle and Foot 2307 47
Figure 47-110. Fracture of the medial sesamoid in a
27-year-old with a several-month history of pain
localized to the head of the first metatarsal. Short-axis
T1-weighted (A) and T2-weighted fat-suppressed (B)
images show normal bone marrow signal in the lateral
sesamoid (white arrow) and bone marrow edema in
the medial sesamoid (black arrow). C, Bone scan, bothfeet-on-detector
view, localizes the increased uptake
to the medial sesamoid of the left foot. This patient
failed to respond to conservative therapy and
ultimately had the medial sesamoid resected.
Infection
Osteomyelitis is always a diagnostic dilemma. The term
osteomyelitis comes from the Greek roots osteon meaning
“bone,” myelos meaning “marrow,” and itis meaning
“inflammation.” Thus, osteomyelitis literally means “inflammation
of bone marrow,” and this is perhaps symbolic of
the dilemma. MRI is extremely sensitive for the detection
of marrow inflammation, but it is not specific for the
inflammation caused by infection. By MRI, the bone
marrow edema caused by infection looks just like the bone
marrow edema caused by a stress response as well as the
edema caused by a nonhealing fracture or even a healing
fracture. For this reason, an MRI for osteomyelitis should
not be read in isolation. It is difficult to arrive at the correct
diagnosis without a thorough clinical workup and
complete understanding of any prior surgical resections or
debridements.
• Imaging Techniques
• Radiography
Radiographs are essential in the workup of osteomyelitis,
and at the UW we insist on having recent radiographs
before we will perform an MRI for infection. Although it
is true that radiographs are insensitive to the bone marrow
and soft tissue edema seen on MRI, they are not without
value. First, radiographs are crucial to screen for the presence
of metal, particularly in the feet of diabetic patients
who may be insensate and thus unknowingly stepped
on pins, not to mention the presence of orthopedic
hardware.
Second, in diabetic feet it is necessary to screen for the
joint-centered collapse that is typically seen with peripheral
neuropathy, the Charcot joint. These radiographic
findings have been described as “the six Ds”: destruction,
increased density, dislocation, debris, distension, and disorganization.
The bone marrow and soft tissue edema seen
with MRI in patients with sterile neuropathic osseous
changes may be indistinguishable from infection, and for
this reason at the UW we recommend that patients who
exhibit radiographic findings of a Charcot joint undergo a
nuclear medicine bone scan and white blood cell scan,
rather than MRI, for the workup of osteomyelitis. And
because neuropathic collapse can occur relatively quickly
and go unnoticed by a patient with an insensate foot (Fig.
47-111), we require that the pre-MRI radiographs be recent,
preferably within the last week.
Third, radiographs may reveal findings that, in the
proper clinical setting, are diagnostic for osteomyelitis.
New cortical erosions (Fig. 47-112) in a bone deep to a
nonhealing ulcer or unresponsive cellulitis are as diagnostic
as MRI for active osteomyelitis. Periosteal reactions,
particularly the aggressive periosteal reaction of acute
osteomyelitis or the thick involucrum of chronic osteomyelitis
(Fig. 47-113), can be diagnostic. Gas in the soft
tissues, such as from a gas-forming organism, is easily
detected on radiographs yet may be hard to interpret on
MRI because it can cause susceptibility artifacts similar to
those caused by metal.
• Magnetic Resonance Imaging
Ultimately, it is easier to rule out osteomyelitis by MRI than
it is to confirm its presence. The absence of increased bone
marrow signal on a good edema-sensitive MRI effectively
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