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

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2306 VII Imaging of the Musculoskeletal System A B D C E Figure 47-109. Fracture of the lateral sesamoid in a 34-year-old who complained of localized pain plantar and lateral to the first metatarsal head, made worse with weight bearing and extension of the great toe, for 1.5 years before the diagnosis was made. Anteroposterior (A), oblique (B), and sesamoid (C) radiographs all clearly show the transverse split across the lateral sesamoid of the great toe. A bipartite lateral sesamoid is an uncommon variant, present in only 1% of the population, and when symptomatic should be interpreted as a fracture. Short-axis T1-weighted (D) and inversion recovery (E) images through the marker (m) indicating the site of maximum pain show normal bone marrow signal in the medial sesamoid (white arrow) and bone marrow edema in the lateral sesamoid (black arrow). medicine bone scan could be obtained if confirmation is necessary. Fractures of the medial sesamoid are more difficult to diagnose radiographically because this sesamoid is not infrequently multipartite in normal people. Here radiographs are of limited value, and more sensitive imaging modalities are often required. Although MRI can demonstrate abnormal marrow signal in the sesamoids, owing to the small size of these bones this may be present on only a single slice, and all imaging planes should be carefully scrutinized. Short-axis images are particularly helpful in comparing the marrow signal from the medial and lateral sesamoids side-by-side (Fig. 47-110). The imaging of sesamoiditis is one of the few instances when we recommend a nuclear medicine bone scan over an MRI. In particular, the both-feet-on-the-detector view is extremely effective for localizing abnormal radiotracer uptake to one of the sesamoids (see Fig. 47-110C). Ch047-A05375.indd 2306 9/9/2008 5:36:08 PM
A B C 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 Ch047-A05375.indd 2307 9/9/2008 5:36:09 PM
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