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

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2312 VII Imaging of the Musculoskeletal System A B C Figure 47-117. Plantar fibroma in a 44-year-old. Coronal T1-weighted (A), proton-density–weighted (B), and T2-weighted (C) images reveal that the lesion (arrows) is relatively dark on all sequences and confined to the fat of the plantar heel pad. and it is not uncommon for cultures of such aspirates to yield no growth. Clinical symptoms are often mild, generally presenting with persistent local pain. Radiographs show an intramedullary lucency with surrounding sclerosis, the density of which depends on the chronicity of the abscess. A thick chronic periosteal reaction may also be present (Fig. 47-116B). MRI after the administration of IV contrast reveals an intraosseous abscess with peripheral but not central enhancement (Fig. 47-116E). 29 Tumors • Soft Tissue Masses Soft tissue tumors of the feet and ankle are common, and MRI is useful in determining the tissue type as well as demonstrating the relationship of the mass to the adjacent anatomic structures. Synovial cysts or ganglia are among the most common soft tissue “masses” found around the foot and ankle. These are uniformly bright on fluidsensitive images and exhibit minimal if any peripheral enhancement after the administration of IVGd-based contrast (see Fig. 47-57). In comparison, nerve sheath tumors such are schwannomas are heterogeneously bright on T2- weighted and inversion recovery images, and they demonstrate heterogeneous contrast enhancement (see Fig. 47-56). Plantar fibromas can have variable signal characteristics but are typically dark on all sequences (Fig. 47-117). These are usually found in the plantar fat adjacent to the aponeurosis, usually close to the calcaneus. Morton’s neuromas usually occur between the heads of the second and third or third and fourth metatarsals and are also usually dark on noncontrast images, although they can exhibit postcontrast enhancement (see Fig. 47-58). Giant cell tumor of the tendon sheath is a localized form of pigmented villonodular synovitis, the latter being a joint-centered synovial proliferative condition. Both diseases show areas of decreased signal on T1-images, protondensity–images, and T2-weighted images, secondary to hemosiderin deposition (Fig. 47-118A to C). The presence of hemosiderin can be detected on gradient echo and precontrast fat-suppressed T1-weighted images (Fig. 47-118D), and the vascularized proliferative synovium should exhibit some contrast enhancement (Fig. 47-118E). • Bone Tumors 28 Osseous tumors are much less common than soft tissue tumors of the foot and ankle. Like all bone lesions, these tumors should be initially evaluated radiographically. MRI, however, is useful in localizing tumors and staging their extent. Because most tumors have nonspecific signal characteristics, MRI is often unable to render a specific preoperative diagnosis. Primary bone tumors of the feet are rare, accounting for only 4% of all bone tumors. 20 Benign bone neoplasms are more common than malignant ones, although in the foot, most bone neoplasms are primary tumors because metastases to the foot are rare. • Benign Tumors The most common benign tumors of the foot are enchondromas and osteoid osteomas. An osteoid osteoma is a relatively common cause of bone pain in adolescents and young adults, accounting for approximately 10% of all benign bone tumors. The classic history is pain at night, relieved by aspirin. Osteoid osteomas are one of the few tumors that are better imaged by CT than MRI. Thin-slice CT well demonstrates the lucent nidus as well as the tiny sclerotic component that is characteristically associated with it (Fig. 47-119A). CT is also used by radiologists for the purpose of percutaneously ablating the nidus. On MRI, osteoid osteomas are seen as a nonspecific edema pattern emanating from a tiny, dark nidus (see Fig. 47-119B to D). Ch047-A05375.indd 2312 9/9/2008 5:36:21 PM
47 Ankle and Foot 2313 47 A B C D E Figure 47-118. Giant cell tumor of the tendon sheath in a 19-year-old with a palpable medial mass. A, Straight axial T1-weighted image through the mass (arrow) demonstrates that it lies within the soft tissues medial to the navicular (N), talus (Ta), and sustentaculum tali (ST) but does not invade the bones. B, The corresponding axial proton-density–weighted image shows that the mass has grown through a split tendon sheath (arrowheads). The tendons themselves—posterior tibial (T), flexor digitorum longus (D), and flexor hallucis longus (H)—are intact and normal in appearance with no evidence of tumor involvement. C, The corresponding axial T2-weighted image shows that the mass (arrow) has heterogeneous signal intensity, consistent with the presence of blood products of varying ages. D, Corresponding axial precontrast fat-suppressed T1-weighted image reveals that some signal in the mass is brighter than the surrounding suppressed fat, consistent with methemoglobin. E, Corresponding axial post–intravenous gadolinium fat-suppressed T1-weighted image demonstrates heterogeneous enhancement, indicative of the vascularity of this synovial proliferation. Chondroblastomas are rare benign cartilaginous neoplasms, and one of the few tumors that arise from the epiphysis in a skeletally immature patient. Chondroblastomas can expand the cortex but should not cross the unfused growth plate (Fig. 47-120A). Radiographically, chondroblastomas can have either a lucent or chondroid matrix. By MRI, these lesions may exhibit considerable edema in the surrounding soft tissues, but the tumor itself should have a sharp, non–aggressive-appearing interface with the normal bone (see Fig. 47-120B to D). Intraosseous lipomas of the calcaneus are rare but have a characteristic radiographic appearance in that they are well circumscribed and nearly totally lucent except for a tiny central sclerotic focus (Fig. 47-121A). By MRI, the intraosseous lipoma is uniformly isointense to fat on all sequences, except for a signal void corresponding to the sclerotic focus (Fig. 47-121C and D). • Malignant Tumors The most common primary malignant tumor of the foot is chondrosarcoma, which has a propensity for the calcaneus (Fig. 47-122). High-grade chondrosarcomas have a calcified matrix that appears radiographically sclerotic (see Fig. 47-122A) and dark on T1- and T2-weighted sequences. Chondrosarcomas are not highly vascularized tumors and Ch047-A05375.indd 2313 9/9/2008 5:36:23 PM
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