British Journal of Radiology (2005) 78, 1104-1115
© 2005 British Institute of Radiology
doi: 10.1259/bjr/73852511
Magnetic resonance imaging appearances of fibrous dysplasia
Z K Shah, MD1,
W C G Peh, FRCPE, FRCPG, FRCR1,
W L Koh, MBBS, FRCR1 and
T W H Shek, MBBS, FRCPA2
1 Department of Diagnostic Radiology, Changi General Hospital, Singapore and 2 Department of Pathology, The University of Hong Kong, Queen Mary Hospital, Hong Kong SAR, China
Correspondence: Professor Wilfred C G Peh, Programme Office, Singapore Health Services, 7 Hospital Drive #02-09, Singapore 169611
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Abstract
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Fibrous dysplasia is a developmental disorder in which normal bone marrow is replaced by fibro-osseous tissue. The radiographic, CT and scintigraphic appearances of this condition are well known. The MRI appearances of fibrous dysplasia have not been widely published. The lesions are largely isointense with areas of hypointensity on T1 weighted images and appear heterogeneously hyperintense on T2 weighted images. The enhancement pattern is patchy central, rim, homogeneous, or a combination. The MRI features reflect the variable tissue components of this entity. This pictorial review aims at highlighting the MRI appearances, with pathological correlation.
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Introduction
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Fibrous dysplasia is a non-inherited developmental disorder in which normal bone marrow is replaced by fibro-osseous tissue. It is often asymptomatic and frequently incidentally detected on radiographs taken for unrelated clinical indications. However, it may be complicated by pathological fracture and, rarely, by malignant degeneration. It can also be associated with aneurysmal bone cysts. It is typically seen in adolescents and young adults. Fibrous dysplasia can be either monostotic or polyostotic and may occur as a component of McCune-Albright syndrome or the rare Mazabraud's syndrome. Uncomplicated monostotic lesions are asymptomatic and do not cause significant deformity. As a rule, they do not convert to the polyostotic form, do not increase in size and become inactive at puberty. The polyostotic variety is more severe with the involvement of multiple sites. It results in significant deformities and, although the lesions become quiescent after puberty, the deformities may progress [1, 2]. This condition has well documented radiographic, scintigraphic and CT findings. The MRI appearances of this condition have not been widely published. To our knowledge, there have been only five papers describing the MRI features of fibrous dysplasia, of which only three had details of all cases [1–5]. Based on our experience of 16 patients with fibrous dysplasia, this pictorial review aims to illustrate the various MRI characteristics of fibrous dysplasia.
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Pathological features
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In fibrous dysplasia, abnormal differentiation of osteoblasts leads to the replacement of normal cancellous bone by immature bone and fibrous stroma. The stroma is characteristically of low cellularity and is composed of collagen and loose myxoid fluid. The involved bone is expanded with either cortical thickening or thinning, but cortical penetration is rare. The tissue is firm, variable, gritty and whitish or reddish [6]. Microscopically, fibrous dysplasia is composed of benign fibrous tissue that contains irregular bony trabeculae of woven bone. The shape and pattern of the bony trabeculae are highly variable, and are likened to "Chinese characters" or "alphabet soup" (Figures 1 and 2
). Haemorrhage and cystic change may occasionally be found. On rare occasions, an enchondromatous component is seen (fibrocartilaginous dysplasia).
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Figure 1. Photomicrograph shows typical features of fibrous dysplasia. There are multiple fibroblasts in a collagenous matrix. The bony trabecular pattern resembles "Chinese characters". Haematoxylin and eosin stain, x 20.
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Figure 2. Photomicrograph shows typical features of fibrous dysplasia. There is haphazard arrangement of the bony trabeculae with background fibroblasts, so-called "alphabet soup". Haematoxylin and eosin stain, x 66.
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In our series of 16 patients, 9 had histological proof of fibrous dysplasia. Not all the patients had a histological correlation, as only those which were clinically and/or radiologically not typical were subjected to biopsy. Open surgical biopsy was done in cases where there was a diagnostic dilemma and surgical resection or curettage with bone grafting was done for those patients who presented with complications of the disease, e.g. pathological fracture (Table 1).
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Radiographic features
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Radiographs show lesions that are medullary in origin. Sometimes, the lesions may be eccentric in location. Replacement of normal cancellous bone with abnormal tissue produces a purely lytic or denser lesion with a typical greyish ground-glass appearance [6] (Figure 3). The lesions are usually bound by a characteristic thick layer of reactive bone that has been described as a "rind".
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Figure 3. (Case 16) Radiograph of the forearm shows a large expanded lesion in the distal shaft of the radius, with homogeneous "ground-glass" matrix and "cloud of smoke" calcification.
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Fibrous dysplasia often causes scalloping of the endosteum. Bony expansion is common. The lesions are typically sharply marginated. Focal nodules of cartilage can produce foci of "pop-corn" like calcification. If the lesion contains a large amount of cartilaginous tissue, the calcification may be denser with a "cloud of smoke" appearance [1] (Figure 3
). The lesion rarely extends into the epiphysis in the immature skeleton, but may extend to the end of the bone after physeal closure. Radiographic appearances of fibrous dysplasia are usually diagnostic. In equivocal cases, histopathological correlation may be required.
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MRI features
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On MRI, fibrous tissue has been described as being hypointense to skeletal muscle on both T1 and T2 weighted images [4–7]. Despite the name, lesions of fibrous dysplasia do not follow the same signal characteristics of pure fibrous tissue. The apparent discrepancy between the theoretical and actual appearance of fibrous dysplasia on MRI can be explained on the basis of the histology of the lesions [2, 6]. This was a common inference in the previous series describing the MR appearances of this condition [1–5] (Table 2).
Ours is a retrospective review of the MRI characteristics of 16 cases proven to be fibrous dysplasia, either by typical radiographic findings or pathological correlation. We found that MRI shows the location and extent of the lesions, better than radiographs. This was well demonstrated in one of our patients, who presented with right hip pain. Radiographs revealed a minimally expansile lesion in the right femur neck and the intertrochanteric region. Further characterization of the lesion was required and hence an MRI was performed. T1 and T2 weighted images showed the lesion to be more extensive than was noticed on the radiographs (Figure 4).
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Figure 4. (a) (Case 14) Radiograph of the pelvis shows a well-defined lesion within the neck and intertrochanteric area of the right femur with a ground-glass appearance and minimal expansion of the bone. (b) (Case 14) Coronal T1 weighted image of the right femur shows the true extent of the lesion within the marrow. The lesion is larger than appreciated on the radiograph. It is predominantly T1 isointense with a more hypointense area in the distal portion. It is minimally expanded with well-defined margins. (c) (Case 14) Coronal fat-suppressed T2 weighted image shows shows a heterogeneously hyperintense lesion with a more markedly hyperintense area at its distal aspect. (d) (Case 14) Coronal contrast-enhanced fat-suppressed T1 weighted image shows heterogeneous enhancement with a non-enhancing area corresponding to the T1 hypointense and T2 markedly-hyperintense area in the distal portion, consistent with cystic change.
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In our experience, most of the lesions are largely isointense to skeletal muscle on T1 weighted images. On T2 weighted images, lesions are typically heterogeneously hyperintense with hypointense, isointense or markedly hyperintense areas within. Depending on the appearance on both the T1 and T2 weighted images, the heterogeneous areas may be due to calcification (Figure 5), cystic changes (Figure 4), fatty areas (Figure 6) or septations (Figure 7). We encountered two cases in which the lesions were homogeneous on both T1 and T2 weighted images (Figures 8 and 9). We could categorise the enhancement characteristics of fibrous dysplasia as: patchy central enhancement (Figures 5d and 7c), rim enhancement (Figure 8c), homogeneous enhancement (Figure 9c), or a combination (Figures 6c and 10c). On both T1 and T2 weighted images, an outer hypointense rim was typically seen and corresponded to the sclerotic rim seen on radiographs.
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Figure 5. (a) (Case 10) Radiograph shows an expansile lesion in the left femoral neck and upper shaft with a thick sclerotic rim and matrix calcifications. (b) (Case 10) Coronal T1 weighted image shows an isointense (to skeletal muscle) lesion with some foci of hypointensity in the distal portion. (c) (Case 10) Coronal fat-suppressed T2 weighted image shows a heterogeneously hyperintense lesion with small hypointense foci distally. (d) (Case 10) Coronal contrast-enhanced fat-suppressed T1 weighted image shows patchy enhancement of most of the lesion. The hypointense foci in the distal aspect of the lesion seen on the T2 weighted image do not enhance and are consistent with calcifications seen on the radiograph. The more proximal area that is T1 and T2 isointense may represent non- or minimally enhancing fibrous tissue.
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Figure 6. (a) (Case 8) Coronal T1 weighted image of the hip shows an expansile isointense lesion within the intertrochanteric region and the shaft of the left femur, with focal hyperintense areas within due to fatty components. (b) (Case 8) Coronal fat-suppressed T2 weighted image shows a heterogeneously hyperintense lesion and multiple linear hypointense areas. (c) (Case 8) Coronal contrast-enhanced fat-suppressed T1 weighted image shows patchy central and peripheral enhancement. The foci of very low signal intensity in the region of the greater trochanter correspond to the fatty areas identified on Figure 6a.
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Figure 7. (a) (Case 7) Lateral radiograph of the knee joint shows an expansile lytic lesion in the distal shaft of the femur with a sclerotic rim. (b) (Case 7) Sagittal T2 weighted image shows a heterogeneous lesion, which is mostly isointense with some hyperintense areas and curvilinear hypointensities representing septations. The central portion of the lesion is more hyperintense compared with the more peripheral portion. (c) (Case 7) Sagittal contrast-enhanced fat-suppressed T1 weighted image shows central enhancement at the site of the hyperintense signal on the T2 weighted image. The peripheral areas that are T2 isointense may represent non or minimally enhancing fibrous tissue.
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Figure 8. (a) (Case 6) Coronal T1 weighted image shows an expansile lesion in the ischial tuberosity which is homogeneously isointense. (b) (Case 6) Coronal fat-suppressed T2 weighted image shows a T2 hyperintense lesion with a small septated area at the inferolateral aspect. (c) (Case 6) Coronal contrast-enhanced fat-suppressed T1 weighted image shows typical rim enhancement pattern.
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Figure 9. (a) (Case 16) Axial T1 weighted image of the right humerus shows a well defined, homogeneously isointense area within the medullary cavity. (b) (Case 16) Axial fat-suppressed T2 weighted image shows the lesion to be homogeneously hyperintense. (c) (Case 16) Axial contrast-enhanced fat-suppressed T1 weighted image shows homogeneous contrast uptake by the lesion. As this image was acquired after the patient had shifted position, the axial images do not exactly match the T1 and T2 weighted images.
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Figure 10. (a) (Case 9) Anteroposterior radiograph shows an osteolytic lesion in the medial aspect of the proximal end of the tibia with a thin sclerotic rim. There is a large area of calcification in the central region. (b) (Case 9) Coronal contrast-enhanced fat-suppressed T1 weighted image shows patchy central enhancement and a peripheral enhancing rim. There is a non-enhancing hypointense area corresponding to the calcification on the radiograph. In addition, there is a focal extension of the lesion through the medial cortex. (c) (Case 9) Axial contrast-enhanced fat-suppressed T1 weighted image confirms a break through the medial cortex at the level of the enhancing lesion, consistent with a pathological fracture.
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Complications
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Complications of fibrous dysplasia include pathological fracture, deformity, recurrence following surgery and associated tumours. Malignant degeneration of fibrous dysplasia complicates less than 1% of all cases, presenting clinically as pain and swelling. Radiographically there is cortical destruction and associated soft tissue masses. One of our patients had a pathological fracture (Figure 10) with adjacent bone marrow oedema seen as diffuse hyperintensity on T2 weighted images. MRI is also useful in the follow-up of partially treated lesions. Recurrence of fibrous dysplasia after treatment was seen in three of the patients and was detected as T1 isointense and T2 hyperintense soft tissue mass which had post-contrast enhancement (Figure 11).
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Figure 11. (a) (Case 12) Radiograph shows an expansile lesion in the right femur involving the lesser trochanter and the proximal shaft. (b) (Case 12) Radiograph, taken 1 year after open surgical curettage and treatment with bone grafting, shows multiple radiodense areas within the lesion corresponding to the areas of bone graft. (c) (Case 12) Coronal T1 weighted image shows a heterogeneous appearance of the lesion, which is predominantly isointense to skeletal muscle, with multiple hypointense foci. A few small hyperintense areas are also seen within it. (d) (Case 12) Coronal contrast-enhanced fat-suppressed T1 weighted image shows multiple areas of enhancement at the periphery of the lesion.
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Conclusion
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Fibrous dysplasia has characteristic MRI appearances, which correlate well with the morphological and histopathological findings. Recognition of these MRI features may be helpful when radiographs are equivocal or unremarkable.
Received for publication October 21, 2004.
Revision received April 20, 2005.
Accepted for publication May 6, 2005.
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References
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Abstract
Introduction
