MR imaging of the painful knee: Correlation with conventional radiography

The images were chosen from a digital archive of more than 150 MR examinations of the knee.

All MR imaging examinations were performed on 1.5 T MR units from GE Medical Systems (one 1.5T Signa MR/i Echospeed-plus unit and one 1.5T Signa MRI) with a dedicated knee coil (all products from GE Medical Systems, Milwaukee, WI). The following parameters were utilized as the knee imaging protocol: sagittal double-echo spin-echo T2-weighted sequence with 16-cm field-of-view (FOV); repetition time (TR) 2175 ms; echo time (TE) 20/90 ms; number of excitations 1; slice thickness/gap 4/1 mm; matrix 256 × 192; axial and coronal oblique fast-spin-echo (FSE) T2-weighted sequences with FOVs of 14 cm and 16 cm, respectively; TR 3000 ms; TE 70 ms; number of excitations 2; echo train length 4; slice thickness/gap 4/1 mm; matrix 320 × 224; coronal spin-echo T1-weighted sequence with FOV of 16 cm; TR 600 ms; TE 12 ms; number of excitations 2; slice thickness/gap 4/1 mm; matrix 512 × 256; coronal cross-hatched gradient-recalled echo (GRE) sequence with FOV of 16 cm; TR 600 ms; TE 15 ms; flip angle 25°; number of excitations 2; slice thickness/gap 4/0 mm; matrix 256 × 192. A frequency-selective fat saturation pulse was used on all sequences (except the T1 and the cross-hatched GRE sequences) to reduce the high signal from surrounding fat. To reduce artifacts, oversampling in the phase-and frequency-encoding axes was performed.

MR imaging can be useful in detecting abnormalities that are radiographically absent even with an optimal radiographic evaluation. An example of this can be seen in early avascular necrosis. Avascular necrosis commonly occurs at the hips due to a variety of factors that give rise to hypercoaguable states or to vessel fragility; but it can also occur around the knee, particularly in patients on steroid therapy for inflammatory disease (ie, systemic lupus erythematosus). Although early avascular necrosis is oftentimes not apparent with radiography, MR imaging may show marrow edema (Figure 1). Another radiographically occult phenomenon, especially in its early stages, is the stress fracture. Early detection of stress fractures can be difficult with radiography. MR imaging, however, can allow visualization of a stress fracture, which is seen as a linear or jagged hypointense area on T1-weighted imaging (T1WI). The linear hypointensity is also seen on T2-weighted images (T2WI) and is characteristically surrounded by marrow edema. This is known as the “stress response.”⁵ The usual stress fracture is short and horizontal to the long axis of the involved bone (Figure 2).⁶ Other pathologic processes that can be radiographically inconspicuous or absent include small osteochondral lesions (Figure 3), various types of sports-induced injuries (Figure 4), and chondromalacia of the patella (Figure 5).

MR imaging, with its excellent soft-tissue contrast, is particularly useful in defining soft-tissue abnormalities and pathology, as they often have a nonspecific radiographic appearance (Figures 6 and 7). Figure 8 presents an a case in which a chondroid lesion detected radio-graphically is shown more clearly on MR imaging to have nonaggressive characteristics, favoring a benign chondroid tumor. Differentiating between enchondromas and chondrosarcomas is paramount, as these two entities have different prognoses and potentially different treatment strategies.⁷ A case of hemophilic arthropathy is another example in which MR imaging can complement the information provided by radiography. In this entity, MR findings of irregular foci of very low signal intensity are due to hemosiderin deposits that help to distinguish degenerative changes secondary to hemophilic arthropathy from other erosive arthritides (Figure 9).

Lipohemarthrosis (Figure 10), pigmented villonodular synovitis (Figure 11), fibroxanthoma (Figure 12), and osteomyelitis are also examples of the complementary roles that MR imaging and radiography can play in diagnosis. Lipohemarthrosis and other fat-containing processes can be difficult to identify with radiography, but are readily diagnosed on MR imaging, due to the predictable signal characteristics of fat on different pulse sequences. In patients with radiographic findings suggestive of osteomyelitis (eg, soft-tissue swelling, osseous erosions), MR imaging may confirm the diagnosis by showing marrow enhancement, abnormally high T2 signal, intermediate to low T1 signal, and demonstration of sinus tracts (Figure 13).

In other instances, a relatively confident diagnosis can be made from radio-graphic findings, and MR findings play only a supportive role, as in a typical case of a giant cell tumor involving the proximal tibial metaphysis (Figure 14).

There are certain instances in which a disease process shows no radiographic findings but is readily detectable on MR imaging. In general, however, conventional radiography and MR imaging play complementary roles in diagnosing many disease processes of the knee. Correlating these two modalities is helpful in synthesizing the information in order to formulate a differential diagnosis or to provide a firm diagnosis. AR