Magnetic resonance imaging (MRI)
Magnetic resonance imaging has recently emerged as the most sensitive, specific, and widely used diagnostic tool for avascular necrosis of femural head. In most reports, MRI can diagnose very early lesions with a greater than 90 percent specificity and sensitivity based on histology or eventual rogression [6,7]. Screening of asymptomatic, high-risk patients may enable early intervention. Imaging findings have been described and have been shown to correlate with the histologic changes within the marrow of the femural head.
In the early stages of the disease, there may not be any alteration of the normal signal intensity of the femural head. The first sign of AVN is nonspecific: diffuse areas of decreased signal intensity are seen in the normally high-signal-intensity fatty marrow on T1-weighted images [8]. This is thought to be due to edema within the marrow. Focal findings along the anterosuperior aspect of the femural head are more specific: low-signal-intensity bands or lines within the femoral head are seen surrounding the area that corresponds to ischemic bone on T1- and T2-weighted images. The band is thick on T1-weighted images and is thinner and accompanied by a second, innerband of high signal intensity on T2-weighted images. The appearance on T2-weighted images is known as the “double-line sign” and is considered highly specific for AVN. This band is believed to represent the reactive interface that separates normal marrow from infarcted marrow.The signal intensity of the central infarcted bone corresponds to areas of bone necrosis seen at histologic examination. High signal intensity on T1-weighted images and low signal intensity on T2-weighted images are seen within areas of necrosis when viable, fatty marrow is still present With prolonged ischemia and necrosis, the necrotic bone has a signal intensity pattern resembling that of fluid, with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Finally, when fibrosis and sclerosis of the involved bone occurs, it is reflected by low signal intensity on both T1- and T2-weighted images. Secondary signs and sequelae of AVN can also be seen at MR imaging. Joint effusion or cartilaginous thinning may be present. Progression of AVN leads to instability of the femural head with fragmentation and eventual collapse.
By comparing the appearance of hips with AVN on MR images and available radiographs, we can demonstrate a chronologic pattern of femoral head necrosis. In early AVN, repair and mechanical failure have not extended into the necrotic segment of bone. Thus, a class A (fatlike) MR appearance is observed (figure 3), with the preservation of normal fat signal except at the sclerotic reactive margin surrounding the lesion. This is similar to the “band pattern” described by Totty et al.[9], who noted this in their three cases with normal radionuclide examinations. When there inssufficient inflammation or vascular engorgement, or if subacute hemorrhage is present, a class B (bloodlike) result (figure 4,5), with highsignal intensity on long and short TR/TE images that is similar to subacute hemorrhage [10]. If enough inflammation, hyperemia, fibrosis, or sclerosis is present to decrease the lipid content of the femoral head, a class C (fluidlike) appearance will be observed (figure 4,5). This consists of low intensity with short TR/TE and high intensity with long TR/TE. Finally, if fibrosis and sclerosis predominate, a class D (fibrouslike) lesion will be seen with low signal intensity regardless of pulse sequence (figure 6,7).
Fig.3  Coronal T1-weighted (T1W) MRI image of the pelvis in a patient with bilateral avascular necrosis of the femoral head shows increased signal within the superior aspect of the femoral head, representing fat, surrounded by a line of decresed signal, representing sclerotic reactive margin. This is an MRI class A (fatlike). Fig.4,5  Patient 39 years old with use of high dose of corticosteroids. Cor T1 and T2-weighted MRI image of the pelvis shows a stage B (blood-like) at the level of right femoral head with increased signal on T1W and T2W; AVN stage C (fluid-like) in left femoral head, with decreased signal intensity on T1W and increased signal on T2W.
Fig.4
Fig.5  Fig.6,7  Cor T1W and T2W MRI in a patient with AVN on the left femoral head with decresed signal intensity on T1W and T2W, representing a stage D (fibrous-like).
Fig.6
Fig.7   Our characterization of the four MR imaging classes or intensity patterns as fatlike, bloodlike, and similar terms are intended as a device for remembering the patterns. For example, isointensity with fat (class A) is not proof that residual fat is present. It is also unknown whether our results will be duplicated at lower magnetic field strengths. Classes A (early) and D (late) AVN are most consistently correlated with radiographic staging. Lesions that are predominantly high in signal intensity with long TR/TE (classes B and C) represent a heterogeneous group, presumably consisting of lesions with proeminent inflammation, hyperemia, and/or hemorrhage, with variable lipid and fibrous contents. The frequent correspondence of the pattern of radionuclide uptake with the distribution of high MR signal intensity could be explained by hyperemia. Rupp et al., in a smaller series, found high intensity with long TR/TE to be correlated with an early stage seen radiographically [11]. The lack of exact correlation between MR image and radiographic staging reflects the different parameters these modalities are measuring; bone marrow changes indicated by MR imaging do not necessarily parallel the changes in bone depicted radiographically.