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. 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.
How can a hip MRI help a diagnosis? | Wake Radiology
In early AVN, repair and mechanical failure have not extended into the necrotic segment of bone. 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. 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.
What does a MRI of the hip show?
Patient 39 years old with use of high dose of corticosteroids. 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. 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. 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. This usually is seen days after the ischemic event. Over a period of weeks to months, increased uptake representing revascularization and repair surrounds and eventually replaces the region of photopenia.
The central region of photopenia with surrounding zone of increased uptake is termed the doughnut sign.
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SPECT provides images of the radioactivity within the target organ in 3 dimensions. With SPECT, overlying and underlying areas of radioactivity may be separated into sequential tomographic planes; this provides increased image contrast and improved lesion detection and localization, as compared with planar scintigraphy. A number of techniques, such as the use of multihead cameras with shorter acquisition times that improve resolution and increase sensitivity, have been advocated, but none has gained universal acceptance.
Initially, uptake is decreased in the perfusion and static phases, which represents the early ischemic event. Later, uptake is decreased within the femoral head in the perfusion phase and increased around the cold region in the static phase. The latter represents the reactive zone around the infarcted segment.
The increased uptake from the reparative zone eventually replaces the photopenic region. Hungerford reported false-negative bone scans in the hips of 14 of 27 patients, 13 of whom had bilateral disease [ 16 ]. In patients with bilateral involvement, the uptake, although symmetric, really is increased bilaterally.
Bone scintigraphy equipped with a pinhole collimator has greater sensitivity for diagnosing AVN than bone scintigraphy using a high-resolution parallel-hole collimator [ 17 , 18 ]. The image obtained is magnified, allowing better visualization of small structures and improving detection of scintigraphic abnormalities. Planar scintigraphic imaging using quantitative bone scan is a technique that provides physiologic data that cannot be obtained with other modalities, including MRI [ 19 ].
It allows quantification of uptake in the perfusion and static phases. It requires correct computer programming This technique is experimental and is not used widely in the clinical setting. The high spatial resolution and contrast resolution of CT allow analysis of morphologic features. CT is more appropriate in evaluating the extent of involvement, such as subchondral lucencies and sclerosis during the reparative stage, before the onset of femoral head collapse and superimposed degenerative disease.
CT scans do not demonstrate the early vascular and marrow abnormalities that result in osteonecrosis [ 21 ]. Osteoporosis is the first visible sign. Later, the central bony asterisk is distorted, appearing as clumping and fusion of the peripheral asterisk rays. This represents changes in the sclerotic interface between necrotic and viable bone and is analogous to the line of low signal surrounding the necrotic bone seen on MRI images. Early signs are caused by microfractures resulting from reduced mechanical load of dead bone trabeculae, altering the shape of the asterisk.
Signs also are related to new bone formation on the dead trabeculae. The lucent cystic region, representing the reparative zone, may be appreciated. Axial CT scan of a patient with avascular necrosis of the femoral head shows clumping and distortion of the central trabeculae representing the asterisk sign arrowhead and an adjacent low-density region arrow is representing the reparative zone.
Unless the asterisk sign is appreciated, articular surface abnormalities may be interpreted as degenerative joint disease. The lucency within the reparative zone may be confused with malignancy, infection, insufficiency fracture, or plasma cell myeloma. Although CT may delineate subtle alterations of bone density when plain radiograph findings are normal, MRI and SPECT scintigraphy are much more sensitive for evaluating early manifestations of the disease, such as bone marrow edema.
CT scans are insensitive for detecting stage 0 and 1 AVN, but are excellent for detecting femoral head collapse, early degenerative joint disease DJD , and the presence of loose bodies. Despite many investigations into AVN, many issues remain unresolved. The pathogenesis in most cases is only speculative and may involve intravascular factors such as microemboli or extravascular factors such as increased interosseous pressure.
MRI has emerged as the diagnostic test of choice for suspected early lesions, and radiographs should be used to diagnose and follow advanced lesions. Bone scanning can be useful for early diagnosis and CT scanning or tomography may help plan surgical procedures.
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The role of the functional exploration of bone is controversial. In conclusion, diagnosing AVN as early as possible is imperative for a greater chance of success of conservative treatment. National Center for Biotechnology Information , U.
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Curr Health Sci J. Zoia Stoica , 1 Daniela Dumitrescu , 1 M. Author information Article notes Copyright and License information Disclaimer. Received Oct 12; Accepted Jan This article has been cited by other articles in PMC. Abstract Avascular necrosis of the femoral head AVN is an increasingly common cause of musculoskeletal disability, and it poses a major diagnostic and therapeutic challenge. Introduction Avascular necrosis of the femoral head AVN is an increasingly common cause of musculoskeletal disability, and it poses a major diagnostic and therapeutic challenge. Open in a separate window.
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. Computed tomography CT The high spatial resolution and contrast resolution of CT allow analysis of morphologic features.
References 1. Yochum T, Rowe L. Essentials of skeletal radiology. Tierney Jr. Current Medical diagnosis and treatment. Resnick D, Niwayama G. Osteonecrosis: diagnostic techniques, special situations and complications. In: Resnick D, editor. Diagnosis of Bone and Joint Disorders. Philadelphia: WB Saunders Co; Ficat RP, Arlet J. Necrosis of the femoral head.
In: Hungerford DS, editor. Ischemia and Necrosis of Bone. A quantitative system for staging avascular necrosis. J Bone Joint Surg Br.
Anatomy of the Pelvis
Bassett L, et al. There is no recovery time, unless you were given a medicine to relax. After an MRI scan, you can resume your normal diet, activity, and medicines.