There are many imaging methods for evaluating osteoporosis. Only a limited assessment of bone density and trabecular architecture can be done by plain radiography. Radiography is most useful in finding atraumatic fractures that are associated with osteoporosis. Radionuclide bone scanning and MRI also are useful in finding these fractures. Evaluation of bone density is most frequently assessed by dual-energy x-ray absorptiometry but can be done by other methods.
Key points
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Several routine imaging modalities are used for evaluation of bone strength, including radiography, CT, MRI, absorptiometry, and quantitative ultrasound densitometry (QUS).
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Detection of atraumatic fractures is important for the detection of decreased bone strength.
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Dual-energy x-ray absorptiometry (DXA) is the standard method for measuring bone density and is used in the World Health Organization (WHO) classification of osteoporosis.
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Advanced imaging techniques using CT, MRI, DXA, and computerized analysis are providing increased information about bone architecture and bone strength.
Introduction
Various imaging techniques are used for evaluating bone density and bone quality. Bone strength is ultimately defined by a combination of bone density and bone quality. Bone quality is a reflection of its component parts, including bone architecture, bone turnover, mineralization, and microfractures. Bone density is a term referring to the amount of mineral matter per volume of bones and accounts for approximately 60% of bone strength. Routinely used imaging modalities, such as radiography, CT, MRI, and DXA, and advanced techniques, which use high-resolution CT and MRI, and computerized quantitative methodology are used to study osteoporosis and bone strength. The advanced laboratory and imaging techniques are used mainly in experimental studies in humans and laboratory animals. As stated by Griffith and Genant, limitations of these advanced techniques include cost, the fact that they are not widely available, their requirement for technical expertise that is limited, and the need for biopsy specimens; all hinder their adoption into mainstream clinical practice. Mainstream techniques and their interpretation are improving, often driven by the improvements in therapy.
Rheumatic diseases are often associated with osteoporosis. Drug therapy, especially corticosteroids, may cause bone loss. Disease and pain in the joints cause decreased mobility, leading to disuse osteoporosis. The disease processes themselves may release proinflammatory cytokines that lead to resorption of bone. The presence of rheumatologic disease should lead to evaluation of bone density earlier than in the otherwise normal population, and there are treatments of diseases, such as rheumatoid arthritis, that control both the disease and the attendant bone loss and risk of fracture.
Osteoporosis itself does not cause pain and is clinically silent except for fracture. Atraumatic or low-impact/fragility fractures are associated with osteoporosis. Vertebral body, femoral neck, or intertrochanteric region of the femur and distal radius are frequent sites of these fractures. If compression fracture of the spine is present, without causal trauma, tumor, or infection, patients can be classified as having osteoporosis, regardless of bone density measurements ( Fig. 1 ). These fractures may cause an accentuation of the thoraic kyphosis. Insufficiency fractures are stress fractures that occur with normal stress on abnormal bone. They often occur in the metaphyses of long bones, pubis and ischium, and femoral neck ( Fig. 2 ). Detection of these fractures may be delayed after the onset of symptoms; thus, imaging methods are crucial for diagnosis of these fractures, with MRI, radionuclide bone scanning, and positron emission tomography (PET) more sensitive than radiography.
