It is well-established that bone mineral loss is a normal physiologic process. Bone mineral density (BMD) peaks between ~20-35 years of age in men and gradually decreases during normal aging. The radiographic findings of vertebral body height loss, trabeculation, cortical thinning, bone lucency/photopenia, and demineralization can occur in the normal processes of aging after peak BMD and do not necessarily indicate an underlying medical condition. Such above radiographic findings lack widely accepted thresholds/criteria for distinguishing normal from abnormal bone mineralization. Reporting of such findings in radiographic reports is based on opinion rather than rooted in quantitative data that can confirm the true degree of mineralization. Additional testing is required to determine if such radiographic findings are normal aging or represent manifestations of a systemic demineralization process.
Dual X-ray Absorptiometry (DXA/DEXA) is the definitive study to determine bone mineral density (BMD) of a patient. Standard x-ray (XR) and computed tomography (CT) that are done as part of routine patient care are insufficient to ascertain the necessary data so, therefore, do not represent the standard of care to make such determinations and diagnoses.
However, if CT is used to evaluate BMD, it can be done via two approaches: 1) visual analysis; and, 2) direct density measurement.
Visual Analysis: The most common method for evaluating BMD on CT is subjective visual analysis. This is typical of a radiology report that refers to bone as appearing “osteopenic,” “demineralized,” “lucent,” or “thinned.” Radiologists know to use caution when drawing conclusions based on visual analysis (i.e., from a study insufficient to make a diagnosis) as it has been proven to: A) not provide any quantitative information; B) have poor accuracy; and, C) have poor interobserver agreement from one radiologist to another. Again, for these reasons, CT does not represent the standard of care to make such determinations and diagnoses.
Direct Density Measurement: The second CT method is by direct density measurement. CT scans can measure density of all tissues, including bone. There is an arbitrary scale called Hounsfield scale (which uses Hounsfield Units (HU)) that theoretically goes from -1000 to +1000. Pure water is 0 HU, and positive numbers correspond to material that is denser (e.g., bone), while negative numbers correspond to material that is less dense (e.g., air). Research has shown that, while not the preferred method of establishing bone mineral density, measurement of some lumbar vertebrae can be used to distinguish normal BMD from low BMD.
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Image courtesy of Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.This file is licensed under the Creative Commons Attribution 3.0 Unported license.