What is the role of CT scanning in the workup of osteoporosis?

Updated: Jan 19, 2021
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Felix S Chew, MD, MBA, MEd  more...
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Answer

Bone-mineral density (BMD) measurements with a CT scanner have the major advantage that the trabecular component can be identified, and thus the measurements can be confined to these parts. For both single- and dual-energy CT methods, careful calibration of the CT unit must be undertaken. In addition, decalcification and fat replacement of the trabeculae not only affect the BMD but also affect the atomic composition of the area. BMD can thus be measured by outlining the trabecular part of the bone being investigated, by calculating the mean Hounsfield number in the area, and by applying the calibration equation to this measured value. For dual-energy CT, the measurement involves 2 scans at different kilovolt peaks; the measured Hounsfield numbers are applied to a more complicated expression. [14, 9, 18, 19]

(See the image below.)

Involutional osteoporosis. Insufficiency fractures Involutional osteoporosis. Insufficiency fractures of the sacrum and the pubic rami are seen on an isotopic bone scan as a characteristic H, or Honda, sign (arrows), which appears as intense radiopharmaceutical uptake at the fracture sites.

Another promising method involves measurement of the amount of radiation from a monoenergetic gamma-ray source that is coherently and incoherently (Compton) scattered by the bone tissue. Because the amount that is coherently scattered is dependent on Z3 (where Z is the atomic number) and because the amount that is incoherently scattered is dependent on Z, the ratio of coherent to incoherent scatter is sensitive to BMD. By using a well-defined collimation of radiation source and detector, the volume investigated can be well defined and positioned in the trabecular part of the bone being investigated. [45]

The advantage of the CT methods is that the result is a true BMD (in milligrams of hydroxyapatite per unit volume) and that it is measured only in the bone tissue of interest (trabecular bone). The precision of the CT methods is high: 1-2% for the single-energy method and 3-5% for the dual-energy method. The accuracy is also high: 4–7% for the single-energy method, and 3–5% for the dual-energy method.

QCT is generally used to measure bone density in the lumbar spine, though it can be applied to other parts of the skeleton, such as the forearm. The accuracy and scanning time depend on the type of CT scanner used. This technique is the only BMD-measurement method that provides a true volumetric measurement of bone density (in milligrams per cubic centimeters) and a separate measurement of trabecular and cortical bone density. [19]

QCT has been used to assess vertebral fracture risk. It has been found to be superior to other methods for assessing age-related bone loss, for distinguishing fractures, and for diagnostic classification.

Developments in CT technology allow 3-dimensional (3D) volumetric BMD analysis of the proximal femur; high-resolution CT (HRCT) allows the analysis of trabecular structure. QCT bone-density measurements of the lumbar spine can be performed on standard CT scanners with provision of specialized software, and peripheral QCT (pQCT) measurements can be obtained on specially designed small-bore CT scanners.

The measurements are accurate and precise and require a comparatively low radiation dose as compared to that needed for a standard diagnostic CT procedure. QCT is more accurate than DXA in measuring BMD, especially in the spine in the older population group, as CT avoids the effects of degenerative disease and extraneous calcification. Developments in 3D QCT allow assessment of the hip and complicated situations in the spine, as when both scoliosis and vertebral fractures are present.

One major disadvantage of QCT is that artifacts hamper the CT data, reducing its accuracy. The usual sources of error include beam hardening, detected scatter, and system drift. The accuracy of QCT readings can be improved with careful attention to detail. Patients should be well centered and scanned using consistent settings. Reference phantoms can be scanned and the results used to correct for deterministic errors. Another limitation of QCT is a significantly higher radiation dose than that of DEXA. For most clinical purposes, DEXA has remained the method of choice over QCT.

The presence of excess fat in the marrow in trabecular bone in the aging population introduces an error in the BMD measurement of 7-15% per 10% of fat. This problem can be resolved by using dual energy, but at the expense of doubling the radiation exposure to the patient. The accuracy error and precision of QCT are 5-8%.


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