What are the limitations of imaging techniques in the diagnosis 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

Answer

Plain radiography is widely available but is not preferred because it is not suitable for the early detection of osteoporosis. Changes on plain radiographs can be seen only after approximately 30% of the bone is lost. However, plain radiographs are useful to rule out osteoporotic fractures and other pathology, such as myeloma. Radiation exposure for an average radiograph is approximately 50 mrem. (See the images below.)

Involutional osteoporosis. Note the overall reduct Involutional osteoporosis. Note the overall reduction in the bone density and moderate kyphosis of the dorsal spine.
Involutional osteoporosis. Note the lateral wedge Involutional osteoporosis. Note the lateral wedge fracture of L3 and a central burst fracture of L5. The patient had a recent fall.

Bone-density measurements are not an effective method to monitor the response to treatment because changes in bone density may not be detected for up to 2 years. Radiation techniques to measure BMD, such as single-photon absorptiometry and DPA, have several limitations. The most important limitation is posed by the inhomogeneity of soft tissues because different components have their own attenuation coefficients. Fat has the lowest attenuation and is generally unevenly distributed in the region of BMD measurement; therefore, it has a variable affect on the accuracy of the measurement. The accuracy of density and attenuation coefficients for the bone mineral and soft-tissue components are also uncertain, though this limitation can partly be overcome with direct DPA and DXA measurement.

The accuracy of photon absorptiometry has been estimated to be 4-8% for SPA and 4-6% for DXA. However, the accuracy can be as low as 11% and is worse for lateral projections, as compared to anteroposterior (AP) projections.

SPA is used to measure forearm bone density, and it may not provide an accurate assessment of bone density of the spine or hip. The procedure takes about half an hour. Radiation exposure from SPA is approximately 5 mrem. DPA is used to measure the density of the spine or hip. The precision of DPA is acceptable for diagnosing osteoporosis but insufficient for detecting changes in individual patients. Radiation exposure from DPA is approximately 5 mrem.

Soft-tissue inhomogeneity affects the accuracy of QCT. The content of yellow marrow in the vertebrae may have a significant effect on the accuracy of BMD measurements. Machine-related artifacts, such as beam hardening, might also affect its accuracy. Overall, the value of single-energy methods is in the range of 5-15%. With 2 effective beam energies, this changes to 3-10%, but at the cost of poor precision. The precision and accuracy of QCT is good, but the radiation involved is relatively high (approximately 200-300 mrem). Therefore, QCT is not a preferred technique when other methods are available.

Ultrasound transmission is attenuated by the thickness and composition of tissues within and surrounding the bone. In trabecular bone, fatty marrow in the intertrabecular spaces influences both broadband ultrasound attenuation (BUA) and velocity. Measurements are determined by means of ultrasonography of the heel. Osteoporosis Australia's Consensus Statement [42] states that this type of measurement of bone strength lacks acceptable measurement precision and long-term stability to be recommended for use in the diagnosis of osteoporosis. If such ultrasound measurements show low bone density, the patient should be referred for DEXA because of its high accuracy and precision.


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