What is the role of ultrasonography 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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Langton first described the measurement of broadband ultrasound attenuation (BUA) in the calcaneus as a potential indicator of hip fracture risk. [48] The concept is based on the knowledge that the speed of sound and attenuation of sound wave are affected by the density, compressibility, viscosity, elasticity, and structure of the material it is traveling through. This technique marked a departure from the conventional methods of bone densitometry that used ionizing radiation. Ionizing radiation is attenuated at the atomic level, whereas ultrasound is attenuated at the macroscopic structural level. Some therefore suggest that BUA depends on the macroscopic structure of cancellous bone in addition to the bone-mineral density (BMD) assessed by using the ionizing radiation techniques. [24, 49, 16, 17]

BUA measurement in the calcaneus requires 1 transducer with 2 broadband ultrasound transducer components: One acts as the transmitter, and the other acts as the receiver. For a given material, ultrasound attenuation is always the same; this is known as the BUA index. To determine the attenuation index of any material (including bone), a broadband of ultrasound frequencies is passed through the full thickness of the material. The amplitude spectrum of the received signal is then compared with the spectrum of a reference material (water). By recording the frequency spectrum through water with and without the heel in position, a plot of attenuation with frequency is achieved. The difference between the 2 spectra is then plotted against frequency, giving a straight-line graph, the slope of which is the BUA index (in decibels per megahertz). The ultrasound frequencies used are in the range of 0.1-1 MHz. This range has become known as BUA.

The relationship between the index and BMD is not straightforward, however. The BUA index is highly influenced by bone structure, not only with regard to the number and thickness of bone trabeculae but also regarding their orientation with respect to the ultrasound beam. A plethora of quantitative ultrasonography (QUS) devices are now available. [12]

QUS for bone analysis is a nonionizing method in which the calcaneus is the measurement site. This technique is both cost-effective and accurate for identifying patients at risk for osteoporotic fracture. QUS has been scientifically validated in both fundamental in vitro studies and clinical in vivo studies. Clinical studies have shown that QUS parameters are sensitive to age-related changes, that they may be useful in distinguishing osteoporotic patients, and that they offer a prospective prediction of fracture risk comparable to that of axial DXA.

Normative data have been defined for several devices. QUS is more diverse than conventional bone densitometry, and both cortical and cancellous bone may be assessed to note their dissimilar pathophysiologic behavior. The 2 fundamental parameters of attenuation and velocity are often device specific and implemented or combined into proprietary parameters.

Comparisons of BMD obtained using BUA methods with BMD using more established techniques have resulted in relatively poor correlation, with r values ranging from 0.36 (BUA vs single-photon absorptiometry [SPA] or quantitative CT [QCT]) in osteoporotic patients to 0.8 (BUA vs SPA) in rheumatoid patients. The poor correlations may be partly attributed to the different sites and to the different physical quantities measured using the 2 techniques.

Whether QUS can be used to monitor treatment has not been conclusively shown. In monitoring the response to treatment, QUS can reliably show differences in responses between individuals. These differences can be predictive of long-term differences in bone mass that are not simply due to measurement error.

A number of factors can affect the accuracy and precision of BUA and produce false-positive or false-negative results. The anatomically incorrect placement of the region to be examined is one of these factors. Other factors are patient specific and may affect bone measurements; these are variability in bone width and soft-tissue thickness or composition; marrow composition; and temperature. Error in measurement can be introduced by diffraction, which affects both attenuation and velocity measurements and is device specific.

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