Quantitative Ultrasound (QUS)
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QUS devices emit inaudible high frequency sound waves in the ultrasonic range, typically between 0.1 and 1.0 megahertz (MHz). The sound waves are produced and detected by means of high-efficiency piezoelectric transducers, which must have good acoustical contact with the skin over the bone being tested. Technical differences among QUS systems are great, with different instruments using variable frequencies, different transducer sizes, and sometimes measuring different regions of interest, even at the same skeletal site. The calcaneus is the skeletal site most often tested, although other bones, including the radius, tibia, and finger phalanges, can be used. Commercial QUS systems usually measure two parameters- the speed of sound (SOS) and broadband ultrasound attenuation (BUA). A proprietary value, such as the “quantitative ultrasound index” (QUI) with the Hologic Sahara or “stiffness index” with the GE Healthcare Achilles Express, may be calculated from a combination of these measurements. SOS varies according to the type of bone, with a typical range of 3000-3600 meters per second (m/sec) with cortical bone and 1650-2300 m/sec for trabecular bone (22). A higher bone density is associated with a higher SOS. BUA, reported as decibels per megahertz (dB/MHz), is a measurement of the loss of energy, or attenuation, of the sound wave as it passes through bone. As with SOS, a higher bone density is associated with a higher BUA. Values obtained from calculations using ultrasound parameters may be used to generate an estimated BMD and a T-score. The T-score derived from a QUS measurement is not the same as a T-score from a DXA. QUS cannot be used for diagnostic classification and is not clinically useful to monitor the effects of therapy (23).
Radiofrequency Echographic Multi Spectrometry (REMS) assesses bone health and fracture risk with an ultrasound scan of the lumbar spine and proximal femur, thereby overcoming the limitation of QUS of only measuring peripheral skeletal sites. REMS technology uses a portable device with a transducer that transmits ultrasound waves to the target axial skeletal site and a receiver that captures the resultant back-scattered waveforms with B-mode image reconstruction of the region of interest. There are studies that support a strong correlation between REMS and DXA measurements of BMD (24). Potential clinical applications include its use in frail patients with limited mobility, bedside measurements in hospitalized patients, and special populations such as pregnant women and children.
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