• 16
    Dec
  • Quantitative ultrasound of bone: calcaneus

Introduction

The measurement of bone density has became an established method to determine the skeletal status for diagnosis and treat­ment of osteoporosis. Diagnosis of osteoporosis today is based mainly on this measurement since bone density at vari­ous anatomic sites has been found to be strongly associated with future fractures. However, both epidemiological and interventional investigations have identified risk factors for fracture other than bone mass. In particular it has been demonstrated that bone structure may play an important role in the determi­nation of fracture. Therefore the ideal diagnostic device should be able to measure bone fragility, whatever the cause is, and not just any decrease in bone mass. QUS seems to provide in­formation that is partly independent from bone density and is able to predict osteoporotic fractures. QUS offer the advantage of small size, relatively quick and simple measurements, no need of ionising radiation and low cost. For these characteris­tics, QUS has continued to be of interest in the last two decades; in 1999, the United States Food and Drug Adminis­tration approved five different ultrasound instruments for the routine diagnosis of osteoporosis, determination of fracture risk and monitoring bone changes.

Ultrasound parameters

Ultrasound is a mechanical wave vibrating at a frequency range from 20,000 waves/s to 100,000 waves/s. The ability of most physical methods of diagnosis (quantitative ultrasound of bone in this case) to provide information on the properties of a particular medium depends on the way in which the ultrasound wave is modified by the medium. With ultrasonic propagation through bone, both the velocity of transmission and the ampli­tude are affected by the medium. Bone tissue therefore may be characterized in terms of ultrasound velocity and ultrasound at­tenuation. From this simple, fundamental starting point, it is in­teresting to note the variety of approach taken in implementing clinical measurements of velocity and attenuation in commer­cial quantitative ultrasound devices. In fact, commercial bone QUS has utilized transit time velocity measurement (the time for the arrival of ultrasound signal at the receiving transducer), with different definition: bone velocity (BV), heel velocity (HV), speed of sound (SOS). For attenuation, more uniformity is pre­sent among commercial devices; in fact attenuation is typically characterized by broadband ultrasound attenuation (BUA), the slope at which attenuation increases with frequencies, general­ly between 0.2 MHz and 0.6 MHz. The velocity of ultrasound wave propagation is determined by the transit time and by the width crossed, and it is expressed as m/s. Speed of sound (SOS) can be related to the mechanical properties by the equation: SOS = (E/r)1/2, where E is the modulus of elasticity and r is the bone density expressed in gr/cm3. Current com­mercial systems for studying the calcaneus use two transduc­ers (a transmitter and a receiver), positioned on each side of the bone to be measured, and three different methods of calcu­lating velocity have been employed: heel velocity (calcaneus + soft tissue), bone velocity (calcaneus only) and time of flight velocity (TOF) (between transducers positioned at a fixed dis­tance and assuming a constant heel thickness). These three approaches to velocity derivation yield slightly different values, but correlate strongly with each other. As an ultrasound wave propagates through the bone some of its energy is lost, and this phenomenon is known as attenuation. Factors con­tributing to the attenuation in bone include scattering, diffrac­tion and absorption. Absorption predominates in cortical bone and scattering in trabecular bone. The increase in attenuation as a function of the frequency is measured by comparing the amplitude spectrum for a reference material with that of the measured sample. The slope of attenuation (BUA) in dB/MHz is given by linear regression of the spectral amplitude differ­ence. Some manufacturers have implemented derived para­meters from BUA and velocity, such as Stiffness, Quantitative Ultrasound Index (QUI), soundness and osteo sono-assess- ment index (OSI). The combination of BUA and SOS into a sin­gle parameter has been shown to improve precision; further­more, from the point of view of clinical interpretation, a single parameter, which combines velocity and attenuation, can sim­plify interpretation. Suffer no more! Buy asthma inhalers online at a price you can afford.

 

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