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					                                              BONE QUANTITATIVE ULTRASOUND IMAGING

                                                                         Pascal Laugier
                                              Laboratoire d’Imagerie Paramétrique CNRS-Université Paris 6, 15
                                                           rue de l'école de Médecine, Paris, France

Introduction                                                    Technol ogical as pects and signal processing

     Osteoporosis, the most frequent bone disease, is a                Although bone properties can be assessed with
systemic skeletal disease characterized by low bone mass        ultrasound at several skeletal sites, the calcaneus is the
and micro -architectural deteriorat ion of bone tissue, with    most popular measurement site and the majority of
a consequent increase in bone fragility and a susceptibility    clin ical reports has focused on this bone. The calcaneus is
to fracture. Over the past 10 years, osteoporosis has           easily accessible, and the medio-lateral surfaces are fairly
emerged as a major clin ical challenge, related to its          flat and parallel, thus potentially reducing measurement
prevalence, the financial burden placed on public health        errors. In addition, several epidemiological studies
resources and the morbidity and mortality of associated         reported that the calcaneus appeared to be among the
fractures. About one third of post-menopausal wo men            optimal BMD measurement site for routine screening of
will have a fracture related to osteoporosis before the end     perimenopausal women to predict the risk of any type of
of their life. In the European commun ity, it contributes to    osteoporotic fractures. The transverse transmission
causing more than 400,000 hip fractures annually.               technique uses two transducers, one acting as a transmitter
     Prevention of osteoporosis has now been recognized         and the second one acting as a receiver. Both transducers
as a major priority in research and health promotion.           are placed on each side of the skeletal site to be tested
Effective prevention and treatment programs require             along its med iolateral axis (i.e. widthwise). The method
quick, inexpensive diagnostic methods suited for wide-          has been applied to the measurement of the slope of the
spread use. For that, it is necessary to evaluate the factors   frequency-dependent attenuation the so-called broadband
of fracture risk, the most important of which is bone           ultrasonic attenuation or BUA in dB/M Hz) and the speed
strength. Bone density accounts for most of the strength        of sound (SoS, m/s) at the calcaneus.
of bone tissue, and today reference methods for skeletal               Ultrasonic measurements are performed using a
status assessment are based on X-ray absorptiometry             substitution method, consisting in the comparison of the
techniques measuring bone mineral density (BM D).               Fourier spectrum of the signal transmitted through bone to
However, bone mass is not sufficient to explain bone            the spectrum of a reference signal transmitted through a
strength. Bone microarch itecture which is not captured by      reference mediu m of known attenuation, e.g. water. The
conventional X-ray-based densitometry also contributes to       frequency-dependent         attenuation    of     ultrasound
bone strength independently of bone mass.                       transmitted through the skeletal site is derived from the
     The alternative to X-ray introduced about fifteen          ratio of the magnitude spectra. As the attenuation varies
years ago is represented by quantitative ultrasound (QUS)       linearly with frequency, the slope of a linear regression fit
methods. As several bone properties contribute                  to the frequency-dependent attenuation between 0.2 and
independently to bone strength, mult iparametric                0.6 MHz y ields the BUA value in d B/MHz. SoS is
approaches combining different ultrasonic parameters            obtained from the difference of time -of-flight of the
reflecting independent bone properties, i.e. BMD and            reference signal and the signal transmitted through bone.
micro -architecture, may offer a mo re co mplete assessment     Given the small amount of soft tissue, the influence of
of bone status, improve the precision of d iagnosis and         soft tissue on BUA and SoS is generally considered as
favor the dissemination of the technique for osteoporosis       being small and consequently neglected.
and other bone related diseases. Both these items have                 Transmission measurements only partially exploit
been addressed in our laboratory and a QUS imaging              the information related to the interaction between the
based technology has been developed to assess BMD and           elastic wave and bone micro-arch itecture. It is known that
bone microarch itecture using a single device. It uses the      ultrasonic backscattered signals depend on the
transmission and the reflection of low frequency                scatterering med iu m micro-structure, in this case the bone
ultrasound (approximately 500 kHz) through the                  trabecular network         micro-architecture.    Ultrasonic
calcaneus (heel bone).                                          backscatter may be useful for the measurement of specific
micro -structural properties such as mean trabecular          adjustment to experimental backscatter coefficient allows
thickness or mean trabecular spacing. In addition, skeletal   the assessment of mean trabecular thickness, a micro -
sites, which are difficult to reach by transmission, could    architectural parameter.
be evaluated by such reflection techniques. Measurements            Currently available imag ing devices measure the
are performed by means of a single transducer used in         heel immersed in a bath. We have recently demonstrated
pulse-echo mode. The simplest method is a substitution        the feasibility of QUS imaging at the wrist (Fig. 1) with
one, in which the signal scattered from the region under      reasonable precision. Waterless contact imaging would
test is compared to the signal fro m a standard reflecting    represent substantial technological progress toward
target. The backscatter coefficient is obtained by dividing   quantitative ultrasonic imaging at mu ltiple skeletal sites.
the average backscatter power spectrum fro m the              The development of QUS techniques to estimate bone
scattering volume by the power spectrum co mputed for         mineral losses during weightlessness of space flights,
the calibration echo. The frequency-averaged backscatter      taking into account the constraints applying to space
coefficient between 0.2 to 0.6 MHz has been termed            equipment, has been supported by the European Space
Broadband Ultrasonic Backscatter (BUB).                       Agency, based on the technical characteristics of the
      Most devices that use fixed probes take the             UBIS device, and has led to the development of an
measurement in a single point without accurate control of     innovative waterless imag ing device using 2D arrays of
the transducers’ position with respect to the individual      transducers.
heel anatomy. Bones are irregularly shaped and have an
inhomogeneous inner structure. For that reason, accuracy
and precision are affected by the location within the bone    Conclusion
interrogated by the ultrasonic beam. The ultrasound bone
imaging system (UBIS) developed for the first time in our          Innovative QUS imaging based devices meet the
laboratory for the calcaneus has the potential to evaluate    need for bone densitometry to assist in the diagnosis of
standardized regions of interest in all patients and to       osteoporosis and in the treatment of osteoporotic patients.
reproduce the analysis of the same reg ion of bone in         It may become an acceptable alternative to existing X-ray
series of scans acquired over a follow-up period of long      based densitometry machines at reduced cost and with no
duration (Fig.1). QUS images are performed by using a         ionizing radiation, thereby contributing to improve overall
mechanically driven pair of single -element focused           quality of life of osteoporotic patients, i.e. reducing
transducers immersed in a water bath (pixel size 1 mm2 ,      fractures incidence, saving life and avoiding pain,
field of v iew 60x60 mm2 ). The acquisition of backscatter    fractures and disability.
radiofrequency signals has been implemented on the
UBIS device. A mean value of QUS parameters (BUA,
SoS, BUB) can be calculated by averaging the values           REFERENCES
within a region of interest (ROI) placed on the image. The
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    Fig. 1: UBIS device developed and commercialized
 by Diagnostic Medical Systems (Montpellier, France);


     Fig. 2: BUA image of the calcaneus (left ) and X-ray
         image of the foot (right) (same subject)

              BUA          BMD

      Fig. 3: BUA (left) and BM D (right) image of the
                 wrist (same subject).

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