• 17
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  • Quantitative ultrasound of bone: calcaneus. QUS and bone

Velocity and BUA provide quantitative information on ultra­sound interaction with the medium; it is currently accepted that QUS parameters are not only influenced by bone density, but also by bone structure. Theory suggests that BUA is deter­mined by bone density and bone microarchitecture, while SOS is influenced by the elasticity of bone as well as by bone densi­ty. However, the exact mechanisms of ultrasound interaction with bone and the physical properties measured remain unde­termined. Considerable data exist showing the positive depen­dence of attenuation and velocity on bone mineral density. The relationship between QUS and BMD are higher in vitro than in vivo, probably also as a consequence of the presence of soft tissue and of anatomic discordance; otherwise this poor association with QUS and BMD has been often attributed to the fact that QUS may measure structure. In particular it has been shown that BUA depends on the trabecular orientation; moreover, Gluer et al. have suggested that SOS is relat­ed to trabecular separation and BUA either to trabecular sepa­ration or connectivity. Other Authors did not find any relation­ship between histomorphometry and QUS parameters after correction for BMD. Nicholson et al. have shown that the ability of QUS to reflect bone structure is also dependent on the direction of the measurement. Recently it has been demon­strated, in human calcaneal specimens, that QUS reflects es­pecially BMD and to a less extent, bone microarchitecture. It has been also shown that SOS, after correction for BMD, is the best predictor of Young modulus, indicating that this para­meter can give information on the mechanical architecture of trabecolar bone. In cadaver studies, calcaneal ultrasound correlates with femoral and vertebral strength, but the predic­tive ability is less than, and it is not independent, from BMD measurements. However, in contrast to these results, Lochmuller et al. found that calcaneal QUS correlates with failure load of the proximal femur similarly to femoral neck BMD.

In conclusion, qualitative evidence for the influence of structure on ultrasound exists, but there are no conclusive data demon­strating that ultrasound provides useful information on specific structural parameter at clinical sites.

Calcaneal QUS devices

Since the pioneering work of Langton et al. in 1984, many clinical quantitative ultrasound machines have been developed and there are currently a multitude of different devices on the market. Ultrasound transducers are coupled to the subject ei­ther with water (wet systems) or gel (dry systems). The sites measured also vary, but most of the available devices measure the calcaneus. The calcaneus is the most studied skeletal site for QUS assessment for several reasons; the high percentage of trabecolar bone (90%), which has a turnover higher than cortical bone, allows early evidence of metabolic changes; the calcaneus is also easily accessible and the mediolateral sur­faces are fairly flat and parallel, thus reducing repositioning er­ror. The choice of the calcaneus as a test site has been sup­ported by Black et al., who reported that the calcaneus ap­peared to be the optimal bone mineral density measurement site in the prediction of any type of osteoporotic fracture in peri- menopausal women. In Table I are reported commercial cal­caneal QUS devices. Although the Walker Sonix UBA 575 is no longer available, it is listed because it provides a back­ground for current systems and also because many studies were carried out using this system. These devices show great technological diversity: coupling, mode of data acquisition, vari­ables, calibration method, hardware performance, analysis al­gorithms, transducer designs.

Figure 1 - Achilles Insight device

Figure 1 – Achilles Insight device (GE, Lunar).

These differences, combined with the fact that no absolute standard exists for ultrasound measurements, cause the impossibility to directly translate the clinical utility of a validated system into that of other technologi­cally different QUS devices. Among water bath systems, the Achilles plus, available in the early 1990s, for its proved ability to predict fragility fractures and comparability to central DXA, is one of the most used ultrasound devices. Compared with wa­ter-coupled systems, gel-coupled systems, such as Sahara, have the advantage of being more portable and having fewer potential concerns about hygiene, disadvantage results from their reduced control over the measurement environment, such as the stability of the temperature and hydration of the tested heel.
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Table I – Calcaneal QUS devices.

Device

Manufacture

Trasmission

Parameter

Precision (cv) (%)

BUA

0.8-4

Achilles plus

GE Lunar

Water

SOS Stiffness

0.2-0.5 1-2.7

BUA

Not available

Achilles Express

GE Lunar

Water and gel

SOS Stiffness

Not available 1-2

BUA

1.4-3.1

Achilles Insight

GE Lunar

Water and alcohol

SOS Stiffness

0.2-0.4 1.9-2.1

CUBA

McCue

Gel

BUA SOS

1.5-4.5 0.2-0.6

DTU-one

Osteometer

Water

BUA SOS

0.8-2.5 0.2-0.4

Paris

Nordland

Gel

BUA SOS

1.8 0.3

QUS-2

Metra

Gel

BUA

0.8-2.0

BUA

0.8-5.0

Sahara

Hologic

Gel

SOS QUI

0.2-0.4 1-3.5

UBIS 5000

DMS

Water

BUA SOS

0.8-2.5 0.2-0.4

UBA575+

Walker Sonix

Water

BUA SOS

2-5 0.2-0.6

These factors could significantly influence the precision of the measurements. GE Lunar has recently released a new ver­sion of Achilles called Achilles Insight: this is an imaging ultra- sonometer, based on the Achilles plus, but water is contained in an inflatable silicone pad and isopropyl alcohol spray is used to provide the coupling (Fig. 1). A good agreement between Achilles plus and Achilles Insight has been recently demon­strated. Regarding the precision, for the same variables, there is moderate difference among different devices; in Table I are reported the values of short-term precision usually report­ed for each device from many studies (Table I). For QUS mea­surements at calcaneus sources of low precision include pres­ence of soft tissue, thickness variation, coupling agent charac­teristics, repositioning error, duration of immersion of the foot and temperature. Foot positioning is considered the primary source of error in BUA measurement because of the lack of homogeneity of the calcaneus.

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