Reducing temperature influence on dry quantitative ultrasound bone assessment with constant temperature control

Abstract

Nowadays, ultrasonic bone assessment is increasingly being used to assess bone status. Therefore, the purpose of this study was to enhance the precision of ultrasonic bone assessment by reducing the influence of temperature in a dry, gel coupled transducer system. A warm airflow generator was designed to make the measurement temperature constant (35 ± 1 °C). Thirty people were recruited for the evaluation of in-vivo performance. The short-term precision was performed 10 times with repositioning during a consecutive measurement session within 20 min. It was expressed as root-mean square average of coefficient of variation, which is abbreviated for CV_RMS. The CV_RMS was 3.84% for broadband ultrasound attenuation, and 0.30% for speed of sound. The Pearson correlations between gel coupled transducer system and dual energy X-ray absorptiometry (DEXA) were 0.808 (p < 0.001) for broadband ultrasound attenuation, and 0.586 (p < 0.005) for speed of sound. The result showed the high performance of reproducibility and the significant (p < 0.005) correlations with DEXA in the dry, gel coupled transducer system.

Introduction

There is an increasing amount of interest in the development of non-invasive diagnostic techniques for the detection of osteoporosis and the prediction of the risk of a bone fracture [1]. As a promising approach, quantitative ultrasound (QUS) is an inexpensive, portable and non-ionizing alternative to dual energy X-ray absorptiometry (DEXA). Normally, the detection is performed on the heel bone, the calcaneus. This bone is well suited for ultrasonic investigations. It is mainly composed of a trabecular bone which is easy to access. Most of QUS devices measure two acoustic properties of the heel bone: broadband ultrasound attenuation (BUA) and speed of sound (SOS) [1].

It is claimed that the precision of ultrasonic bone assessment is affected by temperature because the acoustic properties of biological tissues (soft tissues, adipose, cortical bone and marrow) are generally temperature-dependent [2]. Many experiments have demonstrated the effect. Morris et al. [3] measured calcaneal BUA and SOS in a subject as a function of time after switching the foot from a 40 °C water bath to a 21 °C water bath. They found that BUA decreased by 3.8% and SOS increased by 0.8% over a 30 min interval. Chappard et al. [4] monitored BUA and SOS in three groups (11 pre-menopausal women, 10 postmenopausal women, and 10 men) as functions of time over a 25 min interval after immersion into a 30 °C water bath. Contrary to Morris and coworkers, Chappard and colleagues observed that BUA increased by 20.7% in postmenopausal women, 6.8% in pre-menopausal women, and 2.5% in men, while SOS decreased by 0.8% in both groups of women and increased by 0.4% in men. Wear [5] found the temperature-related variation of defatted human calcaneus was −0.18 dB/cm MHz °C, he concluded that the temperature-related effects needed to be taken into account when performing diagnostic measurements, especially those that required high precision such as monitoring responses to drug intervention. These findings result from the superposition of many complex situations, including diverse sound paths through the calcaneus and various soft tissues. But, to a certainty, the temperature is an important factor to affect the QUS measurements.

Ultrasound transducers may be coupled to the subject with water bath (wet systems), water-filled bladders (smart dry systems) or gel pads (dry systems). With wet systems, BUA and SOS measurements are carried out in a water bath, in which the heel is immersed between two transducers that can be either fixed or moving. Wet systems are soon replaced by smart dry or dry systems because of the easier portability and fewer risks about hygiene in the latter two systems [6]. With smart dry systems, the bladders are filled with water to provide coupling of the ultrasound signal from the transducer to the heel. Although the smart dry systems have fewer risks about hygiene than wet systems, there are two shortcomings in smart dry systems. First, the membranes of bladders are always thin and soft, so they are easy to break, which makes the measurement inconvenient. Second, the bone thickness is difficult to accurately measure due to the irregular shape of bladders, so many devices rely on the assumption that all subjects have the same bone thickness (for example 40 mm) in the SOS calculation. However, the individual bone thickness is not always the same, so this method may produce errors. With dry systems, two transducers are positioned by motors on each side of the heel to maintain constant strain force in direct contact with the subject’s skin, the waterless gel pads provide coupling of the ultrasound signal from the transducers to the heel. As the pads have regular shapes, solid and more durable than bladders, on the one hand, the dry systems possess easier portability, better hygiene than wet systems, on the other hand, the dry systems have better convenience, longer working life, and more accurate measurements of calcaneus thickness than the smart dry systems. For example, in our dry system, the working life of gel pad is several years except man-made sabotage, the measurement error of calcaneus thickness is less than ±0.1 mm. Therefore, the dry systems should enable the significant expansion of QUS measurements to, for example, primary care physicians [7].

In some wet and smart dry systems, the temperature is usually controlled by the way of keeping the water temperature constant. However, we are unable to find a method to reduce temperature influence in dry systems. The reason may be that it is easier to maintain the constant water temperature in wet and smart dry systems in comparison with dry gel pads. Therefore, a dry system was built for QUS bone assessment to reduce temperature influence, the homothermal warm airflow was generated by a special apparatus which is described in the next section. This warm airflow could keep the ambient temperature of measurements constant (35 ± 1 °C) in different parts, including soft tissue, heel, and gel pads.