Determining BMD through Pulse-Echo Ultrasound

SPECIFIC AIMS

Osteoporosis is one of the most common skeletal diseases. It is characterized by thinning of bone tissue and loss of bone mass density over time. Low bone density is one of the important risk factors for osteoporosis and high predictor of fracture. Increase of loss of bone density leads to an increase in bone fragility and susceptibility to fracture.

Currently, bone mass density testing is considered the best way to determine if a person has osteoporosis, osteopenia, or normal bone density for their sex and age. The gold standard BMD test is Dual-energy X-ray absorptiometry (DEXA), is currently the most accurate way measure to test for BMD. DEXA is able to measure BMD at high risk fracture sites in the central skeleton such as the hip and spine. However, it has disadvantages such as high costs, portability, radiation exposure, etc. New techniques for measuring BMD such as ultrasound do not provide a definitive diagnosis due to some limitations. Current ultrasound technique is not able to test in high risk fracture bones such as the spine and hip. Quantitative ultrasound techniques have been shown to offer similar ability to diagnose osteoporosis as DEXA (Hans et al. 1996; Njeh et al. 2000; Frost et al. 2001; Glüer et al. 2004). Most clinical QUS devices are based on the through-transmission measurement of calcaneus (Njeh et al. 1997), which is not a typical fracture site (Hasselman et al. 2003).

BMD at the calcaneus bone is not always a sufficient predictor for bones in the central skeleton. The correlation between the calcaneus’s bone BMD and hip’s bone BMD is not high enough when determining fracture risk at bones of the central skeleton such as the hip.  A new method should help determine the BMD at the fracture site directly rather than using other bone as a predictor. Pulse-echo ultrasound technique sounds promising as it uses only one transducer rather than two as the through transmission technique which could possible test BMD at the hip as the gold standard DEXA diagnosis.

The proposed research will test two hypotheses:

1) Pulse-echo ultrasound has equal or higher accuracy in determining the BMD at the calcaneus bone as the through transmission ultrasound.

2) Demonstrate pulse-echo ultrasound is able to determine BMD of bones surround by soft tissue in the central skeleton such as the hip.

AIM 1: Determine the BMD of the calcaneus bone using a pulse-echo ultrasound technique The pulse-echo technique accuracy is going to be tested in determining the BMD at the calcaneus bone and comparing it to the current through transmission ultrasound technique. If found that the pulse-echo technique determine BMD at the calcaneus bone with the same or better than other techniques, further studies could be made on different fracture sites caused by osteoporosis than the actual ultrasound technique cannot perform.

AIM 2: Determine whether using a pulse-echo ultrasound technique accounts for soft -tissue surrounding the hip and increases accuracy in determining BMD We will test that using a pulse-echo ultrasound technique which uses only one transducer will be able to determine bone BMD at fracture sites that were not possible to evaluate with through transmission ultrasound technique. Based on the advantage of only one transducer and a new technique of ultrasound that take into account the tissue surrounding the bone studied. Results will be compared to the gold standard (axial DEXA) to determine BMD of the hip.

RESEARCH STRATEGY

A. SIGNIFICANTE TO MUSCULOSKETLETAL HEALTH

Osteoporosis is currently one of the highest occurring bone diseases. Osteoporosis means porous bones with increase porosity the bones become more brittle. Osteoporosis is linked to high risk fractures, such fractures occur commonly in spine, hip and wrist.  Unfortunately, osteoporosis commonly remains undiagnosed until a fracture occurs. Previous studies have shown that a bone mineral (BMD) density test is the best way to determine the bone health of an individual. BMD test can identify three key aspects if there is actually osteoporosis, determine the risk for fractures and measure response to osteoporosis treatment.  The gold standard test for BMD is called a dual-energy x-ray absorptiometry, or DEXA test. Although DEXA test gives accurate BMD results it has some drawbacks. Limitations to a DEXA test are it has a high cost, radiation exposure and low portability. Other test currently used to determine BMD include CT and Ultrasound. Ultrasound which is a possible alternative to DEXA has still some limitations which decrease its accuracy to determine BMD.

B. INNOVATION

Based on previous studies using Ultrasound technique to determine BMD we hypothesize that by applying an ultrasound pulse-echo approach we can determine with high accuracy BMD on bones with high risk fracture in central skeleton such as the hip. If found the ultrasound pulse-echo technique has a similar accuracy such as the DEXA we would have a new diagnosing test that has a low-cost, no radiation exposure and high portability compared to the gold standard in BMD determination.

Overlying soft tissue induces significant errors in bone ultrasound measurements

Previous studies have shown that the current clinical ultrasound through transmission technique suffer from measurement uncertainties that are related to soft tissue surround the bone studied. Soft tissues overlying the bone have a major impact on the measurement parameters in a BMD test. The variable thickness and composition of the soft tissue layer overlying the skeletal bones significantly increases uncertainties in bone US measurements (Kotzki et al. 1994; Gomez et al. 1997; Johansen and Stone 1997; Chappard et al. 2000; Riekkinen et al. 2006). Central skeletal bones such as hip which have a high risk fracture are surrounding by moderate quantity of soft tissues causing errors with the information received by an ultrasound transducer. Previous studies have shown that in order to reduce or minimize such errors related to soft tissue surrounding the bone a selection of optimal US frequencies must be determined. Ultrasound pulse-echo technique which only uses one transducer may determine and reduce soft tissue at central skeletal bones such as the hip which is impossible to do with the current through-transmission ultrasound technique.

Most clinical QUS devices are based on the through-transmission measurement of calcaneus

Most bone ultrasound devices are designed for through-transmission measurements of the calcaneus bone. Current ultrasound devices are only able to diagnose bone in the peripheral locations such as the heel. Clinical US devices used currently utilize the through transmission technique which require the use of two transducers placed on opposite ends of the bone studied. This technique accuracy is highly affected by soft tissue surrounding the bone analyzed leading to the heel as the optimal bone for BMD measurement.

Fig.1 Quantitative Ultrasound (QUS) is a high frequency sound wave. We may measure how quickly sound travels through bone, this is termed velocity which is measured as meters per second (m s-1), or how much sound is absorbed by the bone, generally referred to as Broadband Ultrasound Attenuation

Using the pulse-echo ultrasound technique only one transducer would be required and the backscattered wave would be analyzed rather than the attenuation. The flexibility of one transducer lead to predict more accurately by direct measurement at high risk fracture sites such as the hip.

C. APPROACH

Overview

In previous studies show an urge for surpassing the limitations on the current ultrasound technique while still keeping the advantages that promotes ultrasound over DEXA in BMD determination. In the proposed research, we will

1. Determine whether pulse-echo ultrasound has equal or higher accuracy in determining the BMD at the calcaneus bone as the through transmission ultrasound (AIM 1), and
2. determine whether pulse-echo ultrasound is able to determine BMD of bones surrounded by soft tissue in the central skeleton such as the hip (AIM 2).

Experiments AIM 1

These studies will determine the accuracy of using a pulse-echo ultrasound to determine BMD at the calcaneus bone. The results will be compared to a trough-transmission ultrasound. Bone density of the heel bone will be measured with an ultrasound transducer in two different modes through transmission and pulse-echo. This experiment will be carried in-vivo were 100 volunteers both men and women between 40-50 years of age will participate. In large-scale screening studies in the general population, quantitative ultrasound measurement of the bone (QUS) has been used to identify people at risk for developing osteoporosis and fractures (Hollaender et al., 2009; Khaw et al., 2004). The ultrasound transducer beam will angled slightly posteriorly to maintain the beam axis as close to perpendicular to the plantar surface of the os calcis as possible. Three measurements at slightly different locations along the axis of the os calcis will be obtained for the two different ultrasound modes. Experiments are summarized in Table 1.

Table 1: Summary of experiments and analyses for Experiment 1: (N=100/group)

Expected Results and Interpretation of Data

Using the broadband ultrasound attenuation (BUA) and speed of sound (SOS) parameters for through-transmission mode t-score will be derived in order to obtain bone mineral density at calcaneus bone. Same procedure will be completed with pulse-echo mode but using different parameters such as broadband ultrasound backscattering. We expect a high correlation between when comparing BMD obtained with through-transmission mode and pulse-echo mode.

Experiments AIM 2

These studies will determine whether pulse-echo ultrasound is able to determine BMD of bones surrounded by soft tissue in the central skeleton such as the hip. During recent years, numerous studies have focused on developing pulse-echo ultrasonic techniques for the characterization of trabecular bone and for aiding in the diagnostics of osteoporosis (Chaffai et al. 2002; Hakulinen et al. 2004, 2005, 2006; Hoffmeister et al. 2002a, 2002b, 2006; Padilla et al. 2008; Riekkinen et al. 2007a, 2007b; Roberjot et al. 1996; Roux et al. 2001; Wear 1999, 2003, 2008; Wear and Laib 2003; Wear et al. 2005). Based on the results obtained in experiments AIM 1, we will determine if using pulse-echo ultrasound mode for determining BMD at heel is a good indicator when predicting BMD at peripheral bones such as the hip.

However, the main purpose of this aim is to evaluate efficacy of this new ultrasound mode to determine BMD in bones that have a high risk fracture and to compare the results with the gold standard which is axial DEXA when evaluating the BMD at the hip. We will determine the BMD of 30 female volunteers over 50 years old, which is the age were most of the fractures due to osteoporosis or low BMD commonly occur. Previous to evaluation volunteer information would be collected such as BMI and menopause age. Single ultrasound transducer will be used to determine BMD at hip.

Table 2: Summary of parameters analyzed and diagnosing techniques for Experiment 2: (N=30/group)

Expected Results and Interpretation of Data

Using the ultrasound pulse-echo technique we expect to determine BMD at hip with high accuracy similar to DEXA since using the broadband ultrasound backscattering (BUB) parameter we may account for the soft tissue effect that commonly introduces errors when using ultrasound. Soft tissue surrounding the hip would be analyzed by measuring the reflection from the surface of the bone with two frequencies the thickness of lean and adipose tissues.

Methods and Analysis AIM1:

• Ultrasonic measurements will be performed by using two broadband focused transducers in order to compare it to the current ultrasound technique (through transmission). The device could will be used in two different modes: transmission and backscatter
• Before bone density measurement, a questionnaire will be taken for each individual. This questionnaire includes: basic information, body measurements (body weight, body height, WHR), family history of osteoporosis and fracture.
• Both heels left and right would be measured three times for their BMD
• For this study the WHO criteria will be applied, classifying patients with BMD with a standard deviation of over 2.5 lower than the average for a young adult (T-score < -2.5) as osteoporotic and patients with a T-score of between -1 and -2.5 as osteopenic.
• Both T-score and Z-score will be computed. The T-score results from the comparison of the volunteers’ bone status with the average peak value in healthy young people and the Z-score provides a context for a participants’ bone status by comparing individual measurement values with the mean value for people of the same age and gender.

Methods and Analysis AIM2:

Similar methods and analysis as in AIM 1 will be executed. AIM 2 will use a backscattering mode in the ultrasound system to be able to determine the BMD at volunteer’s hip. The backscatter coefficient will be measured as proposed in previous studies by using a substitution technique, in which the signal scattered from the region under test is compared with the signal from a standard reflecting target (Ueda M et al. 1985). Results will be compared to axial DEXA BMD measurements.

KEY PRELIMINARY STUDIES

Previous studies (Riekkinen O, Hakulinen et al. 2008 )introducing pulse-echo ultrasound have shown that soft tissue error present in determining BMD with ultrasound can be removed or decreased with this new technique. Broadband ultrasound backscattered parameter can be used as well as other pulse-echo parameters to reduces the soft tissue error with a numerical method (Riekkinen O et al. 2001).

New ultrasound method for soft tissue correction of bone ultrasound measurements is introduced. The validation with elastomer samples demonstrated significant improvement in accuracy of ultrasound measurements.

In living tissues, the dual-frequency ultrasound technique reduced the mean soft tissue–induced error in BUB and in IRC (at 5.0 MHz) from 58.6% to –4.9% and from 127.4% to 23.8%, respectively. Values (mean +/- SD) of IRC and BUB in human trabecular bone (Table 3)

In Fig.2 (a, c) The mean values of pulse-echo parameters, IRC and BUB, before and after the soft tissue correction and as measured without surrounding soft tissues. (b, d) The soft tissue-induced error increased as a function of US frequency. The error could be reduced by means of numerical correction.