Quantitative Ultrasound (QUS) – Bone Density & Osteoporosis

What is Quantitative Ultrasound?

Quantitative Ultrasound (QUS) is a diagnostic imaging technique that uses sound waves to evaluate the structural properties of bone. Unlike conventional X-ray methods or Dual-Energy X-ray Absorptiometry (DXA), QUS involves no ionizing radiation, making it a safe and accessible option for bone health assessment. It is widely used in the screening and monitoring of osteoporosis and in estimating fracture risk.

How It Works

QUS works by transmitting sound waves through bone tissue and analyzing how those waves travel. Two key parameters are measured:

• Broadband Ultrasound Attenuation (BUA): Measures how much the ultrasound signal is weakened as it passes through bone. Denser, healthier bone absorbs more sound energy, resulting in higher attenuation values.
• Speed of Sound (SOS): Measures how quickly the ultrasound wave travels through the bone. A higher speed generally indicates better bone structure and density.

These two values are often combined into a composite index known as the Stiffness Index or Quantitative Ultrasound Index (QUI), which provides an overall estimate of bone quality.

Clinical Applications

QUS is used across a range of clinical settings:

• Osteoporosis screening: Particularly in postmenopausal women and older men to identify individuals at increased risk of fracture.
• Monitoring bone health: Tracking changes in bone metabolism in patients with known osteoporosis or those undergoing treatment.
• Pediatric use: Assessing bone health in children and adolescents, for example in cases of suspected rickets or metabolic bone disorders.
• Neonatology: Evaluating bone density in premature infants.

Measurement Sites

The most common measurement site is the heel bone (calcaneus), which is easily accessible and contains a high proportion of trabecular (spongy) bone tissue that is particularly sensitive to changes in bone density. Other measurement sites include:

• Proximal phalanges of the fingers
• Radius (forearm bone)
• Tibia (shin bone)

Advantages of QUS

• No radiation exposure – safe for pregnant women and children
• Cost-effective and portable
• Quick to perform (typically just a few minutes)
• Well-suited for large-scale screening programs

Limitations

Despite its many benefits, QUS has notable limitations. Measurements can be influenced by soft tissue surrounding the measurement site, ambient temperature, and precise device positioning. QUS is not a replacement for DXA scanning, which remains the gold standard for diagnosing osteoporosis according to WHO criteria. Abnormal QUS results typically warrant follow-up with a DXA examination for definitive diagnosis.

Interpreting Results

QUS results are often expressed using T-scores and Z-scores, similar to DXA:

• T-score ≥ -1.0: Normal bone density
• T-score between -1.0 and -2.5: Osteopenia (low bone mass)
• T-score ≤ -2.5: Osteoporosis

It is important to note that these thresholds were originally developed for DXA-based measurements. Their direct applicability to QUS results remains a subject of ongoing discussion in the medical literature.

References

1. World Health Organization (WHO): Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. WHO Technical Report Series 843, Geneva, 1994.
2. Njeh C.F., Boivin C.M., Langton C.M.: The role of ultrasound in the assessment of osteoporosis: a review. Osteoporosis International, 1997; 7(1): 7-22. PubMed PMID: 9102061.
3. Kanis J.A. et al.: European guidance for the diagnosis and management of osteoporosis in postmenopausal women. Osteoporosis International, 2019; 30(1): 3-44. PubMed PMID: 30324412.