Quantitative Ultrasound or QUS

Quantitative ultrasound is an ultrasound based modality to measure bone mineral density in osteoporosis.

DXA is considered gold standard for measurement of bone mineral density. In fact quantified definition of osteoporosis is based on measurement of  bone mineral density and its comparison with the healthy young-adult reference population. The values os expressed as a T-score, which represents the number of standard deviations a measured BMD is from the reference population mean.

A T score< -2.5 makes an individual osteoporotic.

In other words, a given individual is said to have osteoporosis if her or his bone mineral density is more than 2.5 standard deviations less than the mean BMD of a healthy, young adult.

Cost, and limited availability of  DXA instruments has led for  search of low cost and widely available devices.

One such device is quantitative ultrasound

As compared to DXA, quantitative ultrasound is are that it is inexpensive, transportable, and ionizing radiation  free.

Low cost and portability has the potential to make quantitative ultrasound  an especially valuable osteoporosis detection tool when access to DXA cannot be made because of nonavailability of the instrument or cost factors.

Studies have suggested quantitative ultrasound to be as effective as axial DXA in predicting hip fractures and all osteoporosis-related fractures in elderly women.

One of the problems with quantitative ultrasound is the required expertise for interpreting US velocity and attenuation and lack of universality across the different manufactures and models

Working of Quantitative Ultrasound

As the name suggests, QUS is based on  ultrasonic waves which are sound  waves outside the threshold of human hearing.  These waves, pass  easily through fluid and other tissues, and are altered  in terms of their shape, intensity and speed.

Study of these changes helps to identify various medical conditions.

Qualitative ultrasound  generates pictures whereas quantitative ultrasound uses ultrasonic waves at lower frequencies to generate empirical measurements.

Quantitative ultrasound can be used  to measure a variety of parameters that  pertain to bone density by measuring values  related to the velocity and attenuation of ultrasound waves as they pass through bone.

Different Quantitative Ultrasound Devices

Three types of devices are available for qualitative ultrasound which vary in the form of US transmission used

Trabecular sound transmission

It is the most commonly utilized category of devices. It is best utilized measuring the heel and currently has the the most clinical applications

Cortical transverse transmission

It is used in phalanges and till date has not found much support for use in osteoporosis

Cortical Axial transmission

These devices are  being investigated for use in phalanges, the radius and the tibia but lack clinical application till now.

With heel devices heel stiffness index or the Quantitative index is measured, which is a composite  score combining the results of broadband ultrasound attenuation  and speed of sound (SOS), as measured  in meters per second.

Application of Quantitative Ultrasound

 Prediction of Fracture Risk

Quantitative ultrasound can be used to  assess fracture risk in some, but not all  populations.

Heel stiffness index, using some but not all devices, is predictive of hip fracture risk in Caucasian  and Asian women over age 55, and of any fracture risk in Caucasian women > 55, level of evidence is weaker for men > 70 years of age.

There is variation of predictability with the device used though.

GE Lunar Achilles and the Hologic Sahara are among the best tested  devices, at the hip, spine and overall, and both seem  effective.

Diagnosis of Osteoporosis

As of now there is no evidence that quantitative ultrasound can be used to diagnose osteoporosis. The problem also lies with definition of osteoporosis.

The T score diagnostic criteria of -2.5 is meant to be used for DXA bone mineral density and using it for any other modality would result in discrepancies  in the numbers of women diagnosed with osteoporosis.

To enable use of quantitative ultrasound for diagnosis of osteoporosis, there is a need for pre-defined, device-specific diagnostic thresholds.

One of the recommended methods is to calibrate quantitative ultrasound measurements with DXA results to be used as gold standard.

For Initiating Osteoporosis Treatment

All currently-published guidelines or recommendations for the initiation of osteoporosis treatment are based upon DXA bone mineral density values except in patients with a low-energy fracture of the hip or spine

A high levels of correlation  between heel trabecular sound transmission and bone mineral density at matched skeletal sites has been suggested though.

It is possible that guidelines could be created using  quantitative ultrasoundto guide when to initiate osteoporosis treatment in conjunction with  the use of clinical risk factors.

Monitor Osteoporosis Treatment

Quantitative ultrasound cannot be recommended for the monitoring of treatment response in patients with osteoporosis. There is not enough evidence.

Further studies are needed to determine if quantitative ultrasound  parameters are sensitive enough to change with treatment.