Quantitative ultrasound is an ultrasound-based modality to measure bone mineral density in osteoporosis.
DXA is considered the gold standard for measurement of bone mineral density. In fact, the quantified definition of osteoporosis is based on the 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 the search of low cost and widely available devices.
One such device is a quantitative ultrasound
As compared to DXA, quantitative ultrasound is are that it is inexpensive, transportable, and ionizing radiation free.
Low cost and portability have 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 a 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 manufacturers 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 the 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 a 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 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 a 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 the 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 the 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 level 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 the quantitative ultrasound to 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.