Bone mineral density assessed by DXA scan is the current gold standard of bone mass measurement; however, DXA does not directly measure bone structure and strength. In fact, the majority of people who sustain a fracture do not have a diagnosis of osteoporosis by DXA. There is considerable overlap in bone density between individuals with and without fractures. Bone density measurements do not identify the changes in bone microstructure that influence true fracture risk. In addition, small improvement in bone density with bone-specific treatment does not explain the large decreases that occur in fracture risk.
I do not want to overemphasize the negatives of DXA, since it is a good measurement tool. However, I am using the limitations of measuring bone density by DXA to point out the factors for development of other imaging tools. Research has focused on determining factors beyond bone density that affect bone strength and fracture risk.
The ideal bone quality measure has so far eluded researchers. Until recently, bone biopsy was the only way to look at bone microstructure. The biopsy procedure involves the removal of a small sample of bone from your pelvis. New methods of bone imaging are being investigated that provide a look at bone's intricate structure without relying on biopsy. The perfect imaging would differentiate type of bone (dense cortical bone versus spongy trabecular bone), provide three-dimensional geometry, and measure properties of bone that could predict how much loading force will likely cause fractures.
QUS: Quantitative Ultrasound
If you are of childbearing age, the mention of ultrasound probably makes you think of the first pictures of your child. If you are older, ultrasound might mean getting a glimpse of your gallbladder or liver. Quantitative ultrasound (QUS) estimates bone mineral density.
No radiation is used. Instead, a pulse of high frequency sound waves is directed across the bone. The speed of sound is used to estimate bone mineral density. In normal, well-connected bone, sound travels through the bone at high speed. Osteoporotic bone with larger spaces and gaps shows a slower speed of transmission across the bone.
Small portable units are designed to make measurements at the heel, shin, or fingers. The heel ultrasound is the only validated device in the United States. The sound waves it generates are unable to penetrate the spine or the hip to make accurate measurements. However, the heel (calcaneus) is similar to the spine in composition, as both are primarily composed of spongy trabecular bone.
Ultrasound machines are attractive because they are small, portable, relatively inexpensive, and do not use radiation. They are used at health fairs, in doctors' offices, and in pharmacies. Ultrasound may be used for screening at-risk individuals, but it cannot be used to diagnose osteoporosis or to monitor response to therapy; however, ultrasound is predictive of fractures in women and men. If DXA imaging is available, it is a more precise way to quantify your bone density.
There was considerable interest in the new ultrasound devices when they were introduced in the 1990s. It was hoped that ultrasound would measure actual bone quality, which is something different from bone density alone. I was part of the ultrasound research boom and had the opportunity to test out the finger ultrasound device manufactured by an Italian company and approved in Europe but not in the United States. Now, many years later, the interest has waned, and the ultrasound has taken a backseat role in the United States.
The exciting diagnostic advances in CAT scan (CT) and magnetic resonance imaging (MRI) are now in the driver's seat of new imaging for bone structure.
QCT: Quantitative Computed Tomography
Total body CTs have become a popular test in some parts of the country. In San Diego, three centers offering whole body CTs are doing a booming business. Usually a bone density is thrown into the package deal. I am referring here to cash-pay, elective whole body tests that are heavily advertised.
The CT bone density consists of the measurement of the first or second lumbar vertebra. In most clinical settings, the test is done using single-energy mode, which does not distinguish between increased bone marrow fat and decreased bone. In addition, results are based on the diagnostic criteria for DXA scans. From my experience, the majority of people are told they have osteoporosis, when in fact they don't. This is the result of a misapplication of the DXA T-scores. The standardized T-score by QCT is about one standard deviation lower than the T-score from a DXA scan. Therefore, the same criteria for diagnosis cannot be applied to the QCT.
Many times the results of the CT scan then lead to the ordering of a DXA scan. So much for the wise use of your healthcare dollars! If you have a whole body CT, with the bone density provided along with your heart calcium scores, just be aware that the interpretation of the results may be incorrect.
On the other hand, QCT densitometry of the spine, properly evaluated, can be used to predict fracture and monitor therapy, although the amount of radiation is significantly higher than the exposure with DXA. CT devices tend to be used more in Europe than in the United States. The CT scans of the forearm (called pQCT) are common in Europe.
Research Tools: High Resolution MRI and QCT
The advances in computer technology are also reflected in imaging. Just like we have high definition (HD) television, the CT and MRI scanners have progressed to high resolution. If you have had the opportunity to watch a movie using Blu-ray technology, which offers an unprecedented HD experience, seeing the vibrant, new images of bone is similar. The detail is amazing! It is like having a noninvasive bone biopsy.
High resolution imaging by QCT or MRI provides a better visualization of structure. Measurements of the microstructure can be taken in much the same way as those of a bone biopsy. So far, the devices are developed for use at the wrist and sites on the leg, but not the sites of major fracture, the hip, and the spine.
FEA: Finite Element Analysis
Engineers using QCT images have modeled bone structure as a collection of finite elements. The models compute strength and can estimate other structural performances just like they would do for bridge construction. Use of FEA may prove to be able to assess fracture risk and predict if you can survive a fall without a fracture. The examination of biomechanical risk of fracture for clinical use should be available in the near future.
Newer imaging techniques that go beyond the DXA scan and can provide doctors with novel information on bone structure are exciting. These technologies hold great promise for predicting fracture, assessing response to treatment, and giving an accurate picture of bone quality. With data from clinical trials becoming available in the coming years, these advanced technologies are anticipated to become common clinical tools for assessment and monitoring of bone health. For now, though, the DXA scan is the best measurement device available for assessment of bone health.
|
The Bare Bones
|