Digital x-ray system provides quicker osteoporosis diagnosis

Osteoporosis, a disease characterized by microstructural changes in bone material, strikes an estimated 50 percent of women and 20 percent of men in the US. Sufferers may experience painful hip and spine fractures, which are associated with significant morbidity and mortality rates among the elderly. A recent report by the National Osteoporosis Foundation pegged the cost of diagnosis and treatment of the disease at $14 billion a year.

A new way to diagnose osteoporosis by measuring bone mass is being developed by a team of scientists and engineers from CompuMed Inc. of Manhattan Beach, Calif., MedOptics Corp. of Tucson, Ariz. [now known as DALSA Life Sciences]; and the University of Massachusetts. Called Digital Radiographic Absorptiometry (DRA), the technique calculates bone density from x-ray images of the hand recorded with a digital camera. This information can be used by a physician to diagnose osteoporosis and recommend treatment strategies.

Emre Toker, president of MedOptics, believes that bone mass measurement is the most accurate method available for diagnosis and for predicting risk of fracture. Although both radiographic and ultrasonic techniques have been used to make these measurements, ultrasonic techniques have not been thoroughly tested in clinical trials.

"The most common radiographic techniques in use are dual energy x-ray absorptiometry (DEXA) and radiographic absorptiometry (RA)," Toker said.

Because it was designed for large areas of the body, DEXA requires imaging the hip or spine at two distinct x-ray wavelengths, either by spectrally filtering the output of a tungsten x-ray source or by applying two different voltages to the tube. The use of two wavelengths allows x-ray absorption by materials other than bone to be discounted. Because this method relies on imaging a projected area of the bone, relative densities rather than true densities are recorded. Relative values are sufficient for diagnosis, since osteoporosis diagnosis and fracture risk assessment are based on deviations from population mean values.

There are problems with this approach, however. Toker pointed out that DEXA does not account for the presence of fatty tissue, which may degrade the accuracy of the measurements. Beam hardening caused by the absorption of lower energy x-rays by soft tissue and beam scattering can also contribute errors.

In radiographic absorptiometry, two x-ray images of the hand are averaged and compared with a reference image of an aluminum alloy wedge. The film is sent to an outside facility for digitization and analysis using Osteogram, CompuMed's proprietary software. The technique subtracts soft tissue effects from the digitized image and compares the intensities of the bone and the aluminum material at specific locations to determine bone density and bone mineral content.

The primary difference between digital radiographic absorptiometry and its non-digital predecessor is that the image capture, display and analysis functions are performed by one system at a physician's site, without the use of x-ray film. In DRA, the patient's hand is placed on a phosphor screen and irradiated by the x-ray source. Visible photons emitted from the phosphor are collected and imaged by a MedOptics charge-coupled device camera. The camera's digital output is sent to a PC, which analyzes the image for bone density and bone mineral content in real-time using software developed by CompuMed.

Simplicity is the key

DRA results correlate well with those obtained by DEXA. Because bones in the hand have have far less surrounding tissue than bones in other areas of the body, images are subject to fewer nonsystematic errors in bone mass measurement from fatty tissue and scatter. "The accuracy of digital radiographic absorptiometry of the hand is the result of simplicity of the structure of the fingers, leading to accurate modeling without the need for dual energy techniques and the relatively low scattering and beam-hardening effects associated with radiographing the hand," Toker explained.

An even more significant advantage of DRA is that it obviates the complexity and expense of dual energy source used in DEXA. The image capture and analysis tools are in one simple unit, reducing cost, enhancing portability and providing real-time measurements of bone density and mineral content. Elimination of uncertainties associated with film processing may also contribute to more accurate results.

Toker believes the market potential for this innovation to be significant, mainly because the cost of the DRA systems should be substantially lower than the cost of conventional bone densitometry units - an important advantage in an age of cost-cutting within the health-care industry.