Bone densitometers offer high-resolution imaging

When bone densitometry was new, it was all about generating numbers, statistical T and Z scores that compare the bone density of individual patients against that of groups of patients in different age ranges. This began to change two years ago, when the makers of bone densitometers added software that postprocessed bone scans into snapshots of the spine.

The images helped physicians identify the most obvious fractures, but the low resolution was not enough to see fractures in the upper thoracic spine or in spines with scoliosis, both of which are strong predictors of future fracture risk.

The new generation of bone densitometers, featured at the 2005 RSNA meeting, addresses that shortcoming, producing high-quality images of bone architecture that will help physicians better predict fracture risk.

"The idea is not to tell a patient that she has a bone density of 0.8 g per square centimeter or that she has a T score of minus 2," said Dr. Kenneth Faulkner, a leading investigator in bone density and chief scientist at GE Healthcare. "It's to tell her she has a fracture risk of 10% over the next five years. We want to be able to assess bone strength as well as bone density, and image quality makes that possible."

High-quality images not only provide a diagnostic boost, but their availability serves as a benchmark for future examinations. With these, physicians can better monitor patient responses to treatment for osteoporosis.

"Many have said that changes in bone density don't correlate well with changes in fracture reduction after treatment for osteoporosis," Faulkner said. "Some new drugs change bone size, not just bone density, and if you don't have image quality, you can miss those parameters."

GE Healthcare has targeted image quality with the new Lunar iDXA. This system achieves a resolution of about 1.2 line pairs/mm, roughly double the 0.6-line-pair/mm range of most current bone densitometry systems. The objective is to get resolution to the size of an end plate where vertebral fractures appear, and no more than a few millimeters wide at the cortical shell of the femur, Faulkner said.

"We use the example of the Mona Lisa," said Jennie D. Hanson, vice president and general manager of GE Lunar. "With our Prodigy, you know what you're seeing and you can appreciate it. With iDXA, you get down to the brushstrokes."

Lunar iDXA builds on the Prodigy. Lunar iDXA and Lunar Prodigy use the same dual-energy source for x-rays. However, the iDXA includes a completely redesigned x-ray tube that permits a smaller focal spot to better delineate bone geometry. The system has a cadmium, zinc, and telluride/high-definition detector that staggers two detector arrays on top of one other and samples the dead space between them. The system also has augmented the number of detector elements from 16 to 64 to increase the precision of bone density measurement.

GE is not the only company to be developing such a system. Hologic revealed at the RSNA meeting the Discovery QDR Series tomo 3D hip imaging, the prototype of a tomographic 3D image reconstruction system that uses the Hologic Discovery line of bone densitometers. The prototype transforms 2D views into volumetric bone density measurements of the hip. The design recognizes that the integrity of the hip is not based solely on an area measurement of the femoral neck in 2D, but rather involves a volumetric evaluation of bone structure as it contributes to bone strength.

Discovery QDR exploits Hologic's patented rotating C-arm to acquire multiple views of the femur at various angles. A series of images are acquired at low x-ray doses, and the images are processed and reconstructed into 3D views in much the way a CT scanner works.

Discovery QDR is expected to be in testing at clinical research sites by the first or second quarter of 2006. It is scheduled to begin shipping, pending FDA clearance, by the end of 2006.