In the bones
At the University of Calgary, AHFMR Scholar Dr. Steven Boyd is applying his mechanical engineering expertise to the problems of bones and joints. A major part of his research combines state-of-the-art imaging technologies with new engineering methods to improve the diagnosis and treatment of osteoporosis. This disease, which is characterized by low bone mass and deterioration of bone tissue, leads to more fragile bones and a greater risk of fracture. As many as two million Canadians suffer from osteoporosis.
One of the challenges of osteoporosis is that bone mass loss occurs without symptoms. A test for bone mineral density can screen for and detect the early stages of osteoporosis. The most commonly used test is a special kind of x-ray called dual-energy x-ray densitometry (DXA). But DXA gives only two-dimensional pictures of the bone, notes Dr. Heather Macdonald, an AHFMR post-doctoral fellow in Dr. Boyd's lab. "And that's a limitation because the skeleton is three-dimensional (3-D). The 3-D images give more detailed structural information. With better information, we might be better able to identify bones that are likely to fracture."
Enter a new 3-D bone imaging technology: the XtremeCT scanner. This computed tomography (CT) scanner uses high-resolution x-ray and image analysis software to produce images that show the 3-D architecture of the bone, not just its density. Unlike, DXA, XtremeCT reveals the bone "microarchitecture" (small structural details) and distinguishes between two different types of bone: the outer shell called the cortical bone and the spongy network that makes up the core, known as the trabecular bone.
Dr. Boyd was one of the first researchers in Canada to acquire an XtremeCT scanner. For her research, Dr. Macdonald is analyzing a set of XtremeCT images from 650 individuals. Most of these people are Calgary-area participants in the Canadian Multicentre Osteoporosis Study (CaMos), a large, long-term study to assess the prevalence of osteoporosis and fracture in Canadian women and men.
"Because XtremeCT is so new, it is important to figure out what is 'normal.' My study is aimed at acquiring normative data," explains Dr. Macdonald. "Once we have the data, we can look at how bone changes with time and how other factors such as nutrition and exercise affect bone architecture. That's why it's important to be part of CaMos, since this information is already being collected as part of that study."
XtremeCT is also central to Dr. Boyd's research. "We get these great 3-D images of bone microarchitecture with technologies like XtremeCT, but what we really want to know is how strong the bone is. Is it likely to break? One way is to take a bone biopsy and crush the bone sample. Although we do this in laboratory research, it's not usually practical or desirable to take bone biopsies in patients. I wondered if there was a virtual way to do this."
That's where Dr. Boyd's engineering expertise kicked in. He turned to finite element analysis, a mathematical technique used to compute where a structure will bend or twist, that allows him to visualize the stresses and movement within the structure. Finite element analysis is used widely in many kinds of engineering work to refine and optimize designs before something is manufactured or constructed.
Dr. Boyd's team has taken XtremeCT images of bone microarchitecture, converted the images to finite element models, and then performed simulations of bone strength tests on the mathematical models. The result is a virtual measure of bone strength without having to do an invasive biopsy. This success was due to the team's ability to refine the finite element models, which are very complex and require huge amounts of time on supercomputers to solve. Now the models can run in less than an hour on a regular computer workstation.
Although the technology is based on images from an Xtreme-CT scanner, which is a research tool and not in clinical use, Dr. Boyd believes that this technology will be available to patients in the future. "It will take work, but I don't think there's any reason why this could not be integrated into any 3-D imaging system like CT or MRI. In the foreseeable future, we could have imaging centres incorporating virtual bone biopsies as part of their basic analysis.
"My whole focus is to move beyond bone mineral density and look at the microarchitecture of the bone. This information will deepen our understanding of diseases like osteoporosis. The analogy we use is that bone mineral density tells you how many bricks are in the building but doesn't tell you how they were put into the building [as a biopsy does]. And the way you build a building has a big impact on strength."