TITLE: Measuring Bone Dynamics
AUTHORS: Tom Beck
Much of medical imaging involves imaging of human tissues that undergo rapid conformational changes due to physiologic functions, such as the heart and lungs. Human bones are considered stationary tissues that seem relatively easy to image but they do undergo subtle conformational changes over time, although over much slower time scales. The technological challenges in reliably detecting subtle changes in an irregular three-dimensional bone are no less challenging than the measurements of moving tissues. Two processes cause changes in bone 1) turnover, where old bone tissue is periodically replaced by new and 2) adaptation, where the amount and distribution of bone tissue adjusts to changes in mechanical load. The importance in characterizing these changes is that they can and do alter the mechanical strength of bones in ways that can make fractures more or less likely. Moreover certain pathologies and pharmacological treatments influence the rates and magnitudes of bone changes. One would like to reliably determine if a disease is reducing strength or a treatment is improving it. Mechanical strength is a function of the stresses generated under a specific loading condition and of the ability of the material to withstand that stress. Bones are fundamentally muscle-activated levers and their strength can be evaluated in much the same way that an engineer assesses any lever in a mechanical device. In those cases a computer model of the lever is generated; stresses are determined from knowledge of its geometry and the loads applied. Information on the material stress limits are then used to determine the load that would cause stresses to exceed those limits. To apply this method to a bone, the imaging method needs to provide information on the lever geometry, how it is loaded and on the material properties. One fundamental limitation is that bone material properties are determined by the composition of the tissue and cannot be reliably measured by any current non-invasive method, although it is possible to measure structural geometry. This is fortunate because studies of bone biology indicate that most changes in human bones alter geometry with relatively little effect on material properties. (Although a reliable non-invasive method for measuring bone material would still have clinical value for some diseases known to alter the tissue composition).
There are two main types of engineering analyses that can be applied to bones and several imaging methods that can generate the geometric data to specify the model. Our work in the past has concentrated on methods to acquire bone geometry from specialized single projection x-ray imaging devices called dual energy x-ray absorptiometry (DXA) scanners. Most work in the past has used either two dimensional x-ray image data or 3-dimensional data from a CT scanner. I will describe several approaches for measuring geometry from these methods and provides some examples of how geometry changes with age, treatment and changes in mechanical load. Limitations of the methods will be described as well as technical approaches for mitigating those limitations.