Accepted for Presentation, 11th International Bone Densitometry Workshop, Gleneden Beach, Oregon, 24-28 September 1995.
Introduction: X-ray micro-CT with 3D voxels of 25 to 100 micrometers (µ m) has been used to investigate the architecture of in-vitro cancellous bone specimens. Application to small animals may permit serial observation of bone changes associated with biochemical or mechanical intervention. We have conducted experimental studies to demonstrate the feasibility of measuring small changes in bone volume for caudal(tail) vertebrae in rats.
Methods: Computer models of micro-CT system performance were used to optimize the bone signal to noise ratio in relation to the dose delivered to rat tail. The dose delivered to the bone marrow region was computed for optimal conditions. TLD experiments using a solid phantom made from embedded rat tail bone were performed to confirm the radiation dose calculations. Image quality measurements were made using the same imbedded bone phantom. An apparatus was constructed to position a live rat in our experimental system and support the rat tail during a scan. Differential precision was observed from scans obtained at 24 hour intervals. 3D data from two studies were rotated and translated to exact alignment. Bone volume was determined by histogram analysis and total bone mass determined by quantitative density analysis.
Results: Using a CsI detector, optimal performance is achieved at 50 kVp (tungsten target) with 3 mm Al filtration . Good signal to noise for bone (20:1) can be obtained with a marrow dose less than 50 rads(.5 Gray) when reconstructed using voxels of 50 µ m dimension. Alignment with subpixel precision is achieved using tri-linear interpolation. Preliminary data suggests that good precision may be achieved and that regional changes may be identified.
Conclusions: X-ray micro-CT systems using a cone beam geometry with a microfocus sources can be used for serial measurements on caudal vertebrae in the rat. The dose levels required are significantly less than those which cause histochemical changes in enzymes associated with bone metabolism.