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The application of dual-energy principles allows accurate measurement of bone density using conventional radiologic equipment. Digital subtraction of two images generated with x-ray beams of different energies yields a bone signal proportional to the amount of bone in the beam path which is free from error due to uncertainty in the amount of overlying soft tissue. In this study the two images are generated simultaneously by selective filtration of the x-ray beam. Computer simulations were performed to optimize the x-ray spectra with respect to noise and separation of high-and low-beam energies. The effects of tube voltage, filter materials, filter thicknesses, soft tissue thickness and bone thickness on measurement uncertainty were examined. Our simulations indicate that, with appropriate filter selection, bone thickness may be measured with a coefficient of variation of less than 1%.

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Most man-made objects provide characteristic straight line edges and, therefore, edge extraction is a commonly used target detection tool. However, noisy images often yield broken edges that lead to missed detections, and extraneous edges that may contribute to false target detections. 1 We present a sliding-block approach for target detection using weighted power spectral analysis. In general, straight line edges appearing at a given frequency are represented as a peak in the Fourier domain at a radius corresponding to that frequency, and a direction corresponding to the orientation of the edges in the spatial domain. 1 Knowing the edge width and spacing between the edges, a band-pass filter is designed to extract the Fourier peaks corresponding to the target edges and suppress image noise. These peaks are then detected by amplitude thresholding. The frequency band width and the subsequent spatial filter mask size are variable parameters to facilitate detection of target objects of different sizes under known imaging geometries. Many military objects, such as trucks, tanks and missile launchers, produce definite signatures with parallel lines and the algorithm proves to be ideal for detecting such objects. Moreover, shadow-casting objects generally provide sharp edges and are readily detected. The block operation procedure offers advantages of significant reduction in noise influence, improved edge detection, faster processing speed and versatility to detect diverse objects of different sizes in the image. With Scud missile launcher replicas as target objects, the method has been successfully tested on terrain board test images under different backgrounds, illumination and imaging geometries with cameras of differing spatial resolution and bit-depth.