New x-ray radiographic systems based on large-area flat-panel technology have revolutionized our capability to produce digital x-ray images. However, these imagers are extraordinarily expensive compared to the systems they are replacing. Hence, there is a need for a low-cost digital imaging system for general applications in radiology. A novel potentially low-cost radiographic imaging system based on established technologies is proposed-the X-Ray Light Valve (XLV). This is a potentially high-quality digital x-ray detector made of a photoconducting layer and a liquid-crystal cell, physically coupled in a sandwich structure. Upon exposure to x rays, charge is collected on the surface of the photoconductor. This causes a change in the optical properties of the liquid-crystal cell and a visible image is generated. Subsequently, it is digitized by a scanned optical imager. The image formation is based on controlled modulation of light from an external source. The operation and practical implementation of the XLV system are described. The potential performance of the complete system and issues related to sensitivity, spatial resolution, noise, and speed are discussed. The feasibility of clinical use of an XLV device based on amorphous selenium (a-Se) as the photoconductor and a reflective electrically controlled birefringence cell is analyzed. The results of our analysis indicate that the XLV can potentially be adapted to a wide variety of radiographic tasks.

Purpose:To compare the effectiveness of three image subtraction algorithms designed to improve arterial conspicuity in first-pass contrast-enhanced magnetic resonance (MR) angiography. Materials and Methods:Three subtraction methods were analyzed through computer simulations, phantom studies, and clinical studies. These algorithms were: complex subtraction, magnitude subtraction, and maximum intensity projection subtraction. Results:In high resolution three-dimensional imaging, maximum intensity projection subtraction generally yields the best background suppression. Complex subtraction is effective in reducing partial volume effects in low resolution imaging. Magnitude subtraction works better in high resolution, low contrast concentration protocols. Conclusion:Choosing the appropriate subtraction method according to the protocol is helpful in optimizing image quality. SUBTRACTION IS A COMMONLY used technique to increase arterial conspicuity in contrast-enhanced magnetic resonance (MR) angiography. Although the importance of complex subtraction has been identified (1), magnitude images reconstructed by the scanner were generally used for the subtraction in most published papers (2-5). In theory, subtracting complex numbers can reduce partial volume effects and recover some vessel signals. However, we found complex subtraction did not always give us the best result in threedimensional MR angiography. On the contrary, subtracting two maximum intensity projection (MIP) images, which was thought to be "not useful" (6), often yielded the best arterial conspicuity in a multiple injection protocol (Fig. 1). This suggests that different subtraction algorithms should be applied in different situations. An analysis of the underlying reason for different results through mathematical modeling and clinical data evaluation is needed. This study will provide a theoretical basis for choosing the appropriate subtraction algorithm for MR angiography. Key THEORY SubtractionTwo characteristics of the MR data sets contribute to the multiplicity of choices for subtraction algorithms. First, MR raw data are complex numbers. We can therefore either subtract the complex numbers of the two corresponding data sets pixel by pixel and then take the magnitude, or we can first take the magnitude of the two data sets and then subtract these magnitude values. In MR imaging, the difference of order could be significant in the presence of partial volume effects (1).The second characteristic is that we use a threedimensional acquisition to cover the vasculature. Thus, some kind of volume rendering algorithm is needed to convert the three-dimensional data set into two-dimensional form to be displayed on a computer screen or film. MIP is the most popular volume rendering algorithm in three-dimensional MR angiography because it is operator independent, free of intensity thresholding, and relatively fast. Although MIP has some potential problems, no other algorithm is currently more practical and objective in the overall sense. We can either first do...