Vision 20/20: Increased image resolution versus reduced radiation exposure

This is a review of methods, currently and potentially, available for significantly reducing x-ray exposure in medical x-ray imaging. It is stimulated by the radiation exposure implications of the growing use of helical scanning, multislice, x-ray computed tomography for screening, such as for coronary artery atherosclerosis and cancer of the colon and lungs. Screening requires high-throughput imaging with high spatial and contrast resolution to meet the need for high sensitivity and specificity of detection and classification of specific imaged features. To achieve this goal beyond what is currently available with x-ray imaging methods requires increased x-ray exposure, which increases the risk of tissue damage and ultimately cancer development. These consequences limit the utility of current x-ray imaging in screening of at-risk subjects who have not yet developed the clinical symptoms of disease. Current methods for reducing x-ray exposure in x-ray imaging, mostly achieved by increasing sensitivity and specificity of the x-ray detection process, may still have potential for an up-to-tenfold decrease. This could be sufficient for doubling the spatial resolution of x-ray CT while maintaining the current x-ray exposure levels. However, a spatial resolution four times what is currently available might be needed to adequately meet the needs for screening. Consequently, for the proposed need to increase spatial resolution, an additional order of magnitude of reduction of x-ray exposure would be needed just to keep the radiation exposure at current levels. This is conceivably achievable if refraction, rather than the currently used attenuation, of x rays is used to generate the images. Existing methods that have potential for imaging the consequences of refracted x ray in a clinical setting are (1) by imaging the edge enhancement that occurs at the interfaces between adjacent tissues of different refractive indices, or (2) by imaging the changes in interference patterns resulting from moving grids which alter the refraction of x rays, that have passed through the body, in a predictable fashion, and (3) theoretically, by an image generated from the change in time-of-flight of x-ray photons passing through the body. Imaging phase shift or change in time-of-flight, rather than attenuation, of x-ray photons through tissues presents formidable technological problems for whole-body 3D imaging. However, if achievable in a routine clinical setting, these approaches have the potential for greatly expanding the use of x-ray imaging for screening. This overview examines the increased contrast resolution and reduced radiation exposure that might be achievable by the above-mentioned methods.