Absorption-edge fluoroscopy using a three-spectrum technique.

We have recently reported on a 1-kVp, two-filter image subtraction method for visualizing low concentrations of elements like iodine which have K-shell absorption edges in the diagnostic x-ray energy range. However, in the application of this technique to human thyroid imaging, superimposed images due to variations in tissue and bone thickness presented serious difficulties. In this paper, a technique is described for implementing a 3-kVp, three-filter approach. Using carefully chosen spectra and logarithmic image processing, images are produced which are compatible with our previously described two-stage storage-tube subtraction device. Proper manipulation of the resulting difference images results in a final absorption-edge image in which only the element of interest is visualized, with unwanted background images suppressed. Computer calculations are presented to illustrate the relative sizes of difference signals arising from the element of interest and from tissue and bone backgrounds. Phantom studies using iodine concentrations as small as 1 mg/cm2, with variations of 10 cm of tissue and 2 g/cm2 of bone, suggest that the theory is sound and that, with straightforward apparatus modifications, images of good quality should be possible.