Spectral considerations for absorption edge fluoroscopy

In our previous reports on absorption edge fluoroscopy, it was not possible to relate fully the subtleties involved in the selection of spectral parameters. This paper is intended as an overview of this important aspect of the technique. It is shown that, by using the 1 kVp, 2 filter technique, it is possible to image certain elements (e.g., iodine and xenon) in the presence of tissue variations of 2 cm about the thickness at which perfect tissue cancellation takes place. Use of logarithmic signal processing extends this range, but bone thickness variations may not be accommodated because only two x ray energies are involved in the imaging process. Use of a 3 kVp, 3 filter technique with logarithmic signal processing is shown to solve this general problem. Computer simulations show that 1 mg/cm² iodine may be imaged in the presence of 10 cm or more tissue variations and 2000 mg/cm² bone variations.