Beam Hardening, Noise, and Contrast Considerations in Selective Iodine Digital Radiography

Temporal and dual beam energy subtraction are discussed in conjunction with selective imaging of iodine. Several different kinds of energy subtraction are described and their relative sensitivities to noise are predicted. Effects due to beam hardening are demonstrated for temporal and energy subtraction. Hardening errors are more severe for the latter case by about a factor of three. A possible hardening correction is presented for energy subtraction and an experimental example is provided. Techniques for choosing optimized spectra and data handling for dual beam energy subtraction are presented. These include qualitative principles for spectral selection, identification of the individual noise contributions from the two beams, use of weighted least squares fits, and beam hardening and exposure reduction through the use of filtration.