Our lab has previously generated selective iodine images with an image intensifier fluoroscopic system using a three-beam K-edge approach. Logarithmically amplified video images Li were linearly combined to yield the final image k1L1 + k2L2 + k3L3. This paper discusses refinements of the K-edge technique. A study is made of the manner in which contrast-reducing effects such as x-ray scatter and image intensifier veiling glare enter into the final image. If such biases can be approximated as multiplicative and independent of the x-ray spectrum, and if the sum of the ki is zero, then the biases are canceled. Experimental data is presented which demonstrates that the inaccuracy due to such biases can be reduced by a factor as large as 10. The theorem that K1 + K2 + K3 is approximately equal to 0 is proven rigorously and discussed. Because the ki add to zero, the final image can be expressed as a linear combination of two of the differences between the Li. A difference-based algorithm which reduces biases and make allowance for nonlinearities such as beam hardening is proposed and discussed.