Three-beam K-edge imaging of iodine using differences between fluoroscopic video images: experimental results

In an earlier article we discussed the rationale for using differences between video images in three-beam selective iodine K-edge imaging. Rather than combining three initial images Li linearly to yield the final image k1L1 + k2L2 + k3L3, differences between the Li were first generated and then combined either to linear or quadratic order. This approach was motivated by the desire to suppress the large multiplicative biases of fluoroscopic imaging and justified by theoretically proving that k1 + k2 + k3 is approximately equal to 0. In this paper we discuss the instrumentation and experimental results obtained from this difference-based technique. A specially-constructed apparatus is described which automatically selects the optimum combination coefficients and combines the difference images up to quadratic order at realtime video rates. Three methods for generating K-edge subtraction images are compared: the former approach in which the Li are linearly combined and combination of differences to linear and quadratic order. In imaging phantoms in which the iodine distribution is known, the resultant subtraction images from all three methods appear similar. Inspection of signal sizes shows that the quadratic difference-based approach provides superior bone and tissue residual suppression by about a factor of 2. In imaging phantoms in which the iodine distribution is unknown, incomplete suppression of x-ray scatter and image intensifier veiling glare prevent a quantitative comparison of performance of the three algorithms. An experiment verification is provided of the theorem which states that k1 + k2 + k3 is approximately equal to 0.