Nonlinear optical coherent gating for medical imaging with lasers

Abstract

It is often desirable to form an image of the object hidden in or behind a scattering medium. For example, the whole new field of transillumination1,2 was established in medicine. Several techniques, relying on the fact that the highly scattered light is delayed in time with respect to the less scattered light, have been used. Various time gating2 techniques with very short laser pulses (<10 psec) have been employed. Recently a chrono-coherent imaging3 technique was developed with long broadband laser pulses. The interference between imaging and reference beams is recorded on the film as a holographic image. One of the disadvantages of this method is the holographic image degradation due to the scattered light. It is also not a real-time imaging technique. We developed a new nonlinear optical cross-correlation technique for hidden object imaging which can be used with long broadband laser pulses. Using nonlinear optical techniques the following advantages are attained: good discrimination against scattered radiation, real-time imaging, image amplification resulting in extremely high sensitivity, up or down conversion of image to a more convenient laser wavelength. This is a general method that can be used with many nonlinear optical processes. We discuss some of the relevant nonlinear optical effects, their advantages, and disadvantages.