Imaging Combines Optical Modalities

Gary Boas

Optical coherence tomography (OCT) and its variant, optical Doppler tomography, have proved useful for the functional imaging of tissue physiology. However, if optical Doppler tomography is to be deployed in clinical settings, it must offer real-time imaging and display of tissue structure and blood flow. To achieve this goal, researchers at Beckman Laser Institute and Center for Biomedical Engineering at the University of California, Irvine, have developed a real-time phase-resolved OCT/optical Doppler tomography system that employs optical Hilbert transformation to overcome the obstacles encountered by conventional systems.

A real-time phase-resolved optical coherence tomography/optical Doppler tomography system developed by researchers at the University of California images the multiple blood vessels in a port-wine stain. Courtesy of Zhongping Chen.

Optical Doppler tomography monitors the Doppler frequency shifts in the measured interference signals caused by interaction of light with moving particles in biological tissue. A spectrogram can detect these shifts, based on the fast Fourier transform algorithm, but it involves trading velocity sensitivity for imaging speed and spatial resolution.

The new system circumvents this by using the phase change between the interference signals for image reconstruction, but this approach also introduces obstacles. Specifically, using phase information requires higher data acquisition rates and signal processing speeds.

To address this difficulty, the researchers incorporated optical Hil-bert transformation into the system. This transformation depends on the combination of a circularly polarized reference and a linearly polarized sample signal to introduce a 90° phase shift in the interference signal. This produced optically an analytical continuation of a complex value that enabled the researchers to generate real-time images.

The advantage of the experimental system over conventional approaches, said researcher Zhongping Chen, is that it can produce real-time OCT and Doppler images simultaneously and with high velocity sensitivity. The disadvantage is that the signal-to-noise ratio is not as high as with conventional systems. The researchers are working on a second-generation system that optimizes the system design and boosts the signal-to-noise ratio.

Two to three years to market

A number of important clinical applications have been suggested for optical Doppler tomography, including monitoring the efficiency of both laser treatment for port-wine stains and photodynamic therapy, screening vasio-active and anti-angiogenic drugs, diagnosing skin cancers and monitoring cortical activity and brain injury. With these applications in mind, the institute is conducting pilot clinical studies of the system at the university.

Chen cautioned, however, that the studies are only the first step toward its deployment in a clinical setting. Full clinical tests may be up to a year away, and it may take two to three years before a commercial system is available.