Simultaneous ultrasound and phase-sensitive optical coherence tomography for improved multimodal imaging

Optical coherence tomography (OCT) and ultrasound are two important modalities for imaging the first few mm of tissue in a range of applications including retinal imaging, skin imaging, intravascular imaging and endoscopic imaging. However, the interaction between OCT and ultrasound in tissue, and the possibility of enhancing the quality of diagnostic data obtained with one modality through its coherent interaction with the other has not previously been investigated. We are proposing to investigate the interaction effects between OCT and ultrasound in tissue with two specific objectives: Direct measurement of acoustic tissue motion: Ultrasound imaging generates contrast from the acoustic backscatter strength in tissue and at tissue boundaries. However, there is currently no way to measure the microscopic acoustic tissue motion induced by the ultrasound or the absolute wavelength of the ultrasound in tissue. We propose to measure these two properties by imaging the optical phase shift induced on light scattered from tissue using phase-sensitive OCT. Tissue would be illuminated by a swept-source OCT laser while a co-axial ultrasound pulse is launched into the tissue. As the ultrasound pulse propagates, it induces an acoustic displacement in the tissue that modulates the optical phase of the reflected light detected by the OCT system at each depth. By applying time-frequency analysis to the signal, the ultrasonic displacement can be sampled at each axial location at each time, and from it the local ultrasonic wavelength in the tissue can be obtained. Acoustic displacement, which is related to tissue stiffness, may be useful in distinguishing pathological from healthy tissue. Wavelength is indicative of speed of sound which can be used to distinguish tissues and to measure temperature. This technique may, therefore, prove useful in obtaining more diagnostically informative ultrasound images in a range of applications. Ultrasound-assisted OCT: A major limitation of OCT is its limited penetration depth, typically 1.5-2mm in optically scattering tissue. The limitation is not caused by a failure of light to penetrate to greater depths, but rather to the singly-scattered light that makes up the desired OCT signal being swamped by multiply-scattered light from near the tissue surface. We propose to investigate whether combining ultrasound with OCT can provide a way to reject multiply-scattered light so as to increase the penetration depth of optical coherence tomography in soft tissue. We will deliver focused ultrasound at a depth in the tissue simultaneously with light from an OCT swept laser source. Within the ultrasonic focus, the ultrasound phase-shifts the light. By processing the OCT signal to only look at the phase shifted light we can generate a signal free from interference from multiple scattering. This would enhance contrast and penetration, allowing OCT imaging at greater depth than is currently possible.