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

光学相干断层扫描 (OCT) 和超声是对组织的前几毫米进行成像的两种重要方式，其应用范围包括视网膜成像、皮肤成像、血管内成像和内窥镜成像。 然而，OCT 和超声在组织中的相互作用，以及通过一种方式与另一种方式的相干相互作用来提高用一种方式获得的诊断数据质量的可能性，此前尚未进行过研究。 我们提议研究 OCT 和超声在组织中的相互作用，有两个具体目标： 直接测量声学组织运动：超声成像根据组织内和组织边界处的声学反向散射强度产生对比度。 然而，目前还没有办法测量超声波引起的微观声学组织运动或超声波在组织中的绝对波长。 我们建议通过使用相敏 OCT 对从组织散射的光引起的光学相移进行成像来测量这两个特性。 当同轴超声脉冲发射到组织中时，扫频 OCT 激光会照射组织。 当超声脉冲传播时，它会在组织中引起声位移，从而调制 OCT 系统在每个深度检测到的反射光的光学相位。 通过对信号进行时频分析，可以在每个时间点对每个轴向位置处的超声位移进行采样，并从中获得组织中的局部超声波长。 与组织硬度相关的声位移可能有助于区分病理组织和健康组织。 波长表示声速，可用于区分组织和测量温度。 因此，该技术可能被证明有助于在一系列应用中获得更具诊断信息的超声图像。 超声辅助 OCT：OCT 的一个主要限制是其有限的穿透深度，在光学散射组织中通常为 1.5-2 毫米。 这种限制并不是由于光无法穿透到更深的深度，而是由于构成所需 OCT 信号的单散射光被来自组织表面附近的多重散射光淹没。 我们建议研究超声与 OCT 相结合是否可以提供一种拒绝多重散射光的方法，从而增加光学相干断层扫描在软组织中的穿透深度。 我们将与来自 OCT 扫频激光源的光同时在组织深处传送聚焦超声波。 在超声波焦点内，超声波使光发生相移。 通过处理 OCT 信号以仅查看相移光，我们可以生成不受多重散射干扰的信号。 这将增强对比度和穿透力，使 OCT 成像的深度比目前可能的更深。

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.