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

光学相干断层扫描（OCT）和超声是用于在包括视网膜成像、皮肤成像、血管内成像和内窥镜成像的一系列应用中对最初几mm的组织进行成像的两种重要模态。 然而，组织中OCT和超声之间的相互作用，以及通过一种模态与另一种模态的相干相互作用来提高用一种模态获得的诊断数据的质量的可能性，以前尚未被研究。 我们建议研究组织中OCT和超声之间的相互作用效应，具有两个特定目标： 声学组织运动的直接测量：超声成像根据组织中和组织边界处的声学反向散射强度生成对比度。 然而，目前还没有办法测量由超声引起的微观声学组织运动或组织中超声的绝对波长。 我们建议测量这两个属性成像的光学相移诱导的光散射组织使用相敏OCT。 当同轴超声脉冲被发射到组织中时，组织将被扫频源OCT激光器照射。 当超声脉冲传播时，它在组织中引起声位移，该声位移调制由OCT系统在每个深度处检测到的反射光的光学相位。 通过对信号进行时频分析，可以在每个时间的每个轴向位置处对超声位移进行采样，并且可以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用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，这种技术可以证明在一系列应用中获得更多诊断信息的超声图像是有用的。 超声辅助OCT：OCT的主要局限性是其穿透深度有限，在光学散射组织中通常为1.5- 2 mm。 该限制不是由光未能穿透到更大的深度引起的，而是由构成所需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引起的。 我们建议调查是否结合超声与OCT可以提供一种方法来拒绝多次散射光，以增加光学相干断层扫描在软组织中的穿透深度。 我们将在组织中的一定深度处同时提供聚焦超声和来自OCT扫描激光源的光。 在超声波焦点内，超声波对光进行相移。 通过处理OCT信号以仅查看相移光，我们可以生成不受来自多次散射的干扰的信号。 这将增强对比度和穿透力，允许在比目前可能的更大深度处进行OCT成像。

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.