Multimodal remote photoacoustic and OCT imaging

The proposed project deals with a combination of remote photoacoustic imaging and optical coherence tomography, allowing remote imaging in both modalities. In particular, both imaging modalities are realized with fiber-optic technology within the same fiber optical network. Optical coherence tomography (OCT) is a high-resolution and contactless imaging method, which allows depth resolved imaging of refractive image changes in turbid media. This technique has been originally developed for ophthalmology, and is currently worldwide intensively pursued for further medical diagnostics of biological tissues. Due to its remote nature, OCT allows remote imaging of refractive index changes, thus making it ideal as an inter- operative imaging tool or for inline material inspection. In contrast to OCT, where changes in the refractive image are measured, photoacoustic imaging (PAI) measures the optical absorption. Imaging relies on the photoacoustic effect, which describes conversion between light and acoustic waves due to absorption of electromagnetic waves and localized thermal expansion. In practice, short laser pulses are used to illuminate a sample. The local absorption of the light is followed by rapid heating, which subsequently leads to thermal expansion and the generation of broadband acoustic waves. By recording the generated ultrasonic waves the initial absorbed energy distribution can be assessed. Therefore, OCT and PAI deliver complementary information. However, conventional ultrasonic transducers which are used for PAI require physical contact to the specimen. This is a limitation for many applications. This limitation can be overcome by remote photoacoustic imaging (rPAI) techniques, which were recently introduced. Hereby, the acoustic waves are recorded directly on the surface of a specimen by means of interferometry. The aim of the proposed project is to demonstrate a combination of rPAI with OCT, thus allowing remote imaging in both modalities. Both imaging techniques, rPAI and OCT, will be realized with fiber-optical components within the same fiber optical network. This allows a relatively simple and robust instrument while providing perfect co- registration of both modalities. For rPAI a low-power optical source is used. Light which is reflected from the surface is amplified by means of optical amplification before detection and demodulation. Due to the combination of a low power optical source and optical amplification before detection, the thermal stresses on the sample can be kept low, while the system provides high spatial resolution. Besides delivering complimentary information, OCT is used to measure the morphology of the specimen`s surface. This information is needed for image reconstruction of photoacoustic images.

Within the project we realized a unique combination of two non-contact, non-invasive imaging methods, namely non-contact photoacoustic imaging (ncPAI) and spectral-domain optical coherence tomography (OCT). Both modalities were integrated within the same fiber-optic network, using the same measurement head to achieve intrinsic co-registration between both imaging modalities. The system was designed by using components from telecommunication technologies (with a wavelength 1310 nm for OCT and 1550 nm for ncPAI detection) and the optical fluence on the examined sample could be kept low by using optical amplification. OCT imaging is based on the interferometric detection of light that is reflected/backscattered at inner subsurface structures, such as e.g. blood vessel walls, cell walls, or layered structures. This technique has been originally developed for examination of the human eye, and is currently worldwide intensively pursued for further medical diagnostics of biological tissues. OCT has, for example, shown potential as a monitoring tool during laser surgery of laryngeal carcinoma. PAI employs an imaging mechanism that is based on the photoacoustic effect: Short laser pulses are used to illuminate a sample. The local absorption of the light (due to molecules such as haemoglobin) is followed by rapid heating, which subsequently leads to thermal expansion and the generation of broadband acoustic waves. By recording the generated ultrasonic waves, the initial absorbed energy distribution can be assessed. Conventional ultrasonic transducers for PAI require physical contact to the specimen, facilitated by using ultrasound gel or water as coupling medium. This limitation could be overcome by non-contact photoacoustic imaging techniques, where the ultrasonic waves impinging onto the sample surface are acquired using interferometric detection. OCT and PAI deliver complementary information. For example, in OCT vessel walls are imaged, but the detection of blood is mainly accomplished by motion of red blood cells. In contrast, PAI images the optical absorption of blood, and is thus also capable of imaging static blood as, e.g., found in hemorrhages, vascular occlusions, etc. As the photoacoustic effect relies on optical absorption, spectroscopic ncPAI was demonstrated using a quantum cascade laser that was swept over its wavelength range.