King’s College London and University College London researchers have developed a miniature, high-speed, photoacoustic endoscope that is small enough to fit inside a 20-gauge medical needle. The needle probe demonstrated the ability to provide functional, molecular, microstructural information about tissue, at subcellular spatial resolution and in real time. It could be used as a forward-viewing endoscopic probe and as tool for guiding minimally invasive surgeries, the developers said. The ultrathin endoscope consists of a multimode fiber for delivering the light for photoacoustic excitation and a single-mode optical fiber, with a microresonator sensor, for detecting ultrasound waves. To enable tissue interrogation, light is tightly focused through the multimode fiber. High-speed wavefront shaping is performed with a digital micromirror device (DMD). The DMD has nearly one million tiny mirrors that can be independently flipped at tens of thousands of frames per second to change the wavefront so the light can be focused and scanned quickly. The high-speed DMD improved imaging speed significantly, compared to the speed that is attained with a liquid crystal spatial light modulator. “The imaging speed of this photoacoustic endomicroscopy probe is two orders of magnitude higher than those previously reported,” researcher Wenfeng Xia said. In addition to increasing imaging speed, the rapid scanning supported by the DMD allowed for a denser spatial sampling (i.e., a sampling of more pixels within a designated area). This enabled subcellular spatial resolution with a fidelity comparable to benchtop photoacoustic microscopy systems. Researchers created a photoacoustic imaging endoscope probe that can fit inside a medical needle with an inner diameter of just 0.6 mm. Researcher Tianrui Zhao holds the imaging probe, which the research team used to acquire high-resolution images of mouse red blood cells. Courtesy of Tianrui Zhao/King’s College London. For optical detection of acoustic waves, the researchers developed an optical microresonator that can be fabricated on the tip of an optical fiber. When sound waves hit the microresonator, its thickness changed, which affected the amount of light that reflected back into the fiber. The fiber optic, plano-concave, microresonantor sensor provided high acoustic sensitivity, broad frequency bandwidth, and a near-omnidirectional response. Until now, instruments used for photoacoustic endoscopy have been either too bulky or too slow for practical use as forward-viewing endoscopes. By combining wavefront-based beam shaping with light-based ultrasound detection and incorporating a fast algorithm for controlling the device, the team devised a miniaturized probe without forfeiting imaging speed. According to Xia, traditional light-based endoscopes only resolve tissue anatomical information on the surface. Additionally, these instruments tend to have large footprints. Contrarily, the endoscope described in the recent work resolved subcellular-scale tissue structural and molecular information in 3D and in real time. It was also small enough to be integrated with interventional medical devices that would allow clinicians to characterize tissue during a procedure, Xia said. The researchers used the endomicroscopic probe to acquire high-resolution photoacoustic images of mouse red blood cells in an area that was 100 µm in diameter at about three frames per second. They also showed that the needle probe could be scanned to enlarge the field of view in real time when consecutive images were stitched together. Imaging performance was not substantially degraded when the probe was scanned, which could indicate that the miniature endoscope is not affected by a modest amount of fiber bending. However, as a step toward clinical use, the team will further investigate how complex fiber bending or semi-rigid configurations could affect imaging performance. Artificial intelligence could be used to improve the probe’s imaging speed by increasing the scanning step size, and therefore reducing the total number of scans without sacrificing the spatial resolution, the team said. The technique used to develop the device could have broader application beyond photoacoustic imaging, such as fluorescence imaging, Raman microscopy, and two-photon microscopy, Xia said. “It could eventually allow 3D characterization of tissue during various minimally invasive procedures such as tumor biopsies. This could help clinicians pinpoint the right area to sample, which would increase the diagnosis accuracy.” The research was published in Biomedical Optics Express (www.doi.org/10.1364/BOE.463057).
