Cellular Imaging from Phonon Imaging on a Fiber Probe

Researchers at the University of Nottingham have developed an ultrasonic imaging system, which can be deployed on the tip of a hair-thin optical fibre, and will be insertable into the human body to visualise cell abnormalities in 3D.

The new technology produces microscopic and nanoscopic resolution images that will one day help clinicians to examine cells inhabiting hard-to-reach parts of the body, such as the gastrointestinal tract, and offer more effective diagnoses for diseases ranging from gastric cancer to bacterial meningitis.

Nature Light Science and Applications- Phonon imaging in 3D with a fibre probe

Abstract
We show for the first time that a single ultrasonic imaging fibre is capable of simultaneously accessing 3D spatial information and mechanical properties from microscopic objects. The novel measurement system consists of two ultrafast lasers that excite and detect high-frequency ultrasound from a nano-transducer that was fabricated onto the tip of a single-mode optical fibre. A signal processing technique was also developed to extract nanometric in-depth spatial measurements from GHz frequency acoustic waves, while still allowing Brillouin spectroscopy in the frequency domain. Label-free and non-contact imaging performance was demonstrated on various polymer microstructures. This singular device is equipped with optical lateral resolution, 2.5 μm, and a depth-profiling precision of 45 nm provided by acoustics. The endoscopic potential for this device is exhibited by extrapolating the single fibre to tens of thousands of fibres in an imaging bundle. Such a device catalyses future phonon endomicroscopy technology that brings the prospect of label-free in vivo histology within reach.

Introduction

In this report we present the first optical fibre-based ultrasonic imaging tool capable of resolving biological cell-sized objects. The device simultaneously accesses topographic and material information from microscopic objects. This is accomplished using a novel signal processing protocol that renders a spatial measurement from the amplitude decay signature of the time-of-flight of a GHz frequency acoustic wave. Proof of concept profilometry and spectroscopy is carried out on a Petri dish and polymer microstructures. It is also demonstrated that the technology is compatible with both single-mode optical fibre and the multi-mode channels of an imaging bundle.

High resolution is achieved by a unique combination of optical lateral resolution and acoustic axial precision. Lateral resolution is set by the mode field diameter of the optical fibre; axial precision is enabled by the phonon wavelength and temporal resolution of the signal processing. Furthermore, the acoustic time-of-flight can be analysed to obtain viscoelastic information by measuring the sound velocity and attenuation of the wave. Due to the partial transparency of the ultrasonic sensor, the device in bundle-format can still be used for brightfield or fluorescence imaging.

The optical fibre and imaging bundle implementations of this technology hold promise for integration into standard endoscopy and endomicroscopy equipment. Sub-cellular resolution provides an opportunity to perform three-dimensional (3D) in vivo histology without the fluorescent labels required by similarly resolved endoscopic techniques. The availability of additional elastic information could also introduce a novel histological metric with which to characterise disease at the point of care. Beyond clinical healthcare, the fields of tissue engineering and precision manufacturing could also utilise this high-resolution tool for superficial diagnostics.

Sub-cellular resolution in optical endoscopy has been most readily achieved by a single optical fibre, which is not limited by the core-to-core spacing of a fibre bundle. In general, a single optical fibre can be used to form images by the following means:
(1) scanning the distal end of the fibre from point-to-point in the object plane or
(2) using a multi-mode fibre to encode spatio-angular information across the range of core modes.

Lacking this latter capability, single-mode optical fibres are typically used in distal scanning or spatially dispersive configurations, and have provided breakthroughs in confocal endomicroscopy and endoscopic optical coherence tomography. On the other hand, a single multi-mode fibre is less dependent on scanning since each mode within the fibre acts as a pixel; the caveat is that mode dispersion scrambles the image information and must be empirically compensated. Once unscrambled, the multi-mode fibre empowers lensless endoscopy with qualities such as high numerical aperture (NA), wide-field, 3D imaging and even super-resolution.

Practically speaking, there are certain limits to the utility of purely optical endoscopy techniques. For example, cellular tissue often exhibits poor optical contrast and specificity, which is typically mitigated within sub-cellular resolution endomicroscopy and endocytoscopy by staining the tissue with fluorescent labels. However, acoustics natively offers a pathway to high contrast imaging within biological media, as demonstrated by an extensive history of imaging modalities.

Compared with optical techniques, acoustics has long been hampered by a lack of resolution which can be attributed to the extreme measures required to reduce the acoustic wavelength, e.g.,
(1) miniaturising piezoelectric transducer systems and
(2) debilitating acoustic attenuation in liquids at high frequencies. Consequently, the same development arc for cellular resolution optical endoscopy has not been duplicated in acoustics. Following the advent of the scanning acoustic microscope in 1974, the most pragmatic breakthroughs in high-resolution acoustics have been provided by opto-acoustics, i.e., the optical detection of acoustic phenomena.

Among these techniques, picosecond ultrasonics (PU) and Brillouin scattering are of particular interest as they offer picosecond temporal resolution and direct read-out of viscoelastic properties (respectively) with optical lateral resolution. Time-resolved Brillouin scattering synergises these concepts and has enabled 3D elastography of biological cells with sub-optical wavelength phonons.

Despite significant progress in these fields, the maximum available resolution of all-optical ultrasonic 3D-imaging fibres is on the order of ~40 μm30, and therefore the reality of cellular resolution acoustic endoscopy remains elusive. Presented here is the first optical fibre-imaging tool for acoustic microscopy. Similar to other state-of-the-art optical fibre-imaging techniques, this 125 μm diameter single-mode phonon probe is arranged in a distal scanning, front-view, and reflection-mode configuration. What follows are principles of operation for the device, results demonstrating label-free parallel profilometry and spectroscopy, and the related performance metrics. Furthermore, it is demonstrated that the single fibre can be extrapolated to a wide-field imaging bundle which enables a new class of phonon endoscopes.