Pencil beam all-optical ultrasound imaging

Abstract

A miniature, directional fibre-optic acoustic source is presented that employs geometrical focussing to generate a nearly-collimated acoustic pencil beam. When paired with a fibre-optic acoustic detector, an all-optical ultrasound probe with an outer diameter of 2.5 mm is obtained that acquires a pulse-echo image line at each probe position without the need for image reconstruction. B-mode images can be acquired by translating the probe and concatenating the image lines, and artefacts resulting from probe positioning uncertainty are shown to be significantly lower than those observed for conventional synthetic aperture scanning of a non-directional acoustic source. The high image quality obtained for excised vascular tissue suggests that the all-optical ultrasound probe is ideally suited for in vivo, interventional applications.

Keywords: (060.2380) Fiber optics sources and detectors; (110.2350) Fiber optics imaging; (110.5125) Photoacoustics; (170.0110) Imaging systems; (170.7170) Ultrasound.

Fig. 1

Schematic (top) and photograph (bottom) of the focussed optical ultrasound source. The inset (top right) shows a photograph of the probe after applying an optically absorbing coating.

Fig. 2

Left: maximum intensity of the transmitted acoustic field measured at an axial distance of 3 mm with the full-width half maximum contour indicated by the dotted blue curve. Middle: spatial extent of the acoustic beam in the elevational (black solid curve) and lateral direction (black dashed curve), together with the peak acoustic amplitude (red dash-dotted curve), as a function of axial depth. The acoustic data were measured at a distance of 3 mm and numerically propagated to the remaining depths. Right: power spectrum of the A-scan (shown in the inset) corresponding to the peak acoustic intensity measured at a distance of 3 mm. The dashed red line indicates the −6 dB level relative to peak power used to measure the bandwidth of the transmitted signal.

Fig. 3

Top row: simulated (first two columns) and measured (middle two columns) all-optical ultrasound images obtained of a phantom using an unfocussed acoustic source. The phantom consisted of the tip of a single rod placed at different depths (right column); the resulting images are compounded into a single synthetic image. Simulations and experiments were performed both in the absence (“0 μm”) and presence (“30 μm”) of deliberate probe positioning errors. Positioning errors were applied to both the axial and the lateral axis and sampled from a uniform random distribution with a range of ±30 μm. Bottom row: the same panels are shown for the case where all-optical pulse-echo ultrasound data were acquired using the focussed acoustic source.

Fig. 4

A. Photograph of the focussed probe positioned above an ex vivo aortic section containing two side-branches (SB1 and SB2). The probe was translated along the dotted line to scan a synthetic aperture. B. Schematic cross-section of the aorta wall and side-branches. C. All-optical ultrasound image acquired using the focussed acoustic source. The image, displayed on a logarithmic scale, was obtained without image reconstruction.

Fig. 5

Cross-sections through a volumetric all-optical ultrasound image of an aorta section containing a side-branch (SB1 in Fig. 4). The geometry of the aorta wall and side-branch veering off to the left are indicated in purple and blue, respectively. All cross-sections are shown on the same logarithmic scale (40 dB dynamic range), and no image reconstruction was applied. The lateral axis was scaled to improve the visibility of the cross-sections.