Wideband Optical Detector of Ultrasound for Medical Imaging Applications

Summary

Optical detection of ultrasound is impractical in many imaging scenarios because it often requires stable environmental conditions. We demonstrate an optical technique for ultrasound sensing in volatile environments with miniaturization and sensitivity levels appropriate for optoacoustic imaging in restrictive scenarios, e.g. intravascular applications.

Abstract

Optical sensors of ultrasound are a promising alternative to piezoelectric techniques, as has been recently demonstrated in the field of optoacoustic imaging. In medical applications, one of the major limitations of optical sensing technology is its susceptibility to environmental conditions, e.g. changes in pressure and temperature, which may saturate the detection. Additionally, the clinical environment often imposes stringent limits on the size and robustness of the sensor. In this work, the combination of pulse interferometry and fiber-based optical sensing is demonstrated for ultrasound detection. Pulse interferometry enables robust performance of the readout system in the presence of rapid variations in the environmental conditions, whereas the use of all-fiber technology leads to a mechanically flexible sensing element compatible with highly demanding medical applications such as intravascular imaging. In order to achieve a short sensor length, a pi-phase-shifted fiber Bragg grating is used, which acts as a resonator trapping light over an effective length of 350 µm. To enable high bandwidth, the sensor is used for sideway detection of ultrasound, which is highly beneficial in circumferential imaging geometries such as intravascular imaging. An optoacoustic imaging setup is used to determine the response of the sensor for acoustic point sources at different positions.

Introduction

Ultrasound detectors play a key role in many imaging applications. Conventionally, ultrasound is detected by piezoelectric transducers, which transform pressure waves into voltage signals¹. In optoacoustic imaging, ultrasound is generated via a process of thermal expansion by illuminating the object with high-power modulated light^2-6. Although piezoelectric transducers are the method of choice in optoacoustic applications, their use often hinders miniaturization mainly because miniaturized piezoelectric transducers are often characterized by low sensitivity. Additionally, since piezoelectric transducers are optically opaque, they may severely int....

Protocol

1. Optoacoustic Characterization of the π-FBG Detector

1. Preparation of a microscopic sphere suspended in agar:
   1. Mix agar powder (1.3% by weight) with distilled water in a glass beaker. Use a hot plate magnetic-stirrer device to heat the solution close to boiling temperature and dissolve the agar powder until the solution becomes clear and free of air bubbles. Alternatively, the agar solution may be heated using a conventional microwave with stirring performed manually using a glass stirrer. Pour the hot solution into a plastic mold, e.g. syringe with its tip cut out.
   2. Sprinkle a small amount of microscopic spheres on ....

Results

Figures 4a and 4b respectively show the signals and their corresponding spectra from the microscopic sphere at a distance of 1 mm from the fiber for three offsets from the center of the π-FBG. The offsets are given in the z direction, as depicted in Figure 3. Clearly, The optical detector’s sensitivity to high frequency ultrasound (f > 6 MHz) is anisotropic and is highest when the center of the π-FBG is directly above the microscopic sphere. Despite the high acoustic .......

Discussion

In conclusion, a new optical method for ultrasound detection is introduced, which is based on a combination of a π-FBG and pulse interferometry. The technique is especially suited for optoacoustic imaging applications owing to the transparency of the sensing element, which enables almost arbitrary object illumination patterns. In contrast, standard piezoelectric based ultrasound detectors are opaque and thus block some of the optical paths to the imaged object, leading to bulky imaging setups. The developed optical .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
π-FBG	Teraxion Inc.		Custom made device
Microscopic spheres	Cospheric LLC	BKPMS 90-106um- 10g	100 µm polyethylene microspheres
Femto-second pulse laser used for interrogation	Menlo Systems GmbH	T-Light Femtosecond Laser
Optical filter	Optoplex Corporation	2-Port Optical Tunable Filter (50 GHz)
Optical amplifier	Amonics	AEDFA-PM-PA-35-B-FC	Benchtop 35dB Gain Pre Amp Polarization Maintaining EDFA
50/50 coupler	OZ-Optics	FUSED-22-1550-8/125-50/ 50-3S3S3S3S-3-0.5-PM	Fused 2 x 2 fiber splitter with 0.5 meter long, 3 mm OD PVC jacketed 1,550 nm 8/125μ PM fiber pigtails, 50/50 split ratio in the slow axes of PM fibers and with super FC/PC connectors on all ports.
Fiber holder	Thorlabs	T711/M-250	Metric, Post-Mountable Fiber Clamp, 250 µm
Agar for microbiology	Sigma Aldrich	05039-500G
Nano-second pulse laser used for generating the optoacoustic signals	Opotek	VIBRANT Arrow 532 type I
Graphite rod	Faber-Castell	120700	Faber-Castell Pencil Leads - 0.7 mm