Wideband Optical Detector of Ultrasound for Medical Imaging Applications

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.

This video will demonstrate the use of an optical sensor of ultrasound based on a PPH phase shifted fiber brag grading, or PI FBG as a sensing element and a pulse interferometry readout system. This is achieved by embedding a dark microscopic sphere in transparent agar and using it to generate a wide band point like acoustic source by illuminating it with high power light pulses. As a second step, the pie FBG is scanned in three dimensions, which enables characterization of its spatially dependent and frequency dependent response to ultrasound.

Next, a water pump is used in order to generate a mechanical disturbance in the vicinity of the ultrasound sensor and evaluate the robustness of the sensor. Results are obtained that show that the optical sensor is appropriate for real world imaging applications based on its spatiotemporal acoustic response and its robustness to mechanical disturbances. The main advantage of this technique over existing optical methods for ultrasound detection like the combination of micro rings with continuous wave interferometry, is that this technique offers a high robustness against mechanical vibrations, which enables its application in the clinical environment.

Additionally, our detectors based on a whole fiber design with a foam factor compatible with the strictest of catheter applications. The implications of this technique extend toward minimally invasive imaging techniques because of the small dimensions of the fiber detector. Generally, individuals new to this method will struggle because of the non-standard interferometric design and the complex acoustic response of the detector at frequencies below six megahertz.

To begin, mix agar powder with distilled water in a glass speaker. Use a hot plate magnetic stir device to heat the solution close to boiling temperature and dissolve the agar powder until the solution becomes clear and free of air bubbles. Sprinkle a small amount of microscopic spheres on the agar solution and wait until the solution fully solidifies.

Take the solid agar phantom out of the mold by pushing the plunger. View the phantom under a stereoscopic microscope. Cut a small piece of agar that contains a single microscopic sphere.

Repeat agar preparation and add the solid agar piece containing the single microscopic sphere to the agar solution. After solidification, cut the agar phantom under the microscope such that the microscopic sphere is located close to the phantom surface. These experiments use a PPH phase shifted fiber Bragg grading as a resonator.

Use two V groove fiber holders to hold the fiber tightly on both sides of the pi.FBG. Connect the holder to a three dimensional translation. Computer operated stage.

Ensure that the fiber is submerged in a water tank. To enable the propagation of ultrasound, find the approximate location of the sensing PI FBG element by illuminating different parts of the fiber with the high power nanosecond pulse laser beam, the optical absorption of the coating. However, weak will create a signal when the illumination is performed on the PI FBG.

Next place the agar embedded microscopic sphere directly underneath the pi FBG. The microscopic sphere should be visible to the naked eye using the translation stage. Perform a 2D scan of the PI FBG in the plain parallel to the ground to find the location where the signal from the microscopic sphere is strongest and its corresponding time delay is shortest.

Perform the last adjustments to the illumination to deliver maximum power to the microscopic sphere. Using the translation stage, perform a 3G scan of the PI FBG and record the signal for each position to obtain the spatially dependent frequency response of the ultrasound detector. Perform the farrier transform on the recorded time domain ultrasound signal.

Use two V groove fiber holders to hold the fiber tightly on both sides of the PI FBG. Place a dark plate sterily in the water tank and illuminate it with the high power nanosecond pulse laser beam to create a strong acoustic field. Then position the PI FBG above the illuminated region.

Place a water pump inside the water tank and turn it on in order to create rapid variations in the environmental conditions to estimate the robustness of the system, measure the output with the locking circuit turned both on and off. When no locking is performed, it is not possible to accurately detect the ultrasound signal. After turning the water pump off, estimate the benefit and sensitivity due to the high coherence of the source by replacing the wideband pulse laser with a low coherence source.

Then repeat the acoustic measurement. A decrease of over an order of magnitude and sensitivity is expected when the low coherent source is used. Shown here are the signals and their corresponding spectra from the microscopic sphere at a distance of one millimeter from the fiber for three offsets from the center of the pi FBG.

The spectra are compared to the spectrum of an ideal spherical source with a diameter of 100 micrometers. Clearly, the optical detector's sensitivity to high frequency ultrasound is a isotropic and is highest when the center of the PI FBG is directly above the microscopic sphere. Despite the high acoustic impedance mismatch between the silica fiber and water, the sensor exhibits a relatively regular resonance free acoustic spectrum at frequencies above six megahertz leading to a well-defined sharp opto acoustic signal suitable for imaging applications.

A comparison between ultrasound signals measured using a low coherent source and a pulse source is shown here. A significant reduction in sensitivity of a factor of 18 is observed for the signals detected with the low coherent source. This lower sensitivity is inherent to the low coherent source as the wideband optical spectrum is generated by a random process as opposed to a deterministic process for the coherent pulse source.

Once mastered, the sensor characterization technique can be done in a few hours if it is performed properly. While attempting this procedure, it's important to remember to distinguish between the acoustic response of the sensor at frequencies below and above six megahertz. Only at tire frequencies is the response relatively resonance free and suitable for imaging applications After its development.

This technique paved the way for researchers in the field of optic acoustic imaging to explore new designs for minimally invasive imaging devices for clinical use.