Medical Lasers and Endoscopy

2D Optical ultrasound array

A 2D optical ultrasound receive array based upon Fabry Perot polymer film sensing technology has been investigated with a view to overcoming some of the limitations associated with 2D piezoelectric arrays. These include difficulties in fabricating acoustically small element sizes for use at MHz frequencies whilst still retaining adequate detection sensitivity. This is particularly so for phased arrays where, for optimum lateral spatial resolution, a near-omnidirectional element response is required necessitating an element size that is small in comparison to the acoustic wavelength. Sub-50µm element sizes are required for an isotropic response at 10 MHz, for example. To achieve this with adequate wideband detection sensitivity (<1kPa) using piezoelectric transducers represents a major challenge. Electrical crosstalk and the complexity that arises from the need to incorporate a large number of electrical connections within the footprint of the array head present further difficulties. A possible solution is to map the incident acoustic field distribution, via a Fabry Perot polymer film sensing interferometer, on to an optical field [8,9]. The spatial discretisation of the acoustic detection process can therefore be removed from the detection plane to a remotely located high-density array of optical detectors such as a photodiode array or CCD array. This offers substantial advantages in terms of the spatial sampling of the acoustic aperture. In particular, substantially smaller element sizes (in principle down to the optical diffraction limit of a few µm) and interelement spacings than can be achieved with piezoelectric arrays are possible. This approach is currently being evaluated for medical and industrial transmission ultrasound imaging, biomedical photoacoustic imaging and ultrasonic field characterisation.

System description

Figure 1 Experimental setup to demonstrate the concept of 2D optical ultrasound array.

A schematic of an experimental system used to demonstrate the concept is shown in figure 1. A laser beam of diameter 12mm illuminates the sensing film. Acoustically-induced changes in the optical thickness of the polymer film produce a corresponding change in the intensity of the reflected beam. In a practical system this would be detected and laterally spatially resolved using a CCD or photodiode array. For the purposes of demonstrating the concept however, a single 25MHz photodiode was mechanically raster scanned over the area of the reflected beam to map the incident acoustic field. To a first approximation, the effective acoustic element size and interelement spacing are simply those of the photodiode area and the scan increment respectively. Sensitivity was obtained by referencing the sensor output with that of a calibrated PVDF membrane hydrophone, giving a noise-equivalent-pressure of approximately 3kPa over a 25MHz measurement bandwidth. The bandwidth of this particular configuration was 12MHz. The advantage of this approach is that by using readily available inexpensive optical detector arrays with very high element densities (e.g. a CCD array with 765x565 elements), the sensitivity-element size limitations and often overwhelming complexity of piezoelectric arrays can be avoided.

The system was evaluated by two methods. Firstly, by obtaining transmission ultrasound images of a variety of spatially calibrated targets. Secondly, by mapping the output of a focussed ultrasound transducer at various distances from the focus.

Transmission ultrasound images

Figure 2 shows a transmission ultrasound image which was obtained by positioning the target, a silicone rubber cross, between the sensor head and a 3.5MHz, 1" dia. planar PZT transducer and scanning the photodiode across the reflected sensor output beam. Figure 3 shows an image of a plastic mesh similarly obtained. The scanned region in both cases was a 12 x 12mm rectangle and the scan increments were 0.1mm. Figures 2 and 3 demonstrate the concept of mapping an acoustic field on to an optical beam showing that a circular "array" aperture in excess of 10mm diameter with appreciable sensitivity is feasible.

Transducer field maps

The experimental arrangement shown in figure 1 was used to map the temporal and lateral field distributions produced by a pulsed 5 MHz focussed piezoelectric PZT transducer. Figure 4 shows the results obtained by scanning the photodiode along a line perpendicular to and passing through the axis of the transducer located above the sensor head in a water bath. At each point of the scan the acoustic waveform detected by the sensor was captured and linearly mapped to a grayscale. Figure 4 shows this with the curvature of the wavefront emitted by the transducer clearly visible.

Figure 4 Line scan through the axis of a pulsed focussed (PZT) 5 MHz transducer. The waveform represents a profile through the centre of the image. A trigger delay (not shown) was used so the vertical axis begins at t=28 ms - i.e. the sensor-transducer separation was approximately 4.5 cm. Photodiode aperture: 0.8 mm, transducer focal length: 6.5 cm, transducer diameter: 25 mm

To obtain the 2D lateral field distribution, the photodiode was raster scanned over a rectangular area of the reflected sensor output beam and the signal amplitude at each step mapped to a grayscale. Figure 5 shows this for three different distances from the focus showing the diverging beam profile of the transducer. The FWHM values obtained from the horizontal and vertical profiles are in good agreement with the calculated 6dB beam widths. For example the calculated beam width at the focus (z=65mm) is 0.67mm. The beam width obtained from figure 5 for z=70.5mm (slightly off the focus) is 0.75mm.

Figure 4 Lateral field distributions at 3 distances z from a 5 MHz focussed ultrasound transducer. From left to right z=70.5 mm, z=77.4 mm, z=84.1 mm. Transducer focal length = 65mm, element diameter = 25mm, photodiode element size = 0.4mm, scan increment = 0.1mm

Applications

Although the system is primarily being developed as part of an instrument for biomedical photoacoustic imaging, it has applications as a receiving array in several other areas. For example it could be used in an acoustic camera to perform high resolution transmission ultrasound imaging. In this approach the object to be imaged is situated between an ultrasound source and a receiving array and variations in the acoustic attenuation of the object are imaged. The high element density and potential for rapid data acquisition (enabling real time imaging) makes the system ideally suited to this. Medical applications include real time imaging of joints and joint cartilage during flexure, imaging the heel for the detection of osteoporosis and tomographic imaging of the breast. Industrial applications include rapid visualisation of faults such as subsurface cracking, voids and delaminations in engineering materials and structures used in aerospace and marine industries. Another possible application is imaging faults in microelectronic circuits during production. Finally, the system could be used to fulfill the need for high speed characterisation of ultrasound transducers and arrays carried out by transducer manufacturers and standards authorities.