The transducer takes incoming electronic signals and uses them to generate ultrasound waves. When these hit their target, they are reflected back to the transducer and transformed back into the corresponding electronic signals, which are used to reconstruct an image of the particular object.
With a conventional piezoelectric transducer, however, it remains difficult to construct the high-frequency arrays that are highly desirable for three-dimensional high-resolution imaging.
Researchers from the University of Michigan in Ann Arbor and from the University of Washington in Seattle are seeking to overcome these limitations. They have designed, fabricated and tested a broadband all-optical transducer for use in a real-time 3-D high-resolution ultrasound imaging system.
Instead of using electronic signals (as piezoelectric transducers do in traditional ultrasound), the system designed by the researchers uses laser beams for input and output. One laser generates ultrasound signals through the transducer, while the reflection of the other laser is used to detect those signals.
Researcher Yang Hou of the University of Michigan said that the key component of the transducer used in the experiments was a two-dimensional nanostructure built with gold nanoparticles, efficient optical absorbers whose optical properties allowed an optoacoustic generator and detector to be integrated into the same transducer.
The transducer was constructed by first laying the two-dimensional gold nanostructure on a glass substrate covered with a 3-μm layer of a polymer bulk and a 30-nm gold layer. An additional polymer layer 0.5 μm thick was added for protection. The gold nanostructure and the gold layer, together with the polymer layer in between, formed an etalon for the optical detection of ultrasound.
During a pulse-echo experiment, a 780-nm beam from a Continuum Inc. pulsed solid-state laser was focused onto the gold transducer. This generated acoustic waves that were reflected back onto the transducer by a mounted reflector. An Agilent Technologies continuous-wave laser focused on the transducer detected the pulse-echo ultrasound signal, and the reflected light was collected by a Newport Corp. InGaAs photodetector.
The signal-to-noise ratio of the pulse-echo signal was more than 10 dB in the far field of the single-element transducer. The center frequency was 40 MHz with –6 dB bandwidth of 57 MHz. Hou said that the results were promising and that, if an array system could be constructed, the signals of all the transducers could be recorded simultaneously. Researchers would be provided with immediate clinical results. The more time-consuming piezoelectric transducer cannot achieve this, making it a less desirable tool for high-resolution clinical studies.
In a second experiment with the technique, the researchers replaced the reflector used in the initial test with a 25-μm-diameter metal wire to determine the imaging resolution of the transducer. The lateral resolution was 38 μm.
Hou said that the system has the potential to image cells — even cellular components — in real time. The transducer possibly even could be attached to a fiber and inserted into the body. It would function much as an endoscope does, capturing internal images with minimally invasive surgery. In a more complex array system, lasers would allow the size and spacing of each array to be precisely controlled.
The researchers did encounter several problems with their technique. The fabrication process is more complicated than with piezoelectric technologies, and each system requires many optical components, which raises the cost. Hou anticipates that the process will be simplified with time, and that costs eventually will be reduced by using other equipment or by modifying the equipment in use.
The biggest hurdle at this stage is expanding from a single-element transducer to an array system. Hou noted that the team plans to design and develop a fiber bundle for simultaneous illumination and detection at an array of spots.
Applied Physics Letters, Aug. 13, 2007, 073507.