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This paper presents the design, fabrication and characterization of a broadband miniature fiber optic ultrasound generator based on photoacoustic (PA) ultrasound generation principle for biomedical ultrasound imaging and ultrasound non-destructive test (NDT) applications. A novel PA generation material, gold nanocomposite, was synthesized by directly reducing gold nanoparticles within polydimethylsiloxane (PDMS) through a one-pot protocol. The fiber optic ultrasound generator was fabricated by coating the gold nanocomposite on the tip of the optical fiber. The efficiency of the PA generation using gold nanocomposite was increased 105 compared to using aluminum thin film and 103 compared to using graphite mixed within epoxy. The ultrasound profile and the acoustic distribution have been characterized. The amplitude of the generated ultrasound signal was as high as 0.64 MPa and the bandwidth was more than 20 MHz. This paper also demonstrated its capability for ultrasound imaging of a tissue specimen.
© 2014 Optical Society of America
1. Introduction
Various studies of ultrasound generators have been conducted to meet the rising challenges of advanced ultrasound applications such as biomedical ultrasound imaging and ultrasound non-destructive test (NDT) [1–3]. Other than conventional piezoelectric ultrasound generators [4–7], optical ultrasound generators, especially fiber optic ultrasound generators are attractive candidates for many advanced ultrasound applications [1, 8, 9]. The most commonly used mechanism to generate ultrasound signals optically is the photoacoustic (PA) principle, which is a wide bandwidth ultrasound generation method because the pulse width of the ultrasound can be tailored by ultra-fast lasers [10, 11]. By taking advantages of PA principle and optical fibers, novel fiber optic ultrasound generators featuring wide bandwidth and compact size for biomedical imaging and NDT applications in limited spaces can be achieved.
The PA principle is an optical approach to generate ultrasound signals [12]. It involves a PA generation material which absorbs the optical energy from the laser and converts it into localized temperature rise. The localized temperature rise will cause the expansion of the PA material due to the thermal expansion effect. The PA material will contract when the laser is shut off. Therefore, the expansion/contraction cycle will generate mechanical waves which are acoustic signals. The most significant advantage of the PA principle is that the profile of the acoustic signals is similar to the profile of the laser, which means that the pulse width of the acoustic signals can be tailored by the laser beam. Ultra-fast lasers, such as nanosecond lasers, have been commercially available. Therefore, the pulse width of the generated acoustic signal can be very short (generally in nanoseconds), which leads to a wide bandwidth of acoustic signals [10, 11, 13, 14].
The key factor of the PA generation principle is the PA generation material which absorbs and converts the optical energy to heat and then acoustic signal. The performances of the PA generation, such as the energy conversion efficiency and the bandwidth, rely on the PA generation material. An ideal PA generation material should feature a high optical energy absorption capability and a high coefficient of thermal expansion (CTE). Recently, various studies have been exerted on developing novel materials to increase the efficiency of the PA generation and it has been reported that materials based on polymer show higher PA generation efficiency than metallic materials [10–15]. Graphite mixed within epoxy and graphite mixed within PDMS were reported by Biagi’s and Buma’s group, respectively. By mixing graphite within epoxy, Biagi’s group reported that the efficiency of the PA generation was increased 2 orders of magnitude compared to the thin aluminum film [10]. By replacing graphite with gold nanostructure, Hou’s group reported a broad bandwidth optical ultrasound transducer [11, 13]. Two dimensional gold nanostructures were fabricated by nanoimprint technique and a layer of PDMS was coated above the gold nanostructure. Recently, Baac’s work used another material, carbon nanotube composite to optically generate ultrasound signals [14, 16].
The optical energy absorption capability of the PA generation material can be further improved by applying noble metal nanoparticles due to their high optical energy absorption capabilities at the plasmon resonant frequencies. It has been proved that gold nanoparticles (Au NPs) show the maximum optical absorption energy at the wavelength of 520 nm when the diameter is around 20 nm [17]. Therefore, the PA generation efficiency can be improved by applying Au NPs.
In this paper, a novel fiber optic PA ultrasound generator based on gold nanocomposite was designed, fabricated, and characterized. The fabrication process of fiber optic PA generator was very easy to operate with relatively low cost. The gold nanocomposite was achieved by directly reducing gold nanoparticles (Au NPs) within Polydimethylsiloxane (PDMS) and was coated on the tip of optical fibers to generate strong ultrasound signals. The gold nanocomposite was synthesized by a one-pot protocol [18] and it has been demonstrated that such material features high optical energy absorption capability which makes it an excellent material for PA generation applications [12].
This paper is organized as follows. Section 2 presents the design and fabrication procedure of a miniature fiber optic ultrasound generator based on the gold nanocomposite. Section 3 describes the ultrasonic pulse generation experiment using proposed generator, based on the experimental results, the PA generation efficiency was determined. In section 4, the ultrasonic field distribution test was characterized; the ultrasound attenuation coefficient and directivity angle range was calculated based on the experimental findings. In section 5 of this paper, an ultrasound image of a tissue specimen was obtained by the proposed generator, and Section 6 concludes the paper. In summary, all those experimental results proved that the fiber optic ultrasound generator with high energy conversion efficiency and wide bandwidth could be used in biomedical imaging and NDT applications.
2. Methodology
2.1 Gold nanocomposite
The gold nanocomposite was prepared by following a one-pot protocol by directly mixing the gold salt (HAuCl4·3H2O) into PDMS [12, 18]. Au NPs were reduced from the gold salt by PDMS molecules. Briefly, the PDMS matrix was prepared by mixing the base and the curing agent with a weight ratio of 10:3. A certain weight of the gold salt was crushed into powder and was mixed within the prepared PDMS matrix. The gold salt/PDMS mixture was subject to an ultrasonic bath for 30 min in ice water and then the mixture was degassed for approximately 20 min in a vacuum chamber. During the ultrasonic bath, the color of the gold salt/PDMS mixture turned to ruby red indicating that Au NPs with a diameter of around 20 nm were reduced and the mixture was turned into gold nanocomposite, which is consistent with the previous study [12]. The concentration of Au NPs within the PDMS can be adjusted by different amount of the gold salt.
The absorption spectra of the gold nanocomposite material have been presented in our previous study [12]. Because the peak optical energy absorption wavelength of gold nanocomposite material is at 530 nm, the optical energy absorption capability is maximized when the wavelength of the optical irradiation is tuned at the similar wavelength. In this paper, the wavelength of the laser is at 532 nm. Therefore, the optical energy absorption can be maximized.
2.2 Fiber optic ultrasound generator
The fiber optic ultrasound generator was fabricated by applying the gold nanocomposite on the tip of an optical fiber. The generator structure is illustrated in Fig. 1. A piece of multi-mode fiber (MMF) with a core diameter of 400 µm was stripped, cleaved, and the end face was polished. The MMF was dipped into the gold nanocomposite perpendicularly and was pulled out of the gold nanocomposite slowly. Due to the high viscosity of the gold nanocomposite, the gold nanocomposite will be attached on the end face of the MMF. Finally, the MMF was mounted at about 5 mm above a hot plate (set at 120 þC) while maintaining the perpendicular position overnight to cure the gold nanocomposite.
The microscopic picture of the fiber optic ultrasound generator coated with the gold nanocomposite is illustrated in Fig. 2. The magnitude of the microscope was 100. The concentration of the gold salt in the gold salt/PDMS mixture was 7.58% by weight. Due to the surface tension of the gold nanocomposite, the gold nanocomposite formed a spherical shape on the tip of the optical fiber. By calculating the ratio between the diameter of the optical fiber and the thickness of the gold nanocomposite, the thickness of the gold nanocomposite was approximately 105 μm at the thickest part.
3. Ultrasonic pulse generation verification
3.1 Experimental setup
An ultrasound pulse generation experiment was performed to evaluate the performance of the broadband fiber optic ultrasound generator. Figure 3(a) and Fig. 3(b) shows the schematic diagram and the experimental setup, respectively. Experiments were conducted under the water. The optical irradiation source was a 532 nm Nd:YLF nanosecond laser (Surelite-I-10, Continuum) with a pulse width of 5 ns and a repetition rate of 10 Hz. The laser beam was coupled into the fiber optic ultrasound generator through a coupler (F810SMA-543, Thorlabs). In Fig. 3(c), a hydrophone (HGL-0200, Onda) with an aperture size of 200 µm was utilized as a receiver 1 mm away from generator to collect the ultrasound signals. Once the laser source was radiated, a trigger signal was sent out from the laser system to trigger a data acquisition card (DAQ) (M2i.4032, Spectrum) with a sampling rate of 50 MHz. The ultrasound signal was collected by the hydrophone and the signal was transferred to the DAQ for process and analysis.
Fig. 3 Experimental setup for the ultrasonic pulse generation and ultrasonic field distribution. (a) Schematic diagram of the experimental setup. (b) The photo of the experimental setup. (c) Zoomed in photo to illustrate the distance between the generator and the hydrophone.
3.2 Results and discussions
Figure 4(a) shows a typical ultrasound signal that was generated by the fiber optic ultrasound generator. The peak to peak amplitude of the ultrasonic pressure was measured to be 0.64 MPa and the distance between the hydrophone and the generator tip was approximately 1 mm. The pulse width was measured to be 160 ns. After performing the Fourier transform, the frequency domain of the generated ultrasound signal is illustrated in Fig. 4(b) (The 0 dB refers to the total signal power). The bandwidth of the signal was at least 20 MHz. It is interesting to note that, the thickness of the gold nanocomposite will affect the bandwidth of the generator [13, 14]. As the ultrasound propagated along the material, high frequency components attenuate faster than low frequency components. Therefore, the extra thickness of the gold nanocomposite may attenuate high frequency components of the generated ultrasound. By taking the advantage of nanofabrication method [19], the fiber optic ultrasound generator with the ultra-thin gold nanocomposite layer can be fabricated to achieve the higher ultrasound bandwidth.
Fig. 4 Ultrasound signal generated by the fiber optic ultrasound generator. (a) The profile of a typical generated ultrasound signal. (b) The frequency domain of the generated ultrasound signal.
The efficiency of the PA generation can be described by the following equation [10]:
where Eoptical is the energy of the laser and Ea is the energy of the ultrasound signal, respectively. Ea can be estimated by the following equation [10]:where c is the sound velocity in the water; B is the bulk modulus of the water; A is the spot area; p is the acoustic pressure.The energy of the laser emitting the optical fiber Eoptical was measured after laser passing through a bare polished MMF with a core diameter of 400 µm, which was 11 μJ/pulse (laser fluence = 8.75 mJ/cm2/pulse).
The energy of the ultrasound signal is Ea = 1.92 nJ via Eq. (2). Therefore, the efficiency of the PA generation was determined as 0.18 × 10−3. The efficiency was approximately 5 orders of magnitude increased comparing to the PA generation efficiency by using aluminum thin film, approximately 103 times increased comparing to using graphite mixed within epoxy [10]. From [16], the PA transmitter using carbon nanotube composite generated the high frequency ultrasound signal with generation efficiency of 1.4 × 10−3 (laser fluence = 42.4 mJ/cm2/pulse). The author’s PA generation efficiency was lower in one order of magnitude comparing to using carbon nanotube composite. This is because the high frequency component in the ultrasound signal was attenuated by the extra thickness of the gold nanocomposite film.
4. Ultrasonic field distribution
4.1 Experimental setup
In this section we presented the experimental activity which led to better understanding of the fiber optic ultrasonic source by characterizing its energy distribution. The ultrasonic field produced by the fiber optic ultrasound generator obeys the physical laws of wave propagation and it can be simply divided into many small PA point sources attached on the fiber tip, and thus producing an interference pattern at any position in the field. The experimental setup was similar to the ultrasonic pulse generation test. The same optical irradiation source was utilized and the experiment was also performed under the water media. In addition, the hydrophone was mounted on a 2-axis stepper motor stage (NRT 100, Thorlabs) to provide accurate scanning capability during the test. For every scanned position, the peak to peak ultrasonic amplitude was recorded by DAQ after averaged 50 times. Figure 3(b) indicates the scanning orientation.
4.2 Results and discussions
A longitudinal section of the ultrasonic field distribution was measured and the results are illustrated in Fig. 5. The ultrasonic field was acquired within a rectangular area (5.0 mm by 4.0 mm) with the resolution of 0.1 mm by the scanning hydrophone. In Fig. 5(a), the color map represented the ultrasonic pressure. It was noted that the fiber optic ultrasound generator was placed at 0 mm of lateral direction and 0 mm of axial direction. In the contour map, the focal point pressure was found to be 0.78 MPa within the position coordinate (0 mm, 1.2 mm). Moreover, at the position coordinate (0 mm, 1 mm), the pressure was found to be 0.64 MPa. This was in good agreement with the result from the ultrasonic pulse generation experiment.
Fig. 5 Ultrasonic field distribution in a longitudinal section generated from fiber optic ultrasound generator. (a) Pressure distribution of ultrasonic field. (b) Normalized magnitude distribution of ultrasonic field.
In Fig. 5(b), the color map represented the normalized magnitude in decibel scale. Figure 6 was extracted from Fig. 5(b) to show the pressure distribution in the cross section along both the axial direction and the lateral direction from the focal point (0 mm, 1.2 mm). From Fig. 6, the focal area at −6 dB was approximately 0.52 mm by 2.10 mm.
Fig. 6 Extracted from Fig. 5(b): pressure distribution along the both axial direction and lateral position from the focal point (0 mm, 1.2 mm). (a) Pressure distribution along the axial direction. (b) Pressure distribution along the lateral direction.
5. Ultrasound imaging
5.1 Experimental setup
To further characterize this fiber optic ultrasound generator, its ultrasound imaging capabilities must be demonstrated. In this section, we present the experimental activity, in which the fiber optic generator was used to image a tissue specimen. The photo of the experimental setup is shown in Fig. 7. This experiment was performed under the water media. The same laser was used as the optical radiation source. The specimen holder was attached with the 2-axis stepper motor stage to provide accurate scanning capability. The hydrophone was fixed and placed at the other side of the specimen holder. For every scanned pixel, the ultrasonic pulse hit the specimen, penetrated through the specimen and the data of that pixel was recorded by hydrophone.
5.2 Results and discussions
In this experiment, the fiber optic ultrasound generator and hydrophone operated in a transmission C-mode. The test specimen was constituted by a slice of pork soft tissue with the thickness of 1 mm. The ultrasound imaging was obtained by incrementally moving the specimen in between the fixed generator and hydrophone, while the ultrasonic wave propagation time between the generator and the hydrophone was measured at each point. Therefore, the speed of sound of each point in the tissue was determined and mapped into the contour figure pixel by pixel using the following equation:
where v is the speed of sound in sample, h = 1 mm is the thickness of sample, c = 1540 ms−1 is the speed of sound in water, t is the measured propagation times from generator to hydrophone, and tw is the propagation time of the standard propagation path with same length when there is only water between generator and hydrophone. The ultrasound image experiment results and the photo of specimen were illustrated in Fig. 8. The resolution of ultrasonic image was 200 µm. It was noted from [20], the ultrasound waves travel more quickly through muscle tissues than fat tissues, therefore the measured ultrasound propagation times through a specimen provides an indication of the tissue’s composition. Compared between Figs. 8(a) and 8(b), it can be clearly observed that the relative propagation time at each point (which is inversely proportional to the speed of sound) is strongly correlated to the muscular-to-fat ratio of tissue at that point. This observation was consisted with reference [20], which suggests the promise of C-mode ultrasound imaging for our system.Fig. 8 The ultrasound imaging of a slice of pork tissue: the ultrasound imaging is obtained by moving the specimen in between the fixed generator and hydrophone. (a) The ultrasound image of a slice of pork tissue. (b) Photo of the tissue specimen (slice of pork tissue).
For high-resolution biomedical ultrasound imaging, one difficulty in this research is the hard-to-controlled thickness of the gold nanocomposite film on the tip of the optical fiber. Thickness of the gold nanocomposite film affects the bandwidth of the generator. As the ultrasound propagate along the material, high frequency components attenuate faster than low frequency components. Therefore, the extra thickness of the gold nanocomposite may attenuate high frequency components of the generated ultrasound. In order to accomplish the high-frequency (> 30 MHz) and ultrahigh-frequency (> 100 MHz) biomedical ultrasound image, future studies in this research will focus on the fabrication of ultra-thin gold nanocomposite film layer by taking the advantage of nanofabrication method, e.g., Focused Ion Beam (FIB) milling [9]. In addition, a picosecond laser or a femtosecond laser system will be utilized to further improve the pulsed laser source and tailor the bandwidth of the generator.
6. Conclusions
In this paper, we have designed, fabricated, and characterized the first fiber optic ultrasound generator based on PA generation technique by using gold nanocomposite as the ultrasound generation material. An optical fiber with a core diameter of 400 μm was coated with the gold nanocomposite. The verification experiment was performed to validate the ultrasound generation capability. The experimental results showed that ultrasound signals with an amplitude of 0.64 MPa was generated by the fiber optic ultrasound generator and bandwidth was more than 20 MHz. The PA generation efficiency was approximately 5 orders of magnitude increased comparing to using aluminum thin film and 103 times increased comparing to using graphite mixed within epoxy.
The ultrasonic field distribution was scanned by a hydrophone attached with 2-axis stepper motor stage. The focal point was approximately 1.2 mm away from the generator with the pressure of 0.78 MPa. Moreover, the first ultrasound image of a tissue specimen was obtained with the resolution of 200 µm by our proposed generator. In summary, the fiber optic ultrasound generator could lead to the development of a new generation of ultrasonic probes featuring high PA efficiency, wide bandwidth, easy fabrication, and miniature size.
Acknowledgments
The authors would like to thank the National Science Foundation for sponsoring this work (CMMI: 1055358 CAREER).
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