Optical and ultrasonic dual-sensitive sensor and its application in photoacoustic microscopy

Multi-modality imaging is significant for biomedical applications. We propose a dual-sensitive sensor to simultaneously detect optical and ultrasonic signals. Based upon the classical piezoelectric structure, we attach a photosensitive layer made of carbon nanotubes-polydimethylsiloxane (CNTs-PDMS) composite to the surface. The photosensitive layer absorbs light and converts it into ultrasound, while allowing acoustic energy to transmit through concurrently. After optimizing the ratio of PDMS to CNTs, we increase the sensor’s light detection sensitivity and maintain the ultrasound detection sensitivity. Finally, the successful implementation in mouse ear optical attenuation–photoacoustic imaging demonstrates the dual-sensitive sensor’s potential application in multi-modality imaging.


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ulti-modality imaging technology is of great importance for biomedical applications because it takes the merits of individual modalities and compensates for their limitations.To name a few, photoacoustic microscopy (PAM) and optical coherence tomography (OCT) have been combined to study neovascularization, [1][2][3] ophthalmology 4) and conduct microsurgeries.5) Photoacoustic (PA) and ultrasound (US) have been combined to provide both structural and chemical information of tissues. Th technology has been successfully applied in studying periodontitis 6) and atherosclerosis. 7)Researchers have proposed an optical scattering-absorption dual-mode imaging system to study the structural and functional properties of tissues at the cell level.[8][9][10] Usually, multi-modality imaging relies on several kinds of detectors to pick up different signals.Photodetectors, including photodiodes, phototransistors, etc., are used to detect light.Ultrasonic transducers are used to detect ultrasound. Howevr, multiple detectors could bring some weaknesses to the multi-modality imaging.For example, multiple detectors could increase the complexity of system design, operation and maintenance, bring the problem of misalignment of multi-modality imaging results and so on.
The photoacoustic effect connects light and sound.][17][18][19][20] This kind of transducer can generate high-frequency ultrasound (>50 MHz) while achieving element size and spacing on the order of several microns.Therefore, they can be applied in areas like all-optical-ultrasound imaging 21) and high-resolution ultrasound imaging. 17,19)n this study, we propose an optical-ultrasonic dual-sensitive sensor, based on the photoacoustic effect.The photosensitive coating made of carbon nanotubes-polydimethylsiloxane (CNTs-PDMS) is the key.We conduct repeated experiments to optimize the coating formula, enhancing the sensor's sensitivity to optical signals and maintaining the sensitivity of ultrasound detection at the same time.Finally, we apply the dual-sensitive sensor to an optical attenuation-photoacoustic dual-mode microscopy, proving its application value.
Figure 1 shows the structural diagram of the proposed dualsensitive sensor.Piezoelectric element, for example, piezoelectric ceramics, can convert mechanical energy (acoustic energy) into electrical energy or conversely.Backing can control the vibration of the transducer by absorbing the energy radiating from the back face of the active element.The acoustic lens is used to focus the sound beam, thus increasing the lateral resolution.][24] These parts are similar to those in a conventional ultrasound transducer.
In addition to the above structures, a photosensitive layer is coated on the surface of the sensor.The photosensitive layer converts light into ultrasound and allows acoustic energy to transmit through.
When the acoustic wave arrives at the sensor, it incidents on the piezoelectric element after penetrating through the photosensitive layer and acoustic matching layer.The piezoelectric element converts acoustic signals into electrical signals due to the piezoelectric effect.Therefore, the sensor can detect ultrasound.When the light arrives at the sensor, it is absorbed and converted into ultrasound by the photosensitive layer due to the photoacoustic effect.The generated ultrasound wave propagates to the piezoelectric element and is converted into electrical signals.In this way, the sensor can detect light.This is the basic principle of the proposed dualsensitive sensor.
The photosensitive coating is the key of the dual-sensitive sensor.On the one hand, the photosensitive coating must have a high light-ultrasound conversion efficiency to ensure the high sensitivity of light detection.On the other hand, the coating must have a low ultrasonic insertion loss to maintain the sensitivity of ultrasound detection.
The light-ultrasound conversion includes two stages.In the first stage, light is absorbed to produce a temperature increase by the absorbers.The efficiency is related to the intensity of light absorption.In the next stage, the heat drives thermal expansion in elastomeric materials, thus producing mechanical vibration.The efficiency of the conversion of heat to pressure can be quantified by Grüneisen coefficient, a dimensionless constant. 25)In order to ensure high lightultrasound conversion efficiency, the material needs to have a high light absorption coefficient and a high Grüneisen coefficient.
The carbon nanotubes-polydimethylsiloxane (CNTs-PDMS) composite well satisfies the above requirements.In this composite material, carbon nanotubes (CNTs) work as light absorbers while the polydimethylsiloxane (PDMS) works as a thermoelastic source.CNTs have a light absorption coefficient of around 0.98-0.99.They have been proven the ability to efficiently transform light into heat. 26,27)Also, CNTs have extraordinary thermal conductivity originated from the nanoscale dimension.Its thermal conductivity is 20-30 times larger than that of typical metal.Thermal energy can efficiently transfer from CNTs to the surrounding PDMS.Also, the Grüneisen coefficient of elastomeric PDMS can reach ∼0.72.It means that PDMS can effectively convert the heat into mechanical vibration. 28)Therefore, the mixture of CNTs and PDMS material can provide strong ultrasound pulses induced by lasers. 16,18,29)urthermore, the acoustic impedance of CNTs-PDMS is between 1.02 and 1.13 MRayls depending on the formulations. 30)Its acoustic impedance is close to the acoustic impedance of water (1.5 MRayls).Consequently, the sound wave can propagate from water into CNTs-PDMS coating with low insertion loss.
PDMS principal agent (PDMS I; DOW SYLGARD 184) and 95% high purity CNTs (Tanfeng Tech TF-25001) are mixed evenly by organic solvents such as acetone.The volume ratio of PDMS I to acetone is 10:7.Acetone itself does not chemically react with PDMS I nor CNTs.Ultrasound is used to evenly disperse CNTs in PDMS for around 6 min.Then we add in the PDMS curing agent (PDMS II; DOW SYLGARD 184), the mass ratio of which to PDMS I is 1:10.The mixture is spun and coated on the molds after manually mixed for about 10 min.The CNTs-PDMS composite will form a gellike substance after about 24 h of standing at room temperature.The vacuum drying oven will effectively speed up this process.
We use pulsed-echo and pulsed-laser measurement to test the performance of dual-sensitive sensor in detecting ultrasound and light.Figure 2(a) is the schematic illustration of the ultrasound test system.The dual-sensitive sensor is driven by a pulser/receiver (SIUI CTS-8077PR) and produces pulsed ultrasound.The ultrasound is then reflected by a smooth glass pane and received by the dual-sensitive sensor.
Figure 2(c) shows the waveform and spectrum of the received ultrasound echo.As shown, the sensor has a central frequency of 9.45 MHz and a bandwidth of 7.25 MHz at −6 dB. Figure 2(b) is the schematic illustration of the optical test system.Laser (Spectra-Physics EXPL-532-2Y) with a wavelength of 532 nm and repetition rate of 1 kHz is directed to the dual-sensitive sensor through a beam expander system.Laser intensity is monitored by a photodiode power sensor (THORLABS S12C) and a power meter console (THORLABS PM100D).Signals from the dual-sensitive sensor are recorded by a digital oscilloscope.The laser power ranges from 0.08 to 1.3 mW.We average the peakto-peak values of detected signals as the strength parameter.Figure 2(d) gives the detected waveform and the signal strength as a function of laser power.We can see that the magnitude of the signals detected by the dual-sensitive sensor is approximately proportional to the laser intensity.These results demonstrate that the proposed dual-sensitive sensor has the ability to detect light and acoustic signals, simultaneously.
The ratio of PDMS to CNTs will not only affect the acoustic impedance of the composite coating, but also the sensitivity of photon detection.Therefore, we then optimize the ratio of the coating mixture to ensure low ultrasound insertion loss and high light detection sensitivity.
We prepare seven coatings with different ratios, as shown in Table I and conduct repeat experiments.Figures 2(e)-2(f) quantifies the amplitudes of the detected ultrasound and light signals.When the mass percentage concentration varies from 3.03% to 0%, both the amplitudes of detected ultrasound and light signals show the trend of increasing first and then decreasing.When the mass percentage concentration ranges from 2.27% to 1.52%, the amplitudes of detected ultrasound signals and light signals both reach near maximum and stabilize.To be more precise, the dual-sensitive sensor with the composite coating of ratio 4 in Table I enables the ultrasound and light signal to have the highest amplitude, respectively.Therefore, in the following experiments, we all use the CNTs-PDMS coating with the mass percentage concentration of 1.52%.
Finally, we apply the dual-sensitive sensor to a dual-mode microscopy, which can achieve optical attenuation-photoacoustic imaging simultaneously.We have reported the detailed description of the dual-mode imaging system previously. 8)hen the laser irradiates the sample, a part of the laser energy is absorbed and converted into ultrasound.Then the remaining photons pass through the sample and reach the dual-sensitive sensor.The two signals can be distinguished according to the different delays of light and sound.Extracting the peak-to-peak value of the two signals as the imaging contrast, we obtain photoacoustic image and optical attenuation image respectively.We apply this system to image the mouse ear in vivo.A male Balb/c mouse aged 8 weeks is anesthetized with isoflurane (3% gas concentration) and its body temperature is kept around 36 °C using a thermostatic blanket during the experiment.The scanning range is 1.5 mm × 1.5 mm with a step of 5 μm.The animal studies are performed in accordance with the institutional guidelines and approved by Nanjing University.images from the same electrical signal, no complex operation is needed to align images exactly points with points.We directly overlay the dual-modality images [Figs.3(e) and 3(f)] to obtain complementary information, as shown in Fig. 3(d).In this way, we can recognize the vascular system from the surrounding tissues and organs with non or weak light absorption (e.g.sebaceous glands) while preserving their relative position information.
For comparison, Figs.3(b) and 3(c) display the images obtained by an US transducer without the photosensitive layer.We could see that the proposed dual-sensitive sensor    In summary, we propose an optical and ultrasonic dualsensitive sensor, which can detect light signals and ultrasound signals at the same time.We attach a photosensitive layer made of CNTs-PDMS to the surface of a classical piezoelectric structure.The photosensitive layer can convert pulsed light into pulsed ultrasound and allow ultrasound to penetrate.We optimize the formula of the photosensitive layer, making it have high ultrasound transmittance and high light-ultrasound conversion.Therefore, the dual-sensitive sensor has high sensitivity of light and ultrasound detection, simultaneously.Finally, we apply the dual-sensitive sensor to  image the mouse ear in vivo.Through a single excitation source and a single detector only, we successfully obtain the optical attenuation and photoacoustic images of organism tissue.It is worth noting that the sensor structure scheme can also be potentially applied in fulfilling optical-ultrasound, ultrasound-photoacoustic and other dual-modality imaging with proper designs.This work provides a promising tool for fulfilling simplification, practicality and portability of multimodality imaging.

Figure 3
Figure 3 shows the imaging results.Figure 3(a) is a picture of the imaging region of the mouse ear.Figure 3(f) is the image reconstructed from the ultrasound signals.This photoacoustic image clearly shows the microvasculature in the mouse ear.Figure 3(e) is the image reconstructed from light signals.It illustrates the optical attenuation image, where the dark areas represent light blockers in the mouse ear, including microvasculature and sebaceous glands.This image reveals the complete morphological structure of the mouse ear in the imaging profile.Because the dual-modality sensor reconstructs optical attenuation and photoacoustic

Figure 3 (
Figure 3 shows the imaging results.Figure 3(a) is a picture of the imaging region of the mouse ear.Figure 3(f) is the image reconstructed from the ultrasound signals.This photoacoustic image clearly shows the microvasculature in the mouse ear.Figure 3(e) is the image reconstructed from light signals.It illustrates the optical attenuation image, where the dark areas represent light blockers in the mouse ear, including microvasculature and sebaceous glands.This image reveals the complete morphological structure of the mouse ear in the imaging profile.Because the dual-modality sensor reconstructs optical attenuation and photoacoustic

Figure 3 (
Figure 3 shows the imaging results.Figure 3(a) is a picture of the imaging region of the mouse ear.Figure 3(f) is the image reconstructed from the ultrasound signals.This photoacoustic image clearly shows the microvasculature in the mouse ear.Figure 3(e) is the image reconstructed from light signals.It illustrates the optical attenuation image, where the dark areas represent light blockers in the mouse ear, including microvasculature and sebaceous glands.This image reveals the complete morphological structure of the mouse ear in the imaging profile.Because the dual-modality sensor reconstructs optical attenuation and photoacoustic Figure 3 shows the imaging results.Figure 3(a) is a picture of the imaging region of the mouse ear.Figure 3(f) is the image reconstructed from the ultrasound signals.This photoacoustic image clearly shows the microvasculature in the mouse ear.Figure 3(e) is the image reconstructed from light signals.It illustrates the optical attenuation image, where the dark areas represent light blockers in the mouse ear, including microvasculature and sebaceous glands.This image reveals the complete morphological structure of the mouse ear in the imaging profile.Because the dual-modality sensor reconstructs optical attenuation and photoacoustic

Fig. 2 . 3 ©
Fig. 2. Performance test of the dual-sensitive sensor.(a) Schematic diagram of pulsed-echo setup for the ultrasound detection testing.(b) Schematic map of experiment setup for the laser detection testing.(c) Waveform (black curve) and spectrum (red curve) of the ultrasound echo detected by the dual-sensitive sensor.(d) Waveform (black curve) of the laser pulse response detected by the dual-sensitive sensor.Blue dots give the magnitude of sensor output as a function of the laser intensity and the red curve is the result of linear fitting.(e) Ultrasound signal amplitude versus mass percentage concentration.(f) Light signal amplitude versus mass percentage concentration.

[Fig. 3 (
e)] remarkably improved the intensity and contrast of the attenuation image.At the same time, the intensity and contrast of optical absorption images are almost the same (Figs.3(c) and 3(f)].Histogram is utilized to conduct quantitative analyses of the imaging results statistically.The horizontal axis represents the pixel intensities.The vertical axis denotes the frequency of each intensity.

Figure 4 (
a) compares the optical attenuation images.The mean gray value of the image acquired by dual-sensitive sensor is 0.4178 and the standard deviation (SD) is 0.1528.Compared with that of US transducer without photosensitive layer, the mean gray value increases 13 times, which represents the sensitive optical detection.The enlarged SD means the enhanced image contrast.Figure 4(b) compares the photoacoustic images.Comparing the images acquired by dual-sensitive sensor and US transducer without photosensitive layer, both the mean gray value (0.1675 and 0.1949) and the SD (0.1262 and 0.1660) are similar.This shows the proposed sensor's sensitivity of ultrasound detection.The successful application in biomedical imaging and the above quantitative analysis demonstrate that the dual-sensitive sensor can detect light signals sensitively and maintain the ultrasound detection sensitivity at the same time.

Fig. 3 .
Fig. 3.The optical attenuation and photoacoustic images of mouse ear achieved by the dual-sensitive sensor.(a) Image taken through an optical microscope.(b) Optical attenuation image from US transducer without photosensitive layer.(c) Photoacoustic image from US transducer without photosensitive layer.(d) Overlay of (e) and (f).(e) Optical attenuation image from dual-sensitive sensor.(f) Photoacoustic image from dual-sensitive sensor.

Fig. 4 . 4 ©
Fig. 4. Quantitative comparison of the images acquired by dual-sensitive senor and US transducer without photosensitive layer in Fig. 3 by using histogram (a) Optical attenuation images (b) Photoacoustic images.017003-4

Table I .
Different ratios of PDMS to CNTs in CNTs-PDM coatings.The mass of PDMS is the total mass of PDMS I and PDMS II.