1. Introduction

Photoacoustic tomography (PAT) imaging is a noninvasive imaging technique for visualizing both structural and functional information about biological tissues. This method has become an active research area in recent years. Compared with the traditional B-Mode ultrasound imaging system, PAT combines a high ultrasonic penetration depth and high optical contrast in tissue imaging [1–4]. In PAT, when electromagnetic energy (such as optical or radio-frequency waves) is absorbed by a sample of biological tissue, a small temperature rise in the tissue generates a thermoelastic expansion, which leads to a photoacoustic (PA) signal. Because the PA pressure is linearly proportional to the optical energy deposited in the biological tissue, the PA signals carry information about the geometrical structure and optical properties of the tissue. Furthermore, since the tissue attenuates and scatters ultrasound much less than light, we can use a wide-band ultrasonic transducer to detect the PA signals and reconstruct the image of the light-absorption distribution in the biological tissue. Hence, the PAT image can display both the structure and function of the tissue. This technique has been applied to imaging skin cancer, breast cancer, brain tumors, blood concentrations and vascular structure [5–8].

Many experiments on PAT images have been carried out in recent years [9–14]. However, most of these PAT imaging systems were based on time-domain PAT, which needs an algorithm to reconstruct the PA images, making it hard to get real-time images. To obtain these, an acoustic lens, which is able to image the initial PA pressure distribution onto an image space in real time without the need for computational reconstruction, has been developed and applied to PAT imaging [15–17]. Furthermore, to display a three-dimensional (3D) real object, a 4f acoustic lens, which guarantees axial and lateral unit magnification of the images, has also been discussed [18].

In our previous study [17–18], a Boxcar was employed to acquire phtoacoustic signals for image reconstruction. The imaging speed was slow. In order to increase the imaging speed, we try to combine an acoustic lens with the parallel imaging characteristic and a linear array detector with the parallel-acquiring ability. However, a linear array detector with 64 detecting elements would need 64 independent acquisition channels to acquire the parallel signals at the same time, which could not be achieved by using a boxcar. One possible solution is using 64 peak-hold modules to acquire the signals from the 64 detectors at the same time, thus making the imaging process more rapid. In this paper, we displayed how one of the peak-hold modules worked successfully in image reconstruction. The filtered results show that the images reconstructed in this experiment agree well with the original samples and are clearer than those from our former system. This work provided the feasibility for fast real-time photoacoustic imaging.

2. Imaging system setup and peak-hold technology

The experimental setup is shown schematically in Fig. 1. A Q-switched Nd:YAG (yttrium aluminum garnet) laser operates at 1064 nm with a pulse duration of 7 ns. The laser beam is expanded to illuminate the object to be imaged. Then the optical absorption of the object generates a proportional distribution of the PA signals. This acoustic pressure distribution is imaged onto the image plane by a 4f acoustic lens in10% milk liquor. A linear array transducer then detects the acoustic pressure distribution on the image plane. This transducer consists of 64 piezoelectric probes with a center frequency of 1 MHz. The radius of each probe is about 0.22 mm, and the central distance between two neighboring probes is 1.5 mm. The linear array transducer is fixed on a computer-controlled scanning stage. The scanning stage drives the linear transducer vertically to detect the planar PA signals. These signals are chosen and amplified in turn (triggered by the Q-switched laser) by the electronic switching of the 64 lines and are input to the peak-hold module, which can hold the peak values of the PA signals. Then the peak-value signal is delivered to a computer via an acquisition card (ADC, model Advantech PCL-818HG) and transformed into 256 corresponding gray levels to display the image. An oscillograph (model: TDS1002) monitors the PA signals and the output peak values from the peak-hold module.

In our former system [17–18], we used a Boxcar for keeping the peak values. Owing to the mechanical scanning of the detector, the distance between the object plane and the detecting plane would not be always the same during the scanning, which would bring the delay-time excursion of the PA signals. As a result, the boxcar could not always catch the peak of photoacoustic signals from one object plane, because the delay time and the width of sampling gate of the boxcar were fixed. Thus the boxcar could acquire the phtoacoustic signals from the near object planes, which could blur the image. In the new system, the peak-hold module had ability to catch the peak value of the PA signals automatically within the gate width when there was delay-time excursion of the PA signals. So it could improve the photoacoustic image obviously.

Because of the long focal depth of the acoustic lens, the PA signals from different object planes in the range of the focal length could precisely image on the same imaging plane. According to the Fourier imaging theory, the PA signals from an object plane needed the same delay time to reach its corresponding imaging plane, while the PA signals from different object planes needed different delay time to reach the same imaging plane. Because the acoustic velocity was much slower than the optical one, we could distinguish the photoacoustic signals from different object planes by the time-resolved technique. Firstly, a peak-hold module that had a fixed delay time was used to acquire the PA signals from one object plane, and then by scanning the linear array photoacoustic detector, we could obtain the photoacoustic image of the object plane. Secondly, by changing the delay time of the peak-hold module and repeating the previous process, we could obtain the photoacoustic image of another object plane. Finally, we can get the PAT images of different object planes on the image planes without moving the transducer.

 

Fig. 1. Experimental setup for PAT imaging based on a 4f acoustic lens and peak-hold technology.

Download Full Size | PDF

3. Experimental results and discussion

3.1. Output of the peak-hold module

The peak-hold module can detect the peak values of the PA signals automatically within the gate width, and its output is a direct-current voltage that is equal to the maximum value of a PA signal. Figure 2 shows a PA signal and its peak-value signal held by the peak-hold module, both of which were observed by an oscilloscope.

 

Fig. 2. A PA signal (a) and its peak-value signal (b) observed by an oscilloscope.

Download Full Size | PDF

We can conclude from Fig. 2 that the peak-hold module can acquire the peak value of the PA signal efficiently. Thus this peak value can be used to reconstruct a 2D PA image and the corresponding image of the sample can be acquired by scanning on the image plane.

3.2. 2D PA imaging

To demonstrate the feasibility of the PAT imaging system, we carried out a series of experiments. First, we tried to use the system for 2D PA imaging. Figure 3(a) shows a sample consisting of two black adhesive tape points stuck to a piece of polymethylmethacrylate and submerged in 10% milk liquor. The sample was heated by the YAG pulsed laser, and PA signals from the sample were imaged on the imaging plane by the 4f acoustic lens. The PA signals were detected by the linear array transducer to reconstruct the corresponding 2D PA image on the imaging plane (see Fig. 3(b)).

 

Fig. 3. Sample consisting of two black adhesive tape points stuck to a piece of polymethylmethacrylate submerged in milk (a) and the corresponding PA image using the new system (b).

Download Full Size | PDF

 

Fig. 4. Sample consisting of two black adhesive tape points stuck to a piece of polymethylmethacrylate submerged in milk (a) and the corresponding PA image using our former system (b).

Download Full Size | PDF

It is obvious that the acoustic image is in perfect agreement with the sample, and that this system possesses 2D imaging ability. Comparing this image with that shown in Fig. 4 from our former system [17–18], it is clear that the image has a sharper edge than before.

3.3. PAT imaging

PAT imaging is realized by combining the long focal depth of the acoustic lens and the time-resolved technique. We previously obtained some experimental results using a Boxcar [17–18]. More careful experiments have now been carried out to test the new system.

Figure 5 shows an example of two different patterns, consisting of three black adhesive tape points stuck to the front and two black adhesive tape points stuck to the back of a piece of polymethylmethacrylate. The polymethylmethacrylate is about 15 mm thick, so the distance between the two patterns is also about 15 mm. The PA signals of the different layers and their peak-value signals, as observed by oscilloscope, are shown in Fig. 6.

 

Fig. 5. Front (a) and side (b) elevations of a sample which consists of two different patterns: three black adhesive tape points stuck to the front and two black adhesive tape points stuck to the back of a piece of polymethylmethacrylate.

Download Full Size | PDF

 

Fig. 6 (a). PA signal and its peak-value signal of the front layer. (b) PA signal and its peak-value signal of the back layer.

Download Full Size | PDF

The results shown in Fig. 6 indicate that the peak-hold module is able to catch and hold the peak values of the different layers by adjusting its delay time. According to the axial unit magnification of the 4f acoustic lens, the distance between the two image planes must be equal to the distance between the two object planes, namely 15 mm. This distance can be expressed as D=ν·Δt, where ν≈2.640mm/µs is the acoustic velocity, and Δt≈5.7µs is the time difference of the two PA signals reaching the detector. So D=ν·Δt≈2.640×5.7≈15.048mm, it is just equal to the distance between the two object planes.

Figure 7 shows the computer-reconstructed image of the two different layers.

 

Fig. 7. PAT images of the two sides of the sample: (a) the front, consisting of three points, and (b) the back, consisting of two points.

Download Full Size | PDF

4. Conclusion

We have presented a novel photoacoustic tomography (PAT) imaging system based on a 4f acoustic lens and peak-hold technology. The experimental results indicate that the system can obtain the PAT images of samples inside strongly scattering media, and the reconstructed images agree well with the samples. In this PAT system, the focusing ability of the 4f acoustic lens greatly enhances the signal–noise ratio, and improves the imaging contrast. As the peak-hold technology can catch and hold the peak values of the PA signals automatically, this new PAT system can has better imaging quality. It still has the potential advantages of forming real-time images and acquiring them more rapidly without any complex algorithms. This method may provide a more convenient method for future in vivo noninvasive imaging of tissues and clinical diagnosis.