photoacoustic imaging using array transducer

Photoacoustic Imaging Using Array Transducer

Suhyun Park, Srivalleesha Mallidi, Andrei B. Karpiouk, Salavat Aglyamov, and Stanislav Y. Emelianov

Department of Biomedical Engineering

University of Texas at Austin, Austin, TX USA 78712

ABSTRACT

To perform ultrasound imaging using an array transducer, a focused ultrasound beam is transmitted in a particular direction within the tissue and the received backscattered ultrasound wave is then dynamically focused at every position along the beam. The ultrasound beam is scanned over the desired region to form an image. The photoacoustic imaging, however, is distinct from conventional ultrasound imaging. In photoacoustic imaging the acoustic transients are generated simultaneously in the entire volume of the irradiated tissue – no transmit focusing is possible due to light scattering in the tissue. The photoacoustic waves are then recorded on every element of the ultrasound transducer array at once and processed to form an image. Therefore, compared to ultrasound imaging, photoacoustic imaging can utilize dynamic receive focusing only. In this paper, we describe the image formation algorithms of the array-based photoacoustic and ultrasound imaging system and present methods to improve the quality of photoacoustic images.

To evaluate the performance of photoacoustic imaging using an array transducer, numerical simulations and phantom experiments were performed. First, to evaluate spatial resolution, a point source was imaged using a combined ultrasound and photoacoustic imaging system. Next, image quality was assessed by imaging tissue imaging phantoms containing a circular inclusion. Finally, the photoacoustic and ultrasound images from the combined imaging system were analyzed.

Keywords: Photoacoustic imaging, ultrasound imaging, array transducer, beamforming, image formation, image reconstruction, delay and sum

1. INTRODUCTION

Ultrasound imaging can visualize the morphology of the tissue but histopathological information is limited. Photoacoustic imaging can be used to identify different functional activity of tissues that may be indistinguishable in ultrasound imaging. Since these imaging methods are complementary, combined photoacoustic and ultrasound imaging can take full advantage of the many synergistic features of these systems1. Integrated together, the combined imaging system can be used in many fundamental and applied, biomedical and clinical applications.

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Fig. 1. Combined ultrasound and photoacoustic imaging system.

Photons Plus Ultrasound: Imaging and Sensing 2007: The Eighth Conference on Biomedical Thermoacoustics,Optoacoustics, and Acousto-optics, edited by Alexander A. Oraevsky, Lihong V. Wang,Proc. of SPIE Vol. 6437, 643714, (2007) · 1605-7422/07/$18 · doi: 10.1117/12.704240

Proc. of SPIE Vol. 6437 643714-1

Combined ultrasound and photoacoustic imaging can be implemented based on a standard ultrasound imaging system interfaced with the pulsed laser source (Fig. 1). Indeed, in the integrated imaging system, several components including acoustic transducer, hardware for signal acquisition, and even some hardware-implemented signal and image processing algorithms can be shared. Generally, image reconstruction algorithm significantly influences the image quality. Furthermore, processing time depends on the particular implementation of the image reconstruction. In this paper, image reconstruction approaches for combined ultrasound and photoacoustic imaging using linear array transducer are analyzed.

1.1 Ultrasound Image Formation

In conventional ultrasound imaging using linear array transducer, as illustrated in Fig. 2(a), each transmitted ultrasound beam is focused at a specified depth and the received beam is dynamically focused at all ranges to form high quality ultrasound image. To interrogate the entire image, many ultrasound beams are required thus reducing the frame rate. However, if frame rate is critical, other ultrasound imaging techniques such as ultrafast ultrasound imaging2,3, parallel receive beamforming imaging4 or explososcan5 are available. Here the transmitted ultrasound beam is broader (slightly focused or even unfocused) and it interrogates the larger volume of tissue. The backscattered ultrasound signal, received on the desired elements of the array transducer at once, is then used to form the ultrasound image. For example, as shown in Fig. 2(b), in ultrafast imaging plane wave is transmitted using the whole aperture of the array transducer and the signals are received using every element of the array transducer aperture at once6.

Most ultrasound imaging systems reconstruct the image using delay and sum beamforming7. The delay and sum beamforming accurately accounts for the delay distances between the transducer elements and the reconstructed point in the image. To improve the image quality, the directivity angle with dynamic receive focusing (fixed F-number), and apodization are considered8,9. Often, delay-and-sum beamforming is hardware implemented in ultrasound system so that the signals can be processed and images can be displayed in real-time.

1.2 Photoacoustic Image Formation

In photoacoustic imaging, the tissue is irradiated by a short laser pulse and the optical absorbers inside the tissue act as acoustic sources. Rapid thermoelastic expansion of optical absorbers generates photoacoustic pressure waves detected using the probe consisting an array of ultrasound transducers. Contrary to conventional ultrasound imaging, however, in photoacoustic imaging, as the laser beam irradiates the tissue, the light quickly spreads throughout the tissue due to the optical scattering. Therefore, laser beam instantly interrogates the entire volume of tissue and the photoacoustic response is simultaneously produced everywhere in tissue as depicted in Fig. 2(c). Consequently, all transducer elements are used to receive these photoacoustic transients at once. Thus, in photoacoustic imaging there is no transmit focusing and only dynamic receive focusing can be used1. Clearly, photoacoustic imaging is similar to ultrafast ultrasound imaging and, therefore, image formation technique from ultrafast ultrasound imaging can be adopted. Generally to form the photoacoustic image, simple backprojection10, Fourier transform reconstruction11, and delay-and-sum12 image reconstruction techniques can be applied.

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Fig. 2. Schematic view of (a) conventional ultrasound, (b) ultrafast ultrasound, and (c) photoacoustic imaging modes.


2. MATERIALS AND METHOD

2.1 Phantoms

Both numerical modeling and experimental studies were performed using point source phantom and phantom models closely imitating the acoustic and optical properties of tissue.

In point source phantom used in numerical analysis, 200 µm diameter optical and ultrasound targets were placed at 10, 20, 30, and 40 mm depth. In experimental study, the point source phantom was assembled using 200 µm diameter glass rod positioned at 30 mm depth in axial direction and orthogonal to the imaging plane. This phantom was imaged using both photoacoustic and ultrasound imaging modes. The geometry of the phantom and location of the point sources relative to the transducer are further outlined in Fig. 3(a).

Tissue phantom was modeled using a homogeneous material having a 10 mm diameter circular inclusion with elevated optical absorption. Small size spherical absorbers were positioned randomly inside of the phantom. The circular inclusion had more optical absorbers than the surrounding material to mimic the characteristic of the tumor with higher level of blood. For experimental studies, similar phantom with a cylindrical inclusion was constructed for photoacoustic imaging. The phantom contained 6% of porcine gelatin mixed with 1% of amberlite in the background, and the circular inclusion was made out of 12% of porcine gelatin, 1% of amberlite and 0.01% of graphite. Here, amberlite particles were acting as the ultrasound scatterers and graphite flakes were primarily optical absorbers needed for photoacoustic imaging. The circular inclusion was positioned in the center of the phantom such that it was 25 mm away from either the top (transducer) or the bottom (laser pulse delivery) surface of the phantom as depicted in Fig. 4(a).

2.2 Numerical Modeling and Experimental Setup

In ultrasound imaging system, the phantom was visualized using 128 element linear array of transducers operating at 5 MHz center frequency and 60% fractional bandwidth. Each element of the array was about 300 µm wide with total aperture of the array, therefore, spanning 40 mm. The same imaging system was modeled in numerical modeling. To image the phantom, 128 ultrasonic beams covering 40 mm lateral range were acquired. The signals from of the transducer elements were acquired and the ultrasound beams were reconstructed using delay-and-sum beamforming. Transmit focus was set at infinity to utilize ultrafast ultrasound imaging and dynamic focusing (F-number = 1) was used in the receive beamforming.

In photoacoustic imaging, a forward detection method was used, i.e., the laser pulse was delivered from the bottom of the phantom. In the forward detection mode, the photoacoustic signal propagates through the absorbing medium and is detected at the rear surface of the irradiated medium. The Nd:YAG pulsed laser (532 nm wavelength, 5 ns pulse duration, and 20 Hz repetition rate) was used to generate thermoelastic expansion. The generation of photoacoustic pressure in an absorbing medium is governed by the thermal equation, the acoustic wave equation, and the thermoelastic expansion equation13. For a spherical absorber, the solution in a form of one-dimensional Green’s function was applied13. The laser induced photoacoustic profile is an N-shaped wave with temporal and spatial characteristics determined by the radius of the absorbing sphere. Using the analytic form of the photoacoustic pressure profile, Beer’s law was applied to simulate the optical attenuation (i.e., scattering and absorption) during the propagation of the light through the medium. The photoacoustic response was then received by all elements of the linear transducer array, and reconstructed by delay-and-sum beamforming used in ultrafast ultrasound imaging.

3. RESULTS AND DISCUSSION

3.1 Photoacoustic Imaging

The photoacoustic image of the phantom with four point sources is shown in Fig. 3(b). This image was reconstructed using the numerically modeled photoacoustic transients detected by the array transducer. All point targets can be easily identified in this image. The close-up view of the target located at 30 mm depth is presented in Fig. 3(c) – this image is compared with experimental photoacoustic image of the point target shown in Fig. 3(d). Both simulated and measured images were reconstructed using the delay-and-sum beamforming approach. The size of the circular target in both


images is slightly larger laterally due to diffraction limited lateral resolution. The artifacts of image reconstruction are also similar in both images. Nevertheless, the delay-and-sum reconstruction produces reasonable images of the targets.

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Fig. 3. (a) Point source phantom and imaging setup, (b) simulated photoacoustic image (inside dotted box - 20 mm laterally by 50 mm axially) of four point sources, (c) the close-up (20 mm by 20 mm) view of the point source located at 30 mm depth, (d) experimental photoacoustic image (20 mm by 20 mm) of point target (glass rod) positioned at 30 mm depth. All images are shown using 40 dB dynamic range.

The simulated and measured photoacoustic images of the phantom with inclusion are shown in Figs. 4(b) and 4(c), respectively. Clearly, the delay-and-sum beamforming, previously utilized in ultrafast ultrasound imaging, properly reconstructs the photoacoustic images of the phantom. Thus, with minimal modifications, the hardware of ultrasound imaging system can be reliably used in photoacoustic imaging.

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Fig. 4. (a) Geometry of the phantom with inclusion and schematic view of the imaging setup, (b) simulated photoacoustic image (inside dotted box - 25 mm by 25 mm area centered around the inclusion) of the phantom, (c) experimental image (25 mm by 25 mm) of inclusion in the homogeneous phantom. All images are displayed using 25 dB dynamic range.

3.2 Combined Photoacoustic and Ultrasound Imaging

As demonstrated in Figs. 3 and 4, to integrate photoacoustic imaging with the standard ultrasound imaging system, the linear array transducer, data acquisition system, and the beamforming hardware can be shared. However, specific signal processing and image formation algorithms do not have to be the same based on fundamental differences in


photoacoustic and ultrasound signals. An example of that is presented in photoacoustic and ultrasound imaging studies using images of a 200 µm diameter glass rod submerged in water. This particular object has both optical absorption and acoustic impedance different from the surrounding material.

Using combined imaging system, both photoacoustic and ultrafast ultrasound images from the glass rod positioned at 30 mm depth were obtained as shown in Fig. 5(a) and Fig. 5(b), respectively. These images are shown using 20 dB display dynamic range and, therefore, the image reconstruction artifacts around the target (see Fig. 3 (d)) are diminished.

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Fig. 5. Experimental (a) photoacoustic and (b) ultrafast ultrasound image of the point source located 30 mm away from the array transducer. Both images, displayed using 20 dB dynamic range, cover 20 mm by 20 mm area centered around the target.

The acquired radiofrequency (RF) photoacoustic and ultrasound signals from the central (64th) transducer array element before the image reconstruction are plotted in Fig. 6. Generally, photoacoustic signal from a spherical absorber is known to be an N-shaped, yet the detected signal is not exactly N-shaped due to the bandpass frequency characteristic of the ultrasound detector used in our study. The duration of the photoacoustic signal is affected by the transducer characteristic and also the size of the absorber, i.e., the size of the absorber determines the extent of the signal in axial direction. In ultrasound imaging, the signal length is not affected by the size of the acoustic scatterer or reflector – the received signal is primarily determined by the frequency characteristics of the ultrasound transducer. As expected, the ultrasound echo from the target is 2-3 cycle sinusoidal signal. This signal closely corresponds to the fractional bandwidth of the transducer.

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Fig. 6. Radiofrequency (a) photoacoustic and (b) ultrasound signals of the target detected on 64th element of array transducer. The target was located at 30 mm depth.


Lastly, the photoacoustic and ultrasound frequency responses were analyzed. The power spectra of the photoacoustic and ultrasound signals are presented in Fig. 7. The center frequency of photoacoustic transient is approximately 3 MHz, and the bandwidth is about 80%. Frequency spectrum of the ultrasound signal is centered around 5 MHz and has 60% bandwidth.

Clearly, the frequency content of the photoacoustic signal depends on the size of the absorber. For transducer operating at 5 MHz center frequency, absorbers on the order of 100-300 µm diameters cause the center frequency of the photoacoustic response to shift to lower frequencies. Smaller absorbers generate higher frequency photoacoustic transients, and the larger absorbers generate detectable signals from the boundaries of the absorbers only. Nevertheless, lower center frequency and broad bandwidth of the photoacoustic signal can be employed to further improve the quality of the photoacoustic image. Indeed, the F-number during dynamic focusing and, therefore, image reconstruction was chosen based on the directivity angle of the ultrasound transducer. However, the same transducer exhibits broader directivity angle for lower frequencies. In turn, broader directivity angle allows to utilize more elements of the transducer thus increasing the receive aperture of the array transducer and improving the lateral resolution.

4. CONCLUSIONS

The results of our numerical and experimental studies demonstrated that photoacoustic imaging system can be implemented using components of the ultrasound imaging system. Ultrasound transducer, data acquisition hardware, and even image reconstruction algorithms, such as delay and sum beamforming, can be shared between ultrasound and photoacoustic imaging. Also, given the differences in ultrasound and photoacoustic signals, the frequency characteristic of the photoacoustic signal can be used to improve the spatial resolution of the photoacoustic imaging.

ACKNOWLEDGMENTS Partial support by National Institutes of Health under grants CA 110079 and EB 004963, and Army Medical Research and Material Command under grant DAMD17-02-1-0097 is gratefully acknowledged.

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Fig. 7. Frequency responses for the RF signal from (a) photoacoustic signal, and (b) ultrasound signal.


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photoacoustic imaging using array transducer

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