Yixiong Su

A photoacoustic tomography system for imaging of biological tissues

Non-invasive laser-induced photoacoustic tomography (PAT) is a promising imaging modality in the biomedical optical imaging field. This technology, based on the intrinsic optical properties of tissue and ultrasonic detection, overcomes the resolution disadvantage of pure-optical imaging caused by strong light scattering and the contrast and speckle disadvantages of pure ultrasonic imaging. Here, we report a PAT experimental system constructed in our laboratory. In our system, a Q-switched Nd : YAG pulse laser operated at 532 nm with a 8 ns pulse width is used to generate a photoacoustic signal. By using this system, the two-dimensional distribution of optical absorption in the tissue-mimicking phantom is reconstructed and has an excellent agreement with the original ones. The spatial resolution of the imaging system approaches 100 µm through about 4 cm of highly scattering medium.

Signal processing using wavelet transform in photoacoustic tomography

In order to improve the imaging contrast and resolution in photoacoustic tomography(PAT), the deconvolution between the transducer impulse response and the recorded photoacoustic(PA) signal of the tissue phantom is often used. The suppression of noise is critical in the deconvolution. Compared with the traditional band-pass filter in Fourier domain, wiener filter is more appropriate for the wide band PA signal. The scaling parameter in wiener filter is hard to determine using the traditional Fourier domain method. To solve the problem, the deconvolution algorithm with wiener filter based on the wavelet transform is presented. The scaling parameter is estimated using discrete wavelet transform(DWT) by its multi-resolution analysis(MRA) ability. The white noise had been effectively suppressed. Both numerical simulation and experimental results demonstrated that the contrast and resolution of PA images had been improved.

The regularization processing of signal deconvolution in photoacoustic signal recovery

In photoacoustic (PA) tomography, a piezoelectrical signal of inner characteristic of interesting object is mainly acquired by a hydrophone. Every piezoelectrical signal as output signal is the convolution of the original input signal that denotes the ultrasonic signal emitting from the substance and the system transfer function. The undistorted input signal is the very physical quantity that we want actually. Therefore an original input signal is computed with the deconvolution of the system transfer function and the output signal. While most practical deconvolution problems are called as blind deconvolution because the system transfer function and the input signal are both unknown and estimated from the output signal in the same time. In common, the deconvolution problem has an important property that it is called ill-condition, which is a special and intractable difficulty that both the theoretic analysis and the numerical computation would meet. For the sake of getting the solution of the deconvolution problem reasonable in physics and responsible for the gained data continually, a package of theory method called regularization to cure the ill-conditioned problems is applied in the PA signal processing.

Simulation on sensitive detection of small absorber in photoacoustic tomography

Noninvasive photoacoustic tomography (PAT) is a novel technique with great potential in biomedical image applications for it combines the merits and most compelling features of light and sound, and has the advantages of providing high contrast and high resolution images in moderate depth below the surface. When the image depth is on the scale of centimeter, the millimeter-scale resolution images still can be obtained. Thus it is a powerful tool for the early-stage breast cancer sensing. In this paper, photoacoustic tomography is studied by using the simulation method. The results show that: (1) the contrast of image increases linearly with respect to the number of measurement position (NMP); (2) the contrast increases exponentially with respect to noise-to-signal ratio.

Simulation study on sensitive detection of small absorbers in photoacoustic tomography

Photoacoustic tomography (PAT), which reconstructs the distribution of light-energy deposition in the tissue, is becoming an increasingly powerful imaging tool. For example, the technique has potential applications in the earlystage breast cancer sensing and the functional imaging of small animal brain. In PAT, the system signal-to-noise ratio (SNR) and the number of measurement positions (NMP) are the two main factors which affect the quality of final reconstructed image. Undoubtedly, the increase of SNR or the numbers of measurement positions will improves image quality. However, one has to pay a cost on the imaging speed for such improvement of image quality. In this paper, the factors influencing the imaging performance of PAT are investigated by means of computer simulations. The result shows that the increase of the number of averaging times in acquiring of acoustic signal and the number of measurement positions are efficient ways to improve image quality. However, there exists a turning point at which the further increase of NMP and averaging times makes the improvement of imaging performance negligible. Thus a tradeoff should be made to achieve the optimal reconstructed image according to the system SNR.

Photoacoustic imaging of blood vessel networks of biotissue

Photoacoustic tomography (PAT) is a powerful medical imaging technique for medicinal diagnosis in that it combines the merits and most compelling features of light and sound to the biological tissue. It can be potentially used for the detection of the first-stage breast cancers and the blood vessel net-works in the deep depth of tissue. In this paper, a PAT experimental system constructed in our laboratory is presented by the use of 532nm wavelength light as an excitation source. By using this system, we demonstrated that it is feasible to image blood vessel networks in highly scattering ex vivo and in vivo tissue samples.

The influence of tissue optical properties on imaging performance of optoacoustic tomography

Recent progress in optoacoustic tomography has shown its enormous potential in biomedical imaging of biological samples in vivo. The greatest advantage in this imaging modality is that it can offer the imaging contrast comparable to the optical techniques and the imaging resolution similar to the ultrasonic techniques. However, its sectioning capability is largely dependent on the scattering properties of the targeted subject. Therefore, the understanding of how the tissue optical properties affect the imaging performance of optoacoustic tomography would be important. In this paper, we systematically investigate the influence of absorption and scattering coefficients of tissue on the optoacoustic imaging resolution, depth and contrast. In addition, we also investigate the influence of spreading photon diffusion in the tissue on its sectioning capability. In the experiments, tissue phantoms were constructed with a range of optical properties. We used the intralipid solution to control the scattering properties, and the indocynane green to control the absorption property of the phantom. The result shows that the scattering coefficient is a major factor that affects the imaging depth and imaging contrast. It also influences greatly the thickness of tomographic imaging slice due to the light broadening inside tissue caused by the scattering property.

Improved performance in diffuse reflectance measurement by polarization discrimination

Previous research has shown that un-contact monitoring and characterization of diffuse reflectance photons emerged at the tissue surface could provide the useful physiological parameters within tissue to aid the diagnosis. Generally within this method, the detected optical signal consists of the specular reflection from the tissue surface and diffusive photons emerging from within tissue, while the latter being the signal of interest. However, the surface reflection signal would degrade the signal-to-noise ratio of the system. Therefore, there is a need to eliminate the effect of specular reflection having on the final measurement. In this paper, a simple method using polarization and cross-polarization pair is presented to improve the system SNR by efficiently removing the surface specular reflection. We are expecting that this simple technique could play an important role in the blood glucose measurement with the NIR approach.

The superfluorescent single-mode optical fibers source for OCT

Optical coherence tomography (OCT) is a new imaging modality that is being actively used in a variety of medical applications. Optical coherence tomography performs cross sectional imaging by measuring the time delay and magnitude of optical echoes at different transverse positions, essentially by the use of a low coherence interferometry to obtain the depth resolved information of a sample. The interference can occur only when the optical path lengths of light in both the sample arm and reference arm are matched to within the coherence length of light source. The most commonly used light sources in the current OCT systems are the superluminescent diodes (SLD). However, the coherence lengths of SLD are typically 10-30 microns that are not sufficient to achieve the resolution required for many medical applications. In the meantime, the moderate irradiance offered by the SLD limits the real time applications for OCT system, which usually require a power with an order of at least 10 milliwatts. Recently the diode-pumped superfluorescent optical fibers sources has been used in a variety of communication and sensor applications. The superfluorescent rare-earth doped optical fibers source is also the very good OCT systematic light source, because of that have a wide bandwidth of fluorescence and high emission power.

Photoacoustic tomography imaging of biological tissues

Non-invasive laser-induced photoacoustic tomography is attracting more and more attentions in the biomedical optical imaging field. This imaging modality takes the advantages in that the tomography image has the optical contrast similar to the optical techniques while enjoying the high spatial resolution comparable to the ultrasound. Currently, its biomedical applications are mainly focused on breast cancer diagnosis and small animal imaging. In this paper, we report in detail a photoacoustic tomography experiment system constructed in our laboratory. In our system, a Q-switched ND:YAG pulse laser operated at 532nm with a 10ns pulse width is employed to generate photoacoustic signal. A tissue-mimicking phantom was built to test the system. When imaged, the phantom and detectors were immersed in a water tank to facilitate the acoustic detection. Based on filtered back-projection process of photoacoustic imaging, the two-dimension distribution of optical absorption in tissue phantom was reconstructed.