Huaimin Gu

Fast multielement phase-controlled photoacoustic imaging based on limited-field-filtered back-projection algorithm

In this paper, the multielement phase-controlled technique and the limited-field -filtered back-projection algorithm are used to investigate the two-dimensional fast noninvasive photoacoustic imaging. By the use of the former to collect photoacoustic signals, which are of high signal-to-noise ratio, one needs not to average the data and can acquire them in less than 5s. The later can greatly improve the lateral resolution of the multielement linear transducer array imaging system from 1.5mmto0.24mm. This method and system can provide a fast and reliable approach to photoacoustic imaging that could be applied to noninvasive imaging and clinic diagnosis.

Real-time optoacoustic monitoring of vascular damage during photodynamic therapy treatment of tumor

The optoacoustic technique is a noninvasive imaging method with high spatial resolution. It potentially can be used to monitor anatomical and physiological changes. Photodynamic therapy (PDT)-induced vascular damage is one of the important mechanisms of tumor destruction, and real-time monitoring of vascular changes can have therapeutic significance. A unique optoacoustic system is developed for neovascular imaging during tumor phototherapy. In this system, a single-pulse laser beam is used as the light source for both PDT and for concurrently generating ultrasound signals for optoacoustic imaging. To demonstrate its feasibility, this system is used to observe vascular changes during PDT treatment of chicken chorioallantoic membrane (CAM) tumors. The photosensitizer used in this study is protoporphyrin IX (PpIX) and the laser wavelength is 532 nm. Neovascularization in tumor angiogenesis is visualized by a series of optoacoustic images at different stages of tumor growth. Damage of the vascular structures by PDT is imaged before, during, and after treatment. Rapid, real-time determination of the size of targeted tumor blood vessels is achieved, using the time difference of positive and negative ultrasound peaks during the PDT treatment. The vascular effects of different PDT doses are also studied. The experimental results show that a pulsed laser can be conveniently used to hybridize PDT treatment and optoacoustic imaging and that this integrated system is capable of quantitatively monitoring the structural change of blood vessels during PDT. This method could be potentially used to guide PDT and other phototherapies using vascular changes during treatment to optimize treatment protocols, by choosing appropriate types and doses of photosensitizers and doses of light.

Integrative prototype B-scan photoacoustic tomography system based on a novel hybridized scanning head

A prototype B-scan photoacoustic tomography system is developed. It integrates pumping fiber, ultrasound coupling medium, and a transducer array into a novel hybridized scanning head. By moving the scanning head a photoacoustic tomography can be obtained in the reflection mode. Concentration-adjustable glycerite is used as the ultrasonic coupling medium to match the ultrasonic velocities in tissues, reducing the acoustic reflection, eliminating the acoustic refraction, and rectifying the acoustic path difference. This system is used to image graphite phantom in tissue and human blood vessels. Our experimental results show that this integrative system has the potential for fast photoacoustic imaging.

High antinoise photoacoustic tomography based on a modified filtered backprojection algorithm with combination wavelet

How to extract the weak photoacoustic signals from the collected signals with high noise is the key to photoacoustic signal processing. We have developed a modified filtered backprojection algorithm based on combination wavelet for high antinoise photoacoustic tomography. A Q-switched-Nd: yttrium-aluminum-garnet laser operating at 532 nm is used as light source. The laser has a pulse width of 7 ns and a repetition frequency of 20 Hz. A needle polyvinylidene fluoride hydrophone with diameter of 1 mm is used to capture photoacoustic signals. The modified algorithm is successfully applied to imaging vascular network of a chick embryo chorioallantoic membrane in situ and brain structure of a rat brain in vivo, respectively. In the reconstructed images, almost all of the capillary vessels and the vascular ramifications of the chick embryo chorioallantoic membrane are accurately resolved, and the detailed brain structures of the rat brain organization are clearly identified with the skull and scalp intact. The experimental results demonstrate that the modified algorithm has much higher antinoise capacity, and can greatly improve the reconstruction image quality. The spatial resolution of the reconstructed images can reach 204 microm. The modified filtered back-projection algorithm based on the combination wavelet has the potential in the practical high-noise signal processing for deeply penetrating photoacoustic tomography.

Thermal coagulation-induced changes of the optical properties of normal and adenomatous human colon tissues in vitro in the spectral range 400–1100 nm

The absorption coefficients, the reduced scattering coefficients and the optical penetration depths for native and coagulated human normal and adenomatous colon tissues in vitro were determined over the range of 400-1,100 nm using a spectrophotometer with an internal integrating sphere system, and the inverse adding-doubling method was applied to calculate the tissue optical properties from diffuse reflectance and total transmittance measurements. The experimental results showed that in the range of 400-1,100 nm there were larger absorption coefficients (P < 0.01) and smaller reduced scattering coefficients (P < 0.01) for adenomatous colon tissues than for normal colon tissues, and there were smaller optical penetration depths for adenomatous colon tissues than for normal colon tissues, especially in the near-infrared wavelength. Thermal coagulation induced significant increase of the absorption coefficients and reduced scattering coefficients for the normal and adenomatous colon tissues, and significantly reduced decrease of the optical penetration depths for the normal and adenomatous colon tissues. The smaller optical penetration depth for coagulated adenomatous colon tissues is a disadvantage for laser-induced thermotherapy (LITT) and photodynamic therapy (PDT). It is necessary to adjust the application parameters of lasers to achieve optimal therapy.

Differences in optical properties between healthy and pathological human colon tissues using a Ti:sapphire laser: an in vitro study using the Monte Carlo inversion technique

The purpose of the study is to analyze and compare differences in the optical properties between normal and adenomatous human colon tissues in vitro at 630-, 680-, 720-, 780-, 850-, and 890-nm wavelengths using a Ti:sapphire laser. The optical parameters of tissue samples are determined using a double integrating sphere setup at seven different laser wavelengths. The inverse Monte Carlo simulation is used to determine the optical properties from the measurements. The results of measurement show that the optical properties and their differences vary with a change of laser wavelength for normal and adenomatous colon mucosa/submucosa and normal and adenomatous colon muscle layer/chorion. The maximum absorption coefficients for normal and adenomatous human colon mucosa/submucosa are 680 nm, and the minimum absorption coefficients for both are 890 nm. The maximum difference of the absorption coefficients between both is 56.8% at 780 nm. The maximum scattering coefficients for normal and adenomatous colon mucosa/submucosa are 890 nm, and the minimum scattering coefficients for both are 780 nm. The maximum difference of the scattering coefficients between both is 10.6% at 780 nm. The maximum absorption coefficients for normal and adenomatous colon muscle layer/chorion are 680 nm, and the minimum absorption coefficients for both are 890 nm. The maximum difference of the absorption coefficients between both is 47.9% at 780 nm. The maximum scattering coefficients for normal and adenomatous colon muscle layer/chorion are 890 nm, and the minimum scattering coefficients for both are 680 nm. The maximum difference of the scattering coefficients between both is 9.61% at 850 nm. The differences in absorption coefficients between normal and adenomatous tissues are more significant than those in scattering coefficients.

Gold nanoshell-based photoacoustic imaging application in biomedicine

Gold nanoshell was used as a new contrast-enhancing agent for photoacoustic tomography. Gold nanoshells are concentric sphere nanoparticles consisting of a dielectric silica core and a gold shell. By varying the relative thickness of the core and shell layers, the plasmon-derived optical resonance of gold can be dramatically shifted in wavelength from the visible region into the infrared over a wavelength range that spans the region of highest physiological transmissivity. In this experimentation, nanoshells with 100 nm silica core diameter and 20 nm gold shells thickness has an optical absorption peak at 800 nm and deep penetrating pulse laser of 800 nm were employed to image the the vasculature architecture of a rat brain in vivo. Here we accurately imaged rat brain structures with gold nanoshell contrast agents. We also mapped the distribution of gold nanoshell in the in vivo rat brain. This experiment results demonstrated that nanoshell accumulation is greatly enhanced NIR optical contrast in the vasculature. Nanoshell-based photoacoustic imaging technique would be applied in biomedicine extensively. The further development of photoacoustic tomography to characterize and monitor the accumulation of nanoshells in vivo will be applied to the detection of tumors in situ as well as to guiding nanoshell-based thermal tumor therapy. Gold nanoshell can potentially provide an accurate noninvasive method, use as imaging contrast agents, to employ in functional photoacoustic at molecular and cellular level.

In vitro study of the effects of ultrasound-mediated glycerol on optical attenuation of human normal and cancerous esophageal tissues with optical coherence tomography

Previous studies from our group have demonstrated that glucose solution can induce optical clearing enhancement of esophageal tissues with optical coherence tomography (OCT). The aims of this study were to evaluate the optical clearing effects of ultrasound-mediated optical clearing agents (OCAs) and to find more effective methods to distinguish human normal esophageal tissues (NE) and cancerous esophageal tissues (CE). Here we used the OCT technique to investigate the optical attenuation of NE and CE in vitro after treatment with 30% glycerol alone and glycerol combined with ultrasound, respectively. Experimental results showed that the averaged attenuation coefficient of CE was significantly larger than that of NE. The maximal decreases of averaged attenuation coefficients of NE and CE were approximately 48.7% and 36.2% after treatment with 30% glycerol alone, and they were significantly lower than those treated with 30% glycerol and ultrasound (57.5% in NE and 44.8% in CE). Moreover, after treatment with 30% glycerol alone, the averaged attenuation coefficients of NE and CE reached their minima in about 80 min and 65 min, respectively. The times were much shorter in NE and CE after treatment with glycerol with ultrasound, being about 62 min and 50 min, respectively. The results suggest that there is a significant difference in the optical properties of NE and CE, and that OCT with an ultrasound–OCAs combination has the ability to distinguish CE from NE.

In Vitro Study of Thermal Changes in Optical Properties of Myocardium Tissue with Diffuse Reflectance Imaging

Optical properties of porcine myocardium tissue in the exposure temperature range of 37-80degC were determined for 700, 750, 800, 850, 900 nm wavelengths of a Ti: sapphire laser. A neural network were trained with diffuse reflectance data from Monte Carlo simulations based on the semi-infinite tissue model, and then it was used to assess the absorption coefficient and reduced scattering coefficient from the spatially resolved relative reflectance which derived from diffuse reflectance image. The result showed that there was slight change in the absorption coefficient; the average for fresh is 0.11 mm-1, which increased to 0.17mm-1 for 80degC. The average reduced scattering coefficient of the five wavelengths for fresh samples is 0.14 mm-1, which increased obviously to 0.66 mm-1 for 80degC, and the average for all samples is 0.13 mm-1. The reduced scattering coefficient at the longer wavelength increased slowly compared with the shorter wavelength in the wavelength range of 700-900 nm. The average optical penetration depth of the five wavelengths for fresh samples is 3.37 mm, which reduced obviously to 1.54 mm for 80degC. In conclusion, thermal effect during photothermal treatment of tissue is an important factor altering optical properties of tissue.

Imaging of gold nanoshell clearance in animal brains in vivo by improved-simultaneous-iterative-based photoacoustic tomography

The high contrast and high resolution photoacoustic tomography was used to image the gold nanoshell clearance in rat brain in vivo. With our current imaging system, the acquisition of photoacoustic signals is realized through a circular scan of a single-element transducer. Therefore, the data acquisition is slow. In this case, an improved simultaneous iterative reconstruction algorithm was developed to reduce the acquisition time by using limited data in the experiments. This algorithm is based on the least square principle; it can be used to reconstruct high quality images from the limited data containing much noise. Furthermore, it is always convergent. So it can improve the imaging quality comparison with conventional filter back-projection algorithm (FBP) and algebraic reconstruction algorithm (ART). Here we accurately mapped rat brain structures with gold nanoshell contrast agents. We also imaged the clearance of gold nanoshell in the rat brain. It provides an accurate non-invasive monitoring method for fluid pathways in biological tissues, which makes photoacoustic tomography as a powerful method for imaging pathologic tumor vessels, delineating neovascularization, and studying global and regional hemodynamic activities in the brain.