Photoacoustic spectrum analysis (PASA) has been found to have the ability to characterize microstructures in phantoms. The ability of PASA technique which is subjected to the ultrasound detector has been reported in identify tissue with hundreds of microns in size. This paper demonstrated the feasibility of micro-ring, an ultrasonic detector with ultra-broad bandwidth, in characterizing microspheres’ sizes ranging from few microns to 100 microns using PASA technique and the results were compared with a commercial hydrophone. In order to further verify the capability of micro-ring, spectrum of single micro-spheres with sizes of tens of microns were measured and compared to simulation result. Our work proves that micro-ring based PASA technique has the ability of differentiating the particles with different size in phantoms.
Sound velocity measurement is of great importance to the application of biomedical especially in the research of acoustic detection and acoustic tomography. Using correct sound velocities in each medium other than one unified sound propagation speed, we can effectively enhance sound based imaging resolution. Photoacoustic tomography (PAT), is defined as cross-sectional or three-dimensional (3D) imaging of a material based on the photoacoustic effect and it is a developing, non-invasive imaging method in biomedical research. This contribution proposes a method to concurrently calculate multiple acoustic speeds in different mediums. Firstly, we get the size of infra-structure of the target by B-mode ultrasonic imaging method. Then we build the photoacoustic (PA) image of the same target with different acoustic speed in different medium. By repeatedly evaluate the quality of reconstruct PA image, we dynamically calibrate the acoustic speeds in different medium to build a finest PA image. Thus, we take these speeds of sound as the correct acoustic propagation velocities in according mediums. Experiments show that our non-invasive method can yield correct speed of sound with less than 0.3% error which might benefit future research in biomedical science.
Photoacoustic (PA) technique involving both ultrasound and light has been explored for potential application in the assessment of bone health. The optical and ultrasound penetration in bone have been studied. The feasibility of conducting 3D PA imaging of bone, and performing quantitative evaluation of bone microstructures by using photoacoustic spectrum analysis (PASA) has also been investigated. The findings from the experiments demonstrate that PA measurement could offer information of bone mineral density and bone microstructure, both relevant to bone health.
Osteoporosis is a progressive bone disease that is characterized by a decrease in bone mass and deterioration in microarchitecture. This study investigates the feasibility of characterizing bone microstructure by analyzing the frequency spectrum of the photoacoustic signals from the bone. Modeling and numerical simulation of photoacoustic signals and their frequency-domain analysis were performed on trabecular bones with different mineral densities. The resulting quasilinear photoacoustic spectra were fit by linear regression, from which spectral parameter slope can be quantified. The modeling demonstrates that, at an optical wavelength of 685 nm, bone specimens with lower mineral densities have higher slope. Preliminary experiment on osteoporosis rat tibia bones with different mineral contents has also been conducted. The finding from the experiment has a good agreement with the modeling, both demonstrating that the frequency-domain analysis of photoacoustic signals can provide objective assessment of bone microstructure and deterioration. Considering that photoacoustic measurement is non-ionizing, non-invasive, and has sufficient penetration in both calcified and noncalcified tissues, this new technology holds unique potential for clinical translation.
The feasibility of an innovative biomedical diagnostic technique, thermal photoacoustic (TPA) measurement, for nonionizing and non-invasive assessment of bone health is investigated. Unlike conventional photoacoustic PA methods which are mostly focused on the measurement of absolute signal intensity, TPA targets the change in PA signal intensity as a function of the sample temperature, i.e. the temperature dependent Grueneisen parameter which is closely relevant to the chemical and molecular properties in the sample. Based on the differentiation measurement, the results from TPA technique is less susceptible to the variations associated with sample and system, and could be quantified with improved accurately. Due to the fact that the PA signal intensity from organic components such as blood changes faster than that from non-organic mineral under the same modulation of temperature, TPA measurement is able to objectively evaluate bone mineral density (BMD) and its loss as a result of osteoporosis. In an experiment on well established rat models of bone loss and preservation, PA measurements of rat tibia bones were conducted over a temperature range from 370 C to 440 C. The slope of PA signal intensity verses temperature was quantified for each specimen. The comparison among three groups of specimens with different BMD shows that bones with lower BMD have higher slopes, demonstrating the potential of the proposed TPA technique in future clinical management of osteoporosis.
Laser-induced thermotherapy (LITT), i.e. tissue destruction induced by a local increase of temperature by means of laser light energy transmission, has been frequently used for minimally invasive treatments of various diseases such as benign thyroid nodules and liver cancer. The emerging photoacoustic (PA) imaging, when integrated with ultrasound (US), could contribute to LITT procedure. PA can enable a good visualization of percutaneous apparatus deep inside tissue and, therefore, can offer accurate guidance of the optical fibers to the target tissue. Our initial experiment demonstrated that, by picking the strong photoacoustic signals generated at the tips of optical fibers as a needle, the trajectory and position of the fibers could be visualized clearly using a commercial available US unit. When working the conventional US Bscan mode, the fibers disappeared when the angle between the fibers and the probe surface was larger than 60 degree; while working on the new PA mode, the fibers could be visualized without any problem even when the angle between the fibers and the probe surface was larger than 75 degree. Moreover, with PA imaging function integrated, the optical fibers positioned into the target tissue, besides delivering optical energy for thermotherapy, can also be used to generate PA signals for on-line evaluation of LITT. Powered by our recently developed PA physio-chemical analysis, PA measurements from the tissue can provide a direct and accurate feedback of the tissue responses to laser ablation, including the changes in not only chemical compositions but also histological microstructures. The initial experiment on the rat liver model has demonstrated the excellent sensitivity of PA imaging to the changes in tissue temperature rise and tissue status (from native to coagulated) when the tissue is treated in vivo with LITT.
The ultimate goal of this work is to develop a novel photoacoustic (QPA) platform for highly-sensitive and quantitative
assessment of bone health. First, the feasibility to perform 3D photoacoustic imaging (PAI) of bone was investigated.
Then another two techniques, including thermal photoacoustic measurement (TPAM) and photoacoustic spectral
analysis (PASA), both being able to achieve quantitative results were investigated for bone characterization. TPAM, by
evaluating the dependence of photoacoustic signal amplitude on the sample temperature, is sensitive to the chemical
constituents in tissue and holds promise for assessment of bone mineral density (BMD). PASA characterizes micron size
physical features in tissue, and has shown feasibility for objective assessment of bone microarchitecture (BMA). This
integrated QPA platform can assess both bone mass and microstructure simultaneously without involving invasive
biopsy or ionizing radiation. Since QPA is non-ionizing, non-invasive, and has sufficient penetration in both soft tissue
and bone, it has unique advantages for clinical translation.
Photoacoustic tomography (PAT) is an effective optical biomedical imaging method which is characterized with noninonizing and noninvasive, presenting good soft tissue contrast with excellent spatial resolution. To build a multi-dimensional breast PAT image, more ultrasound sensors are needed, which brings difficulties to data acquisition. The time complexity for multi-dimensional breast PAT image reconstruction also rises tremendously. Compressive sensing (CS) theory breaks the restriction of Nyquist sampling theorem and is capable to rebuild signals with fewer measurements. In this contribution, we propose an effective optimization method for multi-dimensional breast PAT, which combines the theory of CS and an unevenly, adaptively distributing data acquisition algorithm. With this method, the quality of our reconstructed breast PAT images are better than those using existing multi-dimensional breast PAT system. To build breast PAT images with the same quality, the required number of ultrasound transducers is decreased by using our proposed method. We have verified our method on simulation data and achieved expected results in both two dimensional and three dimensional PAT image reconstruction. In the future, our method can be applied to various aspects of biomedical PAT imaging such as early stage tumor detection and in vivo imaging monitoring.
Most concurrent photoacoustic tomography systems are based on traditional ultrasound measurement regime, which requires the contact or acoustic coupling material between the biological tissue and the ultrasound transducer. This study investigates the feasibility of non-contact measurement of photacoustic signals generated inside biomedical tissues by observing the vibrations at the surface of the tissues with a commercial laser Doppler vibrometer. The vibrometer with 0- 2MHz measurement bandwidth and 5 MHz sampling frequency was integrated to a conventional rotational PAT data acquisition system. The data acquisition of the vibrometer was synchronized to the laser illumination from an Nd:YAG laser with output at 532nm. The laser energy was tuned to 17.5mJ per square centimeter. The PA signals were acquired at 120 angular locations uniformly distributed around the scanned objects. The frequency response of the measurement system was first calibrated. 2-inch-diamater cylindrical phantoms containing small rubber plates and biological tissues were afterwards imaged. The phantoms were made from 5% intralipid solution in 10% porcine gelatin to simulate the light scattering in biological tissue and to backscatter the measurement laser from the vibrometer. Time-domain backprojection method was used for the image reconstruction. Experiments with real-tissue phantoms show that with laser illumination of 17.5 mJ/cm2 at 532 nm, the non-contact photoacoustic (PA) imaging system with 15dB detection bandwidth of 2.5 MHz can resolve spherical optical inclusions with dimension of 500μm and multi-layered structure with optical contrast in strongly scattering medium. The experiment results prompt the potential implementation of the non-contact PAT to achieve “photoacoustic camera”.