Puyun Guo

A real-time photoacoustic tomography system for small animals

A real-time 512-element photoacoustic tomography system for small animal imaging using a ring ultrasound array has been developed. The system, based upon a 5 MHz transducer array formed along a 50 mm circular aperture, achieves sub-200 micron lateral resolution over a 2 cm disk-shaped region. Corresponding elevation resolutions of 0.6 to 2.5 mm over the central volume enable depth-resolved 3D tomographic imaging with linear translation. Using 8:1 electronic multiplexing, imaging at up to 8 frame/sec is demonstrated for both dynamic phantoms and in vivo mouse and brain samples. The real-time, full 2D tomographic capability of the system paves the way for functional photoacoustic tomographic imaging studies in small animals with sub-second time frame.

Simultaneous near-infrared diffusive light and ultrasound imaging

We have constructed a near-real-time combined imager suitable for simultaneous ultrasound and near-infrared diffusive light imaging and coregistration. The imager consists of a combined hand-held probe and the associated electronics for data acquisition. A two-dimensional ultrasound array is deployed at the center of the combined probe, and 12 dual-wavelength laser source fibers (780 and 830 nm) and 8 optical detector fibers are deployed at the periphery. We have experimentally evaluated the effects of missing optical sources in the middle of the combined probe on the accuracy of the reconstructed optical absorption coefficient and assessed the improvements of a reconstructed absorption coefficient with the guidance of the coregistered ultrasound. The results have shown that, when the central ultrasound array area is in the neighborhood of 2 cm x 2 cm, which corresponds to the size of most commercial ultrasound transducers, the optical imaging is not affected. The results have also shown that the iterative inversion algorithm converges quickly with the guidance of a priori three-dimensional target distribution, and only one iteration is needed to reconstruct an accurate optical absorption coefficient.

Coregistered three-dimensional ultrasound and photoacoustic imaging system for ovarian tissue characterization

Ovarian cancer has the highest mortality of all gynecologic cancers, with a five-year survival rate of only 30% or less. Current imaging techniques are limited in sensitivity and specificity in detecting early stage ovarian cancer prior to its widespread metastasis. New imaging techniques that can provide functional and molecular contrasts are needed to reduce the high mortality of this disease. One such promising technique is photoacoustic imaging. We develop a 1280-element coregistered 3-D ultrasound and photoacoustic imaging system based on a 1.75-D acoustic array. Volumetric images over a scan range of 80 deg in azimuth and 20 deg in elevation can be achieved in minutes. The system has been used to image normal porcine ovarian tissue. This is an important step toward better understanding of ovarian cancer optical properties obtained with photoacoustic techniques. To the best of our knowledge, such data are not available in the literature. We present characterization measurements of the system and compare coregistered ultrasound and photoacoustic images of ovarian tissue to histological images. The results show excellent coregistration of ultrasound and photoacoustic images. Strong optical absorption from vasculature, especially highly vascularized corpora lutea and low absorption from follicles, is demonstrated.

Optimal probing of optical contrast of breast lesions of different size located at different depths by US localization

We report a frequency domain optical tomography system utilizing three RF modulation frequencies, which are optimized for probing breast lesions of different size located at different depths. A real-time co-registered ultrasound scanner is used to provide on-site estimation of lesion size and location. Based on the lesion information, an optimal light modulation frequency can be selected, which may yield more accurate estimates of lesion angiogenesis and hypoxia. Phantom experiments have demonstrated that a high modulation frequency, such as 350Mhz, is preferable for probing small lesions closer to the surface while a low modulation frequency, such as 50Mhz, is desirable for imaging deeper and larger lesions. A clinical example of a large invasive carcinoma is presented to demonstrate the application of this novel technique.

Design of near-infrared imaging probe with the assistance of ultrasound localization

A total of 364 optical source-detector pairs were deployed uniformly over a 9 cm x 9 cm probe area initially, and then the total pairs were reduced gradually to 60 in experimental and simulation studies. For each source-detector configuration, three-dimensional (3-D) images of a 1-cm-diameter absorber of different contrasts were reconstructed from the measurements made with a frequency-domain system. The results have shown that more than 160 source-detector pairs are needed to reconstruct the absorption coefficient to within 60% of the true value and appropriate spatial and contrast resolution. However, the error in target depth estimated from 3-D images was more than 1 cm in all source-detector configurations. With the a priori target depth information provided by ultrasound, the accuracy of the reconstructed absorption coefficient was improved by 15% and 30% on average, and the beam width was improved by 24% and 41% on average for high- and low-contrast cases, respectively. The speed of reconstruction was improved by ten times on average.

Co-registered 3-D ultrasound and photoacoustic imaging using a 1.75D 1280-channel ultrasound system

Photoacoustic imaging is a promising non-invasive imaging technology due to its ability to combine the enhanced contrast of optical absorption with the spatial resolution of acoustic imaging. Co-registered three-dimensional (3-D) ultrasound and photoacoustic imaging takes advantage of both modalities to allow visualization of tissue structures within a volume using simultaneous structural and functional information. 1.75D acoustic arrays are well-suited for this application due to their ability to scan in 3-D volumes rapidly and accurately while maintaining a reasonable system complexity and cost. We have designed, fabricated, and tested a 1.75D 1280-ch ultrasound system for co-registered 3-D ultrasound and photoacoustic imaging. The system features a 1.75D 1280-channel ultrasound array with a center frequency of 5MHz and 80% bandwidth. The electronics includes 1280 high-voltage pulsers, 40 32-to-1 multiplexers, amplification circuitry, and a 40-channel data acquisition circuit. The system is able to drive the entire array simultaneously, and each array element independently, to scan a 3-D volume within +/- 40 degrees in azimuth direction and +/- 10 degrees in elevation respectively. System performance including axial and lateral resolution has been characterized and compared with simulations. Co-registered 3-D ultrasound and photoacoustic imaging has been successfully performed on phantoms with different geometries and contrast.

A curved array photoacoustic tomography system for small animal imaging

We have developed and tested a photoacoustic imaging system based on a 128 element curved-phased ultrasonic array, which spans a quarter of a complete circle with a radius of curvature equal to 25mm. The center frequency of the array is 5 MHz with 60% bandwidth. The physical dimensions of the elements are 10x0.3mm (elevation x azimuth) with an elevation focus of 19mm. Earlier we reported acoustic measurements of the axial and lateral resolutions of the system that were limited by the impulse response of the narrowband source used in the test. In this paper we discuss photoacoustic characterization of the system including resolution and sensitivity. The array forms the building block for a 512-element ring designed for complete tomographic imaging of small animals. Imaging results of phantoms will be compared with simulations.

Elevation beamforming performance of a 1.75 D array

1.75 D arrays allow limited beam steering in elevation, which is valuable for three-dimensional (3-D) imaging within a certain field of view. 3-D imaging has important advantage in detecting and visualizing small lesions from multiple viewing angles. Recently, Tetrad Inc has manufactured 1.75 D arrays using the state-of-the-art technologies. The array we have consists of 10 rows in elevation and 128 elements in azimuth, 1280 elements in total. The central frequency of the array is 5 MHz and the bandwidth is 60%. We have constructed 1280 parallel transmission circuitry, 1280 to 40 multiplexing and 40 parallel receiving circuitry. Upon each transmission, 1280-element RF signals were multiplexed to 40 parallel electronic channels, amplified, sampled and stored in the PC. The elevation performance of the array was evaluated through simulations and experiments using a point target. The parameters evaluated were -6 dB, -20 dB beamwidths and grating lobe strength.

A fast 512-element ring array photoacoustic imaging system for small animals

A 512-element photoacoustic tomography system for small animal imaging using a ring ultrasound array has been developed. The system features a 5 MHz piezocomposite transducer array formed into a complete circular aperture. Custom receiver electronics consisting of dedicated preamplifiers, 8:1 multiplexed post-amplifiers, and a 64-channel data acquisition module provide full tomographic imaging at up to 8 frames/second. We present details of the system design along with characterization results of the resolution, imaging volume, and sensitivity. Small animal imaging performance is demonstrated through images of mice brain vasculature at different depths and real-time spectroscopic scans. This system enables real-time tomographic imaging for functional photoacoustic studies for the first time.

Feasibility Study of Three-dimensional Co-registered Ultrasound and Photoacoustic Imaging for Cancer Detection and Visualization

Three-dimensional imaging is very valuable in detecting and visualizing lesions from multiple viewing angles. In addition, co-registered 3D imaging combining conventional ultrasound and photoacoustic tomography allows visualization of tissue structures with simultaneous structural and functional information. We have developed a 1280 element 3D ultrasound imaging system based on a 1.75D acoustic array. Complete volumetric images over the full scanning range can be achieved in a few minutes. In conjunction with a Ti:Sapphire laser, the system has been used for photoacoustic imaging. We present 3D co-registered images obtained with the system. Ultrasound and photoacoustic co-registered images of phantoms with different optical and acoustical properties are shown to demonstrate its advantage in cancer detection.