Near-infrared (NIR) fluorescence is an alternative modality for molecular imaging that has been demonstrated in animals
and recently in humans. Fluorescence-enhanced optical tomography (FEOT) using continuous wave or frequency
domain photon migration techniques could be used to provide quantitative molecular imaging in vivo if it could be
validated against "gold-standard," nuclear imaging modalities, using dual-labeled imaging agents. Unfortunately,
developed FEOT systems are not suitable for incorporation with CT/PET/SPECT scanners because they utilize benchtop
devices and require a large footprint. In this work, we developed a miniaturized fluorescence imaging system installed in
the gantry of the Siemens Inveon PET/CT scanner to enable NIR transillumination measurements. The system consists
of a CCD camera equipped with NIR sensitive intensifier, a diode laser controlled by a single board compact controller,
a 2-axis galvanometer, and RF circuit modules for homodyne detection of the phase and amplitude of fluorescence
signals. The performance of the FEOT system was tested and characterized. A mouse-shaped solid phantom of uniform
optical properties with a fluorescent inclusion was scanned using CT, and NIR fluorescence images at several
projections were collected. The method of high-order approximation to the radioactive transfer equation was then used to
reconstruct the optical images. Dual-labeled agents were also used on a tumor bearing mouse to validate the results of
the FEOT against PET/CT image. The results showed that the location of the fluorophore obtained from the FEOT
matches the location of tumor obtained from the PET/CT images. Besides validation of FEOT, this hybrid system could
allow multimodal molecular imaging (FEOT/PET/CT) for small animal imaging.
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.
Ovarian cancer has the highest mortality of all gynecologic cancers with a five-year
survival rate of only 30%. Because current imaging techniques (ultrasound, CT, MRI, PET) are
not capable of detecting ovarian cancer early, most diagnoses occur in later stages (III/IV). Thus
many women are not correctly diagnosed until the cancer becomes widely metastatic. On the
other hand, while the majority of women with a detectable ultrasound abnormality do not harbor a
cancer, they all undergo unnecessary oophorectomy. Hence, new imaging techniques that can
provide functional and molecular contrasts are needed for improving the specificity of ovarian
cancer detection and characterization. One such technique is photoacoustic imaging, which has
great potential to reveal early tumor angiogenesis through intrinsic optical absorption contrast
from hemoglobin or extrinsic contrast from conjugated agents binding to appropriate molecular
receptors.
To better understand the cancer disease process of ovarian tissue using photoacoustic
imaging, it is necessary to first characterize the properties of normal ovarian tissue. We have
imaged ex-vivo ovarian tissue using a 3D co-registered ultrasound and photoacoustic imaging
system. The system is capable of volumetric imaging by means of electronic focusing. Detecting
and visualizing small features from multiple viewing angles is possible without the need for any
mechanical movement. The results show strong optical absorption from vasculature, especially
highly vascularized corpora lutea, and low absorption from follicles. We will present correlation
of photoacoustic images from animals with histology. Potential application of this technology
would be the noninvasive imaging of the ovaries for screening or diagnostic purposes.
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.
KEYWORDS: Ultrasonography, 3D image processing, Photoacoustic imaging, Imaging systems, Photoacoustic spectroscopy, 3D scanning, Data acquisition, Tissue optics, Acoustics, 3D acquisition
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.
KEYWORDS: Transducers, Signal to noise ratio, Imaging systems, Electronics, Ultrasonics, Photoacoustic imaging, Data acquisition, Photoacoustic spectroscopy, Tumors, Point spread functions
Real-time photoacoustic imaging requires ultrasonic array receivers and parallel data acquisition systems for the simultaneous detection of weak photoacoustic signals. In this paper, we introduce a newly completed ultrasonic receiving array system and report preliminary results of our measured point spread function. The system employs a curved ultrasonic phased array consisting of 128-elements, which span a quarter of a complete circle. The center frequency of the array is 5 MHz and the bandwidth is greater than 60%. In order to maximize the signal-to-noise ratio for photoacoustic signal detection, we utilized special designs for the analog front-end electronics. First, the 128 transducer-element signals were routed out using a 50-Ohm impedance matching PCB board to sustain signal integrity. We also utilize 128 low-noise pre-amplifiers, connected directly to the ultrasonic transducer, to amplify the weak photoacoustic signals before they were multiplexed to a variable-gain multi-stage amplifier chain. All front-end circuits were placed close to the transducer array to minimize signal lose due to cables and therefore improve the signal-to-noise ratio. Sixteen analog-to-digital converters were used to sample signals at a rate of 40 mega-samples per second with a resolution of 10-bits per sample. This allows us to perform a complete electronic scan of all 128 elements using just eight laser pulses.
KEYWORDS: Digital signal processing, Signal processing, Image processing, Optical filters, Filtering (signal processing), Doppler effect, Electronic filtering, Signal detection, Doppler tomography, Data acquisition
We present a real-time data-processing and display unit based on a custom-designed digital signal processor (DSP) module for imaging tissue structure and Doppler blood flow. The DSP module is incorporated into a conventional optical coherence tomography system. We also demonstrate the flexibility of embedding advanced Doppler processing algorithms in the DSP module. Two advanced velocity estimation algorithms previously introduced by us are incorporated in this DSP module. Experiments on Intralipid flow demonstrate that a pulsatile flow of several hundred pulses per minute can be faithfully captured in M-scan mode by this DSP system. In vivo imaging of a rat's abdominal blood flow is also presented.
KEYWORDS: Ultrasonography, 3D image processing, Imaging systems, Digital signal processing, 3D acquisition, Near infrared, 3D image reconstruction, Data acquisition, Multiplexers, Data processing
We have introduced a three-dimensional (3-D) ultrasound imaging system for use in a combined ultrasound and near-infrared (NIR) imager for breast imaging. Compared with commercial ultrasound scanners which can only provide a single 2-D cross-section image at one scan, this 3-D system can provide 3-D co-registered ultrasound images for constraining regions of interest for 3-D NIR imaging reconstruction at one scan. The 3-D high-resolution ultrasound system consists of a state-of-the-art 1.75D ultrasound array of 1280 elements manufactured by Tetrad Inc. We have developed corresponding circuitry to drive the 1.75D array. The circuitry consists of 1280 parallel transmission channels, a 1280-to-40 multiplexer and 40 parallel receiving/data acquisition channels. A Texas Instrument DSP board has been embedded to speed up on-board data processing. We have tested the 3-D ultrasound imaging system with a 3-D ultrasound calibration phantom.
KEYWORDS: Digital signal processing, Signal processing, Image processing, Data acquisition, Signal detection, MATLAB, Blood circulation, Data processing, Electronic filtering, Doppler tomography
Clinical application of ODT requires real-time data acquisition and signal processing. In this paper, we present a real-time signal processing ODT unit based on a custom designed digital signal processor (DSP) module. The DSP is incorporated into a conventional ODT system using a grating-based scanning optical delay line. The newly developed flow velocity algorithms are integrated into the DSP and real-time data processing can be readily achieved.
We have constructed a nearly real-time combined imager suitable for simultaneous ultrasound and near infrared (NIR) diffusive light imaging and co-registration. The imager consists of a combined probe and associated electronics for data acquisition. A 2D ultrasound array occupies the center of the combined probe, while 12 dual wavelength laser source fibers (780 nm and 830 nm) and 8 optical detector fibers are deployed in the periphery. We have experimentally evaluated the effects of missing optical sensors in the middle of the combined probe upon the accuracy of the reconstructed optical absorption coefficient, and assessed the improvements of reconstructed absorption coefficient with the guidance of the co-registered ultrasound. The results have shown that when the central ultrasound array area is in the neighborhood of 2 X 2 cm2, which corresponds to the size of most commercial ultrasound transducers, the quality of optical images will not be degenerated. In addition to the acoustic information for cancer discrimination, NIR image reconstruction becomes much easier and more reliable. According to our results, the iterative inversion algorithm converges very fast with the guidance of a priori target temporal and spatial distributions. Only one iteration is needed to reconstruct accurate optical absorption coefficient.
Coherent sidelobes of a source can severely degrade OCT image quality by introducing false targets if no targets are present at the sidelobe locations. Sidelobes can also add constructively or destructively to the targets that are present at the sidelobe locations. This constructive or destructive interference will result in cancellation of the true targets or display of incorrect echo amplitudes of the targets. We introduce the use of CLEAN, a nonlinear deconvolution algorithm, to cancel coherent sidelobes in OCT images of extracted human teeth. The results show that CLEAN can reduce the coherent artifacts to the noise background, sharpen the air-enamel and enamel-dentin interfaces and improve the image contrast.
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