Anastasios Maurudis

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.

Real-time photoacoustic tomography of cortical hemodynamics in small animals

For the first time, the hemodynamics within the entire cerebral cortex of a mouse were studied by using photoacoustic tomography (PAT) in real time. The PAT system, based on a 512-element full-ring ultrasound array, received photoacoustic signals primarily from a slice of 2-mm thickness. This system can provide high-resolution brain vasculature images. We also monitored the fast wash-in process of a photoacoustic contrast agent in the mouse brain. Our results demonstrated that PAT is a powerful imaging modality that can be potentially used to study small animal neurofunctional activities.

Photoacoustic tomography of small animal brain with a curved array transducer

We present the application of a curved array photoacoustic tomographic imaging system that can provide rapid, high-resolution photoacoustic imaging of small animal brains. The system is optimized to produce a B-mode, 90-deg field-of-view image at sub-200-microm resolution at a frame rate of approximately 1 frame/second when a 10-Hz pulse repetition rate laser is employed. By rotating samples, a complete 360-deg scan can be achieved within 15 s. In previous work, two-dimensional (2-D) ex vivo mouse brain cortex imaging has been reported. We report three-dimensional (3-D) small animal brain imaging obtained with the curved array system. The results are presented as a series of 2-D cross-sectional images. Besides structural imaging, the blood oxygen saturation of the animal brain cortex is also measured in vivo. In addition, the system can measure the time-resolved relative changes in blood oxygen saturation level in the small animal brain cortex. Last, ultrasonic gel coupling, instead of the previously adopted water coupling, is conveniently used in near-real-time 2-D imaging.

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.

Improvements in time resolution of tomographic photoacoustic imaging using a priori information for multiplexed systems

We present results of investigations of the application of a priori information and sparse or limited-view algorithms to simultaneously improve imaging quality and timeresolution in photoacoustic tomography. Modified versions of existing MRI/CT algorithms such as constrained backprojection and keyhole imaging are evaluated as well as a new Wiener estimation methods for extrapolation of missing data from reference data sets. Simulations indicate the effectiveness of the approaches for accurate tracking of dynamic photoacoustic events for data sets with limited views (< 90 degrees) or tomographic views with up to 1/64 of the full data set. We present experimental data of contrast uptake and washout using a 512-element curved transducer with 1:8 electronic multiplexing that demonstrates high-resolution tomographic imaging with a temporal resolution of better than 150 milliseconds using these methods.

Three-dimensional photoacoustic tomography of small animal brain with a curved array transducer

We present the application of an optimized curved array photoacoustic tomographic imaging system, which can provide rapid, high-resolution photoacoustic imaging of small animal brains. The system can produce a B-mode, 90-degree field-of-view image at sub-200 μm resolution at a frame rate of ~1 frame/second when a 10-Hz pulse repetition rate laser is employed. By rotating samples, a complete 360-degree scan can be achieved within 15 seconds. In previous work, two-dimensional ex vivo mouse brain cortex imaging has been reported. In the current work, we report three-dimensional small animal brain imaging obtained with the curved array system. The results are presented as a series of two-dimensional cross-sectional images. Besides structural imaging, the blood oxygen saturation of the animal brain cortex is also measured in vivo. In addition, the system can measure the time-resolved relative changes in blood oxygen saturation level in the small animal brain cortex. Finally, ultrasonic gel coupling, instead of the previously adopted water coupling, is conveniently used in near-real-time 2D imaging.

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.

A fast photoacoustic imaging system based on a curved ultrasound transducer array

Non-invasive laser-induced photoacoustic tomography (PAT) is a promising imaging modality for the biological imaging field due to the intrinsic optical contrast of tissue and the high resolution of ultrasound. In contrast to common experimental setups that employ a single ultrasound transducer rotating around the target, we present a fast PAT system employing a 128-element curved ultrasound transducer array, parallel signal amplifier circuitry, and multi-channel data acquisition. Two-dimensional photoacoustic images of various phantoms and blood tubing were reconstructed based on the delay-and-sum algorithm. The reconstructed images agreed well with the original samples. The spatial resolution of the system reaches 0.2 mm, which matches the theoretical calculation. The device and methodology provide a rapid and reliable approach to imaging biological tissues and may have potential applications in noninvasive imaging and clinical diagnosis.

A novel electronic architecture used to support biomedical photo-acoustic imaging

In this paper, the electronic architecture used in the implementation of a 128-channel biomedical photo-acoustic imaging system is described. The goal of our research was to verify that the electronic system developed is well-suited for our photo-acoustic imaging application. In essence, we investigated the analog front end for our imaging system, including the data acquisition system. The imaging system described herein was designed for use with a 128-element circularly phased transducer array, with an approximate bandwidth of 9 MHz, centered about 5.5 MHz. Our implemented architecture was highly specific for our research application. Specifically, we relaxed any usual transducer probe size limitations to allow us to bring the entire analog front end as close to the transducer insertion point as possible. In our case, each of the 128 transducer elements has a dedicated high-gain, low-noise, pre-amplifier stage within five inches. Even though our end user application is photo-acoustic imaging, other imaging modalities, such as conventional ultrasonic imaging, can be supported

A photoacoustic imaging system employing a curved-phased ultrasonic array and parallel electronics

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.