Svetoslav Ivanov Nikolov

Ultrasound research scanner for real-time synthetic aperture data acquisition

Conventional ultrasound systems acquire ultrasound data sequentially one image line at a time. The architecture of these systems is therefore also sequential in nature and processes most of the data in a sequential pipeline. This often makes it difficult to implement radically different imaging strategies on the platforms and makes the scanners less accessible for research purposes. A system designed for imaging research flexibility is the prime concern. The possibility of sending out arbitrary signals and the storage of data from all transducer elements for 5 to 10 seconds allows clinical evaluation of synthetic aperture and 3D imaging. This paper describes a real-time system specifically designed for research purposes. The system can acquire multichannel data in real-time from multi-element ultrasound transducers, and can perform some real-time processing on the acquired data. The system is capable of performing real-time beamforming for conventional imaging methods using linear, phased, and convex arrays. Image acquisition modes can be intermixed, and this makes it possible to perform initial trials in a clinical environment with new imaging modalities for synthetic aperture imaging, 2D and 3D B-mode, and velocity imaging using advanced coded emissions. The system can be used with 128-element transducers and can excite 128 transducer elements and receive and sample data from 64 channels simultaneously at 40 MHz with 12-bit precision. Two-to-one multiplexing in receive can be used to cover 128 receive channels. Data can be beamformed in real time using the systems 80 signal processing units, or it can be stored directly in RAM. The system has 16 Gbytes RAM and can, thus, store more than 3.4 seconds of multichannel data. It is fully software programmable and its signal processing units can also be reconfigured under software control. The control of the system is done over a 100-Mbits/s Ethernet using C and Matlab. Programs for doing, e.g., B-mode imaging can be written directly in Matlab and executed on the system over the net from any workstation running Matlab. The overall system concept is presented along with its implementation and examples of B-mode and in vivo synthetic aperture flow imaging.

SARUS: A synthetic aperture real-time ultrasound system

The Synthetic Aperture Real-time Ultrasound System (SARUS) for acquiring and processing synthetic aperture (SA) data for research purposes is described. The specifications and design of the system are detailed, along with its performance for SA, nonlinear, and 3-D flow estimation imaging. SARUS acquires individual channel data simultaneously for up to 1024 transducer elements for a couple of heart beats, and is capable of transmitting any kind of excitation. The 64 boards in the system house 16 transmit and 16 receive channels each, where sampled channel data can be stored in 2 GB of RAM and processed using five field-programmable gate arrays (FPGAs). The fully parametric focusing unit calculates delays and apodization values in real time in 3-D space and can produce 350 million complex samples per channel per second for full non-recursive synthetic aperture B-mode imaging at roughly 30 high-resolution images/s. Both RF element data and beamformed data can be stored in the system for later storage and processing. The stored data can be transferred in parallel using the systems sixty-four 1-Gbit Ethernet interfaces at a theoretical rate of 3.2 GB/s to a 144-core Linux cluster.

In-vivo synthetic aperture flow imaging in medical ultrasound

A new method for acquiring flow images using synthetic aperture techniques in medical ultrasound is presented. The new approach makes it possible to have a continuous acquisition of flow data throughout the whole image simultaneously, and this can significantly improve blood velocity estimation. Any type of filter can be used for discrimination between tissue and blood flow without initialization, and the number of lines used for velocity estimation is limited only by the nonstationarity of the flow. The new approach is investigated through both simulations and measurements. A flow rig is used for generating a parabolic laminar flow, and a research scanner is used for acquiring RF data from individual transducer elements. A reference profile is calculated from a mass flow meter. The parabolic velocity profile is estimated using the new approach with a relative standard deviation of 2.2% and a mean relative bias of 3.4% using 24 pulse emissions at a flow angle of 45 degrees. The 24 emissions can be used for making a full-color flow map image. An in-vivo image of flow in the carotid artery for a 29-year-old male also is presented. The full image is acquired using 24 emissions.

Experimental ultrasound system for real-time synthetic imaging

Digital signal processing is being employed more and more in modern ultrasound scanners. This has made it possible to do dynamic receive focusing for each sample and implement other advanced imaging methods. The processing, however, has to be very fast and cost-effective at the same time. Dedicated chips are used in order to do real time processing. This often makes it difficult to implement radically different imaging strategies on one platform and makes the scanners less accessible for research purposes. Here flexibility is the prime concern, and the storage of data from all transducer elements over 5 to 10 seconds is needed to perform clinical evaluation of synthetic and 3D imaging. This paper describes a real-time system specifically designed for research purposes. The purpose of the system is to make it possible to acquire multi-channel data in real-time from clinical multi-element ultrasound transducers, and to enable real-time or near real-time processing of the acquired data. The system will be capable of performing the processing for the currently available imaging methods, and will make it possible to perform initial trials in a clinical environment with new imaging modalities for synthetic aperture imaging, 2D and 3D B-mode and velocity imaging. The system can be used with 128 element transducers and can excite 128 channels and receive and sample data from 64 channels simultaneously at 40 MHz with 12 bits precision. Data can be processed in real time using the systems 80 signal processing units or it can be stored directly in RAM. The system has 24 GBytes RAM and can thus store 8 seconds of multi-channel data. It is fully software programmable and its signal processing units can also be reconfigured under software control. The control of the system is done over an Ethernet using C and Matlab. Programs for doing, e.g. B-mode imaging can directly be written in Matlab and executed on the system over the net from any workstation running Matlab. The overall system concept is presented and an example of a 20 lines script for doing phased array B-mode imaging is presented.

Recursive ultrasound imaging

Presents a new imaging method, applicable for both 2D and 3D imaging. It is based on Synthetic Transmit Aperture Focusing, but unlike previous approaches a new frame is created after every pulse emission. The elements from a linear transducer array emit pulses one after another. The same transducer element is used after N/sub xmt/ emissions. For each emission the signals from the individual elements are beam-formed in parallel for all directions in the image. A new frame is created by adding the new RF lines to the RF lines from the previous frame. The RF data recorded at the previous emission with the same element are subtracted. This yields a new image after each pulse emission and can give a frame rate of, for example, 5000 images/sec. The paper gives a derivation of the recursive imaging technique and compares simulations for fast B-mode imaging with measurements. A low value of N/sub xmt/ is necessary to decrease the motion artifacts and to make flow estimation possible. The simulations show that for N/sub xmt/=13 the level of grating lobes is less than -50 dB from the peak, which is sufficient for B-mode imaging and flow estimation. The measurements made with an off-line experimental system having 64 transmitting channels and 1 receiving channel, confirmed the simulation results. A linear array with a pitch of 208.5 /spl mu/m, central frequency f/sub otr/=7.5 MHz and bandwidth BW=70% was used. The signals from 64 elements were recorded, beam-formed and displayed as a sequence of B-mode frames, using the recursive algorithm. An excitation with a central frequency f/sub otr/=5 MHz (/spl lambda/=297 /spl mu/m in water) was used to obtain the point spread function of the system. The -6 dB width of the PSF is 1.056 mm at axial distance of 39 mm. For a sparse synthetic transmit array with N/sub xmt/=22 the expected grating lobes from the simulations are -53 dB down from the peak value at, positioned at /spl plusmn/28/spl deg/. The measured level was -51 dB at /spl plusmn/27/spl deg/ from the peak. Images obtained with the experimental system are compared to the simulation results for different sparse arrays. The application of the method for 3D real-time imaging and blood-velocity estimations is discussed.

Virtual ultrasound sources in high-resolution ultrasound imaging

This paper investigates the concept of virtual source elements. It suggests a common framework for increasing the resolution, and penetration depth of several imaging modalities by applying synthetic aperture focusing (SAF). SAF is used either as a post focusing procedure on the beamformed data, or directly on the raw signals from the transducer elements. Both approaches increase the resolution. The paper shows that in one imaging situation, there can co-exist different virtual sources for the same scan line - one in the azimuth plane, and another in the elevation. This property is used in a two stage beamforming procedure for 3D ultrasound imaging. The position of the virtual source, and the created waveform are investigated with simulation, and with pulse-echo measurements. There is good agreement between the estimated wavefront and the theoretically fitted one. Several examples of the use of virtual source elements are considered. Using SAF on data acquired for a conventional linear array imaging improves the penetration depth for the particular imaging situation from 80 to 110 mm. The independent use of virtual source elements in the elevation plane decreases the respective size of the point spread function at 100 mm below the transducer from 7mm to 2 mm.

Implementation of a versatile research data acquisition system using a commercially available medical ultrasound scanner

This paper describes the design and implementation of a versatile, open-architecture research data acquisition system using a commercially available medical ultrasound scanner. The open architecture will allow researchers and clinicians to rapidly develop applications and move them relatively easy to the clinic. The system consists of a standard PC equipped with a camera link and an ultrasound scanner equipped with a research interface. The ultrasound scanner is an easy-to-use imaging device that is capable of generating high-quality images. In addition to supporting the acquisition of multiple data types, such as B-mode, M-mode, pulsed Doppler, and color flow imaging, the machine provides users with full control over imaging parameters such as transmit level, excitation waveform, beam angle, and focal depth. Beamformed RF data can be acquired from regions of interest throughout the image plane and stored to a file with a simple button press. For clinical trials and investigational purposes, when an identical image plane is desired for both an experimental and a reference data set, interleaved data can be captured. This form of data acquisition allows switching between multiple setups while maintaining identical transducer, scanner, region of interest, and recording time. Data acquisition is controlled through a graphical user interface running on the PC. This program implements an interface for third-party software to interact with the application. A software development toolkit is developed to give researchers and clinicians the ability to utilize third-party software for data analysis and flexible manipulation of control parameters. Because of the advantages of speed of acquisition and clinical benefit, research projects have successfully used the system to test and implement their customized solutions for different applications. Three examples of system use are presented in this paper: evaluation of synthetic aperture sequential beamformation, transverse oscillation for blood velocity estimation, and acquisition of spectral velocity data for evaluating aortic aneurysms.

3D synthetic aperture imaging using a virtual source element in the elevation plane

The conventional scanning techniques are not directly extendable for 3D real-time imaging because of the time necessary to acquire one volume. Using a linear array and synthetic transmit aperture, the volume can be scanned plane by plane. Up to 1000 planes per second can be scanned for a typical scan depth of 15 cm and speed of sound of 1540 m/s. Only 70 to 90 planes must be acquired per volume, making this method suitable for real-time 3D imaging without compromising the image quality. The resolution in the azimuthal plane has the quality of a dynamically focused image in transmit and receive. However, the resolution in the elevation plane is determined by the fixed mechanical elevation focus. This paper suggests to post-focus the RF lines from several adjacent planes in the elevation direction using the elevation focal point of the transducer as a virtual source element, in order to obtain dynamic focusing in the elevation plane. A 0.1 mm point scatterer was mounted in an agar block and scanned in a water bath. The transducer is a 64 elements linear array with a pitch of 209 /spl mu/m. The transducer height is 4 mm in the elevation plane and it is focused at 20 mm giving a F-number of 5. The point scatterer was positioned 96 mm from the transducer surface. The transducer was translated in the elevation direction from -13 to +13 mm over the scatterer at steps of 0.375 mm. Each of the 70 planes is scanned using synthetic transmit aperture with 8 emissions. The beam-formed RF lines from the planes are passed through a second beamformer, in which the fixed focal points in the elevation plane are treated as virtual sources of spherical waves. Synthetic aperture focusing is applied on them. The -6 dB resolution in the elevation plane is increased from 7 mm to 2 mm. This gives a uniform point spread function, since the resolution in the azimuthal plane is also 2 mm.

Application of different spatial sampling patterns for sparse array transducer design.

In the last few years, the efforts of many researchers have been focused on developing 3D real-time scanners. The use of 2D phased-array transducers makes it possible to steer the ultrasonic beam in all directions in the scanned volume. An unacceptably large amount of transducer channels (more than 4,000) must be used, if the conventional phased array transducers are extrapolated to the 2D case. To decrease the number of channels, sparse arrays with different aperture apodization functions in transmit and receive apertures have to be designed. The design is usually carried out in 1D, and then transferred to a 2D rectangular grid. In this paper, five different 2D array transducers have been considered and their performance was compared with respect to spatial and contrast resolution. An optimization of the element placement along the diagonals using vernier arrays is suggested. The simulation results of the ultrasound fields show a decrease in the grating-lobe level of 10 dB for the diagonally optimized 2D array transducers compared to the previously designed 2D arrays which did not consider the diagonals.

8A-3 System Architecture of an Experimental Synthetic Aperture Real-Time Ultrasound System

Synthetic aperture (SA) ultrasound imaging has many advantages in terms of flexibility and accuracy. One of the major drawbacks is, however, that no system exists, which can implement SA imaging in real time due to the very high number of calculations amounting to roughly 1 billion complex focused samples per second per receive channel. Real time imaging is a key aspect in ultrasound, and to truly demonstrate the many advantages of SA imaging, a system usable in the clinic should be made. The paper describes a system capable of real time SA B-mode and vector flow imaging. The synthetic aperture real-time ultrasound system (SARUS) has been developed through the last 2 years and can perform real time SA imaging and storage of RF channel data for multiple seconds. SARUS consists of a 1024 channel analog front-end and 64 identical digital boards. Each has 16 transmit channels and 16 receive channels both with a sampling frequency of 70 MHz/12 bits for arbitrary waveform emission and reception. The board holds live Virtex 4FX100 FPGAs, where one houses a PowerPC CPU used for control. The remaining four are used for generation of transmit signals, receive storage and matched Alter processing, and focusing and summing of data. Each FPGA can perform 80 billion multiplications/s and the full system can perform 25,600 billion multiplications/s. The FPGAs are connected through multiple 3.2 Gbit Rocket IO links, which makes it possible to send more than 1.6 GBytes/s of data between the FPGAs and between boards. The system can concurrently sample in 1024 channels, thus, generating 140 GBytes/s of data, which also can be processed in real time or stored. The system is controlled over a 1 Gbit/s Ethernet link to each digital board that runs Linux. The control and processing are divided into functional units that are accessed through an IP numbering scheme in a hierarchical order. A single controlling mechanism can, thus, be used to access the whole system from any PC. It is also possible for the controlling PowerPCs to access all other boards, which enables advanced adaptive imaging. The software is written in C++ and runs under Matlab for high level access to the system in a command structure similar to the Field II simulation program. This makes it possible for the user to specify imaging in very few lines of code and the set-up is fast due to the employment of the 64 PowerPCs in parallel. Focusing is done using a parametric beam former. Code synthesized for a Xilinx V4FX100 speed grade 11 FPGA can operate at a maximum clock frequency of 167.8 MHz producing 1 billion I and Q samples/second sufficient for real time SA imaging. The system is currently in production, and all boards have been laid out. VHDL and C++ code for the control has been written and the code for real time beamformation has been made and has obtained a sufficient performance for real-time imaging.