Phased Array in Real Time
Ultrasonic phased arrays have been used for many years for medical imaging, but until recently they have enjoyed limited success in nondestructive evaluation (NDE) due to cost and complexity. Diagnostic Sonar Ltd. (Silicon Glen, Scotland) has been using arrays for real-time imaging in aerospace NDE for three decades. This background has allowed the company to pioneer a major new capability-termed full raw data (FRD) collection and processing-that offers significant benefits to the customer but imposes new challenges for data acquisition hardware and software.
Conventional pulse-echo ultrasound array imaging involves phased excitation of a group of elements from the ultrasonic array, with the differential delays between elements matching the differing propagation paths to generate a beam with specific focus and direction. The analogous reception process involves a recombination of the signals from a similar group of elements after appropriate differential delays have been inserted. These delays can be varied with time so that the receive focus tracks the transmit pulse, a process termed dynamic focusing. The image is composed of a sequence of beams scanned through the area of interest. The operator perceives the performance to be real-time if frame rates exceed 15 hertz. However, area coverage requires significantly greater data rates, with 100 hertz rates routinely achieved.
In contrast, the FRD approach collects pulse-echo data from all transmit and receive element combinations and generates the resulting image by post-processing, permitting dynamic focus on transmit as well as receive for optimum resolution. This new technique also offers several unique processing possibilities, such as non-linear beam-forming and backscatter analysis.
Previous System LimitationsDiagnostic Sonar’s previous imaging system, FlawInspecta, is PCI-based and uses National Instruments’ (NI, Austin, TX) image acquisition cards with -LabVIEW to acquire the nonstandard video format of the ultrasound image data.
Diagnostic Sonar chose LabVIEW because it fulfilled the company’s primary requirements: rapid development of simple but powerful user-interfaces, easy control of both off-the-shelf and custom hardware, and real-time image acquisition and processing offered by the NI Vision Development Module. This hardware and software combination was well-suited to the conventional, real-time pulse-echo imaging application. However, the imaging card’s single channel limits the FRD area coverage rate. The only solution was parallel acquisition, but multiple imaging cards for parallel acquisition was cost-prohibitive.
The NI System SolutionThe solution was to migrate to the new NI PXI-5105 multichannel digitizer/PC-based oscilloscope, which was flexible enough to handle the image format Diagnostic Sonar needed and offered a simple software upgrade path. Each PXI-5105 acquires eight channels, allowing a single module to replace eight individual image acquisition cards and offering a major cost and size reduction. Diagnostic Sonar can implement a 32-channel acquisition system with just four modules. The PXI-5105 also offers a performance increase, with 12-bit resolution at 60 million samples per second compared to Diagnostic Sonar’s previous system’s 10-bit resolution at 40 million samples per second.
It was essential that all the acquisition cards were tightly synchronized, so Diagnostic Sonar used a NI PXI-7830R in the star trigger slot for the critical timing and control functions previously implemented in customized FPGAs.
With eight channels per module, high-resolution sampling, multirecord format and a standardized driver interface for LabVIEW, the flexibility and modularity of the PXI-5105 provided a rugged system with virtually unlimited channel capability so that Diagnostic Sonar could easily configure systems to match customers’ performance requirements and budgets.
FRD ChallengesThe FRD approach introduced two major challenges. The first was the order of magnitude increase in data, which could overwhelm the bus transfer capability. The onboard memory of the PXI-5105 provided a buffer so that the transfer constraint was limited by the average rate rather than peak rate. In the event that Diagnostic Sonar exceeds this rate when performing rapid acquisition over a small area, the onboard memory is sufficient to hold all the data for transfer once the acquisition is over.
The second challenge was the need to reconstruct the data into an image during acquisition. Diagnostic Sonar found the speed of the NI Development Vision Module sufficient to perform this basic imaging on the fly.
NI hardware and LabVIEW software had already proved their worth in the current range of Diagnostic Sonar -FlawInspecta ultrasound phased array imagers for nondestructive evaluation. Diagnostic Sonar’s new FRD acquisition approach offers many benefits to the customer, but the existing single-channel configuration would result in a significant drop in area coverage rate. The company needed multiple acquisition channels, but the additional cost and size of multiple single-channel cards was not feasible. The PXI-5105 offered a solution that was scalable to customer requirements and has a multi-record capability that provides a simple software migration path.