Designing a Medical Ultrasound Test System with NI Hardware and Software

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"National Instruments serves as a single source for support, services, and software development tools for this new test platform. "

- Max Hovila, SonoSite

The Challenge:
Replacing an aging functional test platform based on VXI architecture to automate the functional testing of medical ultrasound systems for transmitter, receiver, and frequency response as well as distortion testing.

The Solution:
Using NI LabWindows™/CVI software and the NI PXI hardware platform to design a modern, precise, and portable solution that meets and exceeds the specifications required for our functional test platform.

Author(s):
Max Hovila - SonoSite

Test System Instrumentation

We developed our previous test system on the VXI hardware platform with NI LabWindows/CVI using Virtual Instrument Software Architecture (VISA) instrument drivers. Our test executive program, a Visual C++ application, communicated using Ethernet control to the test software application in LabWindows/CVI.

We studied the required instrument specifications from the old test platform and found that National Instruments offered PXI instrumentation that meets and exceeds the specifications that we required for older VXI instrumentation testing devices.

Our new test system includes a digitizer, an arbitrary waveform generator, and multiple switching modules to automate ultrasound system testing for production testing at SonoSite and our overseas supplier.

It took approximately four months of software development, test integration, and test system validation to replace the old test system on three of our existing SonoSite products with the new PXI test system. We are implementing the five remaining products within the next three months.

The ultrasound system transmitter test measures unit pulses generated from all channels of the ultrasound system. The NI PXI-5152 digitizer acquires the transmitted waveforms, transfers the transmitter pulse data to the host PC for processing, and limits verification for the transmitter pulse area, rise, fall, and delay characteristics.

To test the receiver, we use the NI PXI-5412 waveform generator to create an input signal to the ultrasound system and verify that all channels in the receiver path of the ultrasound system perform within acceptable limits for receiver gain, frequency response, distortion, and noise performance.

To automate test, the NI PXI-2593 RF multiplexer performs all the switching requirements for the transmitter and receiver testing and completely automates hands-free testing once testing has begun.

While older SonoSite products require more test point measurements, the new test system includes an NI PXI-2527 general-purpose multiplexer for measuring several DC voltages and an NI PXI-2566 relay module to switch in a load circuit to the ultrasound system to test the system battery charger circuit.

Software Development and Integration

Software conversion from the old platform to the new PXI platform was easy because most of our code from the VXI software implementation was reusable and applicable to the new platform. We had to replace only the VISA driver calls with the newer instrument function calls.

The test tools are virtually identical in their implementation when performing functional testing. Therefore, production testing for the newer system required minimal retraining because the test software controls and user interface to the operators did not change for this new test platform.

The functional test software developed with LabWindows/CVI includes several virtual UI panels for test development and debugging/troubleshooting the ultrasound hardware. The test software also runs in production mode, which suppresses any user interface resource (UIR) panel pop-ups and provides full automated test execution.

During software development, we used the virtual UI panels’ graphing capabilities and LabWindows/CVI advanced math libraries to analyze fast Fourier transform performance; examine output amplitude, harmonic distortion, and receiver noise testing; and analyze detailed test measurement performance of SonoSite ultrasound systems.

Cost Savings

We evaluated continued production testing with the existing VXI test platform versus upgrading to a newer test platform. Our annual cost to continue using the old test platform and replace instruments with devices from the used test equipment market was approximately $30,000 USD. This system did not perform to specifications and failed instrument calibrations over time. Upgrades to the PXI test platform showed return on investment in two and a half years. National Instruments serves as a single source for support, services, and software development tools for this new test platform.

Additional Benefits

Solving measured result variation among multiple test stations – One big problem with the older VXI test platform was inconsistent digitizer performance across several stations for measuring transmitter pulses. Some stations measured high for pulse measurements while other stations measured low. This resulted in undesirable performance when different test stations reported inconsistent pass/fail results.

We compared four PXI-5152 digitizers on the new PXI platform and found more consistent results within the same transmitter pulse measurements, and we expect consistent performance with the test stations on the new platform.

Figure 2 shows two overlaid transmitter pulse waveforms for two PXI digitizers. The red and yellow traces on the graph represent one transmitter pulse sweep for each digitizer. All four digitizers displayed this equivalent measured performance, and results were very close in their measured values.

The older VXI digitizers produced variation in the pulse characteristics and the digitizers did not measure consistently from station to station. Figure 3 shows the transmitter pulse measurements on one VXI station compared to another that produced variation in its measured results (see the two sweeps: one in red and the second in yellow). The amplitudes clearly mark different measurements causing different measured results. One digitizer appears to have noise on its signal line that is presumably due to the age of the instrument.

Portability – The new equipment is portable and easy to ship to the supplier. Individual instruments are compact and require minimal storage.

Calibration services – We can obtain quick support for our new architecture calibration services because both our overseas supplier and a National Instruments calibration facility are located in Singapore.

Eliminating some special test equipment (STE) – The PXI instrumentation specifications eliminate some of the older custom test equipment and require less cabling and custom equipment maintenance such as an external bandpass filter circuit. The PXI-5412 waveform generator has sufficiently low harmonic levels to use for distortion testing without any of the special circuitry that we previously required for the older test platform.

Author Information:
Max Hovila
SonoSite
21919 30th Dr. SE
Bothell, WA 98021
Tel: (425) 951-1200
max.hovila@sonosite.com

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