Prototyping Micro-Ultrasound Instruments for Medical Imaging

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"The extensive range of example code for NI hardware built into LabVIEW made it easy to integrate the new digitizer into the existing project. Readily available technical support and fast responses from an NI application engineer helped us move the project forward within our time limits."

- Florence Ndum, University of Dundee

The Challenge:
Upgrading the Diagnostic Sonar Ltd FI Toolbox for medical ultrasound imaging from imaging at 15 MHz to the micro-ultrasound regime of 50 MHz with multichannel transmit and receive.

The Solution:
Using a new NI PXI digitizer card to replace the original digitizer to attain the high sampling rate required to capture 50 MHz pulses accurately, and using the scalable nature of LabVIEW to easily integrate the new card by adapting the existing FI Toolbox code.

Author(s):
Florence Ndum - University of Dundee
Romans Poltarjonoks - University of Dundee
Dave Lines - Diagnostic Sonar Ltd
Sandy Cochran - University of Dundee
Holly Lay - University of Dundee

Background to Micro-Ultrasound Imaging

Ultrasound imaging is used extensively in the medical profession and accounts for approximately 13 percent of all preclinical imaging. Increasing the frequency of the ultrasound wave increases the resolution of resulting images. The micro-ultrasound regime is defined as frequencies above 15 MHz, which delivers a resolution below 50 micrometers. Because of the high resolution and rapid frame rate, the technique has been used in a wide variety of medical applications such as imaging blood flow for cardiovascular research and tumours for cancer research. It is an invaluable tool for medicine and can be used for non-destructive testing (NDT) of materials.

Diagnostic Sonar Ltd (DSL) designed and now manufactures the FI Toolbox, which is a flexible, modular, and scalable PC-based ultrasound system for medical, NDT, and sonar data collection and research. This high-performance platform can operate in a number of different ultrasound modes, including array imaging. The FI Toolbox multichannel digitizer limits the ultrasound frequency to around 15 MHz, which covers most conventional imaging applications but limits the use for micro ultrasound.

The current FI Toolbox transmitter design can generate programmable excitation pulses or sequences to extend control over the bandwidth, but is still fundamentally limited by the integrated high-voltage pulser hardware to an upper limit of around 20 MHz. In comparison, conventional micro-ultrasound hardware uses an impulse excitation method to drive single element transducers that are designed to resonate at much higher frequencies. The wide range of frequencies contained within that impulse means the transducer element produces an ultrasound pulse with a wide bandwidth and high resolution.

The Institute for Medical Science and Technology (IMSaT) at the University of Dundee is an interdisciplinary institute that brings together physics, engineering, and clinical and life sciences. One key area of research for IMSaT is medical ultrasound technology. This led us to collaborate with DSL to upgrade the FI Toolbox to the micro-ultrasound regime so it could generate and receive the required high-frequency signals.

The FI Toolbox

Diagnostic Sonar Ltd produces the FI Toolbox system as an ultrasound imaging and data acquisition platform, which is based on the NI PXI platform and custom FlexRIO FPGA cards.

Figure 1. The FI Toolbox System

Figure 2. Block Diagram of the FI Toolbox. Units in red were identified to be replaced after characterisation to achieve micro-ultrasound capability.

Figure 2 shows the full FI Toolbox system diagram. The modules that could be impacted by the proposed increase in frequency range are:

DSL32T transmit adapter module: This is a standard FlexRIO adapter module with additional pulser and high-voltage interfaces, providing 32 independent ultrasound excitation channels. It typically generates high-voltage pulses at the frequency specified by the user through a LabVIEW interface. The FlexRIO FPGA card generates the signals before they are amplified in the DSL32T transmitter and then sent either to the DSL32M or directly to the ultrasound array.

DSL32R receive adapter module: The module receives 32 channels of returning ultrasound signals from the array or DSL32M. The DSL32R is built into a standard FlexRIO adapter module housing and delivers high-voltage protection and optimised low noise preamplification.

NI 5752 digitizer adapter module: This attaches to a FlexRIO FPGA card and converts the analogue signals from the DSL32R into digital signals that are passed on to the FPGA for preprocessing. Each of the 32 channels connects through a programmable swept-gain amplifier to an anti-aliasing filter (AAF) and A/D converter. The AAFs have a programmable cut-off frequency of 7.5 MHz, 10 MHz, or 14 MHz and can perform 12-bit digitizing at up to 50 MS/s.

DSL32M multiplexer adapter module: This module has 32 independent high-voltage bidirectional analogue switches. It receives as inputs the 32 individual transmit signals from the DSL32T. It has a small signal frequency range of DC to 20 MHz.

Upgrade Analysis

We characterized the FI Toolbox and found its limitations with respect to micro-ultrasound imaging at 50 MHz. The receiver showed good performance up to 100 MHz; therefore, we could use it in micro-ultrasound applications. The transmitter generates pulses up to 24 MHz and +/-90 V amplitude. The NI 5752 delivers a maximum AAF cut-off of 14 MHz and the multiplexer works up to 20 MHz. Thus, we need to replace the DSL32T, DSL32M, and NI 5752 to achieve the new frequency requirements.

Figure 3. Upgraded Version of FI Toolbox

Proposed Solution

Based on the outcome of the characterisation of the FI Toolbox and after research on suitable hardware, we proposed a new system as illustrated in the block diagram of Figure 4.

 

Figure 4. Block Diagram of Single Channel Evaluation Block of Micro-Ultrasound Imaging System

Transmit End

We created a transmit board to prototype a replacement unit for the original transmit adapter module. We could then integrate this into the system design above to achieve micro-ultrasound pulse generation.

Designed for single channel operation, the transmit board’s requirement is to generate a single pulse at a desired frequency with amplitude between 30 V and 50 V. A pulse width modulation chip generates the pulse. This chip produces pulses with a width, and thus frequency, determined by an input value that the FlexRIO FPGA obtains. The amplitude of the generated pulse is 5 V, so a combination of an op amp and a power PMOS performs pulse amplification. We draw power for the PWM chip, the op amp rails, and the source of the PMOS from the backplane of the PXI chassis. We designed a voltage tripler circuit to boost the +12 V from the backplane to about 36 V to give the amplitude of the final output pulse from the transmit board. Figure 5 describes the unit.

Figure 5. Diagram of the Transmit Board

Receive End

Figure 6 shows the diagram for the new receive end of the FI Toolbox with the components necessary to function in the micro-ultrasound range. As seen during the characterization phase, the DSL32R has adequate specifications to operate at 100 MHz, so we used it within this prototype. The multiplexers in the FI Toolbox are limited to signals below 20 MHz, so the high-frequency multiplexers replaced them with the same functionality. The original NI 5752 digitizer could not meet the requirements, so we replaced it with the NI PXIe-5170R digitizer card.

Figure 6. Block Diagram for Receive End of FI Toolbox

The new NI PXIe-5170R card has eight simultaneously sampled 14-bit channels, which is a 2-bit improvement over the previous digitizer resolution. We need the DSL32Rs with a swept-gain control to match that in the NI 5752 to achieve a similar improvement in instantaneous dynamic range. The new FI Toolbox digitizer needs a minimum sampling rate of 100 MS/s to capture signals with frequencies up to 50 MHz. However, the signals could have frequency components above 50 MHz, so the high 250 MS/s sampling rate of the new digitizer can help us fully capture the frequency content of interest. The reconfigurable functionality of the NI PXIe-5170R allows for the multiplexing and beamforming, needed for the eventual imaging system, to be implemented within the FPGA.

Conclusion

Because we had already based the FI Toolbox system on LabVIEW and NI hardware, we could easily implement the new NI PXIe-5170R digitizer due to the modular nature of the PXI platform. The extensive range of example code for NI hardware built into LabVIEW made it easy to integrate the new digitizer into the existing project. Readily available technical support and fast responses from an NI application engineer helped us move the project forward within our time limits.

We developed the system proposed here for micro-ultrasound imaging using the FI Toolbox as independent transmit and receive sections. The next step is to improve the current design for the transmit unit prototype so that it integrates smoothly with the FlexRIO FPGA and the PXI backplane to produce pulses reliably at the desired micro-ultrasound frequency and amplitude. We then need to integrate it with the receive end section and extend the whole system to multichannel operation by using high-frequency multiplexers on transmit and receive. Finally, we can test it with transducer elements and arrays to get full images. After testing, researchers can use it to probe brand new materials or to increase their understanding of life-threatening diseases.

Author Information:
Florence Ndum
University of Dundee
Dundee
United Kingdom

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