Thomas Carpenter

Time-division multiplexing for cable reduction in ultrasound imaging catheters

In ultrasound imaging catheter applications, gathering the data from multi-element transducer arrays is difficult as there is a restriction on cable count due to the diameter of the catheter. In such applications, CMUT-on-CMOS technology allows for 2D arrays with many elements to be designed and bonded directly onto CMOS circuitry. This allows for complex electronics to be placed at the tip of the catheter which leads to the possibility to include electronic multiplexing techniques to greatly reduce the cable count required for a large element array. Current approaches to cable reduction tend to rely on area and power hungry circuits to function, making them unsuitable for use in catheters. Furthermore the length requirement for catheters and lack of power available to on-chip cable drivers leads to limited signal strength at the receiver end. In this paper an alternative approach using Analogue Time Division Multiplexing (TDM) is presented, which addresses the cable restrictions of the catheter and, using a novel digital demultiplexing technique, allows for a reduction in the number of analogue signal processing stages required.

Direct Digital Demultiplexing of Analog TDM Signals for Cable Reduction in Ultrasound Imaging Catheters

In real-time catheter-based 3-D ultrasound imaging applications, gathering data from the transducer arrays is difficult, as there is a restriction on cable count due to the diameter of the catheter. Although area and power hungry multiplexing circuits integrated at the catheter tip are used in some applications, these are unsuitable for use in small sized catheters for applications, such as intracardiac imaging. Furthermore, the length requirement for catheters and limited power available to on-chip cable drivers leads to limited signal strength at the receiver end. In this paper, an alternative approach using analog time-division multiplexing (TDM) is presented, which addresses the cable restrictions of ultrasound catheters. A novel digital demultiplexing technique is also described, which allows for a reduction in the number of analog signal processing stages required. The TDM and digital demultiplexing schemes are demonstrated for an intracardiac imaging system that would operate in the 4- to 11-MHz range. A TDM integrated circuit (IC) with an 8:1 multiplexer is interfaced with a fast analog-to-digital converter (ADC) through a microcoaxial catheter cable bundle, and processed with a field-programmable gate array register-transfer level simulation. Input signals to the TDM IC are recovered with -40-dB crosstalk between the channels on the same microcoax, showing the feasibility of this system for ultrasound imaging applications.

Real-time imaging system using a 12-MHz forward-looking catheter with single chip CMUT-on-CMOS array

Forward looking (FL) imaging catheters would be an important tool for several intravascular ultrasound (IVUS) and intracardiac echocardiography (ICE) applications. Single chip capacitive micromachined ultrasonic transducer (CMUT) arrays fabricated on front-end CMOS electronics with simplified electrical interconnect have been previously developed for highly flexible and compact catheters. In this study, we present a custom built real time imaging system utilizing catheters with single chip CMUT-on-CMOS arrays and show initial imaging results. The fabricated array has a dual-ring structure with 64 transmit (Tx) and 56 receive (Rx) elements. The CMUT arrays fit on a 2.1 mm diameter circular region with all the required front-end electronics. The device operates at 12 MHz center frequency and has around 20 V collapse voltage. The single-chip system requires 13 external connections including 4 Rx channels and power lines. The electrical connections to micro cables in the catheter are made from the top side of the chip using polyimide flex tapes. The device is placed on a 6-Fr catheter shaft and secured with a medical grade silicon rubber. For real time data acquisition, we developed a custom design FPGA based imaging platform to generate digital control sequences for the chip and collect RF data from Rx outputs. We performed imaging experiments using wire phantoms immersed in water to test the real time imaging system. The system has the potential to generate images at 32 fps rate with the particular catheter. The overall system is fully functional and shows promising image performance.

Excitation of leaky lamb waves in cranial bone using a phased array transducer in a concave therapeutic configuration

Ultrasonic therapeutic transducers that consist of large numbers of unfocused, low power elements have begun to replace single, focused, high power elements. This allows the operator to use phased array techniques to change the focal position in the tissue during therapy. In transcranial therapy, this phased array configuration is essential to reduce local heating at the highly attenuating bone. Recently, Dual Mode Ultrasound Arrays (DMUAs) have been developed which leverage existing elements for imaging during therapy. DMUAs have the benefit of both the therapeutic and imaging systems being co-registered. This improves upon the existing approach of using a separate ultrasound system for guidance, as the acoustic beam path is the same for both. Unfortunately, the highly reflective nature of bone means that DMUAs have not been applied to transcranial therapy. However the recent near-field observation of lamb waves in cranial bone opens the possibility for DMUAs to be applied to a guided wave scan of the sk...

Performance of switched mode arbitrary excitation using Harmonic Reduction Pulse Width Modulation (HRPWM) in array imaging applications

Switched excitation allows the miniaturisation of excitation circuitry for transducer integrated front ends, high channel count and portable ultrasound systems. Harmonic Reduction Pulse Width Modulation (HRPWM) provides a method to design five level switched mode excitation signals with control of instantaneous amplitude, frequency and phase plus minimised third harmonics for advanced ultrasound applications. This paper details the application of HRPWM using commercial transmit front end integrated circuits and linear array transducers. The ability of HRPWM to control the pressure of the ultrasound wave is investigated. A full scale error between desired and measured pressure of 3.5% at 4.1 MHz is demonstrated. The temporal windowing of linear frequency modulated excitation signals using HRPWM is demonstrated. Pulse compression linear imaging of a tissue phantom is demonstrated where an improvement in the -20 dB axial resolution of a nylon mono-filament target from 2.14 mm using bipolar excitation to 1.88 mm using HRPWM is shown.

Acoustic microbubble trapping for enhanced targeted drug delivery

Systemic drug administration can result in low-efficiency therapeutic delivery (less than 1% to solid tumors) and potential side-effects by subjecting all tissues to toxic agents. Recently, targeted drug delivery and localized payload release has been a key goal of biomedical research. By combining ultrasound contrast agents (UCAs) with a therapeutic payload, promises potential for an enhanced therapeutic outcome. We have proposed a ultrasonic trap technique that is able to accumulate large populations of UCAs for enhanced localized release of a therapeutic agent. Sequences for UCA manipulation, fast imaging, and UCA fragmentation are interleaved with a same linear array medical ultrasound transducer. Once UCA clusters are halted in the trap within flow channels through custom-designed beam profiles, destruction pulses are transmitted to deliver therapeutic payload and simultaneous fast imaging depicts the flow dynamics. Verification of the simultaneous operation of trapping, destruction and imaging sequences is performed with a tissue-mimicking flow phantom and microscopy, respectively. Optical imaging clarifies UCAs can still be retained within the low pressure trap during the fast switch between different excitation schemes.Systemic drug administration can result in low-efficiency therapeutic delivery (less than 1% to solid tumors) and potential side-effects by subjecting all tissues to toxic agents. Recently, targeted drug delivery and localized payload release has been a key goal of biomedical research. By combining ultrasound contrast agents (UCAs) with a therapeutic payload, promises potential for an enhanced therapeutic outcome. We have proposed a ultrasonic trap technique that is able to accumulate large populations of UCAs for enhanced localized release of a therapeutic agent. Sequences for UCA manipulation, fast imaging, and UCA fragmentation are interleaved with a same linear array medical ultrasound transducer. Once UCA clusters are halted in the trap within flow channels through custom-designed beam profiles, destruction pulses are transmitted to deliver therapeutic payload and simultaneous fast imaging depicts the flow dynamics. Verification of the simultaneous operation of trapping, destruction and imaging seque...

An Adaptive Array Excitation Scheme for the Unidirectional Enhancement of Guided Waves.

Control over the direction of wave propagation allows an engineer to spatially locate defects. When imaging with longitudinal waves, time delays can be applied to each element of a phased array transducer to steer a beam. Because of the highly dispersive nature of guided waves, this beamsteering approach is sub-optimal. More appropriate time delays can be chosen to direct a guided wave if the dispersion relation of the material is known. Existing techniques however need a priori knowledge of material thickness and acoustic velocity, which changes as a function of temperature and strain. The scheme presented here does not require prior knowledge of the dispersion relation or properties of the specimen to direct a guided wave. Initially, a guided wave is generated using a single element of an array transducer. The acquired waveforms from the remaining elements are then processed and re-transmitted; constructively interfering with the wave as it travels across the spatial influence of the transducer. The scheme intrinsically compensates for the dispersion of the waves and thus can adapt to changes in material thickness and acoustic velocity. The proposed technique is demonstrated in simulation and experimentally. Dispersion curves from either side of the array are acquired to demonstrate the schemes ability to direct a guided wave in an aluminium plate. Results show that uni-directional enhancement is possible without a priori knowledge of the specimen using an arbitrary pitch array transducer. Experimental results show a 34 dB enhancement in one direction compared with the other.Control over the direction of wave propagation allows an engineer to spatially locate defects. When imaging with longitudinal waves, time delays can be applied to each element of a phased array transducer to steer a beam. Because of the highly dispersive nature of guided waves (GWs), this beamsteering approach is suboptimal. More appropriate time delays can be chosen to direct a GW if the dispersion relation of the material is known. Existing techniques, however, need a priori knowledge of material thickness and acoustic velocity, which change as a function of temperature and strain. The scheme presented here does not require prior knowledge of the dispersion relation or properties of the specimen to direct a GW. Initially, a GW is generated using a single element of an array transducer. The acquired waveforms from the remaining elements are then processed and retransmitted, constructively interfering with the wave as it travels across the spatial influence of the transducer. The scheme intrinsically compensates for the dispersion of the waves, and thus can adapt to changes in material thickness and acoustic velocity. The proposed technique is demonstrated in simulation and experimentally. Dispersion curves from either side of the array are acquired to demonstrate the scheme’s ability to direct a GW in an aluminum plate. The results show that unidirectional enhancement is possible without a priori knowledge of the specimen using an arbitrary pitch array transducer. The experimental results show a 34-dB enhancement in one direction compared with the other.

Simultaneous trapping and imaging of microbubbles at clinically relevant flow rates

Mechanisms for non-invasive target drug delivery using microbubbles and ultrasound have attracted growing interest. Microbubbles can be loaded with a therapeutic payload and tracked via ultrasound imaging to selectively release their payload at ultrasound-targeted locations. In this study, an ultrasonic trapping method is proposed for simultaneously imaging and controlling the location of microbubbles in flow by using acoustic radiation force. Targeted drug delivery methods are expected to benefit from the use of the ultrasonic trap, since trapping will increase the MB concentration at a desired location in human body. The ultrasonic trap was generated by using an ultrasound research system UARP II and a linear array transducer. The trap was designed asymmetrically to produces a weaker radiation force at the inlet of the trap to further facilitate microbubble entrance. A pulse sequence was generated that can switch between a long duration trapping waveform and short duration imaging waveform. High frame rate plane wave imaging was chosen for monitoring trapped microbubbles at 1 kHz. The working principle of the ultrasonic trap was explained and demonstrated in an ultrasound phantom by injecting SonoVue microbubbles flowing at 80 mL/min flow rate in a 3.5 mm diameter vessel.

Elevation resolution enhancement in 3D photoacoustic imaging using FDMAS beamforming

Photoacoustic imaging is a non-invasive and non-ionizing imaging technique that combines the spectral selectivity of laser excitation with the high resolution of ultrasound imaging. It is possible to identity the vascular structure of the cancerous tissue using this imaging modality. However, elevation and lateral resolution of photoacoustic imaging is usually poor for imaging target. In this study, three dimension filter delay multiply and sum beamforming technique (FDMAS(3D)) is used to improve the resolution and enhance the signal to noise ratio (SNR) of the 3D photoacoustic image that is created by using linear array transducer. This beamforming technique showed improvement in the elevation by 36% when its compared with three dimension delay and sum beamforming technique (DAS(3D)). In addition, it enhanced the SNR by 13 dB compared with DAS (3D).

In-situ measurement of transducer impedance using AFE active termination through analysis of ultrasound echoes

Measurement of transducer impedance in ultrasound systems indicates both the performance of a transducer and the presence of damaged elements and cables. Load impedance analysis is typically performed with dedicated test equipment and is a time consuming and thus expensive process, especially for high channel count systems. This paper proposes a method for the measurement of channel source impedance (transducer, coaxial cable and T/R switch) through the use of an integrated receiver analogue front end with configurable termination resistance. The proposed method is first demonstrated using a pitch-catch configuration using a separate source transducer to excite the transducer under test. Subsequently the method is demonstrated the Ultrasound Array Research Platform (UARP) and a linear array transducer in a pulse echo mode where the transducer impedance is determined through the analysis of two ultrasound echoes. The proposed method is especially suited to rapid transducer testing in the field, such as in a hospital, or where access to the transducer is not possible as in many industrial processes.