John Castellucci

Real-time Three-dimensional Echocardiography for Determining Right Ventricular Stroke Volume in an Animal Model of Chronic Right Ventricular Volume Overload

BACKGROUND The lack of a suitable noninvasive method for assessing right ventricular (RV) volume and function has been a major deficiency of two-dimensional (2D) echocardiography. The aim of our animal study was to test a new real-time three-dimensional (3D) echo imaging system for evaluating RV stroke volumes. METHODS AND RESULTS Three to 6 months before hemodynamic and 3D ultrasonic study, the pulmonary valve was excised from 6 sheep (31 to 59 kg) to induce RV volume overload. At the subsequent session, a total of 14 different steady-state hemodynamic conditions were studied. Electromagnetic (EM) flow probes were used for obtaining aortic and pulmonic flows. A unique phased-array volumetric 3D imaging system developed at the Duke University Center for Emerging Cardiovascular Technology was used for ultrasonic imaging. Real-time volumetric images of the RV were digitally stored, and RV stroke volumes were determined by use of parallel slices of the 3D RV data set and subtraction of end-systolic cavity volumes from end-diastolic cavity volumes. Multiple regression analyses showed a good correlation and agreement between the EM-obtained RV stroke volumes (range, 16 to 42 mL/beat) and those obtained by the new real-time 3D method (r=0.80; mean difference, -2.7+/-6.4 mL/beat). CONCLUSIONS The real-time 3D system provided good estimation of strictly quantified reference RV stroke volumes, suggesting an important application of this new 3D method.

Theory and operation of 2-D array piezoelectric micromachined ultrasound transducers

Piezoelectric micromachined ultrasound transducers (pMUTs) are a new approach for the construction of 2-D arrays for forward-looking 3-D intravascular (IVUS) and intracardiac (ICE) imaging. Two-dimensional pMUT test arrays containing 25 elements (5 times 5 arrays) were bulk micromachined in silicon substrates. The devices consisted of lead zirconate titanate (PZT) thin film membranes formed by deep reactive ion etching of the silicon substrate. Element widths ranged from 50 to 200 mum with pitch from 100 to 300 mum. Acoustic transmit properties were measured in de-ionized water with a calibrated hydrophone placed at a range of 20 mm. Measured transmit frequencies for the pMUT elements ranged from 4 to 13 MHz, and mode of vibration differed for the various element sizes. Element capacitance varied from 30 to over 400 pF depending on element size and PZT thickness. Smaller element sizes generally produced higher acoustic transmit output as well as higher frequency than larger elements. Thicker PZT layers also produced higher transmit output per unit electric field applied. Due to flexure mode operation above the PZT coercive voltage, transmit output increased nonlinearly with increased drive voltage. The pMUT arrays were attached directly to the Duke University T5 phased array scanner to produce real-time pulse-echo B-mode images with the 2-D pMUT arrays.

In Vivo Real-Time 3-D Intracardiac Echo Using PMUT Arrays

Piezoelectric micromachined ultrasound transducer (PMUT) matrix arrays were fabricated containing novel through-silicon interconnects and integrated into intracardiac catheters for in vivo real-time 3-D imaging. PMUT arrays with rectangular apertures containing 256 and 512 active elements were fabricated and operated at 5 MHz. The arrays were bulk micromachined in silicon-on-insulator substrates, and contained flexural unimorph membranes comprising the device silicon, lead zirconate titanate (PZT), and electrode layers. Through-silicon interconnects were fabricated by depositing a thin-film conformal copper layer in the bulk micromachined via under each PMUT membrane and photolithographically patterning this copper layer on the back of the substrate to facilitate contact with the individually addressable matrix array elements. Cable assemblies containing insulated 45-AWG copper wires and a termination silicon substrate were thermocompression bonded to the PMUT substrate for signal wire interconnection to the PMUT array. Side-viewing 14-Fr catheters were fabricated and introduced through the femoral vein in an adult porcine model. Real-time 3-D images were acquired from the right atrium using a prototype ultrasound scanner. Full 60° × 60° volume sectors were obtained with penetration depth of 8 to 10 cm at frame rates of 26 to 31 volumes per second.

5I-4 Piezoelectric Micromachined Ultrasound Transducer (pMUT) Arrays for 3D Imaging Probes

Piezoelectric micromachined ultrasound transducers (pMUTs) have been fabricated and characterized in this work as a new approach for manufacture of 2D arrays for forward-looking 3D intravascular (IVUS) and intracardiac (ICE) imaging. Two-dimensional pMUT arrays containing 25 elements (5 times 5 arrays) were bulk micromachined in silicon substrates. The devices consisted of suspended PZT thin film membranes formed by deep reactive ion etching. Membrane widths ranged from 50 to 200 mum, with element pitch from 100 to 300 mum. Acoustic transmit properties were measured in de-ionized water with a calibrated hydrophone placed at a range of 20 mm. Measured transmit frequencies for the pMUT elements ranged from 4-11 MHz. Element capacitance for pMUTs ranged from 30 to over 400 pF depending on element size. Smaller element sizes generally produced higher acoustic transmit output than larger elements, and thicker PZT layers also produced higher output per unit electric field applied. Due to flexure mode operation above the PZT coercive voltage, transmit output also increased nonlinearly with increased drive voltage. The pMUT arrays were attached directly to the system amplifiers of the Duke University T5 Phased Array Scanner by a modified cable assembly in order to produce real-time pulse-echo B-mode images with the 2D pMUT arrays

Improved pulse-echo imaging performance for flexure-mode pMUT arrays

Piezoelectric micromachined ultrasound transducers (pMUTs) are potential candidates for catheter-based ultrasound phased arrays. pMUTs consist of lead zirconate titanate (PZT) thin film membranes formed on silicon substrates and are operated in flexure mode by driving the PZT film above its coercive field to induce flextensional motion. The fundamental operation of pMUT devices has been demonstrated; however, pulse-echo imaging has been limited to date. The objective of this work was to optimize transducer design for improved pulse-echo imaging performance. Flexure mode operation was optimized by (1) increasing transmit voltage above the PZT coercive field to induce ferroelectric domain switching, and (2) using partial cycle transmit pulses to increase the polarization in the PZT thin film and increase receive signal. As a result, pulse-echo images of tissue were obtained. 1-D arrays operating at 5 MHz were capable of resolving targets in a commercial tissue phantom as well as human anatomy. Real-time 3-D imaging was also demonstrated using 2-D arrays at 5 and 12.5 MHz. These results suggest that pMUTs have sufficient performance for application in ultrasound imaging with frequency range suitable for catheter-based phased-array transducers.

Interactive volume rendering of real-time three-dimensional ultrasound images

Real-time, three-dimensional (RT3D) ultrasound allows video frame rate volumetric imaging. The ability to acquire full three-dimensional (3-D) image data in real-time is particularly helpful for applications such as cardiac imaging, which require visualization of complex and dynamic 3-D anatomy. Volume rendering provides a method for intuitive graphical display of the 3-D image data, but capturing the RT3D echo data and performing the necessary processing to generate a volumetric image in real time poses a significant technical challenge. We present a data capture and rendering implementation that uses off-the-shelf components to real-time volume render RT3D ultrasound images. Our approach allowed live, interactive volume rendering of RT3D ultrasound scans

C-Mode Real Time Tomographic Reflection for a Matrix Array Ultrasound Sonic Flashlight

RATIONALE AND OBJECTIVES Real-time tomographic reflection (RTTR) permits in situ visualization of tomographic images so that natural hand-eye coordination can be used directly during invasive procedures. The method uses a half-silvered mirror to merge the visual outer surface of the patient with a simultaneous scan of the patients interior without requiring a head-mounted display or tracking. A viewpoint-independent virtual image is reflected precisely into its actual location. When applied to ultrasound, we call the resulting RTTR device the sonic flashlight. We previously implemented the sonic flashlight using conventional two-dimensional ultrasound scanners that produce B-mode slices. Real-time three-dimensional (RT3D) ultrasound scanners recently have been developed that permit RTTR to be applied to slices with other orientations, including C-mode (parallel to the face of the transducer). Such slice orientation may offer advantages for image-guided intervention. MATERIALS AND METHODS Using a prototype scanner developed at Duke University (Durham, NC) with a matrix array that electronically steers an ultrasound beam at high speed in 3D, we implemented a sonic flashlight capable of displaying C-mode images in situ in real time. RESULTS We present the first images from the C-mode sonic flashlight, showing bones in the hand and the cardiac ventricles. CONCLUSION The extension of RTTR to matrix array RT3D ultrasound offers the ability to visualize in situ slices other than the conventional B-mode slice, including C-mode slices parallel to the face of the transducer. This orientation may provide a broader target, facilitating certain interventional procedures. Future work is discussed, including display of slices with arbitrary orientation and use of a holographic optical element instead of a mirror.

64-channel ultrasound transducer amplifier

In this paper we present a 64-channel ultrasound preamplifier device that is used to amplify and filter pulsed echo transducer signals sourced from a real time three dimensional (RT3DU) non-invasive ultrasound system. Schematics, simulation data, and layout for each of the broadband sub-circuit macros are described which include a high gain preamplifier, a linear output buffer, and bias circuits. This device was implemented using AMI Semiconductors, 0.5 /spl mu/m, double poly, triple level metal CMOS technology. The device floorplan, circuit schematics and layout, specifications and measured test data are presented.

Live high-frame-rate echocardiography

We describe an advanced real-time high-speed echocardiographic system with live display while scanning. Images are acquired at rates up to 1000 per second for adult cardiac applications and are stored in computer memory. Images may be played back in slow motion or frame by frame to analyze cardiac motion at the millisecond time scale. Images are acquired using the T5 Duke University Phased Array Scanner that allows 32:1 hardware parallel processing in receive and uses a defocused transmit beam. Clinical scans of 70 patients at rates of 240 to 1000 fps showed adequate image quality for diagnostic purpose. We anticipate that high temporal resolution cardiac images will enable the realization of more accurate and new quantitative descriptors of cardiac function in disease and health.

11F-3 Performance of Flexure-Mode pMUT 2D Arrays

We report on the performance characteristics of 2D pMUT arrays. 2D arrays suitable for diagnostic ultrasound imaging containing 81 elements were bulk micromachined in silicon substrates. The devices consisted of suspended PZT thin film membranes formed by deep reactive ion etching. Membrane widths ranged from 50 to 200 mum, with element pitch from 100 to 300 mum. Center frequency, fractional bandwidth, acoustic pressure output and pulse-echo insertion loss were measured in a de-ionized water tank. Pulse-echo images were made using the Duke University T5 Phased Array Scanner. Measured resonance frequencies for the flexure-mode pMUT elements in transmit mode were in the range of 4-13 MHz, and different vibrational modes were observed. Larger membrane widths tended to operate with standard plate-mode vibration, whereas smaller membranes were relatively thickness independent and operated in a transverse resonant mode proportional to membrane length. Acoustic pressure output was 322 Pa/V for a 75 mum pMUT single element measured at 8.6 MHz by a calibrated hydrophone at 22 mm range. This compared to 382 Pa/V for a commercial 2D bulk ceramic array with 350 mum elements operating at 3.5 MHz at the same distance. Pulse-echo insertion loss measured by reflection from an aluminum block at 22 mm range in the water tank was -106 to -124 dB per element for 75 mum elements. It was observed that the pMUTs were more efficient in transmit than receive mode, as transmit insertion loss was -38 dB per element. Receive sensitivity was increased by applying a dc bias to the pMUT elements prior to receiving acoustic reflection. Because flexure-mode pMUTs operate above the coercive voltage in transmit mode, receive bias increased the polarization of the piezoelectric film prior to transducer receive. Receive sensitivity increased by as much as 18 dB with up to 20 V dc bias. Pulse- echo B-mode images were produced with 9x9 element (1.1 mm x 1.1 mm) pMUT arrays operating at 7 MHz, producing axial resolution of 1 mm at 10 mm depth in a tissue phantom.