Real-time 3-D ultrasound guidance of interventional devices

We have previously developed 2-D array transducers for many real-time volumetric imaging applications. These applications include transducers operating up to 7 MHz for transthoracic imaging, up to 15 MHz for intracardiac echocardiography (ICE), 5 MHz for transesophageal echocardiography (TEE) and intracranial imaging, and 7 MHz for laparoscopic ultrasound imaging (LUS). Now we have developed a new generation of miniature ring-array transducers integrated into the catheter deployment kits of interventional devices to enable real-time 3-D ultrasound scanning for improved guidance of minimally invasive procedures. We have constructed 3 new ring transducers. The first consists of 54 elements operating at 5 MHz. Typical measured transducer element bandwidth was 25%, and the 50 Ohm round trip insertion loss was -65 dB. Average nearest neighbor cross talk was -23.8 dB. The second is a prototype 108-element transducer operating at 5 MHz. The third is a prototype 108-element ring array with a transducer center frequency of 8.9 MHz and a -6 dB bandwidth of 25%. All transducers were integrated with an 8.5 French catheter sheath of a Cook Medical, Inc. vena cava filter deployment device.     

Real-time, 3-D ultrasound with multiple transducer arrays

Modifications were made to a commercial real-time, three-dimensional (3-D) ultrasound system for near simultaneous 3-D scanning with two matrix array transducers. As a first illustration, a transducer cable assembly was modified to incorporate two independent, 3-D intra-cardiac echo catheters, a 7 Fr (2.3 mm O.D.) side scanning catheter and a 14 Fr (4.7 mm O.D) forward viewing catheter with accessory port, each catheter using 85 channels operating at 5 MHz. For applications in treatment of atrial fibrillation, the goal is to place the sideviewing catheter within the coronary sinus to view the whole left atrium, including a pulmonary vein. Meanwhile, the forward-viewing catheter inserted within the left atrium is directed toward the ostium of a pulmonary vein for therapy using the integrated accessory port. Using preloaded, phasing data, the scanner switches between catheters automatically, at the push of a button, with a delay of about 1 second, so that the clinician can view the therapy catheter with the coronary sinus catheter and vice versa. Preliminary imaging studies in a tissue phantom and in vivo show that our system successfully guided the forward-viewing catheter toward a target while being imaged with the sideviewing catheter. The forward-viewing catheter then was activated to monitor the target while we mimicked therapy delivery. In the future, the system will switch between 3-D probes on a line-by-line basis and display both volumes simultaneously.     

Intracardiac catheter 2-D arrays on a silicon substrate

The design, fabrication, and characterization of a 7 MHz, two-dimensional (2-D) array transducer built on a silicon substrate is described. The array fits inside a 9-French (2.9 mm O.D.) catheter for use in real-time intracardiac volumetric imaging. The -6 dB fractional bandwidth of the transducer is 30%, the 50 /spl Omega/ pitch-catch insertion loss is 78 dB, and the interelement crosstalk is -25 dB. Realtime volumetric images in phantoms and in-vitro images of a sheep heart have been acquired yielding measured spatial resolution of 2 mm at a depth of 1 cm. The cardiac structures imaged include ventricular chambers, interventricular septum, mitral and tricuspid valves and real-time 3-D rendered volumes of the tricuspid valve in the open and closed position.     

3-D ultrasound imaging using a forward-looking CMUT ring array for intravascular/intracardiac applications

Forward-viewing ring arrays can enable new applications in intravascular and intracardiac ultrasound. This work presents compelling, full-synthetic, phased-array volumetric images from a forward-viewing capacitive micromachined ultrasonic transducer (CMUT) ring array wire bonded to a custom integrated circuit front end. The CMUT ring array has a diameter of 2 mm and 64 elements each 100 microm x 100 microm in size. In conventional mode, echo signals received from a plane reflector at 5 mm had 70% fractional bandwidth around a center frequency of 8.3 MHz. In collapse mode, 69% fractional bandwidth is measured around 19 MHz. Measured signal-to-noise ratio (SNR) of the echo averaged 16 times was 29 dB for conventional operation and 35 dB for collapse mode. B-scans were generated of a target consisting of steel wires 0.3 mm in diameter to determine resolution performance. The 6 dB axial and lateral resolutions for the B-scan of the wire target are 189 microm and 0.112 radians for 8 MHz, and 78 microm and 0.051 radians for 19 MHz. A reduced firing set suitable for real-time, intravascular applications was generated and shown to produce acceptable images. Rendered three-dimensional (3-D) images of a Palmaz-Schatz stent also are shown, demonstrating that the imaging quality is sufficient for practical applications.     

Finite-element analysis of temperature rise and lesion formation from catheter ultrasound ablation transducers

A model using finite-element analysis (FEA) has been developed to calculate the temperature rise in tissue from intracardiac ultrasound ablation catheters and to predict if this temperature rise is adequate for producing a lesion in the tissue. In the model, acoustic fields are simulated with Field II, and heat transfer is modeled with an FEA program. To validate the model, we compare its results to experimental results from an integrated, real-time three-dimensional (3-D) ultrasound imaging and ultrasound ablation catheter. The ultrasound ablation transducer is a ring transmitting at 10 MHz capable of producing an acoustic intensity of 16 W/cm2. It was used to ablate four lesions in tissue, and temperature rise as a function of time was monitored by embedded thermocouples. The average absolute difference between final temperatures predicted by FEA and those measured is 1.95 +/- 0.72 degrees C. Additionally, model and experimental lesion size are in good agreement. The model then is used to design a new ultrasound catheter with a 7.5 MHz linear phased array for ablation. Eight designs are modeled, and acoustic intensity, temperature rise, and ablation ability are compared.     

An integrated circuit with transmit beamforming flip-chip bonded to a 2-D CMUT array for 3-D ultrasound imaging

State-of-the-art 3-D medical ultrasound imaging requires transmitting and receiving ultrasound using a 2-D array of ultrasound transducers with hundreds or thousands of elements. A tight combination of the transducer array with integrated circuitry eliminates bulky cables connecting the elements of the transducer array to a separate system of electronics. Furthermore, preamplifiers located close to the array can lead to improved receive sensitivity. A combined IC and transducer array can lead to a portable, high-performance, and inexpensive 3-D ultrasound imaging system. This paper presents an IC flip-chip bonded to a 16 x 16-element capacitive micromachined ultrasonic transducer (CMUT) array for 3-D ultrasound imaging. The IC includes a transmit beamformer that generates 25-V unipolar pulses with programmable focusing delays to 224 of the 256 transducer elements. One-shot circuits allow adjustment of the pulse widths for different ultrasound transducer center frequencies. For receiving reflected ultrasound signals, the IC uses the 32-elements along the array diagonals. The IC provides each receiving element with a low-noise 25-MHz-bandwidth transimpedance amplifier. Using a field-programmable gate array (FPGA) clocked at 100 MHz to operate the IC, the IC generated properly timed transmit pulses with 5-ns accuracy. With the IC flip-chip bonded to a CMUT array, we show that the IC can produce steered and focused ultrasound beams. We present 2-D and 3-D images of a wire phantom and 2-D orthogonal cross-sectional images (Bscans) of a latex heart phantom.     

Characterization of a broadband all-optical ultrasound transducer-from optical and acoustical properties to imaging

A broadband all-optical ultrasound transducer has been designed, fabricated, and evaluated for high- frequency ultrasound imaging. The device consists of a 2-D gold nanostructure imprinted on top of a glass substrate, followed by a 3 microm PDMS layer and a 30 nm gold layer. A laser pulse at the resonance wavelength of the gold nanostructure is focused onto the surface for ultrasound generation, while the gold nanostructure, together with the 30 nm thick gold layer and the PDMS layer in between, forms an etalon for ultrasound detection, which uses a CW laser at a wavelength far from resonance as the probing beam. The center frequency of a pulse-echo signal recorded in the far field of the transducer is 40 MHz with -6 dB bandwidth of 57 MHz. The signal to noise ratio (SNR) from a 70 microm diameter transmit element combined with a 20 microm diameter receive element probing a near perfect reflector positioned 1.5 mm from the transducer surface is more than 10 dB and has the potential to be improved by at least another 40 dB. A high-frequency ultrasound array has been emulated using multiple measurements from the transducer while mechanically scanning an imaging target. Characterization of the device's optical and acoustical properties, as well as preliminary imaging results, strongly suggest that all-optical ultrasound transducers can be used to build high-frequency arrays for real-time high-resolution ultrasound imaging.     

A miniaturized catheter 2-D array for real-time, 3-D intracardiac echocardiography

The design, fabrication, and characterization of a 112 channel, 5 MHz, two-dimensional (2-D) array transducer constructed on a six layer flexible polyimide interconnect circuit is described. The transducer was mounted in a 7 Fr (2.33 mm outside diameter) catheter for use in real-time intracardiac volumetric imaging. Two transducers were constructed: one with a single silver epoxy matching layer and the other without a matching layer. The center frequency and -6 dB fractional bandwidth of the transducer with a matching layer were 4.9 MHz and 31%, respectively. The 50 omega pitch-catch insertion loss was 80 dB, and the typical interelement crosstalk was -30 dB. The final element yield was greater than 97% for both transducers. The transducers were used to acquire real-time, 3-D images in an in vivo sheep model. We present in vivo images of cardiac anatomy obtained from within the coronary sinus, including the left and right atria, aorta, coronary arteries, and pulmonary veins. We also present images showing the manipulation of a separate electrophysiological catheter into the coronary sinus.     

Annular phased-array high-intensity focused ultrasound device for image-guided therapy of uterine fibroids

An ultrasound (US), image-guided high-intensity focused ultrasound (HIFU) device was developed for noninvasive ablation of uterine fibroids. The HIFU device was an annular phased array, with a focal depth range of 30-60 mm, a natural focus of 50 mm, and a resonant frequency of 3 MHz. The in-house control software was developed to operate the HIFU electronics drive system for inducing tissue coagulation at different distances from the array. A novel imaging algorithm was developed to minimize the HIFU-induced noise in the US images. The device was able to produce lesions in bovine serum albumin-embedded polyacrylamide gels and excised pig liver. The lesions could be seen on the US images as hyperechoic regions. Depths ranging from 30 to 60 mm were sonicated at acoustic intensities of 4100 and 6100 W/cm2 for 15 s each, with the latter producing average lesion volumes at least 63% larger than the former. Tissue sonication patterns that began distal to the transducer produced longer lesions than those that began proximally. The variation in lesion dimensions indicates the possible development of HIFU protocols that increase HIFU throughput and shorten tumor treatment times.     

Noninvasive transesophageal cardiac thermal ablation using a 2-D focused, ultrasound phased array: a simulation study

This simulation study proposes a noninvasive, transesophageal cardiac-thermal ablation using a planar ultrasound phased array (1 MHz, 60 x 10 mm2, 0.525 mm interelement spacing, 114 x 20 elements). Thirty-nine foci in cardiac muscle were defined at 20, 40, and 60-mm distances and at various angles from the transducer surface to simulate the accessible posterior left atrial wall through the esophageal wall window. The ultrasound pressure distribution and the resulting thermal effect in a volume of 60 x 80 x 80 mm3, including esophagus and cardiac muscle, were simulated for each focus. For 1, 10, and 20-s sonications with 60 degrees C and 70 degrees C peak temperatures in cardiac muscle and without thermal damage in esophageal wall, the transducer acoustic powers were 105-727, 28-117, 21-79 W and 151-1044, 40-167, 30-114 W, respectively. The simulated lesions (thermal dose in equivalent minutes at 43 degrees C > or = 240 minutes) at these foci had lengths of 1-6, 3-11, 3-13 mm and 3-15, 5-19, 6-23 mm, respectively, and widths of 1-4, 2-7, 3-9 mm and 3-9, 4-13, 4-17 mm, respectively. As a first step toward feasibility, controllable tissue coagulation in cardiac tissue without damage to the esophagus was demonstrated numerically.     

A 256 x 256 2-D array transducer with row-column addressing for 3-D rectilinear imaging

We present simulation and experimental results from a 5-MHz, 256times256 2-D (65536 elements, 38.4times38.4 mm) 2-D array transducer with row-column addressing. The main benefits of this design are a reduced number of interconnects, a modified transmit/receive switching scheme with a simple diode circuit, and an ability to perform volumetric imaging of targets near the transducer with transmit beamforming in azimuth and receive beamforming in elevation. The final dimensions of the transducer were 38.4 mm times 38.4 mm times 300 mum. After a row-column transducer was prototyped, the series resonance impedance was 104 Omega at 5.4 MHz. The measured -6 dB fractional bandwidth was 53% with a center frequency of 5.3 MHz. The SNR at the transmit focus was measured to be 30 dB. At 5 MHz, the average nearest neighbor crosstalk was -25 dB. In this paper, we present 3-D images of both 5 pairs of nylon wires embedded in a clear gelatin phantom and an 8 mm diameter cylindrical anechoic cyst phantom acquired from a 256 times 256 2-D array transducer made from a 1-3 composite. We display the azimuth and elevation B-scans as well as the C-scan for each image. The cross-section of the wires is visible in the azimuth B-scan, and the long axes can be seen in the elevation B-scan and C-scans. The pair of wires with 1-mm axial separation is discernible in the elevational B-scan. When a single wire from the wire target phantom was used, the measured lateral beamwidth was 0.68 mm and 0.70 mm at 30 mm depth in transmit beamforming and receive beamforming, respectively, compared with the simulated beamwidth of 0.55 mm. The cross-section of the cyst is visible in the azimuth B-scan whereas the long axes can be seen as a rectangle in the elevation B-scan and C-scans.     

Two-dimensional ultrasound receive array using an angle-tuned Fabry-Perot polymer film sensor for transducer field characterization and transmission ultrasound imaging

A 2-D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically induced changes in the optical thickness of a thin polymer film acting as a Fabry-Perot sensing interferometer (FPI). By illuminating the sensor with a large-area laser beam and mechanically scanning a photodiode across the reflected output beam, while using a novel angle-tuned phase bias control system to optimally set the FPI working point, a notional 2-D ultrasound array was synthesized. To demonstrate the concept, 1-D and 2-D ultrasound field distributions produced by planar 3.5-MHz and focused 5-MHz PZT ultrasound transducers were mapped. The system was also evaluated by performing transmission ultrasound imaging of a spatially calibrated target. The "array" aperture, defined by the dimensions of the incident optical field, was elliptical, of dimensions 16 x 12 mm and spatially sampled in steps of 0.1 mm or 0.2 mm. Element sizes, defined by the photodiode aperture, of 0.8, 0.4, and 0.2 mm were variously used for these experiments. Two types of sensor were evaluated. One was a discrete 75-microm-thick polyethylene terephthalate FPI bonded to a polymer backing stub which had a wideband peak noise-equivalent pressure of 6.5 kPa and an acoustic bandwidth 12 MHz. The other was a 40-microm Parylene film FPI which was directly vacuum-deposited onto a glass backing stub and had an NEP of 8 kPa and an acoustic bandwidth of 17.5 MHz. It is considered that this approach offers an alternative to piezoelectric ultrasound arrays for transducer field characterization, transmission medical and industrial ultrasound imaging, biomedical photoacoustic imaging, and ultrasonic nondestructive testing.     

Integration of 2D CMUT arrays with front-end electronics for volumetric ultrasound imaging

For three-dimensional (3D) ultrasound imaging, connecting elements of a two-dimensional (2D) transducer array to the imaging system's front-end electronics is a challenge because of the large number of array elements and the small element size. To compactly connect the transducer array with electronics, we flip-chip bond a 2D 16 x 16-element capacitive micromachined ultrasonic transducer (CMUT) array to a custom-designed integrated circuit (IC). Through-wafer interconnects are used to connect the CMUT elements on the top side of the array with flip-chip bond pads on the back side. The IC provides a 25-V pulser and a transimpedance preamplifier to each element of the array. For each of three characterized devices, the element yield is excellent (99 to 100% of the elements are functional). Center frequencies range from 2.6 MHz to 5.1 MHz. For pulse echo operation, the average - 6-dB fractional bandwidth is as high as 125%. Transmit pressures normalized to the face of the transducer are as high as 339 kPa and input-referred receiver noise is typically 1.2 to 2.1 mPa/pHz. The flip-chip bonded devices were used to acquire 3D synthetic aperture images of a wire-target phantom. Combining the transducer array and IC, as shown in this paper, allows for better utilization of large arrays, improves receive sensitivity, and may lead to new imaging techniques that depend on transducer arrays that are closely coupled to IC electronics.     

Characterization of the spatial resolution of different high-frequency imaging systems using a novel anechoic-sphere phantom

The spatial resolution of high-frequency ultrasound (HFU, >20 MHz) imaging systems is usually determined using wires perpendicular to the beam. Recently, two tissue-mimicking phantoms (TMPs) were developed to estimate three-dimensional (3-D) resolution. Each TMP consists of nine 1-cm-wide slabs of tissue-mimicking material containing randomly distributed anechoic spheres. All anechoic spheres in one slab have the same dimensions, and their diameter is increased from 0.1 mm in the first slab to 1.09 mm in the last. The scattering background for one set of slabs was fabricated using 3.5-μm glass beads; the second set used 6.4-μm glass beads. The ability of a HFU system to detect these spheres against a speckle background provides a realistic estimation of its 3-D spatial resolution. In the present study, these TMPs were used with HFU systems using single-element transducers, linear arrays, and annular arrays. The TMPs were immersed in water and each slab was scanned using two commercial imaging systems and a custom HFU system based on a 5-element annular array. The annular array had a nominal center frequency of 40 MHz, a focal length of 12 mm, and a total aperture of 6 mm. A synthetic-focusing algorithm was used to form images with an increased depth-of-field. The penetration depth was increased by using a linear-chirp signal spanning 15 to 65 MHz over 4 μs. Results obtained with the custom system were compared with those of the commercial systems (40-MHz probes) in terms of sphere detection, i.e., 3-D spatial resolution, and contrast-to-noise ratio (CNR). Resulting B-mode images indicated that only the linear-array transducer failed to clearly resolve the 0.2-mm spheres, which showed that the 3-D spatial resolution of the single-element and annular-array transducers was superior to that of the linear array. The single-element transducer could only detect these spheres over a narrow 1.5 mm depth-of-field, whereas the annular array was able to detect them to depths of at least 7 mm. For any size of the anechoic spheres, the annular array excited by a chirp-coded signal provided images of the highest contrast, with a maximum CNR of 1.8 at the focus, compared with 1.3 when using impulse excitation and 1.6 with the single-element transducer and linear array. This imaging configuration also provided CNRs above 1.2 over a wide depth range of 8 mm, whereas CNRs would quickly drop below 1 outside the focal zone of the other configurations.     

Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.     

Two-dimensional arrays for medical ultrasound using multilayer flexible circuit interconnection

The development of 2-D array transducers has received much recent interest. Unfortunately, fabrication of high density 2-D arrays is difficult due to the large number of electrical interconnections which must be made to the back side of the elements. A typical array operating at 2.2 MHz may have 256 or more connections within a 16.4 mm circular footprint. Interconnection becomes even more challenging as operating frequencies increase. To solve this problem, we have developed a multilayer flexible (MLF) circuit interconnect consisting of a polyimide dielectric with inter-laminar vias routing signals vertically and etched metal traces routing signals horizontally. A transducer is fabricated from an MLF by bonding a PZT chip to its surface and dicing the chip into individual elements, with the saw kerf extending partially into the top polyimide layer to ensure physical and electrical isolation of the elements. The KLM model was used to compare the performance of an MLF 2-D array to a conventional hand wired 2-D array. MLF and wire guide transducers were fabricated, each with 256 active elements, 0.4 mm interelement spacing, and 2.2 MHz center frequency. Vector impedance, pulse length, bandwidth, angular response, and cross-coupling were found to be comparable in both types of arrays. Using the MLF, however, fabrication time was reduced dramatically. More importantly, MLF technology may be used to increase 2-D array connection density beyond the limitations of current of hand wired fabrication techniques. Thus MLF circuits provide a means for the interconnection of current and future high frequency 2-D arrays.     

Annular-ring CMUT arrays for forward-looking IVUS: transducer characterization and imaging

In this study, a 64-element, 1.15-mm diameter annular-ring capacitive micromachined ultrasonic transducer (CMUT) array was characterized and used for forward-looking intravascular ultrasound (IVUS) imaging tests. The array was manufactured using low-temperature processes suitable for CMOS electronics integration on a single chip. The measured radiation pattern of a 43 X 140-microm2 array element depicts a 40 degrees view angle for forward-looking imaging around a 15-MHz center frequency in agreement with theoretical models. Pulse-echo measurements show a -10-dB fractional bandwidth of 104% around 17 MHz for wire targets 2.5 mm away from the array in vegetable oil. For imaging and SNR measurements, RF A-scan data sets from various targets were collected using an interconnect scheme forming a 32-element array configuration. An experimental point spread function was obtained and compared with simulated and theoretical array responses, showing good agreement. Therefore, this study demonstrates that annular-ring CMUT arrays fabricated with CMOS-compatible processes are capable of forward-looking IVUS imaging, and the developed modeling tools can be used to design improved IVUS imaging arrays.     

Producing bowtie limited diffraction beams with synthetic arrayexperiment

Limited diffraction beams have a large depth of field and could have applications in medical ultrasound and other wave related areas such as electromagnetics and optics. However, these beams have higher sidelobes than conventional focused beams at their focuses. Recently, a new type of beam, called bowtie limited diffraction beams, was developed. These beams can achieve both low sidelobes and a large depth of field in medical imaging. In this paper, the production of bowtie beams in water with a synthetic array experiment is reported. A broad-band PZT ceramic/polymer composite transducer of about 1 mm diameter and 2.5 MHz central frequency was scanned in a raster format and placed at the centers of elements of an equivalent two-dimensional array of 50 mm diameter aperture. A polyvinylidene fluoride (PVDF) needle hydrophone of 0.5 mm diameter was used to receive the waves produced by the transducer. Proper weighting functions were applied to the received signals to produce various beams. Results show that the bowtie beams produced with the synthetic array experiment are in good agreement with those derived from theory and obtained by computer simulations. The depth of field of these beams is about 216 mm and sidelobes of a tenth derivative bowtie X wave in pulse-echo imaging are about 30 dB lower than those of rotary symmetric limited diffraction beams such as the zeroth-order X wave discovered previously     

Multifunctional catheters combining intracardiac ultrasound imaging and electrophysiology sensing

A family of 3 multifunctional intracardiac imaging and electrophysiology (EP) mapping catheters has been in development to help guide diagnostic and therapeutic intracardiac EP procedures. The catheter tip on the first device includes a 7.5 MHz, 64-element, side-looking phased array for high resolution sector scanning. The second device is a forward-looking catheter with a 24-element 14 MHz phased array. Both of these catheters operate on a commercial imaging system with standard software. Multiple EP mapping sensors were mounted as ring electrodes near the arrays for electrocardiographic synchronization of ultrasound images and used for unique integration with EP mapping technologies. To help establish the catheters' ability for integration with EP interventional procedures, tests were performed in vivo in a porcine animal model to demonstrate both useful intracardiac echocardiographic (ICE) visualization and simultaneous 3-D positional information using integrated electroanatomical mapping techniques. The catheters also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures. The companion paper of this work discusses the catheter design of the side-looking catheter with special attention to acoustic lens design. The third device in development is a 10 MHz forward-looking ring array that is to be mounted at the distal tip of a 9F catheter to permit use of the available catheter lumen for adjunctive therapy tools.     

A 32 x 32 capacitive micromachined ultrasonic transducer array manufactured in standard CMOS

As ultrasound imagers become increasingly portable and lower cost, breakthroughs in transducer technology will be needed to provide high-resolution, real-time 3-D imaging while maintaining the affordability needed for portable systems. This paper presents a 32 x 32 ultrasound array prototype, manufactured using a CMUT-in-CMOS approach whereby ultrasonic transducer elements and readout circuits are integrated on a single chip using a standard integrated circuit manufacturing process in a commercial CMOS foundry. Only blanket wet-etch and sealing steps are added to complete the MEMS devices after the CMOS process. This process typically yields better than 99% working elements per array, with less than ±1.5 dB variation in receive sensitivity among the 1024 individually addressable elements. The CMUT pulseecho frequency response is typically centered at 2.1 MHz with a -6 dB fractional bandwidth of 60%, and elements are arranged on a 250 μm hexagonal grid (less than half-wavelength pitch). Multiplexers and CMOS buffers within the array are used to make on-chip routing manageable, reduce the number of physical output leads, and drive the transducer cable. The array has been interfaced to a commercial imager as well as a set of custom transmit and receive electronics, and volumetric images of nylon fishing line targets have been produced.