Jeremy A. Johnson

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.

Comparative tracking error analysis of five different optical tracking systems

OBJECTIVE Effective utilization of an optical tracking system for image-based surgical guidance requires optimal placement of the dynamic reference frame (DRF) with respect to the tracking camera. Unlike other studies that measure the overall accuracy of a particular navigation system, this study investigates the precision of one component of the navigation system: the optical tracking system (OTS). The precision of OTS measurements is quantified as jitter. By measuring jitter, one can better understand how system inaccuracies depend on the position of the DRF with respect to the camera. MATERIALS AND METHODS Both FlashPointtrade mark (Image Guided Technologies, Inc., Boulder, Colorado) and Polaristrade mark (Northern Digital Inc., Ontario, Canada) optical tracking systems were tested in five different camera and DRF configurations. A linear testing apparatus with a software interface was designed to facilitate data collection. Jitter measurements were collected over a single quadrant within the camera viewing volume, as symmetry was assumed about the horizontal and vertical axes. RESULTS Excluding the highest 5% of jitter, the FlashPoint cameras had an RMS jitter range of 0.028 +/- 0.012 mm for the 300 mm model, 0.051 +/- 0.038 mm for the 580 mm model, and 0.059 +/- 0.047 mm for the 1 m model. The Polaris camera had an RMS jitter range of 0.058 +/- 0.037 mm with an active DRF and 0.115 +/- 0.075 mm with a passive DRF. CONCLUSION Both FlashPoint and Polaris have jitter less than 0.11 mm, although the error distributions differ significantly. Total jitter for all systems is dominated by the component measured in the axis directed away from the camera.

Implementation, calibration and accuracy testing of an image-enhanced endoscopy system

This paper presents a new method for image-guided surgery called image-enhanced endoscopy. Registered real and virtual endoscopic images (perspective volume renderings generated from the same view as the endoscope camera using a preoperative image) are displayed simultaneously; when combined with the ability to vary tissue transparency in the virtual images, this provides surgeons with the ability to see beyond visible surfaces and, thus, provides additional exposure during surgery. A mount with four photoreflective spheres is rigidly attached to the endoscope and its position and orientation is tracked using an optical position sensor. Generation of virtual images that are accurately registered to the real endoscopic images requires calibration of the tracked endoscope. The calibration process determines intrinsic parameters (that represent the projection of three-dimensional points onto the two-dimensional endoscope camera imaging plane) and extrinsic parameters (that represent the transformation from the coordinate system of the tracker mount attached to the endoscope to the coordinate system of the endoscope camera), and determines radial lens distortion. The calibration routine is fast, automatic, accurate and reliable, and is insensitive to rotational orientation of the endoscope. The routine automatically detects, localizes, and identifies dots in a video image snapshot of the calibration target grid and determines the calibration parameters from the sets of known physical coordinates and localized image coordinates of the target grid dots. Using nonlinear lens-distortion correction, which can be performed at real-time rates (30 frames per second), the mean projection error is less than 0.5 mm at distances up to 25 mm from the endoscope tip, and less than 1.0 mm up to 45 mm. Experimental measurements and point-based registration error theory show that the tracking error is about 0.5-0.7 mm at the tip of the endoscope and less than 0.9 mm for all points in the field of view of the endoscope camera at a distance of up to 65 mm from the tip. It is probable that much of the projection error is due to endoscope tracking error rather than calibration error. Two examples of clinical applications are presented to illustrate the usefulness of image-enhanced endoscopy. This method is a useful addition to conventional image-guidance systems, which generally show only the position of the tip (and sometimes the orientation) of a surgical instrument or probe on reformatted image slices.

Volumetric ultrasound imaging using 2-D CMUT arrays

Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128/spl times/128-element 2-D CMUT array with through-wafer via interconnects and a 420-/spl mu/m element pitch. As an experimental prototype, a 32/spl times/64-element portion of the 128/spl times/128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16/spl times/16-element portion of the array to surrounding bondpads. An 8/spl times/16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4/spl times/4 group of elements in the middle of the 8/spl times/16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.

Coherent-array imaging using phased subarrays. Part I: basic principles

The front-end hardware complexity of a coherent array imaging system scales with the number of active array elements that are simultaneously used for transmission or reception of signals. Different imaging methods use different numbers of active channels and data collection strategies. Conventional full phased array (FPA) imaging produces the best image quality using all elements for both transmission and reception, and it has high front-end hardware complexity. In contrast, classical synthetic aperture (CSA) imaging only transmits on and receives from a single element at a time, minimizing the hardware complexity but achieving poor image quality. We propose a new coherent array imaging method - phased subarray (PSA) imaging - that performs partial transmit and receive beam-forming using a subset of adjacent elements at each firing step. This method reduces the number of active channels to the number of subarray elements; these channels are multiplexed across the full array and a reduced number of beams are acquired from each subarray. The low-resolution subarray images are laterally upsampled, interpolated, weighted, and coherently summed to form the final high-resolution PSA image. The PSA imaging reduces the complexity of the front-end hardware while achieving image quality approaching that of FPA imaging

Medical imaging using capacitive micromachined ultrasonic transducer arrays

We are investigating the use of capacitive micromachined ultrasonic transducers (cMUTs) for use in medical imaging. We propose an ultrasound probe architecture designed to provide volumetric ultrasound imaging from within an endoscope channel. A complete automated experimental system has been implemented for testing the imaging performance of cMUT arrays. This PC-based system includes custom-designed circuit boards, a software interface, and resolution test phantoms. We have already fabricated 1D and 2D cMUT arrays, and tested the pulse-echo imaging characteristics of 1D arrays. Beamforming and image formation algorithms that aim to reduce the complexity of data acquisition hardware are tested via numerical simulations and using real data acquired from our system.

Broadband capacitive micromachined ultrasonic transducers ranging from 10 kHz to 60 MHz for imaging arrays and more

Capacitive micromachined ultrasonic transducers (CMUTs) have long been studied. Past research has shown that CMUTs indeed have remarkable features such as wide bandwidth and high efficiency. This paper introduces an inclusion to the CMUT technology that uses the wafer-bonding technique to fabricate membranes on silicon. This new technology enables the fabrication of large membranes with large gaps, and expands the frequency span of CMUTs to 10 kHz in the low end. CMUT devices with different frequency spans are fabricated using both technologies, and tested. Electromechanical coupling efficiency, k/sub T//sup 2/, value as high as 0.85 and fractional immersion bandwidth as wide as 175 % are measured.

A real-time freehand 3D ultrasound system for image-guided surgery

Current freehand 3D ultrasound techniques separate the scanning or acquisition step from the visualization step. The process leads to a single image volume dataset that can be rendered for viewing later. While satisfactory for diagnostic purposes, the method is not useful for surgical guidance where the anatomy must be visualized in real time. The Image Guidance Laboratories are currently developing a freehand 3D ultrasound system that will allow real-time updates to the scanned volume data as well as the capability to simultaneously view cross-sections through the volume and a volume-rendered perspective view. The equipment used is not unlike other freehand 3D ultrasound systems: an optical tracking system for locating the position and orientation of the ultrasound probe, a video frame grabber for capturing ultrasound frames, and a high-performance computer for performing real-time volume updates and volume rendering. The system incorporates novel methods for inserting new frames into, and removing expired frames from, the volume dataset in real time. This paper reports on current work in progress, and focuses on methods unique to achieving real-time 3D visualization using freehand 3D ultrasound.

Volumetric imaging using 2D capacitive micromachined ultrasonic transducer arrays (CMUTs): initial results

This paper presents the first volumetric images obtained using a 2D CMUT array with through-wafer via interconnects. An 8/spl times/6-element portion of a 32/spl times/64-element array flip-chip bonded onto a glass fanout chip was used in the experiments. This study experimentally demonstrates that 2D CMUT arrays can be fabricated with high yield using silicon micromachining processes, individual electrical connections can be provided using through-wafer interconnects, and the flip-chip bonding technique can be used to integrate the dense 2D arrays with electronic circuits for practical imaging applications.

Coherent array imaging using phased subarrays. Part II: simulations and experimental results

For pt.I, see ibid., vol.52, no.1, p.37-50 (2005). The basic principles and theory of phased subarray (PSA) imaging provides the flexibility of reducing the number of front-end hardware channels between that of classical synthetic aperture (CSA) imaging - which uses only one element per firing event - and full-phased array (FPA) imaging - which uses all elements for each firing. The performance of PSA generally ranges between that obtained by CSA and FPA using the same array, and depends on the amount of hardware complexity reduction. For the work described in this paper, we performed FPA, CSA, and PSA imaging of a resolution phantom using both simulated and experimental data from a 3-MHz, 3.2-cm, 128-element capacitive micromachined ultrasound transducer (CMUT) array. The simulated system point responses in the spatial and frequency domains are presented as a means of studying the effects of signal bandwidth, reconstruction filter size, and subsampling rate on the PSA system performance. The PSA and FPA sector-scanned images were reconstructed using the wideband experimental data with 80% fractional bandwidth, with seven 32-element subarrays used for PSA imaging. The measurements on the experimental sector images indicate that, at the transmit focal zone, the PSA method provides a 10% improvement in the 6-dB lateral resolution, and the axial point resolution of PSA imaging is identical to that of FPA imaging. The signal-to-noise ratio (SNR) of PSA image was 58.3 dB, 4.9 dB below that of the FPA image, and the contrast-to-noise ratio (CNR) is reduced by 10%. The simulated and experimental test results presented in this paper validate theoretical expectations and illustrate the flexibility of PSA imaging as a way to exchange SNR and frame rate for simplified front-end hardware