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 /spl times/ 140-/spl mu/m array element depicts a 40/spl deg/ 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.

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

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 /spl mu/m /spl times/ 100 /spl mu/m 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 /spl mu/m and 0.112 radians for 8 MHz, and 78 /spl mu/m 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.

/pdf/3-d-ultrasound-imaging-using-a-forward-looking-cmut-ring-106u3zhqbq.pdf

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

Intravascular ultrasound (IVUS) imaging systems using circumferential arrays mounted on cardiac catheter tips fire beams orthogonal to the principal axis of the catheter. The system produces high resolution cross-sectional images but must be guided by conventional angioscopy. A real-time forward-viewing array, integrated into the same catheter, could greatly reduce radiation exposure by decreasing angiographic guidance. Unfortunately, the mounting requirement of a catheter guide wire prohibits a full-disk imaging aperture. Given only an annulus of array elements, prior theoretical investigations have only considered a circular ring of point transceivers and focusing strategies using all elements in the highly dense array, both impractical assumptions. In this paper, we consider a practical array geometry and signal processing architecture for a forward-viewing IVUS system. Our specific design uses a total of 210 transceiver firings with synthetic reconstruction for a given 3-D image frame. Simulation results demonstrate this design can achieve side-lobes under -40 dB for on-axis situations and under -30 dB for steering to the edge of a 60/spl deg/ cone.

Optimizing the beam pattern of a forward-viewing ring-annular ultrasound array for intravascular imaging

In this paper, a scheme called coded excitation with spectrum inversion (CEXSI) is presented. An established optimal binary code whose spectrum has no nulls and possesses the least variation is encoded as a burst for transmission. Using this optimal code, the decoding filter can be derived directly from its inverse spectrum. Various transmission techniques can be used to improve energy coupling within the system pass-band. We demonstrate its potential to achieve excellent decoding with very low (<80 dB) side-lobes. For a 2.6 /spl mu/s code, an array element with a center frequency of 10 MHz and fractional bandwidth of 38%, range side-lobes of about 40 dB have been achieved experimentally with little compromise in range resolution. The signal-to-noise ratio (SNR) improvement also has been characterized at about 14 dB. Along with simulations and experimental data, we present a formulation of the scheme, according to which CEXSI can be extended to improve SNR in sparse array imaging in general.

Coded EXcitation with spectrum inversion (CEXSI) for ultrasound array imaging

Capacitive Micromachined Transducers (cMUTs) manufactured on silicon substrates using integrated circuit processing techniques have a great potential for fabrication of forward looking intravascular ultrasound (IVUS) imaging arrays with integrated electronics. To realize this potential for these annular arrays, in which an array element consists of a small number of cMUT membranes, the individual elements have to be modeled in detail and optimized for performance. In this paper, we use finite element analysis (FEA) to calculate the electromechanical coupling coefficient, k/sup 2/, using the fundamental, energy based definition. We then improve the cMUT equivalent circuit model by incorporating the FEA results in the circuit elements. Our results show that existing equivalent circuit models based on radiation impedance of a piston transducer overestimate the bandwidth by more than 40%. We show that the cMUT geometry can be optimized for maximum coupling coefficient, allowing cMUT IVUS array elements to be designed with more than 60 dBN dynamic range with 10MHz bandwidth around 20MHz. We also discuss fabrication of 64 element ring annular cMUT arrays using surface micromachining technology.

Capacitive micromachined ultrasonic transducers for forward looking intravascular imaging arrays

We have investigated IVUS array design and image synthesis methods allowing simultaneous sideward imaging at 20MHz and forward imaging at 10MHz. This design is appropriate for over-the-wire delivery but limits the forward-viewing aperture to a ring-annular array. In particular, a 1200/spl mu/m diameter ring-annular array with 64 elements has been constructed. The measured forward-viewing radiation pattern of an individual element indicates an acceptance angle range of over 90/spl deg/. Our beam synthesis involves a total of 210 transceiver firings in a synthetic imaging mode for a given 3D frame. Broadband simulations demonstrated this design can achieve side-lobes under -40dB for on-axis situations and under -30dB for steering to the edge of a 60/spl deg/ cone.

A forward-viewing ring-annular array for intravascular ultrasound imaging

In this paper, we present some initial results from our proposed forward viewing ring-annular array for intravascular ultrasound imaging. We have investigated array design and image synthesis allowing simultaneous sideward imaging at 20 MHz and forward imaging at 10 MHz from the same array. This design is appropriate for over-the-wire delivery but limits the forward-looking aperture to a ring-annular array. In particular, a 1200 /spl mu/m diameter ring-annular array with 64 elements has been constructed. The measured forward- viewing radiation pattern of an individual element indicates an acceptance angle range of over 90/spl deg/. Currently, there are significant sensitivity and center frequency variations among the elements. Nevertheless, images of point targets are obtained as a proof-of-concept. Future improvements of the array will be discussed.

http://ieeexplore.ieee.org/iel5/9075/28792/01293391.pdf

Initial results from a forward-viewing ring-annular ultrasound array for intravascular imaging

We have proposed a forward-viewing ring-annular array for intravascular ultrasound imaging. Our proposed beamformer involves a total of 210 transceiver firings per frame. To maintain adequate signal to noise ratio (SNR) in such sparseness, an additional 10 dB SNR improvement is required. Conventional correlation-based coded excitation leads to substantial side-lobes due to small time bandwidth product. A scheme called Coded EXcitation with Spectrum Inversion (CEXSI) is presented. An established optimal binary code whose spectrum has no nulls and possesses the least variation is used for transmission. Decoding is a simple matter of spectrum inversion. Code transmission can be closely coupled to the transducer pass-band. For a 2.6 /spl mu/s code, an array element with a center frequency of 10 MHz and fractional bandwidth of 38%, range side-lobes of about 40 dB have been achieved experimentally with little compromise in range resolution. SNR improvement also has been characterized at 14.5 dB. CEXSI can be extended to improve SNR in general sparse array imaging.

Yao Wang

Papers

A forward-viewing ring-annular array for intravascular ultrasound imaging

Initial results from a forward-viewing ring-annular ultrasound array for intravascular imaging

Coded EXcitation with Spectrum Inversion (CEXSI) to improve SNR in sparse array imaging