3D Beamformer Design

In the design of an ultrasound imaging system, image resolution and signal-to-noise ratio improve as the transducer array area is increased. Thus, large area arrays are desired for high quality ultrasound imaging. To avoid aliasing in the image, the elements of the array must be on the order of one-half the wavelength of ultrasound in the tissue (typically hundreds of microns). The result is that large area arrays require many elements, particularly for 3D imaging where 2D transducer arrays are used. In a conventional phased array system, each element of the array needs to be connected to transmit and receive beamforming hardware. Such a system is impractical for large 2D arrays because of the large number of cables and beamforming channels needed. Common approaches for reducing the hardware complexity for 3D imaging include using only a subset of the array for transmit and receive (sparse arrays), increasing the information acquired for a given transmit and receive sequence, and multiplexing multiple array elements to a single beamforming channel.

For real-time 3-D imaging, in addition to difficulties in fabricating and interconnecting 2-D transducer arrays, there are also challenges in acquiring and processing data from a large number of ultrasound channels. To reduce the system complexity and cost, synthetic aperture [1], sparse array [2, 3] and subaperture processing [4] techniques have been proposed. As part of our recent research, we have developed several beamforming schemes to reduce the frontend hardware complexity while keeping the image quality as close as possible to the gold-standard phased array imaging. The phased subarray (PSA) approach we proposed combines the principles of phased-array and synthetic-aperture imaging methods to reduce the system cost and size by decreasing the number of active channels while maintaining high image quality. This method is most useful for cost or size-constrained real-time acoustic imaging systems. The phased-subarray technique developed extends the capabilities of the earlier subarray imaging approach [4, 5] by contributing in two ways. First, the transmit subarray is not fixed in the center of the array but is placed at a number of positions across the entire array. This allows the realization of a significantly wider effective aperture compared to the earlier methods using a fixed-transmit subarray, resulting in improved lateral resolution. Second, a new reconstruction method is developed that uses a set of subarray-dependent, 2-D filters for wideband imaging. This extends the capabilities of previous reconstruction methods that used 1-D lateral filters suitable for narrowband imaging (Fig. 1). Point spread functions (PSFs) obtained using experimental data are shown in Fig. 2 comparing FPA and PSA. Details of this method and experimental results can be found in [6, 7].

Recently, we investigated alternative beamforming methods to alleviate the frontend hardware complexity. The three major designs we concentrated on use separate elements of a 2D array to transmit and receive as shown in Fig. 3. These configurations are: plus-transmit x-receive, boundary-transmit x-receive with no common elements, and full-transmit x-receive with no common elements. Each design is compared with classical synthetic aperture and classical phased array imaging. Based on simulated PSFs, hardware complexity, SNR, and frame rate, the FT-XR-NC design was chosen for implementation in our next-generation system. This design uses almost the entire aperture on transmit, leveraging the many transmit channels made possible by integrated electronics. Elements along the diagonals are used in receive to achieve acceptable frame rates with a 16-channel data acquisitions system. An IC implementing this design is being developed. Details of this work can be found in [Wygant06]. For photoacoustic/pulse-echo applications TX and RX apertures can be interchanged, so that photoacoustic image quality will improve at the cost of reduced frame rates.

 

 

FIGURE 1. Phased subarray image reconstruction.

 

 

 

FIGURE 2. Comparison of FPA and PSA imaging using experimental data. For both methods, the fixed transmit focus corresponds to the fourth wire target, and dynamic focusing is used on receive. (a) FPA image formed with 361 beams using all 128 CMUT elements. (b) PSA image formed using 5, 32-element subarrays, each acquiring 91 beams. (c) and (d) Magnification of the fourth wire target shows that the 2-D PSF for PSA imaging closely approximates that of FPA imaging.

 

 

 

FIGURE 3. Four different beamforming schemes: (clockwise starting from top left) conventional full phased array (CFPA), X-Transmit and Full-Receive (XT-FR), X-Transmit and Boundary-Receive (XT-BR), Two-Rows-Transmit and Two-Columns-Receive (2RT-2CR)

 

 

 

References

 

[1] Karaman M, Li PC and O’Donnell M, ”Synthetic aperture imaging for small scale systems,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 42, pp. 429-442, May 1995.

[2] Lockwood GR and Foster FS, ”Optimizing the radiation pattern of sparse periodic two-dimensional arrays,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 43 pp. 15-19, Jan 1996.

[3] Austeng A and Holm S, “Sparse 2-D arrays for 3-D phased array imaging - design methods,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 49, pp. 1073-1086, Aug. 2002.

[4] Karaman M and O’Donnell M, ” Subaperture processing for ultrasonic imaging,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 45, pp. 429-442, Jan. 1998.

[5] Karaman M and Tavli B, “Efficient ultrasonic synthetic aperture imaging,” Electronics Letters, vol. 355, no. 6, pp. 1319-20, Aug. 1999.

[6] Johnson JA, Karaman M, and Khuri-Yakub BT, “Coherent-array imaging using phased subarrays. Part I: basic principles,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 52, no. 1, pp. 37-50, Jan. 2005.

[7] Johnson JA, Oralkan Ö, Ergun AS, Demirci U, Karaman M, and Khuri-Yakub BT, “ Coherent Array Imaging Using Phased Subarrays. Part II: Simulations and Experimental Results,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, v. 52, no.1, pp. 51-64, Jan. 2005.

[8] I. O. Wygant, H. J. Lee, A. Nikoozadeh, Ö. Oralkan, M. Karaman, and B. T. Khuri-Yakub, “An integrated circuit with transmit beamforming and parallel receive channels for real-time three-dimensional ultrasound imaging,” presented at the IEEE Ultrason. Symp., Vancouver, BC, Canada, 2006.

 

 

Acknowledgements

 

This project was funded by National Institutes of Health under grant CA99059.