Karthik Ranganathan

Direct sampled I/Q beamforming for compact and very low-cost ultrasound imaging

A wide variety of beamforming approaches are applied in modern ultrasound scanners, ranging from optimal time domain beamforming strategies at one end to rudimentary narrowband schemes at the other. Although significant research has been devoted to improving image quality, usually at the expense of beamformer complexity, we are interested in investigating strategies that sacrifice some image quality in exchange for reduced cost and ease in implementation. This paper describes the direct sampled in-phase/quadrature (DSIQ) beamformer, which is one such low-cost, extremely simple, and compact approach. DSIQ beamforming relies on phase rotation of I/Q data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional demodulation. We describe an efficient hardware implementation of the beam-former, which results in significant reductions in beam-former size and cost. We present the results of simulations and experiments that compare the DSIQ beamformer to more conventional approaches, namely, time delay beamforming and traditional complex demodulated I/Q beam-forming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are also presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for low end scanners. We also describe implementation of the DSIQ beamformer in an inexpensive hand-held ultrasound system being developed in our laboratory.

A novel beamformer design method for medical ultrasound. Part I: Theory

The design of transmit and receive aperture weightings is a critical step in the development of ultrasound imaging systems. Current design methods are generally iterative, and consequently time consuming and inexact. We describe a new and general ultrasound beamformer design method, the minimum sum squared error (MSSE) technique. The MSSE technique enables aperture design for arbitrary beam patterns (within fundamental limitations imposed by diffraction). It uses a linear algebra formulation to describe the system point spread function (psf) as a function of the aperture weightings. The sum squared error (SSE) between the system psf and the desired or goal psf is minimized, yielding the optimal aperture weightings. We present detailed analysis for continuous wave (CW) and broadband systems. We also discuss several possible applications of the technique, such as the design of aperture weightings that improve the system depth of field, generate limited diffraction transmit beams, and improve the correlation depth of field in translated aperture system geometries. Simulation results are presented in an accompanying paper.

Cystic resolution: A performance metric for ultrasound imaging systems

This paper describes a metric that can be used to characterize the resolution of arbitrary broadband coherent imaging systems. The metric is particularly suited to medical ultrasound because it characterizes scanner performance using the contrast obtained by imaging anechoic cysts of various sizes that are embedded in a speckle-generating background, accounting for the effect of electronic noise. We present the theoretical derivation of the metric and provide simulation examples that demonstrate its utility. We use the metric to compare a low-cost, handheld, C-scan system under development in our laboratory to conventional ultrasound scanners. We also present the results of simulations that were designed to evaluate and optimize various parameters in our system, including the f/# and apodization windows. We investigate the impact of electronic noise on our system and quantify the tradeoffs associated with quantization in the analog to digital converter. Results indicate that an f/1 receive aperture combined with 10-bit precision and a signal-to-noise ratio (SNR) of 0 dB per channel would result in adequate image quality

A novel beamformer design method for medical ultrasound. Part II: Simulation results

For pt.I see ibid., vol.50, no.1, p.15 (2003). In the first part of this work, we introduced the minimum sum squared error (MSSE) technique of ultrasound beamformer design. This technique enables the optimal design of apertures to achieve arbitrary system responses. In the MSSE technique, aperture weights are calculated and applied to minimize the sum squared error (SSE) between the desired and actual system responses. In this paper, we present the results of simulations performed to illustrate the implementation and validity of the MSSE technique. Continuous wave (CW) and broadband simulations are presented to demonstrate the application of the MSSE method to obtain arbitrary system responses (within fundamental physical limitations of the system). We also describe CW and broadband simulations that implement the MSSE method for improved conventional depth of field (DOF) and for improved correlation DOF in translated aperture geometries. Using the MSSE technique, we improved the conventional DOF by more than 200% in CW simulations and more than 100% in broadband simulations. The correlation DOF in translated aperture geometries was improved by more than 700% in both CW and broadband simulations.

Portable, low-cost medical ultrasound device prototype

The first generation prototype of a portable, low-cost medical ultrasound device is described along with experimental results. The prototype system consists of a fully sampled 2D transducer array, sixteen custom receive circuitry chips multiplexed into two bandpass filter channels, and an onboard programmable logic device. A PC fitted with a commercially available data acquisition card is used for data collection and analysis. Beamforming is performed using the direct-sampled in-phase/quadrature method. Pulse-echo images obtained with the prototype are presented and future work is discussed.

Direct sampled I/Q beamforming: a strategy for very low cost ultrasound

We describe a new beamforming strategy designed to support the development of an extremely low cost hand held ultrasound system. Our size and cost constraints for this system are too aggressive to make conventional beamforming feasible. We propose a radical beamforming scheme, direct sampled I/Q (DSIQ) beamforming that applies focal delays only via complex rotation of data acquired by directly digitizing the RF echo data. This strategy has a very low cost in terms of digital and analog hardware. We describe the DSIQ beamformer, and discuss efficiencies in hardware implementation. We show results of experiments performed using a Philips SONOS 5500 imaging system. Theses results show that image quality is acceptable for routine applications such as intravenous line insertion.

A prototype low-cost handheld ultrasound imaging system

We describe a very low cost handheld ultrasound system that we are currently developing for routine applications such as image guided needle insertion. We provide a system overview and focus discussion on our beamforming strategy, direct sampled I/Q (DSIQ) beamforming. DSIQ beamforming is a low cost approach that relies on phase rotation of in-phase/quadrature (I/Q) data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional baseband demodulation. We describe our efficient hardware implementation of the beamformer, which results in significant reductions in beamformer size and cost. We also present the results of experiments and simulations that compare the DSIQ beamformer to more conventional approaches, namely time delay beamforming and traditional complex demodulated I/Q beamforming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for routine applications.

The minimum sum squared error (MSSE) beamformer design technique: initial results

The design of transmit and receive aperture weightings is a critical step in the development of ultrasound imaging systems. Current design methods are generally iterative, and consequently time-consuming and inexact. We have previously described a general ultrasound beamformer design method, the minimum sum squared error (MSSE) technique, that addresses these issues. We provide a brief review of the design method, and present results of simulations that investigate the performance of the technique. We also provide an example of application by applying the technique to improve the depth of field in CW and broadband ultrasound systems.

Synthetic aperture angular scatter imaging: system refinement

Angular scatter imaging has been proposed as a new source of image contrast in medical ultrasound and as a parameter for tissue characterization. We describe a new method that combines the translating apertures algorithm (TAA) with synthetic aperture methods to coherently obtain angular scatter information with high resolution in both space and scattering angle. This method, which we term synthetic aperture angular scatter (SAAS) imaging effectively applies the TAA to single array elements and then focuses the data synthetically to form high resolution images at precisely defined scattering angles. In this paper, we present experimental results implementing SAAS to form angular scatter images of a 5-wire depth of field phantom, a tissue mimicking 3-wire phantom, and in vivo human thyroid. We discuss the degree of uniformity necessary in element response for successful SAAS imaging. These experiments show new image information previously unavailable in conventional B-mode images and suggest that angular scatter imaging may have applications in the breast, thyroid, and peripheral vasculature.

Ultrasonic synthetic aperture angular scatter imaging

Conventional coherent imaging systems map the energy which is reflected directly back towards the transducer. While extremely useful, these systems fail to utilize information in the energy field which has been scattered at other angles. Angular scatter imaging attempts to form images from the scattered energy field at angles other than the 180/spl deg/ backscattered path. We propose a synthetic aperture based imaging scheme for acquiring angular scatter data in medical ultrasound. We describe this technique in k-space and provide an intuitive explanation of the imaging systems behavior. This method, which we term Synthetic Aperture Angular Scatter (SAAS) imaging effectively uses single element geometries to acquire data at a range of scattering angles. In this paper, we present experimental results implementing SAAS on a GE Logiq 700MR system. We applied the SAAS method to form angular scatter images of a 5-wire depth of field (DOF) phantom and a tissue mimicking 3-wire phantom (steel, nylon and cotton). We present results from this data and discuss the degree of uniformity necessary in element response for successful SAAS imaging. Results from these experiments show new image information previously unavailable in conventional B-mode images and suggest that angular scatter imaging may have applications in the breast, thyroid and peripheral vasculature.