Delta-sigma beamformers with minimal dynamic focusing artifacts

Details Delta-sigma beamformers with minimal dynamic focusing artifacts Delta-sigma beamformers with minimal dynamic focusing artifacts

2000-03-28

2002-04-02

Young, Brian (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C341S155000

Reexamination Certificate

active

06366227

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to beamforming in ultrasound imaging systems and, more particularly, to dynamically focused digital beamformers which use delta-sigma modulators.
BACKGROUND OF THE INVENTION
A conventional ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or time delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. Resolution of a scan line is a result of directivity of the associated transmit and receive beam pair.
In a typical ultrasound imaging system, the output signals of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Conventional ultrasound beamformers use dynamic focusing during reception of echoes. With this method, the beamformation process is optimized for each depth to achieve as good a beamshape (i.e., narrow beamwidth with low sidelobes) as possible. In most systems, a single fixed focus is used during transmit beamformation to try to maintain a good combined beamshape. In areas away from the transmit focus, the beamwidth of the resultant beam enlarges and the sidelobes increase. In the manufacture of an ultrasound system, the beamformer control is installed in either algorithmic or tabulated form.
In many conventional ultrasound imaging systems, time delay resolution in the beamforming circuitry of the receiver requires a large amount of hardware and consumes a large amount of power. The most recent designs sample each transducer element output signal using very accurate analog-to-digital converters (ADCs) which produce multi-bit digital numbers. These multi-bit numbers are delayed by separate circuitry, including first-in/first-out (FIFO) registers, decimators, and interpolators, before being summed with the separately delayed, multi-bit signals from each of the other transducer elements. This is a considerable amount of hardware for a conventional 128-element transducer array, and is an enormous amount of hardware when a two-dimensional transducer array having 512 or more elements is considered.
A delta-sigma modulator or converter is a circuit which converts an analog signal into a digital signal stream. Delta-sigma (&Dgr;-&Sgr;) analog-to-digital converters have been proposed to radically reduce the size, cost and power consumption of digital ultrasound beamformers. Noujaim et al. U.S. Pat. No. 5,203,335, issued Apr. 20, 1993 and assigned to the instant assignee, sets forth advantages of the &Dgr;-&Sgr; converter for digital beamforming. The &Dgr;-&Sgr; converter produces a data stream with a small number of bits—in its purest form, a single bit at a data rate much higher than the Nyquist sampling frequency of the input signal. The converter output signal can be converted into a more familiar multiple-bit data stream, such as that produced by the traditional ADC, by filtering and decimating in time. To a reasonable approximation, the capacity of a digital data stream is nR, where n is the number of bits in the data and R is the data sample rate. Thus a 1-bit &Dgr;-&Sgr; converter running at 320 MHz is roughly the equivalent of an 8-bit, 40-MHz ADC.
The relatively high data rate of the &Dgr;-&Sgr; converter is attractive for digital time-delay beamforming. In its simplest form, a digital beamformer delays the digital data stream of each channel according to a receive focusing schedule, then sums over all the delayed data streams to produce a focused “beamsummed” signal. In this simple form, the resolution of the time delays which the beamformer can generate is the data sampling time interval. The 8-bit, 40-MHz ADC used in the example above is representative of what is currently available for ultrasound imagers. A sampling rate of 1/(40×10
6
MHz)= 25 nsec is inadequate for all but the lowest imaging frequencies, so that various interpolation schemes are required. These increase the cost and complexity of the integrated circuit which implements the digital delay.
The sampling rate of the equivalent one-bit &Dgr;-&Sgr; converter is eight times higher, which is adequate for imaging at center frequencies up to about 10 MHz. A beamformer could simply delay, without interpolation, the &Dgr;-&Sgr; data streams. The beamsummed data would be filtered and decimated to reconstruct a multiple-bit signal which would be passed to the display processor of the imager.
The simplicity of the &Dgr;-&Sgr; beamformer architecture enables a reduced size with respect to the equivalent beamformer based upon ADCs. This means that the beamformer hardware could be moved from the ultrasound console into the probe itself. This has a number of important advantages. Currently a shielded signal line is required to connect each beamformer channel to the console. The bulk of these cables is a serious obstacle to increasing the channel count in ultrasound imagers. With a &Dgr;-&Sgr; beamformer in the probe, the channel signals are not brought to the console, so that the cable count is dramatically reduced. This reduces the transducer cable complexity (size and weight). Noise pickup in the cable is a serious concern currently, since the cable carries low-level analog signals. In a &Dgr;-&Sgr; beamformer, only the beamsummed digital signal need be transmitted to the console.
However, &Dgr;-&Sgr; beamformers are beset by a difficulty in practice. Delaying a digital signal by one sample, for example, requires insertion of some sample into the data stream. With a multiple-bit data stream, a sample could simply be repeated when the data stream is to be delayed. This introduces some waveform distortion on the scale of the change in the waveform over the sampling time interval, which is a relatively small error when the sampling time interval is sufficiently small. (In practice, traditional ultrasound machines implement delays using a combination of repeated sample and interpolation to relax the requirement on sampling time interval.)
The artifact produced by repeating a single-bit &Dgr;-&Sgr; sample is more severe, however. The reason is that the single-bit &Dgr;-&Sgr; data stream must at some point be low-pass-filtered to reconstruct a multiple-bit waveform. The relative error introduced by repeating a &Dgr;-&Sgr; sample is of order one divided by the filter length. The filter length will be of order of the oversampling ratio, which is currently limited by circuit speed to about 32 for a 10-MHz transducer frequency. This inserted sample, therefore, produces a distortion in the output signal of order 20log({fraction (1/32)}) or about −30 dB. This is unacceptable for all but the least expensive of beamformers.
Freeman et al., in a paper entitled “An Ultrasound Beamformer Using Oversampling,” 1997 IEEE Ultrasonics Symposium Proceedings, proposed three methods to correct the dynamic focus artifact. In one method, they proposed using a two-bit &Dgr;-&Sgr; converter, which produces a zero level in addition to the +1 and −1 levels of a one-bit &Dgr;-&Sgr; converter. This zero level is used when a sample must be inserted, which reduces the size of the artifact that survives the reconstruction filter. In a second method, they interpret the four levels of a two-bit converter as −1, {fraction (−1/2)}, +½, and +1. They divide in half the sample to be repeated and spread it over two inserted samples. In the third method, they manipulate the g

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