Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Having specific delay in producing output waveform
Reexamination Certificate
1999-07-20
2001-03-27
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Having specific delay in producing output waveform
C327S284000, C327S293000
Reexamination Certificate
active
06208189
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to ultrasonic imaging and more particularly to delta-sigma modulation of an ultrasonic imaging signal.
I. BACKGROUND
Many coherent array systems (acoustic or electromagnetic) use some form of dynamic focusing to generate images with diffraction limited resolution. Examples include ultrasound, sonar, and RADAR. The remainder of this disclosure will be focused primarily on ultrasound applications; however, the principles can be applied to sonar, RADAR or any coherent array imaging system as well.
Current clinical ultrasound systems generate images of soft tissue within the body by launching a vibratory pulse and then receiving and processing the reflected energy. The transmitted vibratory pulse is often limited to a single focus along a particular steering angle for each firing. In contrast, reflected signals are continuously recorded permitting array refocusing on receive. Dynamic receive focusing is accomplished by changing individual channel delays with time (range) prior to summing the RF signal over all elements to form the received beam.
A complete state-of-the-art ultrasound imaging system uses a large collection of application specific integrated circuits (ASICs), digital signal processors (DSPs), microcontrollers (&mgr;C), memory buffers, etc . . . integrated onto a set of printed circuit boards connected by a modified communications bus (usually a VME bus).
FIG. 1
generally shows a block diagram of the various processing elements providing the wide ranging capabilities clinicians expect today from a high quality ultrasound imager. The front-end processor, and more specifically the beamformer, will be the primary focus of this disclosure. Significant prior art exists for different beamforming architectures as well as different implementations of downstream processing elements, such as Doppler and color flow processors.
State-of-the-art systems employ a beamforming scheme similar to that shown in
FIG. 2
, where a high speed, multi-bit analog to digital converter on each channel samples the incoming ultrasound signal. These samples are then delayed by one of several means before being summed within a pipelined set of digital adders. The delay structure compensates for the channel's geometric position relative to the desired receive focus. Properly delayed signals yield coherent interference when summed across the array. These delays, however, must change as the transmitted pulse propagates into tissue. Dynamically changing delays are difficult to implement, and there is considerable prior art which documents various methods used to date. Older systems (until 1980 or so) used analog delays and sums which suffered from signal to noise and temperature drift problems adversely affecting image quality. Current fully digital systems provide greatly improved quality; however, the required beamforming and processing hardware is extensive, expensive, and consumes significant power.
The system proposed under this invention solves these problems using oversampled delta-sigma modulation and dynamic delay for beamforming a received image. Feedback control within the delta-sigma modulator or recoding the digital outputs reduces distortion introduced by changes in dynamic delay.
The basic oversampled approach of the invention has been further improved through premodulation, whereby bandwidth can be effectively traded-off with quantization noise. Also, multiple stages of beamforming are included so that two dimensional arrays can be used effectively. One delay stage is used for elevational beamforming, and the other for azimuthal. Finally, correct transmit phasing can, for the first time, be performed using existing receive phasing circuitry, thus reducing system complexity and power consumption.
II. SUMMARY OF THE INVENTION
An apparatus and method are provided for compensating a dynamically delayed signal stream for distortion in a delta-sigma (&Dgr;&Sgr;) modulator of an imaging system. The method includes the steps of changing a length of a portion of the bit stream being generated by the delta-sigma modulator and either adjusting a feedback magnitude of the delta-sigma processor or recoding the manipulated digital signal sample to compensate for the changed delay.
An overview of the system will be presented first describing the components and operation of the oversampled receive beamformer. Using these components for transmit purposes will also be briefly discussed. Issues related to the &Dgr;&Sgr; analog to digital converter (A/D) and its use in the system will be presented in detail. Measures to improve its performance will also be presented. Other important details of the system will be described including methods to apodize the array, delay the sample stream, and perform necessary arithmetic.
This section will repeatedly refer to
FIG. 3
, showing a system-level schematic of the proposed beamformer. A general discussion will be provided here of transmit and receive operation. It will be expanded in the following two sections to include a detailed description of each of the functional elements. For illustration, we assume the active transducer is a 1.5-D array of 64×8 elements sequentially stepped in azimuth across a total array of 192×8 elements, thereby sweeping out a linear sector (for a flat array) or an offset sector (for a curved array). Please note that the specific strategy presented for this system can be easily modified for any arbitrary array geometry.
In the discussions presented throughout this disclosure, there are specifics presented that could easily be modified. The number of elevational elements in the array, for instance, is variable, so that 7 elements could be used instead of 8. The following is a list of system parameters that should be considered variable:
Array geometry and configuration—affects the scanning modes and magnitude of delays required for proper beamforming.
Transmit sample rate—affects signal to quantization noise (SQNR) of the transmitted signal as well as pattern memory size and datapath bandwidth requirements
Receive sample rate—affects the SQNR of the digitized signal, set by the &Dgr;&Sgr; modulator. Also affects the clock rates and datapath width of the system.
Parallel-Serial and Serial-Parallel—circuits are used throughout the system to change the clock rates and bit-widths of the data. All such circuits could be implemented to provide different clock ratios of parallel to serial conversion, and visa-versa.
&Dgr;&Sgr; modulator order—affects the SQNR of the digitized signal. A higher order modulator has better noise shaping but involves more complicated circuitry.
&Dgr;&Sgr; quantizer bits—affects the SQNR of the digitized signal as well as the stability of the modulator. The datapath bandwidth also depends on this.
All of these items will be discussed with a specified embodiment in mind; however, all of them can be changed depending on design tradeoffs.
II.1. Transmit Beamformer (Tx)
Generating an ultrasound transmit (Tx) beam requires that a transmit pulse waveform be appropriately delayed to drive each transducer element in the 1.5-D array. In our system, the waveform is stored in a transmit pattern memory common to the entire system. The pulse waveform is coded using a 2
nd
order, two level, delta-sigma digitization scheme operating at a nominal 320 MHz sampling rate, where data can be represented (and stored) using only one bit per sample. Data are read out of the memory several samples (e.g., 16) at a time at {fraction (1/16)} the Tx sampling rate and fed to a 64:1 splitter buffering it to 64 different digital delay structures. Data are shifted at {fraction (1/16)} the sampling rate into the delay structure. Each azimuthal channel delay structure has an independent setting allowing 4096 different delays to be applied to the transmit waveform, for azimuthal steering and focusing for example. Delay granularity is 16 times the Tx sampling period because changing the input tap position by one sample actually changes the transmit delay by sixteen 1-bit samples.
Followin
Desilets Charles S.
Freeman Steven R.
Linnenbrink Thomas E.
Morin Marc A.
O'Donnell Matthew
Christensen Jon P.
Cox Cassandra
Schnayer Jerold B.
The Regents of the University of Michigan
Wells Kenneth B.
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