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EP-3282951-B1 - METHOD AND SYSTEM FOR CODED EXCITATION IMAGING BY IMPULSE RESPONSE ESTIMATION AND RETROSPECTIVE ACQUISITION

EP3282951B1EP 3282951 B1EP3282951 B1EP 3282951B1EP-3282951-B1

Inventors

  • FLYNN, John, A.
  • PFLUGRATH, LAUREN, S.

Dates

Publication Date
20260506
Application Date
20160330

Claims (10)

  1. A method of ultrasonic imaging by coded excitation reconstruction, CER, comprising: emitting acoustic signals into a medium simultaneously from multiple transducer elements in a transducer using coded excitation transmission over a time sequence to achieve simultaneous transmission of code sequences on M multiple channels, each channel corresponding to a respective transducer element; receiving echo signals on receive channels at the multiple transducer elements in response to the emitting; the method being characterized by further comprising: processing the received echo signals to estimate a multi-input-single-output, MISO, system for each receiver channel independently and based on the simultaneous transmission of code sequences on the M multiple channels and collection of the echo signals of the transmission sequence in accordance with one of the models: Gauss-Markov, mixed-effects, or regression, thereby giving an impulse response for each transmit-receive transducer element pair; acquiring virtual data through excitation of the MISO systems repeatedly as a proxy for the imaged media present; and producing a visual image on a display device using the virtual data.
  2. The method of claim 1, wherein the emitting comprises emitting the acoustic signals using coded excitation transmission waveforms to produce a best linear unbiased estimate, BLUE, of impulse response; and wherein the acquiring virtual data and subsequent image producing steps use a subset of the MISO systems.
  3. The method of claim 1, wherein the emitting comprises producing beam-steered or phased-array imaging with lambda-spaced transducer arrays while using the coded excitation reconstruction, CER, imaging technique to suppress grating lobe artifacts and thereby obtain improved lateral resolution of the visual image on the display device.
  4. The method of Claim 1, wherein ultrasonic imaging of moving tissue is accomplished by executing a CER imaging technique with sliding-window reconstruction, SWR, SWR-CER, and processing sequential, overlapping, contiguous intervals of MISO subsets to generate a CER-derived image sequence representative of motion present during an entire image acquisition sequence.
  5. The method of Claim 4, wherein the SWR-CER execution produces an image sequence representative of motion present in the medium during an entire acquisition sequence; and subsequently using pixel-wise processing to accomplish wall-motion or stationary tissue signal rejection and subsequent Doppler frequency estimation for effecting Doppler flow imaging or spectral Doppler analysis.
  6. The method of Claim 5, wherein the SWR-CER execution and pixel-wise processing provide Doppler frequency estimation and Doppler tissue motion imaging.
  7. The method of claim 5, wherein the SWR-CER execution pixel-wise processing provide shear-wave detection and shear-wave imaging.
  8. The method of claim 1, wherein the emitting comprises transmitting a plurality of distinct codes simultaneously over a set of transducer elements that is configured to transmit on a subarray of the transducer elements that are excited identically to effect steering or focusing at a spatial point in the medium.
  9. The method of claim 1, wherein the emitting comprises transmitting a plurality of distinct codes simultaneously over a set of transducer elements, each distinct code utilizing all available transducer elements in the set, and each distinct code defined by focusing delays to obtain a focus of the emitted acoustic signals at least one physical focus location in the medium, so that multiple simultaneous wavefronts modulated by distinct codes are generated.
  10. The method of claim 9 wherein the modulating by distinct codes comprises generating acoustic array transmissions that are sums of individual array signals of individual aperture coded modulations in the case of linear continuous level transmit amplifiers or are discrete-state encodings of the array signal summations in the case of discrete-level amplifiers.

Description

BACKGROUND Technical Field The present disclosure pertains to methods for encoding arbitrary waveforms into a sequence suitable for control of a tri-state RF ultrasonic transmitter, under various fidelity criteria, and to a related system. Description of the Related Art Coded pulse excitation provides the opportunity to design waveforms with large time-bandwidth products, which is a means to improve signal penetration while maintaining resolution in coherent imaging. For instance, F. Gran and J. A. Jensen: "Identification of pulse echo impulse responses for multi source transmission", Signals, Systems and Computers, vol. 1, November 07, 2004, pages 168-172, discloses a method of ultrasonic imaging based on coded excitation. Acoustic signals are simultaneously emitted into a medium from multiple transducer elements using coded excitation transmission over a time sequence. Echo signals are received at the multiple transducer elements in response to the emission. The received echo signals are processed to provide an impulse response for each transmit-receive transducer element pair. The impulse response is used to calculate an amplitude for each image point. However, with the distributed scattering typical of ultrasonic imaging, clutter induced by pulse compression side lobes, and crosschannel correlation cause reduced contrast resolution. BRIEF SUMMARY The present disclosure is directed to a system and process that implements a two-step method for coded excitation imaging that first estimates the medium's impulse response (IR), then by retrospective transmission through the IR set, synthesizes virtual wavefronts for beamforming. In step one, coded waveforms are transmitted simultaneously on multiple elements for several frames. A multi-input, single output (MISO) system is constructed from the codes to model transmit-receive paths. The system and RF data observation are solved by linear model theory, giving an IR set for the medium. In step 2, the estimates are subsequently applied to a secondary MISO system, constructed by analogy to the first, but with pulses convenient for beamforming, e.g., a focused set of single-cycle pulses for ideal focused reconstruction. Thus, the probing sequence can be optimally designed for IR characterization under acoustic time budget, while the retrospective acquisitions are synthesized outside of acoustic time. The implementation of the method demonstrates that under typical imaging system constraints and assumptions, this approach provides a framework to achieve optimal unbiased estimation of beamformed pixels. Using a research ultrasound platform, the method I demonstrated on a phantom and in-vivo. The invention provides a method of ultrasonic imaging by coded excitation reconstruction, CER, as specified in claim 1. Advantageous implementations are specified in the dependent claims. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS The foregoing and other features and advantages of the present disclosure will be more readily appreciated as the same become better understood from the following detailed description when taken in conjunction with the accompanying drawings, wherein: Figure 1 is a first embodiment of an ultrasound imaging system formed in accordance with the present disclosure;Figure 2 is an illustration of the Coded Excitation Reconstruction Architecture formed in accordance with the present disclosure;Figure 3 is a schematic relation between reconstructed pixel (top panel) and impulse response lags (bottom panel) in accordance with the present disclosure;Figure 4 is a schematic form and sparsity structure of a Toeplitz matrix operator for fully-convolved impulse response in accordance with the present disclosure;Figure 5 is a schematic form of a sparsity structure and partitioning of a matrix operator for convolution in the mixed-effects model of the present disclosure;Figure 6 is a schematic of a convolution operator sparsity structure for pulse-stacking (high-Doppler) model for a single acquisition; andFigure 7 is a schematic of a sparsity structure for a high-Doppler model (two acquisition intervals) in accordance with the present disclosure;Figure 8 is illustrates a geometry and definitions for motion effects on observed Doppler in an impulse response;Figure 9 is a schematic of a sparse transmit element configuration;Figure 10 is a schematic of a sparse transmit aperture with extended elements;Figure 11 is a schematic of a sparse transmit aperture with virtual apex source locations;Figure 12 are screen shots of a simulation example of coded excitation impulse response estimation of a moving target (low-flow STR algorithm);Figure 13 is an illustration of architecture for CER algorithm for stationary tissue rejection and blood flow imaging;Figure 14 illustrates the virtual source locations for simultaneous wavefront synthesis;Figure 15 illustrates the steps of the steps for CER-based vector motion imaging;Figure 16 illustrates a transducer compensation e