Lauren S. Pflugrath

High framerate vector velocity blood flow imaging using a single planewave transmission angle

Vector velocity blood flow imaging gives speed and direction of blood flow at each pixel. An imaging algorithm proposed earlier [2] requires multiple angles of planewave (PW) transmissions to construct a robustly invertible model for vector velocity estimates. Here we demonstrate a vector velocity estimation approach that requires only a single planewave transmission angle. The proposed algorithm uses PW transmission and reconstruction to generate a blood motion image sequence in the B-mode flow (B-Flow) modality, at frame rates in the Doppler PRF regime. Pixel ensembles in the image sequence at point p = [x, z] and pulse t are comprised of IQ magnitude values, computed from the IQ data at each pixel p after wall filtering the ensemble. The sequence of values thus captures motion at a framerate equal to the PRF, revealing fine-scale flow dynamics as a moving texture in the blood reflectivity. Using the chain rule, spatial and temporal derivatives resulting from the space-time gradient of the image sequence couple to the texture flow velocity vector field [vx(x, z, t), vz(x, z, t)] at each pixel p and PRI t. The resulting Gauss-Markov models are solved by least squares to give the vector velocity estimates, which are formulated in the model to be constant over the estimation window. We also evaluate variants that include in the observation, lag-product samples (autocorrelation summands) at non-zero lags, as well as instantaneous Doppler-derived axial velocity estimates. Compared to the multi-angle planewave algorithm presented in [2], this approach allows for a longer time interval for wall filtering, as the frame is not partitioned into separate segments for different planewave angles. This permits wall filters with steeper transition bands, and allows flexibility in balancing framerate and sensitivity, suggesting application to vector flow imaging of deep tissue where efficiently achieving planewave angle diversity at the target becomes difficult. Using a Philips L7-4 transducer and a Verasonics (TM) acquisition system, we evaluate single-angle PWT vector velocity imaging on a Doppler string phantom, and demonstrate it successfully on a carotid artery.

Coded excitation reconstruction by impulse response estimation and retrospective acquisition

Coded excitation (CE) imaging enables large time-bandwidth waveform transmissions, which are often pulse-compressed by matched filtering (MF). Much research investigates schemes for reducing MF sidelobes to decrease clutter with distributed targets. The goal of this presentation is to harmonize CE transmission with the image quality of Synthetic Transmit Aperture (STA) imaging at high frame rates. We propose a two-step process that estimates medium impulse responses (IR), then retrospectively images using the IR set. In this way, the probing code sequence is used optimally yet is decoupled from the imaging step, circumventing the sidelobe problem. The method first estimates transmit-receive (TR) pair IR of the acoustic medium, using random codes transmitted simultaneously across channels and acquisition intervals. Linear model theory solves for unbiased estimates of the IR set. In the second step, retrospective STA imaging excites the estimated IR set, using it as a simulation of the medium. Retrospective transmit signals and apodizing can differ at each pixel. We show the resulting beamformed pixels are unbiased estimates of STA pixels, and that time diversity in codes improves estimation error over static codes. To address tissue motion, the model is extended to polynomial and Fourier basis. We also demonstrate retrospective transmission of a transducer-compensated pulse in the imaging step, bypassing the limited precision of tri-state transmitter hardware.