Abstract
Medical ultrasound usually implements ray-based imaging algorithms, in which the most severe limitation involves the implicit assumption of constant-velocity media. When there are tissues with different velocities---typical for the human body---, the image of the underlying targets is strongly degraded in placement and resolution, due to $p\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}e$ $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$. To address this problem, the authors look to concepts developed in the context of seismic prospecting, relying upon an undulatory description of the physical process. Laboratory assessment of this imaging strategy, even in the presence of an aberrant layer, reveals remarkable spatial resolution and highly accurate target placement.
Highlights
This work presents an original ultrasound-imaging strategy, operating in the frequency domain, which relies on the undulatory description of the physical process, differing from the simplified geometric model of most of the current ultrasound systems.Conventional ultrasound systems for medical imaging are based on the impulse-echo technology known as beamforming [1]
A test is performed on experimental raw data acquired in the synthetic aperture mode, with a single element transmitting and all elements receiving in a repeating cycle
As the exact Hilbert-transformed intensity data cannot be retrieved from the ultrasound-system output, to qualify the synthetic aperture imaging of Algorithm 1, signal-to-noise ratio (SNR) evaluations are performed with reference to a theoretical formulation on the statistics of the speckle intensity
Summary
This work presents an original ultrasound-imaging strategy, operating in the frequency domain, which relies on the undulatory description of the physical process, differing from the simplified geometric model of most of the current ultrasound systems. The potential round-trip time of the ultrasound pulse, sensed by a subset of elements around the view line, can be a priori evaluated, stored in a table, and used to form an image by properly delaying and summing the captured signals It is by sequentially insonifying all the view lines that the illuminated portion of the region of interest can be entirely imaged with emitted frequencies that, depending on the target depth, range anywhere between 1 and 20 MHz. Above and below the focused target, the beamformed backscattered signals are plagued by out-of-focus echoes that compromise the lateral resolution and the contrast of the image. The final reconstruction is formed by stacking on a single image all the partial results obtained from common-source gathers corresponding to different source locations The insertion in this processing of the downward wave propagator, operating forward and backward in time, produces a simple, accurate, and potentially fast numerical engine.
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