Full-waveform inversion imaging of the human brain

Abstract

Magnetic resonance imaging and X-ray computed tomography provide the two principal methods available for imaging the brain at high spatial resolution, but these methods are not easily portable and cannot be applied safely to all patients. Ultrasound imaging is portable and universally safe, but existing modalities cannot image usefully inside the adult human skull. We use in silico simulations to demonstrate that full-waveform inversion, a computational technique originally developed in geophysics, is able to generate accurate three-dimensional images of the brain with sub-millimetre resolution. This approach overcomes the familiar problems of conventional ultrasound neuroimaging by using the following: transcranial ultrasound that is not obscured by strong reflections from the skull, low frequencies that are readily transmitted with good signal-to-noise ratio, an accurate wave equation that properly accounts for the physics of wave propagation, and adaptive waveform inversion that is able to create an accurate model of the skull that then compensates properly for wavefront distortion. Laboratory ultrasound data, using ex vivo human skulls and in vivo transcranial signals, demonstrate that our computational experiments mimic the penetration and signal-to-noise ratios expected in clinical applications. This form of non-invasive neuroimaging has the potential for the rapid diagnosis of stroke and head trauma, and for the provision of routine monitoring of a wide range of neurological conditions.

Full wave 3D inverse scattering transmission ultrasound tomography in the presence of high contrast

“…We present here a data acquisition system and reconstruction algorithm which gives the required reconstructions from laboratory data (not simulations) establishing the feasibility of ultrasound tomography even in the presence of bone. We note the ~ 200 times speed up that Guasch et al 1 require with a reconstruction time of ~ 24 min for the algorithm presented here. The data were acquired in ~ 12-14 min (depending on the rate of rotation of the arrays).…”

“…The difficulty with this is the elliptic nature of the Helmholtz partial differential equation (PDE). Such PDE's, when discretized for solution on the computer, involve the inversion of very large matrices, which is a notoriously time consuming, and clinically relevant images must be available in tens of minutes-not days 1 .…”

“…ent technique is equivalent to 5 forward problems: Two forward problems for the analytic Jacobian calculation (also known as Frechet derivative) and two equivalent forward problems ('adjoint field and backpropagated www.nature.com/scientificreports/ field) for the gradient calculation and a forward problem to check the residual with the new value. The efficiency of the forward problem and gradient formation lead to image creation in ~ 20 min, which is the speed up asked for in Guasch et al 1 . Further this is done with laboratory data, which addresses the calibration problem which is handled as in Wiskin et al 13 .…”

“…However, there is presently no ultrasound tomographic system for orthopaedic or whole-body imaging in the presence of high impedance contrast (bone). Recent work with simulations has shown promise 1 . However, they bypass critical issues of calibration when dealing with in vivo data and the image reconstruction times given are not feasible clinically.…”

Reflection-Mode Ultrasound Computed Tomography Based on Wavelet Processing for High-Contrast Anatomical and Morphometric Imaging

“…Diffraction-mode USCT is effective for imaging the soft tissues of an organ, for example a breast, for which a fluid-like model is workable, and provides promising benefits [7]. However, USCT can be hampered by the acquisition of a large volume of recorded data, long processing times, or high computation costs, especially for iterative nonlinear approaches [8][9][10][11][12][13][14][15][16][17][18]. The computational cost issue tends to be less limiting nowadays thanks to the continuous increase in the power of computers and high-performance computing systems, so that USCT could become, in the medium term, an interesting modality for complete organ imaging.…”