3D ultrafast ultrasound imaging in vivo

Abstract

Very high frame rate ultrasound imaging has recently allowed for the extension of the applications of echography to new fields of study such as the functional imaging of the brain, cardiac electrophysiology, and the quantitative imaging of the intrinsic mechanical properties of tumors, to name a few, non-invasively and in real time. In this study, we present the first implementation of Ultrafast Ultrasound Imaging in 3D based on the use of either diverging or plane waves emanating from a sparse virtual array located behind the probe. It achieves high contrast and resolution while maintaining imaging rates of thousands of volumes per second. A customized portable ultrasound system was developed to sample 1024 independent channels and to drive a 32 × 32 matrix-array probe. Its ability to track in 3D transient phenomena occurring in the millisecond range within a single ultrafast acquisition was demonstrated for 3D Shear-Wave Imaging, 3D Ultrafast Doppler Imaging, and, finally, 3D Ultrafast combined Tissue and Flow Doppler Imaging. The propagation of shear waves was tracked in a phantom and used to characterize its stiffness. 3D Ultrafast Doppler was used to obtain 3D maps of Pulsed Doppler, Color Doppler, and Power Doppler quantities in a single acquisition and revealed, at thousands of volumes per second, the complex 3D flow patterns occurring in the ventricles of the human heart during an entire cardiac cycle, as well as the 3D in vivo interaction of blood flow and wall motion during the pulse wave in the carotid at the bifurcation. This study demonstrates the potential of 3D Ultrafast Ultrasound Imaging for the 3D mapping of stiffness, tissue motion, and flow in humans in vivo and promises new clinical applications of ultrasound with reduced intra--and inter-observer variability.

Figure 1. 3-D Ultrafast Ultrasound Imaging Framework

A Acquisitions are defined by a virtual array located behind the probe, which is then used to synthetically form an entire volume. B For each individual source, delays are computed and a sub-aperture is defined. C When virtual sources are located near the physical probe, the sub-aperture used is smaller and the curvature of the emitted waveform is increased, which results into the insonification of a large field-of-view at the cost of a lower propagated energy. D When sources are located far behind the probe, larger sub-apertures, and thus, emitted energy, at the cost of a smaller field of view. In the extreme case of sources located at infinity behind the probe, tilted plane waves are obtained.

Figure 2. Resolution and Contrast

A An improvement in resolution is obtained when a larger virtual array is used. Indeed, as the number of virtual sources increases, the focal spot narrows and converges toward the optimal case of a focused emission in transmit and receive. Note that transmit-receive dynamic focusing does not lead to a spectacular improvement of the resolution compared to receive only focusing. The improvement in terms of contrast is more important; even a small number of emissions can significantly increase the contrast both in phantoms (B) and in the carotid of a human subject in vivo (C).

Figure 3. Motion estimation for 3-D Shear-Wave Imaging

Propagation of a shear-wave generated using radiation force from an A approximately isometric view, from B a top view and C a side view. The velocity of the wave was found to be approximately 0.8 m/s and corresponds to the stiffness of the phantom used (1.92 kPa).

Figure 4. 3-D Ultrafast Doppler Imaging in the Heart

Blood flow in the left ventricle of a healthy volunteer during an entire cardiac cycle. After segmentation using the Power Doppler date, different well-known phases can be identified in the Color Flow Doppler ciné-loops such as ejection and rapid filling.

Figure 5. 3-D Ultrafast Doppler of the Carotid Bifurcation

Blood flow and tissue motion in the bifurcation of the carotid of a healthy volunteer overlaid onto a high quality B-mode volume during an entire cardiac cycle, along with quantitative blood flow shown for three regions at the entrance and exit of the bifurcation.