We present a magnetic resonance elastography approach for tissue characterization that is inspired by seismic noise correlation and time reversal. The idea consists of extracting the elasticity from the natural shear waves in living tissues that are caused by cardiac motion, blood pulsatility, and any muscle activity. In contrast to other magnetic resonance elastography techniques, this noise-based approach is, thus, passive and broadband and does not need any synchronization with sources. The experimental demonstration is conducted in a calibrated phantom and in vivo in the brain of two healthy volunteers. Potential applications of this "brain palpation" approach for characterizing brain anomalies and diseases are foreseen.elastography | brain | correlation T he complexity of wave fields can sometimes be an advantage for imaging. Such is the case in multiple scattering or reverberating media, where wave fields contain information about their sources and the medium itself. Turning this wave noise into useful measurements through correlation techniques has provided a breakthrough in a wide variety of domains, which range from seismology (1) to acoustics (2, 3) and electromagnetism (4). Living tissue is also full of unexploited vibrations. Their detection with ultrafast ultrasound scanners that can reach thousands of frames per second (5-7) has recently opened up the medical field to correlation techniques and therefore, passive elastography (8). However, ultrasound is not suitable for brain imaging. MRI can image the brain, but its relatively low acquisition rate of a few frames per second is an issue. Synchronization with the shear-wave source is, thus, necessary (9, 10), which penalizes its potential implementation based on natural shear waves.We describe a magnetic resonance elastography (MRE) -based method that is free from the need for synchronization and any controlled source. This approach extracts information related to the mechanical properties of the soft tissue from hundreds of snapshots of randomly fluctuating shear-wave fields. The key to decrypting the complex field is correlation or similarly but from a physical point of view, time reversal (11, 12). Not only does this wide-band approach maximize the signal-to-noise ratio, as any matched filter would, but it also avoids the Nyquist-Shannon problem that is inherent to slow imaging devices. Indeed, although time information is definitely lost, the spatial information is still present and allows shear-wavelength tomography to be conducted. This wavelength is closely related to the shear elasticity and thus, the intuitive estimation of the stiffness felt by physicians during palpation examination. To start, this concept is shown using MRI in a calibrated elastography phantom under randomly sampled vibrations. Arterial pulsation can produce motion in the brain as high as 1 mm (13); the resulting natural shear-wave field is analyzed through correlation algorithms, and passive brain palpation reconstructions are presented. ResultsPhantom Experiments. As s...

Optical coherence tomography (OCT) can map the stiffness of biological tissue by imaging mechanical perturbations (shear waves) propagating in the tissue. Most shear wave elastography (SWE) techniques rely on active shear sources to generate controlled displacements that are tracked at ultrafast imaging rates. Here, we propose a noise-correlation approach to retrieve stiffness information from the imaging of diffuse displacement fields using low-frame rate spectral-domain OCT. We demonstrated the method on tissue-mimicking phantoms and validated the results by comparison with classic ultrafast SWE. Then we investigated the in vivo feasibility on the eye of an anesthetized rat by applying noise correlation to naturally occurring displacements. The results suggest a great potential for passive elastography based on the detection of natural pulsatile motions using conventional spectral-domain OCT systems. This would facilitate the transfer of OCT-elastography to clinical practice, in particular, in ophthalmology or dermatology.

SignificanceIn wave physics, and especially seismology, uncorrelated vibrations could be exploited using “noise correlation” tools to reconstruct images of a medium. By using a high-frequency vibration, a high-speed tracking device, and a reconstruction technique based on temporal correlations of travelling waves we conceptualized an optical microelastography technique to map elasticity of internal cellular structures. This technique, unlike other methods, can provide an elasticity image in less than a millisecond, thus opening the possibility of studying dynamic cellular processes and elucidating new mechanocellular properties. We call this proposed technique “cell quake elastography.”

When a wave field is measured within a propagative medium, it is widely accepted that the resulting image resolution depends on the measuring point density, and no longer on the wavelength. Indeed, in situ measurements allow the near-field details needed for super-resolution to be retrieved. Rarely studied in elastography, this is supported here by experiments. A passive elastography imaging of two inclusions in a tissue mimicking phantom is shown with a resolution down to 1/45 of a shear wavelength.

This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of 1 µm. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample. The displacement of a conductor in a magnetic field induces eddy currents. Conversely, the application of an electrical current in a conductor placed in a magnetic field induces a displacement due to Lorentz force [1]. These two phenomena are currently investigated to produce medical images [2]. In the technique called Lorentz Force Electrical Impedance Tomography [3], also known as Magneto-Acoustical Electrical Tomography [4], an ultrasound beam is focused in a tissue placed in a magnetic field. The displacement of the tissue due to ultrasound in a magnetic field induces an electrical current. The current is measured using electrodes and has been used to produce tissue electrical conductivity interface images. In a "reverse" mode, injecting an electrical current in a tissue placed in a magnetic field induces a displacement due to Lorentz force. As in the megahertz range, shear waves decay over a few micrometers, the displacement propagates only through compression waves. These waves can be detected using ultrasound transducers to produce electrical conductivity images. One implementation of this method is called Magneto-Acoustic Tomography with Magnetic Induction [5].We hypothesized in this study that applying a low frequency (10-1000 Hz) electrical current through a tissue placed in a magnetic field would produce a shear wave within the medium. This could notably have applications in shear wave elastography [6], [7], [8], a medical imaging technique used to map the mechanical properties of biological tissues. The mechanical properties of biological tissues are known to be viscoelastic (hence frequency-dependent) [9], [10], [11], often anisotropic, e.g. along muscle fibers [12], and nonlinear (changing with pre-stress). However, in the field of medical imaging, most applications rely on a simple model, assuming an elastic isotropic linear solid. The viscoelasticity effect has been shown to have only second effect orders [13] and the synthetic phantoms as used in this study can reasonably be considered as fully isotropic and linear [14], [15]. Under these assumptions, tissue elasticity can be described by two parameters only, e.g. the shear modulus µ and Poisson's ratio. The shear modulus ...

The local application of ultrasound is known to improve drug intake by tumors. Cavitating bubbles are one of the contributing effects. A setup in which two ultrasound transducers are placed confocally is used to generate cavitation in ex vivo tissue. As the transducers emit a series of short excitation bursts, the evolution of the cavitation activity is monitored using an ultrafast ultrasound imaging system. The frame rate of the system is several thousands of images per second, which provides several tens of images between consecutive excitation bursts. Using the correlation between consecutive images for speckle tracking, a decorrelation of the imaging signal appears due to the creation, fast movement, and dissolution of the bubbles in the cavitation cloud. By analyzing this area of decorrelation, the cavitation cloud can be localized and the spatial extent of the cavitation activity characterized.

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Water electrolysis was discovered in 1800, with the famous experiment investigated here within soft tissue from an elastic-wave point of view. Indeed, we report that the rapid formation of hydrogen bubbles after transient (10 ms) electrolysis in water-based gels produces elastic waves. These bubbles are observed using an ultrafast optical camera. As the bubbles are trapped between the rigid electrode and the soft matter, they act as a source of elastic waves that are measured in the bulk using an ultrafast ultrasound scanner. The elastic-wave amplitude is shown to be in good agreement with a simple bubble model.