Elastography, sometimes referred as seismology of the human body, is an imaging modality recently implemented on medical ultrasound systems. It allows to measure shear waves within soft tissues and gives a tomography reconstruction of the shear elasticity. This elasticity map is useful for early cancer detection. A general overview of this field is given in the first part of the presentation as well as latest developments. The second part, is devoted to the application of time reversal or noise correlation technique in the field of elastography. The idea, as in seismology, is to take advantage of shear waves naturally present in the human body due to muscles activities to construct shear elasticity map of soft tissues. It is thus a passive elastography approach since no shear wave sources are used. In the third part some examples are provided using ultrasounds, MRI or optic to detect shear waves and reconstruct a speed tomography in a human liver, thyroid, brain, in a mouse eye and a single cell.
Background and motivation -
Conventional Optical Coherence Elastography (OCE) methods consist in launching controlled shear waves in tissues, and measuring their propagation speed using an ultrafast imaging system. However, the use of external shear sources limits transfer to clinical practice, especially for ophthalmic applications. Here, we propose a totally passive OCE method for ocular tissues based on time-reversal of the natural vibrations.
Experiments were first conducted on a tissue-mimicking phantom containing a stiff inclusion. Pulsatile motions were reproduced by stimulating the phantom surface with two piezoelectric actuators excited asynchronously at low frequencies (50-500 Hz). The resulting random displacements were tracked at 190 frames/sec using spectral-domain optical coherence tomography (SD-OCT), with a 10x5µm² resolution over a 3x2mm² field-of-view (lateral x depth). The shear wavefield was numerically refocused (i.e. time-reversed) at each pixel using noise-correlation algorithms. The focal spot size yields the shear wavelength. Results were validated by comparison with shear wave speed measurements obtained from conventional active OCE. In vivo tests were then conducted on anesthetized rats.
The stiff inclusion of the phantom was delineated on the wavelength map with a wavelength ratio between the inclusion and the background (1.6) consistent with the speed ratio (1.7). This validates the wavelength measurements. In vivo, natural shear waves were detected in the eye and wavelength maps of the anterior segment showed a clear elastic contrast between the cornea, the sclera and the iris.
We validated the time-reversal approach for passive elastography using SD-OCT imaging at low frame-rate. This method could accelerate the clinical transfer of ocular elastography.
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.
Time-resolved 2D Pulsed Elastography is a new elastographic technique for imaging the shear modulus of soft tissues. A low-frequency transient shear wave is sent in the medium while an ultra-fast ultrasonic imaging system acquires frames at a very high frame rate (up to 10,000 frames/s). This ultra-fast ultrasonic imaging system has been specifically developed for this application. It is based on a time-reversal mirror of 128 channels sampled at 50 MHz and having 2 Mbytes random access memory. Displacements induced by the slowly propagating shear wave are measured using the standard cross-correlation technique. The low-frequency excitation is obtained with a device composed of two rods that are placed around the ultrasonic transducer linear array. The rods vibrate perpendicularly to the surface of the tissues. They may be either parallel or perpendicular to the active surface of the array. With this device, large amplitude displacements are observed in the ultrasonic image area. We have measured the spatio- temporal evolution of the displacements induced by the low- frequency (20-100 Hz) shear wave in tissue-equivalent phantoms and breast in-vivo. A direct local inversion is used to recover the shear modulus distribution map in phantoms containing hard regions.