Imaging mechanical shear waves induced by piezoelectric ceramics in magnetic resonance elastography

Abstract

Magnetic Resonance Elastography (MRE) is a noninvasive technique to measure elasticity of tissues in vivo. In this paper, a mechanical shear wave MR imaging system experiment is set for MRE. A novel actuator is proposed to generate mechanical shear waves propagating inside a gel phantom. The actuator is made of piezoelectric ceramics, and fixed on a plexiglass bracket. Both of the gel phantom and the actuator are put into a head coil inside the MR scanner's bore. The actuator works synchronously with an MR imaging sequence running on the MR scanner. The sequence is modified from a FLASH sequence into a motion-sensitizing phasecontrast sequence for shear wave MR imaging. Shear wave images are presented, and these effects on the shear wave MR imaging system, including the stiffness of phantoms, the frequency of the actuator, the parameters of the motion-sensitizing gradient, and the oscillation of the patient bed, are discussed.MRI is a noninvasive medical imaging modality for stimulating the development of medical diagnosis technology. Scientists and engineers are now trying to extend the use of traditional MRI scanner to a wider field. The magnetic resonance elastography (MRE) technique is one of the instances [1] . It is one of the several medical elasticity imaging methods including ultrasound elastography [2 -5] , optical elastography [6,7] . MRE is an analog to the palpation, which has been used by doctors for thousands of years, and is still used now in modern hospitals as a very useful technique to detect diseases such as breast diseases. Doctors use palpation to detect manually the changes of biomechanical properties of pathological tissues. It is proved that the biomechanical properties, such as shear modulus and Yong's modulus can serve as parameters to detect diseases such as breast cancer. For example, the Yong's modulus of pathological tissues are obviously larger than that of normal tissues [8] . The advantages of MRE are that the technique can evaluate the biomechanical properties of tissues quantitatively but not by experience, and can enable us to observe the tissues inside the body profoundly that cannot be palpated by doctors. MRE is proposed to be a very potential technique in medical diagnosis, especially for breast diseases [9][10][11][12][13] . The other medical application fields of MRE include the muscle [14][15][16] and the prostate [17] diseases diagnosis. MRE needs to be implemented via an MRI scanner. An additional mechanical oscillation actuator is always set on the surface of the tissue, and the waves produced by that oscillation will spread from the surface to the inner, which is to be imaged via the MRI scanner. On the way of the wave propagation, when the stiffness of tissue changes, the wave changes its parameters, such as wavelength. So from the wave images of the tissue, the change of the tissue stiffness can be detected, and the map of the tissue elasticity can be calculated via elasticity reconstruction algorithms [18][19][20][21][22][23] .For MRE, the actuator is an importa...

Magnetic resonance elastography: A review

“…Figure 3b shows an electromechanical driver that works via the Lorentz force and utilizes the magnetic field of the main MRI magnet (Braun et al, 2003; Muthupillai et al, 1995). A piezoelectric stack driver system is shown in Figure 3c, where the motion created is based on the piezoelectric property of certain materials (Chen et al, 2006; Othman et al, 2005). Focused-ultrasound-based (FUS-based) radiation force has also been investigated as a means to create mechanical motion for various elasticity imaging strategies including MRE, where shear waves are created directly within tissue with externally placed ultrasound transducers (Bercoff et al, 2004; Nightingale et al, 2001; Wu et al, 2000).…”

Tuneable Resonance Actuators for Magnetic Resonance Elastography

“…Piezoelectric materials are MRI compatible, and MR safe actuators can be made from these materials, and safely placed in close proximity to patients with minimal image distortion [10]. Lever systems for displacement amplification combined with a piezo actuator [10], a piezoelectric bending element with needle positioned parallel to the plane surface [11], piezoelectric ceramic connected to a phantom surface with a slim plexiglass plate [12], and a piezoceramic actuator with a compliant mechanical amplifier [13] are few actuator setups that have been used for MRE measurements. The main problem with piezoelectric actuators is their effective displacement gets smaller as actuation frequency is increased.…”

Magnetic Resonance Elastography

“…Although simple in design and flexible in its ability to produce a pure shearing motion, these devices typically provide limited power and can introduce imaging artifacts during sensitive imaging sequences, such as echo planar imaging (EPI). Various configurations of piezoelectric elements have also been used to mechanically excite tissue, including piezoelectric benders, extenders and stacks [36–38]. Like electromagnetic actuators, piezoelectric devices can also produce well-controlled motion, but they tend to be fragile, low-power (high-voltage) and not widely used due to low relative displacements and high acoustic impedance.…”