Elasticity is an important physical attribute of biological tissue. Elasticity of normal tissue can differ significantly with pathological tissue in its stiffness. It is important to develop a clinical viable method to determine and visualize the elasticity of histological tissue. Magnetic resonance (MR) elastography is a promising noninvasive method to quantitatively measure the viscoelastic properties of biological tissue. We developed a new quasi-static actuation system with the spatial modulation of magnetization (SPAMM) imaging pulse sequence. The MR compatible actuation system consists of an ultrasonic motor, a force sensor, motion-transmission mechanism, ECG signal generation, and controller. The high torque ultrasonic motor, USR60-E3 by Shinsei Corporation, Japan, can function well in the magnetic environment. The simulated ECG signal is generated using function generator, and outputted to both MR scanner and controller respectively. SPAMM imaging sequence is utilized to acquire tagging images of the deformed tissue. The actuation is synchronized with the SPAMM sequence via simulated ECG signal. Indentation force is measured by MR compatible force sensor for the subsequent stiffness inversion procedure. Imaging experiments were conducted using this actuation system and a GE SIGNA 1.5T MR scanner. Clear tag-deformed images of agar gel phantom have been acquired. The results demonstrated the feasibility of utilizing SPAMM sequence and the proposed actuation system for tissue characterization. In comparison with dynamic method of MR elastography, this quasi-static method is relatively simple to implement since no motion sensitive gradient or gradient synchronization is required. A limitation of indentation method is the difficulty to obtain significant deformation of tumor inside the liver organ. Since radio frequency (RF) needle is inserted into liver organ during tumor ablation, the proposed system with needle-based actuation making use of this inevitable insertion will be able to intra-operatively image the liver tumor during ablation. The RF needle can directly vibrate the pathological tissue within the liver organ.

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