286 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 13, NO. 3, JUNE 2008 Evaluation of Electrorheological Fluid Dampers for Applications at 3-T MRI Environment Azadeh Khanicheh, Dionyssios Mintzopoulos, Brian Weinberg, A. Aria Tzika, and Constantinos Mavroidis, Member, IEEE Abstract—This paper evaluates the use of electrorheological flu- ids (ERFs) within a magnetic resonance imaging (MRI) environ- ment. ERF is a semiactive variable impedance material, which could be used as an alternative type of resistive force/torque gen- eration or in combination with other actuators as a damper/clutch to modulate the output force/torque of the actuator. In this paper, an ERF damper/brake is introduced and its magnetic resonance (MR) compatibility is examined at a 3-T MR imaging environment by measuring the output performance of the damper and the SNR of the MRI images. The experimental results showed that damper’s resistive force generation while positioned within the MRI is almost the same as that in normal operation. The signal-to-noise investiga- tion was performed both with a phantom and human. The results indicated that the ERF damper did not affect the MRI images when it was operated over 30 cm away from the MRI’s RF coil. We hope that the synthesis and tables presented in this paper can facilitate the choice of ERF brake actuation principle to various applications in an MR environment. Index Terms—Actuator, brake, damper, electrorheological fluid (ERF), functional magnetic resonance imaging (fMRI), magnetic resonance (MR) compatible robotic/mechatronic systems, mag- netic resonance imaging (MRI). I. INTRODUCTION M AGNETIC resonance imaging (MRI) is one of the most versatile modalities in modern diagnostic medicine and an indispensable tool in a wide range of clinical and basic sci- ence research. Among the emerging biomedical engineering areas of current development in MRI are the performance of image-guided interventions (IGIs), neuroscience studies during functional MRI (fMRI), diagnostic fMRI, and the monitoring of the progress of rehabilitation therapies [1]–[7]. The use of MRI in IGI, neuroscience studies, and monitoring of rehabili- tation procedures share common benefits and challenges, both originating from the inherent properties of this modality. The MRI scanner employs extreme magnetic fields, rapidly changing magnetic field gradients, and RF pulses. As a result, the com- Manuscript received December 5, 2007; revised February 27, 2008. Recom- mended by Guest Editor N. Tsekos. This work was supported by the National Institutes of Health (NIH) under Grant 5R21EB004665-02. A. Khanicheh, B. Weinberg, and C. Mavroidis are with the Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115 USA (e-mail: azadeh@coe.neu.edu; weinberg@coe.neu.edu; mavro@coe.neu.edu). D. Mintzopoulos and A. A. Tzika are with the Department of Surgery, Mas- sachusetts General Hospital and Shriners Burns Institute, Harvard Medical School, Boston, MA 02114 USA (e-mail: dionyssi@nmr.mgh.harvard.edu; aaria tzika@hms.harvard.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMECH.2008.924043 monly used mechatronic and robotic systems are not suitable for MRI applications, resulting in a need for viable alternatives to the conventional materials, sensors, and actuators. The choice of an adequate actuation method is a primary is- sue in the development of any mechatronic device and strongly determines the dynamic performances of the system. This choice is particularly difficult for mechatronic systems work- ing within a magnetic resonance (MR) environment because of the safety and compatibility constraints imposed by the MR environment. These constraints are even more severe in the case of fMRI, where actuation is also required during imaging, whereas current MR-compatible interventional systems are typ- ically moved between imaging phases. The actuators must work safely within the MR environment, not disturb the imaging, and, in certain cases, allow safe physical interaction with a human subject. Conventional actuation principles involving electromagnetic actuators are generally not compatible with MR environment owing to their principle of operation. The generated electro- magnetic fields would disturb the imaging, and the permanent magnets would pose a safety threat as the ferromagnetic com- ponents would be strongly attracted by the high static magnetic field of the scanner (missile effect) [8], [9]. Therefore, alternative actuation principles must be investigated. A detailed overview of actuation principles for applications in MR environments is given in [8] and [9]. These principles include hydraulic, pneu- matic, belt, and cable transmissions as well as electrostatic and piezoelectric actuators. This paper presents another nonmagnetic type of actuation principle based on electrorheological fluids (ERFs). Electrorhe- ological (ER) fluids are smart materials whose rheological prop- erties (viscosity, yield stress, shear modulus, etc.) can be readily controlled using an external electric field. They can switch from a liquid-like material to a solid-like material within a millisec- ond with the aid of an electric field, and this phenomenon is called the ER effect. The unique feature of the ER effect is that ER fluids can reversibly and continuously change from a liquid state to a solid state. Using the electrically controlled rheolog- ical properties of ERFs, compact resistive elements capable of generating tunable and controllable forces/torques can be de- veloped. In this paper, a rotary ER fluid brake and a linear ER fluid damper is introduced. Two mechatronic systems that have utilized the ER brake/damper for fMRI in order to investigate human motor control and related dysfunctions are briefly ex- plained [10], [11]. This paper focused on the MR-compatibility evaluation of the ER fluid brakes and dampers at 3-T MR en- vironment. The MR compatibility in fMRI environments is 1083-4435/$25.00 © 2008 IEEE