A convertible pneumatic actuator for brain and phantom elastography

Introduction Here we present a pneumatic actuator design developed for human brain magnetic resonance elastography (MRE). The MRE is part of the MR examination protocol, which provides input data for a neurosurgical oncology rehearsal and training system. Authentic information on mechanical properties of healthy and tumor tissue makes realistic emulation of haptic perception in the neurosurgical simulations possible. Methods The following criteria are important in actuator design: • minimize possible interference of the actuator with other MR examinations e.g. fMRI, DTI etc.,which are included in the protocol. For example, susceptibility artifacts from the actuator could corrupt fMRI data, mechanical actuator parts could visually obscure the projection of images during fMRI, etc. • easy and fast setup of MRE examination in order to fit within the time restrictions available for the examination, • provide maximal convenience and safety for the patient, • flexible design, which allows reconfiguration of the actuator for experiments using phantoms. Considering these criteria, a pneumatic actuator concept was chosen [1], as an alternative to an electromechanical actuator used in previous MRE brain studies [2;3]. After experimenting with different shapes and configurations of pneumatic drivers, we found that some of the polyethylene containers commonly used for food or personal care products could be exploited for this purpose. We used a pair of liquid honey (noname® brand) bottles because of their shape and flexible walls. A homemade fitting was used to connect the bottleneck to the hard-walled flexible tubes of 8 m length and 32 mm diameter. Two modified active subwoofers placed outside the magnet room were used to deliver acoustic pressure waves into the drivers (see Fig.1(a)). The hoses from the speakers enter the magnet room trough the waveguide filters installed on the shielding cage in order to eliminate any eventual electromagnetic interferences. One speaker is set to operate with a phase difference of 180, using a toggle switch. The arbitrary function generator (Tektronix AFG3022B), triggered from the MRE pulse sequence was used to generate the desired acoustic waveforms. The system is capable to operate in a frequency range of 25 -150Hz. Three different MRE actuator configurations are available: Head actuator thermally molded tray, which could be slit into the head coil, serves as the base for the head actuator (see Fig. 2B). Velcro® strips are used to attach the drivers to the tray. The drivers are placed in a “V” shape around the back of the head. Foam-padding installed between them provides additional support for the head. A small exchangeable towel covers the bottles and foam pad during the exam for hygienic reasons. Probe actuator – the drivers are placed in homemade brackets and attached to the coil instead of head restrainers (see Fig. 2C). The T-bar placed between the drivers is secured with Velcro® strips. The height of the actuator contact plate is adjustable, using a screw. Various contact plates can be used with the actuator according the phantom’s size and shape. Bed type actuator – when a large contact area with the source of oscillations is desirable or when the sample is enclosed in a container, the configuration can be changed to a so called bed-type actuator [4]. The holder, equipped with four rollers, is positioned into the bottom part of the head coil. The sample cart is placed on the