Preliminary Investigation into the Vibration Characteristics and Isolation Requirements of a Prototype MRI Scanner

Abstract

Magnetic Resonance Imaging (MRI) is a powerful diagnostic and research tool, used extensively in diagnoses of brain diseases, musculoskeletal disorders or damage, and an- giography. A persistent issue with MRI machines is inadequate vibration isolation. The vibration isolation issue is two-faceted. First, the scanner produces significant noise and vibrations during operation which, when transmitted to the machines environment, have negative repercussions such as the production of acoustic noise. Second, any motion of the scanner due to environmental factors will produce noise in the image, thereby limiting the resolution that can be achieved by the scanner. The objective of the present work is to implement passive vibration isolation in the base structure of a prototype MRI scanner. The first step to accomplishing this is the creation and validation of a finite element (FE) model of the MRI base, which requires computational and experimental modal analysis. The natural frequencies and mode shapes of a model of the MRI base up to 600 Hz were calculated computationally using ANSYS and experimentally with a shaker modal test. Seven mode pairs were visually correlated between experimental and computational data. Thus far, subjectively good correlation has been achieved. Following this, the model is used to simulate isolator performance in conjunction with further experimental testing of vibration isolator alternatives. A small scale test of the pneumatic isolator demonstrates that the specific isolator type performs well under the MRI loading conditions, and establishes a testing protocol for future testing. Furthermore, on-site testing that compares the performance of the full-sized pneumatic isolator installed on a scanner against a scanner mounted on aluminium blocks. The isolators were found to perform well, but do not meet the design requirements. Lastly, the pneumatic isolators are modelled in ANSYS to evaluate their performance in the worst case scenario (isolator resonance). A Matlab script and the ANSYS model are used in conjunction to calculate optimal stiffness and damping coefficients.