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Development and Evaluation of an Actuator System based on Centrifugal Force for Magnetic Resonance Elastography

Neumann, Wiebke

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Abstract

Magnetic resonance elastography (MRE) serves as an important diagnostic tool. It represents one of numerous approaches to monitor tissue stiffness. The most fundamental challenges that MRE face are posed by two linking factors: Constructing a mechanical device that induces tissue motion to the depth of interest and meaningfully resolving said movement in the complex magnetic resonance imaging (MRI) signal. This work aims to address these challenges by improving the quantification of tissue stiffness through the development of a new actuation system for MRE. Firstly, a 3D printed pneumatic turbine vibrator was developed to induce sinusoidal mechanical waves. It used an eccentrically rotating mass generating a centrifugal force in the turbine. Contrary to conventionally used acoustic pressure drivers, the pneumatic turbine was capable of producing wave amplitudes in the range of appropriate shear waves in human tissue - especially at higher frequencies due to the centrifugal force increasing quadratically in relation to the rotational frequency. A technical assessment showed that the turbine generated vibrations in the range of 30 Hz to 150 Hz. The extent of artifacts caused by the materials brought into the field of view was restricted to the proximity of the actuator. It did not affect image quality in the region of interest. The turbine was MR-safe and an in-house certification according to §3 MPG was conducted, which enabled in-house clinical in vivo studies. The actuation system was additionally extended to a dual turbine actuator in order to investigate if the attenuation of shear waves could be further compensated by using two wave sources. Secondly, a motion encoding sequence was developed to meaningfully encode the tissue motion in the MRI signal. It was a spin-echo echo-planar-imaging sequence (SE-EPI) and contained a motion encoding gradient (MEG) adjustable for actuation frequencies ranging from 40 Hz to 120 Hz. To accurately reconstruct the wave velocities, i.e tissue elasticity, a trigger was implemented that synchronized the motion encoding sequence to the mechanical waves. Thirdly, the actuator system was evaluated regarding its performance for MRE image acquisition in a clinical MRI scanner. Silicone-based tissue elasticity mimicking phantoms were developed as test objects with known elasticity. Their shear moduli were in the range of 1.47 kPa to 7.29 kPa, which corresponds to the range of human soft tissue elasticities. A prostate phantom and an anthropomorphic abdominal phantom were manufactured. MR images were acquired with the SE-EPI sequence and were sufficient in terms of signal to noise ration (liver: SNR = 71.5) and contrast to noise ratio (liver: CNR = 16.5). The phantoms may also be used for multi-modal imaging; besides MRI, computed tomography (liver: 106+/-6 HU) and ultrasound imaging by adding scatter particles is feasible. The actuator did not interfere with the imaging procedure and could be integrated into existing clinic equipment. Three actuation set-ups were evaluated: a single, a large surface and a dual source actuation. For each, the strength of the MEG was varied from 5 mT/m to 20 mT/m for actuation frequencies ranging from 50 Hz to 80 Hz. The dual source actuation demonstrated a more uniform penetration of a larger volume of interest, especially in the peripheral region of the abdominal phantom. The obtained elasticity maps showed elasticity values (liver: 1.12+/-0.16 kPa, filling material: 4.37+/-0.52 kPa) in accordance to the results obtained by rheometric testing of the silicone samples. Additionally, an in vivo MRE examination was conducted, which served as a proof-of-principle for the successful implementation of the first developed MRE actuator system in our clinic. For both liver and prostate MRE, the actuator was well tolerated by the volunteer. Since the developed actuation technique is non-invasive, its incorporation into routine MRI protocols will facilitate patient acceptance, while its short additional set-up time will also increase clinical acceptance. MRE is a unique technique for the identification of various pathologies and the quantification of the shear modulus has the potential to become a further independent parameter for MRI diagnostics in a variety of clinical applications.

Item Type: Dissertation
Supervisor: Zöllner, adj. Prof. Dr. Frank G.
Place of Publication: Heidelberg
Date of thesis defense: 4 April 2019
Date Deposited: 02 May 2019 11:34
Date: 2019
Faculties / Institutes: Medizinische Fakultät Mannheim > Zentrum für Biomedizin und Medizintechnik (CBTM)
Subjects: 610 Medical sciences Medicine
620 Engineering and allied operations
Controlled Keywords: Kernspintomografie, Elastographie
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