Vesicle Biomechanics in a Time-Varying Magnetic Field

Background Cells exhibit distortion when exposed to a strong electric field, suggesting that the field imposes control over cellular biomechanics. Closed pure lipid bilayer membranes (vesicles) have been widely used for the experimental and theoretical studies of cellular biomechanics under this electrodeformation. An alternative method used to generate an electric field is by electromagnetic induction with a time-varying magnetic field. References reporting the magnetic control of cellular mechanics have recently emerged. However, theoretical analysis of the cellular mechanics under a time-varying magnetic field is inadequate. We developed an analytical theory to investigate the biomechanics of a modeled vesicle under a time-varying magnetic field. Following previous publications and to simplify the calculation, this model treated the inner and suspending media as lossy dielectrics, the membrane thickness set at zero, and the electric resistance of the membrane assumed to be negligible. This work provided the first analytical solutions for the surface charges, electric field, radial pressure, overall translational forces, and rotational torques introduced on a vesicle by the time-varying magnetic field. Frequency responses of these measures were analyzed, particularly the frequency used clinically by transcranial magnetic stimulation (TMS). Results The induced surface charges interacted with the electric field to produce a biomechanical impact upon the vesicle. The distribution of the induced surface charges depended on the orientation of the coil and field frequency. The densities of these charges were trivial at low frequency ranges, but significant at high frequency ranges. The direction of the radial force on the vesicle was dependent on the conductivity ratio between the vesicle and the medium. At relatively low frequencies (\u3c200 \u3eKHz), including the frequency used in TMS, the computed radial pressure and translational forces on the vesicle were both negligible. Conclusions This work provides an analytical framework and insight into factors affecting cellular biomechanics under a time-varying magnetic field. Biological effects of clinical TMS are not likely to occur via alteration of the biomechanics of brain cells

Deformation but Not Migration and Rotation – A Model Study on Vesicle Biomechanics in a Uniform DC Electric Field

Background: Biological cells migrate, deform and rotate in various types of electric fields, which have significant impact on the normal cellular physiology. To investigate electrically-induced deformation, researchers have used artificial giant vesicles that mimic the phospholipid bilayer cell membrane. Containing primarily the neutral molecule phosphatidylcholine, these vesicles deformed under evenly distributed, strong direct current (DC) electric fields. Interestingly, they did not migrate or rotate. A biophysical mechanism underlying the kinematic differences between the biological cells and the vesicles under electric stimulation has not been worked out. Methods: We modeled the vesicle as a leaky, dielectric sphere and computed the surface pressure, rotation torques and translation forces applied on the vesicle by a DC electric field. We compared these measurements with those in a biological cell that contains non-zero, intrinsic charges (carried by the functional groups on the membrane). Results: For both the vesicle and the cell, the electrically-induced charges interacted with the local electric field to generate radial pressure for deformation. However, due to the symmetrical distribution of both the charges and the electric field on the vesicle/cell surface, the electric field could not generate net translation force or rotational torques. For a biological cell, the intrinsic charges carried by the cell membrane could account for its migration and rotation in a DC electric field. Conclusions: Results from this work suggests an interesting control diagram of cellular kinematics and movements by the electric field: cell deformation and migration can be manipulated by directly targeting different charged groups on the membrane. Fate of the cell in an electric field depends not only on the delicately controlled field parameters, but also on the biological properties of the cell

Advancements in Magnetic Resonance Image Guided Radiotherapy

Magnetic resonance image guided radiation therapy (MRgRT) devices are a recently developed technology that integrate the excellent soft tissue contrast and real-time imaging capabilities of MRI with a medical linear accelerator (Linac). This provides an unprecedented ability to guide and adapt radiation therapy treatments based on real-time cine imaging. However, the merging of these technologies has come with unique challenges. MRI lacks the geometric fidelity of computed tomography (CT). Spatial inaccuracies in MRI can result from magnetic field (B0) or center frequency variations, gradient-induced eddy currents, and magnetic field gradient imperfections (e.g., nonlinearities, poor calibration, concomitant fields, and unsatisfactory electronic fidelity). Previous work identified gantry angle dependent shifts in the imaging isocenter of a commercial 0.35 T MRI-Linac. Additionally, the balanced steady state free precession (bSSFP) sequences used in MRgRT offer excellent signal to noise ratios (SNRs) and temporal resolution, but require high levels of B0 homogeneity, B0 stability, and precise control over the gradient systems. Banding artifacts appear in the resulting images if these stipulations are violated and intravoxel dephasing approaches an odd multiple of π. Rotation of the radiation therapy gantry also results electromagnetic interference (EMI). The gantry-related EMI causes banding artifacts on images collected during that time. Similarly, cardiac implanted electronic devices (CIEDs) result in magnetic susceptibility artifacts primarily due to ferromagnetic components. These artifacts manifest as banding artifacts in bSSFP images and make tracking structures in or near the heart challenging during treatment imaging. The work presented in this dissertation investigates and quantifies the causes of imaging isocenter shifts, develops a method for real-time B0 compensation during rotation of the radiation therapy gantry, and introduces a deep learning solution to CIED induced artifacts on a commercial low-field MRgRT system

MRI quality control for low-field MR-IGRT systems: Lessons learned

PURPOSE: To present lessons learned from magnetic resonance imaging (MRI) quality control (QC) tests for low-field MRI-guided radiation therapy (MR-IGRT) systems. METHODS: MRI QC programs were established for low-field MRI- RESULTS: Image noise and artifacts were attributed to room noise sources, unsatisfactory system cabling, and broken RF receiver coils. Gantry angle-dependent magnetic field inhomogeneities were more prominent on the MRI-Linac due to the high volume of steel shielding in the gantry. B CONCLUSIONS: There are significant technological challenges associated with implementing and maintaining MR-IGRT systems. Most of the performance issues were identified and resolved during commissioning