Mechanical properties of interfacial films formed by lysozyme self-assembly at the air-water interface

We present the first characterization of the mechanical properties of lysozyme films formed by self-assembly at the air-water interface using the Cambridge interfacial tensiometer (CIT), an apparatus capable of subjecting protein films to a much higher level of extensional strain than traditional dilatational techniques. CIT analysis, which is insensitive to surface pressure, provides a direct measure of the extensional stress-strain behavior of an interfacial film without the need to assume a mechanical model (e.g., viscoelastic), and without requiring difficult-to-test assumptions regarding low-strain material linearity. This testing method has revealed that the bulk solution pH from which assembly of an interfacial lysozyme film occurs influences the mechanical properties of the film more significantly than is suggested by the observed differences in elastic moduli or surface pressure. We have also identified a previously undescribed pH dependency in the effect of solution ionic strength on the mechanical strength of the lysozyme films formed at the air-water interface. Increasing solution ionic strength was found to increase lysozyme film strength when assembly occurred at pH 7, but it caused a decrease in film strength at pH 11, close to the pI of lysozyme. This result is discussed in terms of the significant contribution made to protein film strength by both electrostatic interactions and the hydrophobic effect. Washout experiments to remove protein from the bulk phase have shown that a small percentage of the interfacially adsorbed lysozyme molecules are reversibly adsorbed. Finally, the washout tests have probed the role played by additional adsorption to the fresh interface formed by the application of a large strain to the lysozyme film and have suggested the movement of reversibly bound lysozyme molecules from a subinterfacial layer to the interface.

An inverse methodology for high frequency RF head coil design with pre-emphasized B1 field in MRI

An inverse methodology to assist in the design of radio-frequency (RF) head coils for high field MRI application is described in this work. Free space time-harmonic electromagnetic Green's functions and preemphasized B1 field are used to calculate the current density on the coil cylinder. With B1 field preemphasized and lowered in the middle of the RF transverse plane, the calculated current distribution can generate an internal magnetic field that can reduce the EM field/tissue interactions at high frequencies. The current distribution of a head coil operating at 4 T is calculated using inverse methodology with preemphasized B1 fields. FDTD is employed to calculate B1 field and signal intensity inside a homogenous cylindrical phantom and human head. A comparison with conventional RF birdcage coil is reported here and demonstrated that inverse-method designed coil with preemphasized B1 field can help in decreasing the notorious bright region caused by EM field/tissue interactions in the human head images at 4 T.

Tailoring magnetic field gradient design to magnet cryostat geometry

Eddy currents induced within a magnetic resonance imaging (MRI) cryostat bore during pulsing of gradient coils can be applied constructively together with the gradient currents that generate them, to obtain good quality gradient uniformities within a specified imaging volume over time. This can be achieved by simultaneously optimizing the spatial distribution and temporal pre-emphasis of the gradient coil current, to account for the spatial and temporal variation of the secondary magnetic fields due to the induced eddy currents. This method allows the tailored design of gradient coil/magnet configurations and consequent engineering trade-offs. To compute the transient eddy currents within a realistic cryostat vessel, a low-frequency finite-difference time-domain (FDTD) method using total-field scattered-field (TFSF) scheme has been performed and validated