Current applied to wire coils wound at the tip of an endovascular catheter can be used to remotely steer a catheter under magnetic resonance imaging guidance. In this study, we derive and validate an equation that characterizes the relationship between deflection and a number of physical factors: theta/sin(gamma-theta) = nIABL/EI(A) where theta is the deflection angle, n is the number of solenoidal turns, I is the current, A is the cross-sectional area of the catheter tip, B is the magnetic resonance (MR) scanner main magnetic field, L is the unconstrained catheter length, E is Young's Modulus for the catheter material, and I(A) is the area moment of inertia, and y is the initial angle between the catheter tip and B. Solenoids of 50, 100, or 150 turns were wound on 1.8 F and 5 F catheters. Varying currents were applied remotely using a DC power supply in the MRI control room. The distal catheter tip was suspended within a phantom at varying lengths. Images were obtained with a 1.5 T or a 3 T MR scanner using "real-time" MR pulse sequences. Deflection angles were measured on acquired images. Catheter bending stiffess was determined using a tensile testing apparatus and a stereomicroscope. Predicted relationships between deflection and various physical factors were observed (R2 = 0.98-0.99). The derived equation provides a framework for modeling of the behavior of the specialized catheter tip. Each physical factor studied has implications for catheter design and device implementation.
The high-pressure magnetic properties of the heavy lanthanide elements Gd, Tb, Dy, Ho, Er, and Tm have been investigated using ac-magnetic susceptibility with a diamond anvil cell. It is found that the magnetic transition temperatures monotonically decrease with increasing pressure. In addition, the amplitudes of the magnetic transition signals decrease with increasing pressure, with the signals all eventually disappearing at pressures of 20 GPa. In contrast to previous studies, we see no evidence of any pressure-induced transitions from one magnetically ordered phase to another in Gd, Tb, Dy, or Ho. The transition temperatures T crit are all found to drop at a rate proportional to their de Gennes factor, and the values of T crit / T crit ͑P =0͒ vs P / P crit , where P crit is the pressure at which the magnetic transition disappears, all sit on a single phase diagram.
High-pressure electrical conductivity experiments have been performed on the Mott insulator MnO to a maximum pressure of 106 GPa. We observe a steady decrease in resistivity to 90 GPa, followed by a large, rapid decrease by a factor of 10 5 between 90 and 106 GPa. Temperature cycling the sample at 87 and 106 GPa shows insulating and metallic behavior at these pressures, respectively. Our observations provide strong evidence for a pressure-induced insulator-to-metal transition beginning at 90 GPa.
An RF low phase noise voltage-controlled oscillator is implemented with micromachined IC-compatible variable capacitors and three-dimensional coil inductor. Unlike conventional on-chip passive devices, the micromachined variable capacitors achieve a high-Q value above 60 at 1 GHz with a 15% tuning range for a nominal 2 pF capacitance with 3 V tuning voltage. Three-dimensional inductors minimize the substrate loss and achieve a Q of 30 at 1 GHz with a 4.8 nH inductance. Both passive components are fabricated on silicon substrates and thus amenable to monolithic integration with standard IC process. The prototype VCO exhibits −136 dBc/Hz phase noise at 3 MHz offset frequency from the carrier, suitable for most wireless communication applications, in particular GSM. The VCO is tunable from 855 Mhz to 863 MHz, limited by the test set-up.
High pressure magnetic susceptibility experiments can yield valuable insights into the changes in magnetic behavior and electron correlation properties which can accompany extreme compressions of matter. However, magnetic susceptibility experiments with ultrahigh pressure diamond anvil cells are extremely challenging due to the very small size of the high-pressure sample ͑Ϸ75 m diameter͒ and the difficulty of obtaining good coupling between the sample and the sensing coil. As a result, measurement sensitivity and poor signal-to-background ratios tend to be serious concerns which limit the applicability of these experiments. We present here a new approach to high-pressure ac magnetic susceptibility experiments that involve specially fabricated diamond anvils with diamond encapsulated sensing microcoils which are located just 10-20 m from the high-pressure sample. We also present some test results taken with a gadolinium sample in order to demonstrate the viability of this high-pressure ac susceptibility technique.
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