William Daniel Barber

Comparison of linear and circular polarization for magnetic resonance imaging

Abstract A comparison of experimental imaging results obtained with linearly polarized and circularly polarized radiofrequency excitation and reception is presented. Simulation images in good agreement with the experimental scans are described. The simulations are calculated with a model in which a homogeneous, isotropic cylinder of lossy dielectric material and infinite axial extent is immersed in a uniform rf magnetic field perpendicular to the axis. It is found that with the usual linear polarization, reconstructions of uniform objects have regions of decreased intensity. These artifacts are shown to arise from dielectric standing wave effects and eddy currents. The effects become more severe as the frequency or object size is increased, and depend upon the complex conductivity of the object. Results indicate that a significant reduction in the artifact intensity is achieved when circular polarization is employed for both transmission and reception. The expected benefits of circular polarization over linear polarization in reduction of excitation power (up to 50% reduction) and signal-to-noise advantage (√2) have been realized in practice with cylindrical objects and human subjects.

A New Time Domain Technique for Velocity Measurements Using Doppler Ultrasound

A new technique for determining the Doppler frequency shift in a phase-coherent pulsed Doppler system is presented. In the new approach, the Doppler frequency shift is given directly in the time domain in terms of the measured I and Q components of the measured Doppler signal. The algorithm is based on an expression for the instantaneous rate of change of phase which separates rapidly varying from slowly varying terms. It permits noise smoothing in each term separately. Since the technique relies solely on signal processing in the time domain, it is significantly simpler to implement than the classic Fourier transform approach. In addition, the algorithm can be shown to give rigorously accurate values for instantaneous frequency and outperform the Fourier transform approach in poor signal-to-noise environments. Experimental results are presented which confirm the superiority of the new domain technique.

Radiofrequency power deposition utilizing thermal imaging

Wavelength effects influence radiofrequency (RF) power deposition distributions and limit magnetic resonance (MR) medical applications at very high magnetic fields. The power depositions in spherical saline gel phantoms were deduced from proton resonance shift thermal maps at both 1.5 T and 3.0 T over a range of conductivities. Phase differences before and after RF heating were measured for both a quadrature head coil and a circular surface coil. A long echo time (TE) pulse sequence with a 3D phase unwrap algorithm provided increased thermal sensitivity. The measured thermal maps agreed with a model of eddy‐current heating by circularly polarized oscillating RF fields in a conducting dielectric sphere. At 3.0 T, thermal maps were acquired with a <0.32°C temperature rise at 4 W. Proton resonance shift thermal maps provided a measure of hot spots in very‐high‐field MR imaging (MRI), in which both the phase sensitivity and signal‐to‐noise ratio (SNR) were increased. The method provides a means of studying the heat distribution generated by RF coils excited by clinical pulse sequences. Magn Reson Med 51:1129–1137, 2004.

Transient Perturbation to Permanent Magnetic Field by Gradient Pulses in MRI Magnets

Open MRI magnets are generally designed with ferromagnetic poles to contain and shape the magnetic flux and to reduce conductor cost. Permanent magnet MR magnets have blocks of PM and bulk ferromagnetic materials on or close to the pole face. These electrically conducting regions are sources of eddy currents that affect the image quality because of their relatively long time constants and close proximity to the imaging volume. The impact on image quality can be minimized by appropriate segmentation and/or lamination of these components. Detailed eddy current diffusion models are necessary to quantify the field distortion and time constants of the resulting field to perform design studies. The three dimensional frequency or time domain models required to accurately predict effects of eddy currents due to gradient fields are not computationally economical. This paper describes modeling of a PM imaging system using simplified 2D models with appropriate assumptions to evaluate the impact of these eddy currents. Experimental validation of some of the results with a prototype magnet is provided

Memory characteristics of 2.2-mil plated-wire memory

To obtain lower drive requirements and higher densities, plated-wire memory must use thinner films on smaller diameter substrate. The memory element characteristics of thin (1000-3000 A) Permalloy film on 2.2-mil tungsten wire substrate are reported here. Nondestructive readout as well as destructive read-out properties and the effect of a simple keeper structure will be described. The determination of a valid test program in terms of pulse aud Belsen tests will also be discussed.

Low-frequency measurement of anisotropy field

A number of different methods of measuring the anisotropy field H K of magnetic films were investigated for application to the automation of low-frequency measurements. These methods included torque, the large signal susceptibility method recommended by the IEEE,[5] the small signal susceptibility, and a new method using the second harmonic component of the magnetization signal. The agreement between the IEEE method and the second harmonic method was found to be good. Both these methods also gave good agreement with values obtained for torque measurements.

Some factors controlling the uniformity and properties of NiFe plated wires

The changes in composition of NiFe for a change in flow, temperature, or current in the NiFe electrolyte are determined. The local uniformity of composition is controlled by varying these coefficients of composition through changes in the Ni++concentration and temperature of deposition. At low concentrations of Ni++and low temperatures, it is concluded that a local, or short-term, fluctuation in flow is the principal factor affecting the uniformity of the NiFe composition deposited, At higher Ni++concentrations the temperature fluctuations become the principal factor affecting the uniformity. The trends in magnetic properties and memory quality are also related t o t h e Ni++concentration and temperature of deposition. These trends are interpreted in terms of the model previously described for the origin of the nondestructive read-out properties of plated wire.