Cecil E. Hayes

An efficient, highly homogeneous radiofrequency coil for whole-body NMR imaging at 1.5 T

Optimisation de lhomogeneite du champ radiofrequence necessaire pour produire des sequences dimpulsions a multichocs et du rapport signal sur bruit

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.

Shield for decoupling RF and gradient coils in an NMR apparatus

A shield for decoupling radio frequency (RF) and magnetic field gradient coils in an NMR apparatus is made up of first and second arrays of electrically conductive regions separated from one another by relatively narrow non-conductive regions. The arrays are disposed on opposite surfaces of a member formed of a high dielectric material. The conductive regions of one array are offset relative to the conductive regions of the other array such that the conductive regions of the one array bridge the non-conductive regions of the other array to form a plurality of capacitive elements. In operation, the capacitive elements act essentially as electrical short-circuits at radio frequencies, but transmit substantially unaffected the homogeneous magnetic field and audio frequencies associated with pulsed magnetic field gradients.

Nuclear magnetic resonance imaging at microscopic resolution

Abstract Resolution limits in NMR imaging are imposed by bandwidth considerations, available magnetic gradients for spatial encoding, and signal to noise. This work reports modification of a clinical NMR imaging device with picture elements of 500 × 500 × 5000 μm to yield picture elements of 50 × 50 × 1000 μm. Resolution has been increased by using smaller gradient coils permitting gradient fields >0.4 mT/cm. Significant improvements in signal to noise are achieved with smaller rf coils, close attention to choice of bandwidth, and signal averaging. These improvements permit visualization of anatomical structures in the rat brain with an effective diameter of 1 cm with the same definition as is seen in human imaging. The techniques and instrumentation should open a number of basic sciences such as embryology, plant sciences, and teratology to the potentials of NMR imaging.

Noise performance of surface coils for magnetic resonance imaging at 1.5 T

In this paper we analyze the signal-to-noise ratio (SNR) for surface coil magnetic resonance imaging at 1.5 T. We have applied the treatment of Hoult and Lauterbur to determine the factors that most affect coil performance. We have imaged lossy phantoms with 8-, 10-, and 14-cm-diam circular surface coils and compared the results to body and head coil images. Surface coils can improve SNR by a factor of 4 or more for regions close to the surface. Surface coils are effective for regions up to 6 cm deep in the head and about 12 cm deep in the body. Nonuniformity of image intensity is a necessary requirement for improved SNR in surface coils. Coil losses make only a small contribution to image noise compared to tissue losses at 1.5 T. Surface coils need not be placed in close contact with the patient at 1.5 T.

Apparatus and method for enhanced multiple coil nuclear magnetic resonance (NMR) imaging

A nuclear magnetic resonance (NMR) signal acquisition apparatus includes a cylindrical array of overlapping coils. Coupling of currents between coils due to re-radiation of received signals, in particular noise currents, is reduced by presenting a high impedance to each coil, thereby reducing the current circulating in each coil. A PREDAMP circuit is disclosed which utilizes the input impedance of a preamplifier, transformed through a quarter-wavelength transmission line segment, to achieve the high input impedance for the coil. As a result, multiple images, each with a high signal-to-noise ratio (SNR), can be simultaneously obtained. A method is disclosed for combining the multiple images into a composite image with optimum SNR, taking into account the phase shifts between the images resulting from the spatial orientation of the coils.

Pulmonary Mr Angiography Utilizing Phased-array Surface Coils

Magnetic resonance angiography of the pulmonary vasculature was evaluated in 12 subjects using breath-hold gradient echo scans and surface coils at 1.5 T. Flow-compensated GRASS, spoiled GRASS (SPGR), and WARP-SPGR sequences were utilized. Comparisons were made among flip angles of 10-60 degrees, slice thicknesses of 3-10 mm, and body coil as well as Helmholtz pair and phased-array multiple coils. With 30-40 contiguous slices encompassing the lung, intrathoracic vasculature was segmented using a UNIX/X-windows based package dubbed VIDA. Three-dimensional anatomy was visualized by a brightest voxel projection algorithm, following reduction of chest wall pixel intensities by an operator-interactive module. Both SPGR (30 degrees flip angle, 4 mm slice thickness) and WARPSPGR (15 degrees flip angle, 5 mm slice thickness) in combination with phased-array multiple coils provided the most satisfactory images, based upon observations by three radiologists and signal-to-noise ratio measurements. The MR angiograms visualized vessels as distal as sixth to seventh order branches. The technique was successfully applied to three patients with pulmonary embolism. The results of this study demonstrate that the pulmonary vascular tree can be imaged by MR angiography combining a high resolution technique utilizing phased-array multiple coils, fast gradient echo sequences with breath-holding, and postprocessing of the volumetric image data. The technique is attractive since it is noninvasive and provides a full three-dimensional portrayal of the pulmonary vasculature.

An adjustable RF coil loading device

This paper describes an adjustable loading device that can substitute for the tissue losses of various sized patients in a whole-body MR imager. It resembles a lowpass birdcage resonator but with resistors replacing the capacitors on the cylindrical surface. Power dissipated is a monotonic function of the total surface conductance of the loader. The loader can be used in conjunction with a low loss water filled phantom to measure signal-to-noise ratios (SNR). The impact on measured SNR due to screening by the loader and changing the phantom size or composition are also briefly discussed.

DESIGN AND ANALYSIS OF SURFACE COILS FOR NMR IMAGING.

The parameters for designing surface coils for NMR imaging have been identified by introducing an equivalent circuit for it. Also a method of evaluation and testing of surface coils has been discussed in the lab environment.