Abstract: An MRI system includes a noise filter which receives each acquired NMR signal during a scan and detects short-duration noise pulses by sensing the signal level in a band of frequencies outside the imaging bandwidth. A blanking circuit suppresses the NMR signal during the time interval each noise pulse is detected to remove the noise pulse prior to image reconstruction.

Abstract: A radio frequency coil for use in MR imaging applications includes a series of conducting segments aligned with the longitudinal, static magnetic field of the MR magnet. A hybrid power splitter/combiner feeds each axial segment with a phased signal to generate a transverse circularly polarized magnetic field. The power splitter combiner isolates the axial segments to reduce magnetic field distortions produced by capacitive coupling between the imaged object and the radio frequency coil. The axial segments are tuned to series resonance to further reduce capacitive coupling. A stepped shield design permits separation of the power splitter combiner from the axial segments without piercing the bore tube.

Abstract: A method of tuning a radio frequency coil for use in MR imaging applications uses reactive shunts having a reactance in parallel with a switch to connect sections of the coil to ground thereby constraining the resonant mode of the coil to a known orientation. A plurality of measurements of the frequency of the resonances along different such constrained orientations is used to deduce the frequencies of the two unconstrained orthogonal resonant modes of the coil and its orientation. The shunts may also be used to tune the coil according to the deduced frequencies of the unconstrained resonance and the desired frequency and orientation by adjusting the reactance of the shunts. A shunt having a variable reactance and a switch for shorting the RF coil as required uses a PIN diode in combination with a variable reactance.

Abstract: A method for providing plural coaxial cable connections, each to a different portion of a single radio-frequency (RF) antenna without requiring the use of isolation means at any RF connection, determines at least a point within the antenna having a desired common potential adjacent to each of the different portions to which one of the coaxial cable connections is to be made; Then forms a separate segment of each different portion which is located substantially at the common potential and is reactively separated from adjacent segments of the associated portion; and connects a shield conductor of an associated coaxial cable to the separate segment, while connecting a center conductor of that same associated coaxial cable to a selected one of the adjacent segments of that different portion.

Abstract: A detunable coil assembly for an NMR imaging system has a cylindrical cage coil comprising two spaced-apart conductive end loops with a plurality of conductive segments extending between the loops. A grounded shield is disposed around the cage coil. Each of the end loops is coupled to the shield by four switchable impedance circuits that are equidistantly spaced around the loop. Each impedance means includes a coaxial cable having a length equal to one-quarter the wavelength of the resonant frequency of the cage coil with the central conductor coupled at one end to the cage coil and the other conductor connected to the shield. A PIN diode terminates the other end of the coaxial cable. A voltage supply provides potentials for alternately forward and reverse biasing the PIN diode.

Abstract: A coil assembly for radio frequency NMR imaging is designed to resonate at a desired frequency. The assembly includes a cylindrical field coil having a pair of conductive end loops disposed in a spaced-apart relation along a central axis. A plurality of conductive segments electrically interconnect the conductive end loops at periodic points around their periphery. These conductive segments include reactive elements which cause the field coil to resonate at a given frequency. A magnetic shield is disposed about the field coil. A plurality of reactive shunts are coupled between different locations about the periphery of the field coil and a common electrical node, such as the shield. Each of the reactive shunts is variable to increase or decrease the resonant frequency of the field coil, thereby tuning it to the desired frequency.

Abstract: 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.

Abstract: A matching device uses mutual inductance to couple an NMR radio frequency (RF) coil to an RF power amplifier and RF receiver preamplifier. The device includes a matching coil connected to the RF power amplifier and preamplifier and which is positioned between the NMR RF coil and a decoupling shield in a region where RF flux density, produced by the NMR RF coil, is maximum. This ensures effective coupling to the NMR RF coil and provides a way for adjusting the degree of coupling (hence, input impedance) by varying the area of the matching coil available for intercepting the RF flux.