Abstract: A table-top MRI system with substantially reduced capital requirements enables various applications to be used on education and research institution and various laboratory use for MRI applications. A magnetic structure is disclosed with two rectangular permanent magnets, four right triangular permanent magnets, and four right trapezoidal permanent magnets. The MRI system further comprises an RF assembly distinguished by its tuning system.

Abstract: Switched magnetic fields (in addition to the usual pulsed magnetic gradient fields) aiding and/or opposing the usual constant static magnetic field B.sub.o are utilized so as to increase the signal-to-noise ratio for given available imaging sequence times and/or to provide special imaging effects.

Abstract: A control interface for a magnetic resonance imaging (MRI) system provides extended, improved linker syntax/techniques that maximize reuse of program segments, allow optimal compromise between detailed representation and efficient sequence program design and maintenance, and permit uniform application of the loop syntax to link commands for MRI sequencers with a wide range of hardware implementations.

Abstract: A multi-rate sample MRI (magnetic resonance imaging) digital receiver maintains synchronization of digitally processed signals. This multi-rate sample data system is used to demodulate and filter a digitized MRI RF signal. A phase/timing relationship is established between the signal received and processed by the digital receiver, and the physical nuclear magnetic resonance (NMR) process the body being imaged is undergoing. Once established, the phase/timing relationship is maintained for the duration of the particular NMR experiment being performed. Special logic in the digital system ensures that the data output is synchronized with an external synchronous signal controlling data acquisition within the MRI system. Optimum signal processing is performed to minimize the time between the end of the acquisition control signal and the last output from the digital signal processing system.

Abstract: An all digital controlled current driver is used for each pulsed electromagnet (e.g., gradient coils) in a magnetic resonance imaging (MRI) system. Such an all digital current controller may be advantageously employed in either closed loop or open loop gradient coil control circuits. The elimination of analog components decreases cost, increases operating efficiency and improves operating characteristics of the MRI system.

Abstract: A local frequency generator employing a single crystal oscillator, latches and direct digital synthesizer circuits digitally produces all signals needed in the transmitter channel of a MRI system to generate MRI transmitter RF pulses. The local frequency generator is operable in both the single side band and double side band modes and has the capability of switching between the modes. The generator is constructed with a phase resetting capability for providing the plural output frequencies needed for making plural MRI slices.

Abstract: The effective Q of a capacitively coupled RF coil in a magnetic resonance imaging (or magnetic resonance spectroscopic imaging) system is controllably lowered from its intrinsic maximum value by controlling the impedance reflected across the coil via a tuning/matching network associated and located with the coil. The Q may be advantageously lowered during transmit time as compared with receive times (during which the Q may be relatively increased) by effecting controlled impedance mismatches within the RF feed network used to supply RF signals to/from the RF coil.

Abstract: Static B.sub.o field strength is measured during each TR interval of an MRI sequence providing field calibration data used to compensate for rapid variations in B.sub.o during the MRI sequence.

Abstract: At least one extra NMR measurement cycle is performed without any imposed magnetic gradients during readout and recordation of the NMR RF response. Calibration data derived from this extra measurement cycle or cycles can be used for resetting the RF transmitter frequency and/or for phase shifting other conventionally acquired NMR RF response data to compensate for spurious changes in magnetic fields experienced during the NMR data measuring processes. Some such spurious fields may be due to drifting of the nominally static magnetic field. Another source of spurious fields are due to remnant eddy currents induced in surrounding conductive structures by magnetic gradient pulses employed prior to the occurrence of the NMR RF response signal. Special procedures can be employed to permit the compensation data itself to be substantially unaffected by relatively static inhomogeneities in the magnetic field and/or by differences in NMR spectra of fat and water types of nuclei in imaged volumes containing both.

Abstract: In a gradient coil set for a magnetic resonance system, the y gradient coils are located substantially closer to the patient than are the x and z gradient coils. As a result, one may design the y gradient coils to produce a stronger y gradient, to have reduced inductance or otherwise better tailor the magnetic/electrical properties of the gradient coil set for MRI imaging sequences. In the exemplary embodiment, at least portions of the y gradient coils have a first spacing from the z-axis while the x and z gradient coils have a second substantially larger spacing from the z-axis. Furthermore, while the x and z gradient coils are centered about the z-axis in the patient access space, alternate sides of the y gradient coil set are centered about respectively off-set centers vertically displaced from the z-axis center of the patient access area.

Abstract: An RF coil for a magnetic resonance imaging device using nuclear magnetic resonance phenomena includes self-tracking ganged coupling capacitors which provide impedance matching and also tune out coil inductance to establish resonance at the RF operating frequency. At least one variable capacitor connected in parallel with the RF coil is mechanically ganged to at least one variable capacitor connected in series to the RF coil. The parallel and series capacitances are initially set to achieve resonance at a desired operating frequency. As the shaft of one variable capacitor is rotated in a first direction through a given angle, the shaft of the other variable capacitor rotates in the opposite direction through the same angle, thereby varying the ratio (but not the sum) of parallel and series capacitances. A servo controller connected to a standing wave ratio detector automatically adjusts the ratio of parallel to series capacitances to optimize impedance matching between the source and the RF coil.

Abstract: A detuning/decoupling arrangement for a Magnetic Resonance Imaging (MRI) system RF coil arrangement (of the typing using the nuclear magnetic resonance, or NMR, phenomenon) uses switching diodes to selectively connect and disconnect portions of an RF resonant circuit in response to a DC control signal. The DC control signal selectively forward biases and reverse biases the switching diodes. The DC control current is fed to the resonant circuit along the same RF transmission line used to feed RF signals to/from the circuit. An in-line coaxial shielded RF choke connected to the RF transmission line isolates the DC control signals from the RF signals flowing on the same transmission line--reducing the number and complexity of isolation devices required on the ends of the transmission line to separate the RF and DC signals.

Abstract: A quadrature detection MRI radio frequency coil includes plural axially-extending conductive legs. At least two of the legs include breaks in conductivity at their mid-point with circumferentially extending inner-bridge conductors connected thereacross to which RF input/output feed connections are conveniently made so as to achieve a center fed coil structure, open at both ends. Other axially-extending legs may also include conductive breaks with capacitive coupling thereacross and circumferentially extending outer-bridges between the outer or distal ends of the legs may also include breaks in conductivity with capacitive coupling thereacross.

Abstract: In a multi-slice magnetic resonance imaging system which employs a train of plural RF NMR nutation pulses, the frequency spectrum and/or magnetic gradient employed for succesive pulses is controlled so as to effect more nearly equal full-width-half-magnitude (FWHM) or other spatial dimensions of actual nuclei nutation variation versus distance curves for all of the slice selective nutation pulses. The result is a reduction of any "gap" of non-imaged volume disposed between the succession of selected MRI slice volumes.

Abstract: NMR imaging apparatus and method is arranged so as elicit NMR image response data in a predetermined order which provides the more significant lower spatial frequency image data during an initial portion of a relatively long complete image data acquisition cycle. The remaining higher spatial frequency image data is captured during subsequent portions of the overall image data acquisition cycle. In this manner, apparent motion artifact in the resulting image is reduced. Furthermore, such a special data acquisition sequence permits image reconstruction processes to produce a recognizable image at an earlier time in the complete data gathering cycle thus permitting a more timely image display for the apparatus operator to use in monitoring and/or controlling the NMR imaging procedure.

Abstract: An assembler/linker process utilizes predetermined control program segments ("templates" defining a sub-sequence of magnetic resonance imaging (MRI) states) together with predetermined program-change tables of MRI parameter values. At least some of the program segments are referenced by predetermined symbolic addresses and include pointer-references to corresponding ones of the table entries. The assembler/linker process machine replicates a predetermined set of such slice/specific program segments in a predetermined order while also simultaneously indexing corresponding symbolic addresses and referenced table entries in a predetermined sequence so as to maintain proper correspondence between slice-specific main programs and subroutines for each replicated program segment.

Abstract: NMR response data usable to form a T1 NMR image is obtained by using constant repetition rate data acquisition cycles but wherein different initial nuclei nutation angles are employed to elicit NMR spin echo signal responses. Because of the different initial nutation angles employed to elicit the NMR responses, those responses will differ in accordance with an exponential function of the T1 spin-lattice relaxation time constant of the region under test. Once such T1-dependent data is captured, the T1 time constant may be calculated or otherwise derived (e.g. by an empirical best fit to an exponential curve of known time constant) to produce a T1 value for each incremental volume element (voxel) of the volume under test (e.g. a planar volume). In this manner a visual image of the T1 NMR parameter is created when the complete array of derived T1 values is converted to gray-scale (or color) values and displayed as corresponding picture elements (pixels) in a raster-scanned CRT display or the like.

Abstract: First and second co-located axially extending RF coils are displaced spatially with respect to one another by 90 degrees so as to produce a pair of RF signals from the enclosed volume which are in phase quadrature. The same coils or one of them may also be utilized for transmitting NMR RF signals into the same volume. The coils have a common open-end through which objects to be imaged may be inserted and a common "closed" end for convenient location of RF coupling and feeding structures. Each of the coils has an RF feedline structure which extends across a diameter of common closed end and the two feedline structures are disposed substantially perpendicular with respect to one another so as to minimize cross coupling. The feedlines are also spatially configured to facilitate mounting of associated tuning and coupling capacitors.

Abstract: NMR imaging apparatus having at least one pulsed magnetic field gradient coil generating pulsed magnetic gradient fields and having at least one other coil structure magnetically linked thereto (and therefore subject to induced pulsed currents which create opposing magnetic fields if permitted to flow) includes apparatus connected in circuit with the other coil structure for suppressing such induced currents. The suppression is effected by causing the other coil structure to have increased electrical impedance with respect to such induced currents. Where the other coil structure is a radio frequency transmit/receive coil having plural parallel connected turns, the suppression apparatus includes at least one capacitor in series with at least some of the turns so as to present a substantially open circuit condition at the relatively lower frequencies of the unwanted induced currents.

Abstract: An imaging NMR scanner obtains plural spin echo signals during each of successive measurement cycles permitting determination of the T2 parameter for each display pixel after but a single measurement sequence. The amplitude of the NMR spin echo responses is dependent on an "a" machine parameter (the elapsed time between initiation of a given measurement cycle and the occurrence of the NMR response) and upon a "b" machine parameter (the elapsed time between initiation of successive measurement cycles). These a and b machine time parameters are selectively controlled to enhance resultant image contrast between different types of tissue or other internal structures of an object under examination. Special phase control circuits ensure the repeatability of relative phasing between successive NMR responses from the same measured volume and/or of reference RF signals utilized to frequency translate and synchronously demodulate the NMR responses in the successive measurement cycles of a complete measurement sequence.