The present invention pertains to the use of magnetic materials for the suppression of eddy currents, and the use of highly conductive materials to enhance signal-to-noise performance as related to radio frequency antennas of magnetic resonance (NMR) imaging systems.
Magnetic resonance imaging techniques in present use generally employ pulsed magnetic field gradients to spatially encode the nuclear magnetic resonance (NMR)signal from various portions of an anatomical region of interest. The pulsed magnetic field gradients together with radio frequency excitation of the nuclear spins, and acquisition of signal information are commonly referred to as a pulse sequence.
Pulsing current through a set of conductors will produce a magnetic field external to the conductor; the magnetic field generally has the same time course of development as the current flow in the conductors. The conductors may be distributed in space to produce three orthogonal gradients X, Y, Z. Each of the gradients may be independently pulsed by a separate time dependent current waveform. When the set of conductors is in close proximity to an electrically conductive object, such as the ferromagnetic pole of a primary field magnet assembly, pulsing magnetic field gradients will in turn generate currents and their associated magnetic fields in the electrically conductive object. These secondary magnetic fields oppose the establishment of the gradient magnetic fields. Such eddy currents thus create a delay in the establishment of steady state levels of the magnetic field gradients.
A typical imaging procedure involves the use of three orthogonal magnetic field gradients, Z, X and Y, which are pulsed coordinately along with bursts of radio frequency energy. An example of this is as follows: the Z gradient is pulsed on for two brief time periods during which a 90.degree. radio frequency pulse in the first time period and a 180.degree. radio frequency pulse in the second time period are used to select a slice of anatomy of interest and to induce the nuclear spin system within that slice to generate an NMR signal. Once the slice is selected by the Z gradient, the two remaining orthogonal gradients are used to confer spatial encoding on the NMR signal in the two orthogonal directions. Thus, the Y-gradient will encode on the basis of phase advances imparted on a series of signal responses by using a pulsed gradient waveform of progressively increasing area; and the X gradient, which is pulsed on during the signal collection period, will frequency encode the NMR signal in the third orthogonal direction.
The NMR signal will be processed to yield images which give an accurate representation of the anatomical features in the selected slice, as well as provide excellent soft tissue contrast. NMR signals may be processed using various algorithms depending upon the precise nature of the data acquisition procedure; however, all methods employed rely on the ability to spatially encode the signal information by making use of magnetic field gradients which are time modulated and sequentially pulsed in various modes to effect the desired result.
Since most of the aspects of a pulse sequence, such as radio frequency excitation of the nuclear spin system, and acquisition of spatially encoded information are predicated on the existence of stable, steady state magnetic field gradients, the existence of eddy currents lengthens the time course of the pulse sequence, and thus the imaging process. Also, eddy currents inhibit the ability to follow a faster imaging regimen which might yield potentially more valuable diagnostic information. A reduction or suppression in eddy currents is therefore a desirable goal in NMR imaging and is the subject of this invention.
The pulsed gradient sequence described above is by no means an exhaustive treatment of pulse sequences used in magnetic resonance imaging. More complete descriptions of pulse sequences and how they are varied to yield medically diagnostic information may be found in numerous publications, for example "Magnetic Resonance Imaging" by STARK, DAVID, D., and BRADLEY Jr., WILLIAM, G., (C.V. MOSBY COMPANY, 1988).
A traditional method used to overcome eddy currents is to overdrive the gradient voltage waveform during the early phases of establishment of desired magnetic field gradient levels. This method has proven effective in suppressing the effects of eddy currents. However, the method still limits the execution of pulse sequences with more rapidly switching magnetic field gradients, and must be tailored to the amplitude and duration of the individual waveforms. Furthermore, such methods do not address the problem of spatial dependence of the eddy currents, leaving only best compromise solutions to cope with the variations in eddy current magnitude and duration across the imaging region of interest.