1020253035404550[0001] This invention relates generally to methods and apparatus for correction of distortion of signals ina nuclearmagnetic resonance (NMR) system, and more particularly to methods and apparatus for correction of distortion in suchNMR systems caused by switching of gradient magnetic fields when such switching results in oscillatory B0 magneticfields.[0002] In at least one known NMR imaging device, nuclear spins are subjected to magnetic fields and excited byaspin excitation signal froma radio frequency transmitter. The magnetic field is uniform and homogeneous.The frequencyofthe spin excitation signal is such thata resonant matching occurs toa natural Larmor precession frequency for thosemagnetic spins to be excited. The excited nuclearspins precess abouta direction ofthe homogeneous magnetic fieldvector at an angle that depends uponthe strength and duration ofthe spin excitation field. If the homogeneous magneticfield varies with time, the precession frequency will also vary.[0003] In addition, at least one known NMR and imaging device utilizes gradient magnetic fields for volume selectivespectroscopy or imaging. Gradient magnetic fields are appliedto encode volume regions ofa sample and thereby allowfor position sensitive measurements of the nuclear magnetic resonance signal. However, whenthe gradient magneticfields are switched on and off, inductive coupling produces current flow in conductive elements of the device. Thesecurrents produce undesirable time-dependentmagnetic fields that adversely affect signal measurement. An undesirableeddy current field component, i.e.,a uniform B0 component, is one result of necessary gradient field changes. Uncom-pensatedB0 eddy currentscan leadto image quality problems suchas ghostingor to degradedMR (magnetic resonance)spectroscopy performance.[0004] In known systems, only exponentially decaying gradient and B0 eddy current errors have been recognized.However,the introduction of shielded magnets with shortened axial extenthas resulted in drastically higherstatic magneticfields nearthe edges ofthe gradient coil. These magnetic fields, in turn, produce higher forces and significant oscillatoryeddy currents. Oscillatory B0 eddy currents cause unwanted side lobes in MR spectroscopy and artifacts in MR imaging.[0005] S. Crozier et at. in "Correction forthe Effect of Induced B e Shifts..."; J. Magn. Res., seriesB 1 03,1 15 - 119(1994) describea method anda correction system for compensating for the effects of eddy currents due to pulsedgradients in an NMR system, the method comprising the steps of measuring the temporal magnetic fields due to eddycurrents and shifting the frequency ofthe NMR transmitter and receiver by an amount proportional to the B e shift dueto the eddy currents.[0006] A change ofthe gradient waveform, necessitatesa new measurement to be carried out, in order to assesstheeddy currents and the temporal magnetic fields which also have changed. Hence, this method cannot adapt in real-timetoa change ofthe applied gradients.[0007] It would be desirable to provide apparatus and methods for correcting for oscillatory B0 eddy currents in thepresence of changes in the gradient field.[0008] In one exemplary embodiment ofthe present invention, an NMR apparatus havinga transmitter generatingaspin excitation signal anda receiverdetecting an NMR signal analyzes gradient signals to estimatea resulting oscillatoryB0 eddy currentB e (t).A frequency of either the transmitter or the receiver of the NMR apparatus, or both, is shifted infrequency by an amount proportional toB e (I) to compensate for the oscillatory B0 eddy current. The applied gradientsignals are digitized and filtered usinga recursive filter derived from an oscillatory model ofthe eddy currentB e st). Therecursive filter has a complex-valued output, and the eddy currentB e (I) estimate is the real part ofthe complex-valuedoutput ofthe recursive filter.[0009] The above described embodiment and others that are described herein effectively compensate for oscillatoryB0 eddy currents to provide enhanced NMR image quality and MR spectroscopy performance.Embodiments ofthe invention will now be described, by way of eKample, with reference to the accompanying drawings,in which:-Figure1 isa graph illustrating an observed B0 behavior in response to an application ofa gradient magnetic field.Figure2 isa graph showinga measured oscillatory B0 eddy current ina NMR system.Figure3 isa block diagram ofan embodiment ofa single channel recursive filter for estimatinga correction frequencyin accordance with the invention.Figure4 isa graph showing measuredB0 eddy currents inthe sameNMR system from which Figure2 was obtained,but in which frequency shift compensation in accordance with the invention was applied.Figure5 isa block diagram ofan embodiment ofa three-channel recursive filter for estimatinga correction frequencyin accordance with the invention.[0010] The present invention, in one embodiment, isa filter that is utilized in an NMR system (not shown)to providereal-time compensation of B0 error for arbitrary gradient inputs. In this embodiment, theNMR system includesa radiofrequency transmitter (not shown) that excites nuclear spins subjected toa uniform and homogeneous magnetic field2
