Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1998-03-25
2001-01-16
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06175236
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention disclosed and claimed herein generally pertains to an improved method for acquiring spatially and spectrally localized magnetic resonance (MR) images. More particularly, the invention pertains to a method of such type which corrects or compensates for undesirable effects caused by a perturbation magnetic field, which is produced by an oscillatory slice-selection gradient required for data acquisition.
As is well known by those of skill in the arts relating to MR imaging, a sequence known as a spatial-spectral (SPSP) pulse sequence, or SPSP pulse, can be used to excite a selected spectral species, in a spatially localized region. That is, the SPSP pulse sequence selects a particular slice through a subject (spatial localization) and at the same time selects material lying within the slice which has a particular spectral frequency range, such as water or fat. Such pulse sequences have been found to be especially useful in fast imaging sequences, for suppressing fat signals. Use of the SPSP pulse in an MR imaging sequence is described in further detail in the prior art, such as U.S. Pat. No. 4,999,580, issued Mar. 12, 1991 to Meyer et al.
As is further well known, an SPSP pulse sequence includes a slice-selection gradient field G(t), which oscillates as the SPSP pulse is being played out, so that two-dimensional (k,t)-space can be traversed to simultaneously achieve spatial and spectral localization. The SPSP pulse sequence further includes an RF pulse, comprising a succession of RF sub-pulse components having an envelope defined by the peaks of respective sub-pulses. The shape of the RF sub-pulses and the gradient determine the slice profile. If an MR image is to be acquired from a slice offset by a distance z from the iso-center of an associated MR system (i.e., the center of the MR main magnet), the SPSP pulse has an associated frequency modulation function f
z
(t) related to the gradient G(t), for an ideal arrangement, by the expression f
z
(t)=&ggr;G(t)z/2&pgr;, where &ggr; is the gyromagnetic ratio for a given spin species, e.g., protons. It is noted that since the gradient oscillates, the frequency modulation function f
z
(t) oscillates as well.
Unfortunately, the SPSP pulse sequence typically results in an arrangement which is not ideal. The oscillatory gradient, required for use with the SPSP pulse as stated above, often produces undesirable effects, such as eddy currents and non-linear gradient amplifier response. These effects, in turn, generate perturbation magnetic fields, which result in considerable signal intensity loss for large values of z, that is, for slices distant from the MR magnet iso-center.
SUMMARY OF THE INVENTION
The invention is directed to a method for acquiring spatially and spectrally selective MR images from a subject positioned proximate to an MR imaging system. The method includes the step of selecting an SPSP pulse sequence, comprising a succession of RF sub-pulses and an oscillatory gradient magnetic field, the SPSP sequence being disposed to select a slice through the subject for imaging. Specified parameters of a perturbation magnetic field associated with the imaging system are measured, and an expression for the perturbation field is derived from the parameters and from the SPSP gradient magnetic field. The method further includes specifying a frequency modulation function associated with the SPSP sequence which is disposed to offset the selected slice to a particular spatially localized region of the subject. The SPSP pulse sequence is modified by adjusting the frequency modulation function thereof, in specififed corresponding relationship with the derived expression for the perturbed oscillatory gradient, and the modified SPSP pulse is applied to the subject to excite a selected spectral species in the spatially localized region.
A useful embodiment of the invention is directed to a situation wherein the perturbation field comprises a time-dependent magnetic field induced by eddy currents produced by the oscillatory gradient. The measuring step for such embodiment comprises measuring amplitudes and time constants respectively associated with the eddy currents. The amplitudes and time constants are then employed to derive the perturbation field expression.
In another useful embodiment, wherein the perturbation field results from a non-ideal response of the gradient amplifier or the like, the expression for the perturbation field can be derived by means of a self-encoding technique. Such technique is described, for example, in an article entitled “A Fourier-transform approach for k-space trajectory measurement”, Alley et al., Proceedings of International Society of Magnetic Resonance in Medicine, Abstracts, p. 1406 (1996).
In a preferred embodiment of the invention, the expression for the perturbation field comprises a time varying function, and the SPSP pulse sequence modifying step comprises adding such function to the specified frequency modulation function.
OBJECTS OF THE INVENTION
An object of the invention is to provide an improved MR imaging sequence of the type employing an SPSP pulse sequence, wherein signal intensity loss resulting from a perturbation magnetic field, produced by the oscillatory gradient of the sequence, is substantially reduced.
Another object is to provide an arrangement of the above type, wherein signal intensity loss is reduced by determining parameters associated with the perturbation magnetic field, and then using the parameters to analytically derive an expression for the perturbation field.
Another object is to provide an arrangement of the above type, wherein the perturbation field expression is employed to adjust the frequency modulation function of the SPSP pulse sequence, in order to match the actual time-dependent magnetic field.
These and other objects of the invention will become more readily apparent from the ensuing specification, taken together with the accompanying drawings.
REFERENCES:
patent: 4698591 (1987-10-01), Glover et al.
patent: 4999580 (1991-03-01), Meyer et al.
patent: 5729138 (1998-03-01), Purdy et al.
patent: 5856744 (1999-01-01), Block et al.
patent: 5864233 (1999-01-01), Zhou et al.
“A Fourier-transform Approach for k-space Trajectory Measurement” Alley et al, Proceedings of International Soc. of Magn. Res. in Med., Abstract 1996, p. 1406.
Bernstein Matthew A.
Zhou Xiaohong
Cabou Christian G.
General Electric Company
Oda Christine K.
Price Phyllis Y.
Skarasten Law Offices
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