Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-03-10
2001-08-14
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06275038
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to magnetic resonance imaging (MRI) techniques and in particular to mapping magnetic fields used in MRI imaging.
BACKGROUND OF THE INVENTION
The quality and resolution of an MRI image of a subject is sensitive to inhomogeneities in a static polarizing magnetic field that is used in the imaging process to polarize nuclei in the subject. The polarizing field is ideally uniform over the volume of the subject and deviations from a desired constant value can degrade the image.
Particularly susceptible to the effects of inhomogeneities in the polarizing field are MRI images of subjects produced using gradient echo (GE) imaging, water and fat separated imaging or echo planar (EPI) imaging. Often, inhomogeneities in the polarizing field are produced as a result of magnetic susceptibility of the material or tissue of a subject imaged. Inhomogeneities resulting from magnetic susceptibility of material imaged can be especially problematic in in-vivo imaging for which strong magnetic fields are generally required.
In order to compensate for distortions in an MRI image of a subject caused by inhomogeneities in the polarizing magnetic field, the inhomogeneities are measured as a function of position. The measured inhomogeneities are used to correct the image distortions mathematically and/or to determine currents for shimming coils used to produce shimming fields to moderate the inhomogeneities.
Measurements of the magnetic field inhomogeneities are often made by acquiring two sets, hereinafter referred to as “k-space scans”, of data in k-space that characterize the subject in k-space. An MRI imaging sequence is used to acquire a first k-space scan of the subject at a first time. Subsequently, following an accurately determined delay time, the imaging sequence is repeated to acquire a second k-space scan of the subject at a second time. Each of the k-space scans is used to generate a spatial image of the subject. The delay time results in phase differences between values of the two generated spatial images, which phase differences are functions of magnetic field inhomogeneities. The phase difference at a given position is proportional to the delay time and the magnitude of an inhomogeneity in the magnetic field at the position. By dividing the phase difference at the position by the time delay, the inhomogeneity in the field at the position is determined.
The acquisition of data for two images using conventional magnetic field inhomogeneity measurement procedures generally requires a time period of a few seconds or tens of seconds. In an article entitled “In Vivo Rapid Magnetic Field Measurement and Shimming Using Single Scan Differential Phase Mapping” by Kanayama et al, in MRM 36:637-642, (1996), which is incorporated herein be reference, a procedure for measuring inhomogeneities in a polarizing magnetic field from two images comprising 128×128×16 pixels each is described. In the article a data acquisition time of over a hundred seconds is reported for acquiring the two images. An article entitled “Automated Shimming at 1.5 Tesla Using Echo-Planar Image Frequency Maps”, by Reese et al, in JMRI 5:739-745, (1995), which is incorporated herein by reference, also describes measuring inhomogeneities using two images of a subject. As reported in this article, each image is generated from 25 images of slices of the subject and ten seconds are required to acquire data for the two images.
In many situations long data acquisition times for magnetic field measurements compromise the usefulness of the field measurements. For example, when imaging biological processes in human organs, changes often take place in less than a second and field inhomogeneities are a function of the magnetic susceptibility of tissues in the organs. For these situations, field measurement times on the order of seconds or more are often of limited usefulness.
SUMMARY OF THE INVENTION
An aspect of preferred embodiments of the present invention is related to providing a method for rapidly acquiring data for determining inhomogeneities in a polarizing magnetic field used to provide an MRI image of a subject.
In accordance with one aspect of preferred embodiments of the present invention, two k-space scans of the subject are acquired using a single application of an MRI pulse sequence, hereinafter referred to as a “field mapping sequence”, instead of using an MRI pulse sequence applied twice sequentially, as in prior art.
A field mapping sequence, in accordance with a preferred embodiment of the present invention, comprises a plurality of data acquisition cycles. In a first part of each data acquisition cycle a portion of a total amount of data required for a first k-space scan of the subject is acquired. During a subsequent second part of the data acquisition cycle a portion of a total amount of data required for a second k-space scan of the subject is acquired. The second part of each data acquisition cycle starts after the end of the first part of the acquisition cycle following an accurately determined delay time after the start of the first part of the data acquisition cycle. There are enough data acquisition cycles and the amount of data acquired for each k-space scan in each data acquisition cycle is large enough so that in a single application of the field mapping sequence the total amount of data for each of the first and second k-space scans is acquired.
As a result of the “interleaved” method of data acquisition described above, in accordance with a preferred embodiment of the present invention, both images are rapidly acquired in a single application of the field mapping sequence. Furthermore, the second k-space scan is “time displaced” with respect to the first k-space scan by an accurately known time delay, hereinafter referred to as an “acquisition delay”, that is equal to the time delay between the starts of first and second parts of each of the data acquisition cycles. The acquisition delay is used, in accordance with a preferred embodiment of the present invention, to determine field inhomogeneities in the polarizing field.
When data acquisition for the two k-space scans is completed, first and second spatial images of the subject are generated respectively from the first and second k-space scans. If there are neither field inhomogeneities in the polarizing field nor errors, hereinafter referred to as “instrumental errors”, in acquired k-space data caused by instrumental inaccuracies, timing inaccuracies or eddy currents, the first and second spatial images are real and identical. In the presence of field inhomogeneities and instrumental errors, values of both images have non-zero phases, which phases at same points in space are generally different for the two images. If contributions to the phases from instrumental errors are removed, a difference in phases for the two images at a point in space is substantially proportional to the product of the acquisition time delay and the magnitude of an inhomogeneity in the polarizing field at the point.
In accordance with another aspect of a preferred embodiment of the present invention, phase accumulations from instrumental errors are removed from the spatial images. The field inhomogeneity at the point is then determined, in accordance with preferred embodiments of the present invention, by dividing the phase difference by the acquisition delay.
Phase accumulations in the values of the spatial images produced by instrumental errors are evaluated, in accordance with a preferred embodiment of the present invention, from data acquired by imaging the subject with a calibration pulse sequence. Preferably, the calibration pulse sequence is incorporated in the field mapping sequence. In some preferred embodiments of the present invention the calibration pulse sequence is separate from the field mapping sequence.
There is therefore provided in accordance with a preferred embodiment of the present invention, a method for evaluating an inhomogeneity in a magnetic polarizing field used to acquire an MRI image of a slice o
Cowan Liebowitz & Latman P.C.
Dippert William H.
Fetzner Tiffany A.
Oda Christine
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