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Magnetic field mapping during ssfp using phase-incremented or frequency-shifted magnitude imagesMagnetic field mapping during ssfp using phase-incremented or frequency-shifted magnitude images description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060220645, Magnetic field mapping during ssfp using phase-incremented or frequency-shifted magnitude images. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] Conventional magnetic resonance imaging (MRI) methods rely on a linear relationship between magnetic fields and spatial positions of the nuclear spins within the object being imaged. This relationship is realized by using a combination of homogeneous magnets and linear gradient coils. Deviations from this linear relationship can cause undesired image distortions and sometimes artifacts. [0002] Field anomalies are deviations of magnetic fields from prescribed values. Anomalies can be static as well as sequence-induced. Static anomalies may occur from, for example, field inhomogeneities produced by the magnet used to generate the main static magnetic field. Sequence-induced anomalies may occur from, for example, eddy-currents or the Maxwell fields. [0003] Two remedies often used for correcting anomalies in magnetic field are: (i) minimization of inhomogeneities by shimming the magnetic field using special shimming hardware, and (ii) post data acquisition image correction to compensate for the effects of the field anomalies. These remedies require accurate information about the spatial distribution of the magnetic field at the time when the image data is acquired. [0004] Conventionally, maps of the magnetic field are generated using dedicated field-mapping sequences. Since images are acquired using a different sequence, such field maps contain no information about sequence-induced field anomalies relevant to the imaging sequence. In cases when the sequence-induced changes are significant, such as during balanced steady-state free precession (SSFP) sequences, it is preferable that the field maps be generated using the exact same sequence that is used for imaging. [0005] In addition, field maps are often generated from the phases of the acquired images. As anyone skilled in the art understands, phases of acquired magnetic resonance (MR) signals can be affected by factors other than the static magnetic field. Such factors include, but not limited to, chemical shifts and radio-frequency (RF) field orientation. For example, in the single quadrature Dixon (SVD) method recently introduced for water-fat separated SSFP, maps of magnetic field are obtained from the phases of the isolated echo images. Since the phases of the echo signals are influenced by field inhomogeneities as well as chemical shifts, the magnetic field maps so generated are susceptible to interference from chemical shifts. [0006] There is therefore a need for improved methods to generate magnetic field maps that are relevant to imaging sequences, wherein the maps are not degraded by interferences such as that from chemical shifts. BRIEF DESCRIPTION OF THE INVENTION [0007] A system has been developed for mapping the magnetic field during a balanced steady-state free precession (SSFP) sequence. A field map is generated using the data acquired during the same phase-incremented or frequency-shifted SSFP sequences used to generate the images. The system maps the magnetic field by analyzing the magnitude images pixel-by-pixel. The system need not rely on phase information. [0008] A field map is generated by analyzing the magnitude images acquired using a phase-incremented or frequency-shifted balanced SSFP sequence. Pixel intensities are surveyed as a function of the phase increment or frequency shift used during the data acquisition. The precession angles of the nuclear spins during each time of repetition (TR) period are determined for each pixel from the radio frequency (RF) phase increment or frequency shift that yields a minimum pixel intensity. The map of the precession angles is unwrapped to form a field map of the magnetic field. [0009] The magnetic field map so generated includes information regarding the static magnetic field inhomogeneities and any effects of sequence-induced field changes. The magnetic field map can be used to automatically shim the magnetic field so as to compensate for the static field inhomogeneities as well as to compensate for the sequence-induced field anomalies during SSFP. The magnetic field map can also be used during post-data acquisition processing to correct for the effects of the field anomalies, such as in the processes for separation of water and fat signals. BRIEF DESCRIPTION OF THE DRAWINGS [0010] FIG. 1 is a schematic diagram of an MRI system. [0011] FIG. 2 is an exemplary balanced SSFP imaging sequence. [0012] FIG. 3 is a graph of the theoretical spectrum of the magnitude of the steady-state transverse magnetization as a function of the precession angle acquired in a balanced SSFP sequence. [0013] FIG. 4 is a flow chart of steps for acquiring phase-incremented or frequency-shifted SSFP data and generating maps of the magnetic field. [0014] FIG. 5 is a schematic diagram of non-uniformly sampled k-space data that is sorted into data for imaging and data for field mapping. [0015] FIG. 6 is a flow chart of steps for acquiring and processing phase-incremented or frequency-shifted SSFP data for water-fat separation. DETAILED DESCRIPTION OF THE INVENTION [0016] As shown in FIG. 1, a magnetic resonance imaging (MRI) system typically includes a large magnet 10 to impose a static magnetic field (B.sub.0), gradient coils 14 for imposing spatially distributed gradient magnetic fields (G.sub.x, G.sub.y, and G.sub.z) along three orthogonal coordinates, and RF coils 15 and 16 to transmit and receive RF signals to and from the selected nuclei of the object being imaged. The object 13 lies on a movable table 12 such that a portion of the object to be imaged is moved, in three-dimensions, in an "imaging volume" 11 between the magnet and coils, which defines a field of view (FOV) of the MRI system. [0017] To acquire MRI data, the MRI system generates magnetic gradient and RF nutation pulses via MRI pulse sequence controllers 17 and 18 that operate under the control of a programmable processor 19, e.g., a workstation computer 24. In addition, the processor 19 controls a gradient pulse amplifier 20, and RF source and amplifier circuits 21 and 22. The MR signal circuits (RF detector) 25 are suitably interfaced with MR signal RF coils 15, 16 located within the shielded MRI system gantry. The received MR RF echo signal responses are digitized by a digitizer 23 and passed to the processor 19, which may include an array processors or the like for image processing and suitable computer program storage media (not shown) wherein programs are stored and selectively utilized so as to control the acquisition and processing of MR signal data and to produce images on a display of control terminal 24. The MRI system control terminal 24 may include a suitable keyboard, touch screen or other input devices for exerting operator control. Images may also be recorded directly on film, stored electronically or printed on a suitable media by a printing device. [0018] FIG. 2 schematically shows a three-dimensional (3D) magnetic resonance imaging sequence with fully refocused steady-state free precession (SSFP). A radio frequency (RF) excitation pulse 30 is applied to the object being scanned. The RF pulse 30 is repeatedly applied with a time of repetition (TR). An RF echo signal (S) is received during a data acquisition (ADC) period 32. The received echo signal occurs after a time to echo (TE) period that begins at the center of the RF pulse 30. Gradient magnetic fields are applied in the x, y and z directions to select and encode a set of slices (Gslice) of the object for imaging and to spatially phase encode (Gphase) the excited nuclei of the selected slices. A readout magnetic field gradient (Gread) is applied during the ADC period to frequency-encode the received echo signal. [0019] Steady-state free precession (SSFP) is a technique used to generate MRI signals from precessing hydrogen nuclei that do not completely return to their thermal equilibrium state. The SSFP sequence uses a series of RF excitation pulses and magnetic gradient pulses applied at repetition times (TR) significantly shorter than the spin-lattice (T1) and the spin-spin (T2) relaxation times of hydrogen nuclei within the object being imaged. In a fully refocused SSFP sequence, the magnetic gradient pulses are fully balanced, i.e., the total area (gradient moments) of all gradient pulses in each TR period is zero for each gradient channel. [0020] Theory of Magnetic Field Mapping During SSFP: Continue reading about Magnetic field mapping during ssfp using phase-incremented or frequency-shifted magnitude images... 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