US20080272778A1 - Enhanced spectral selectivity for steady-state free precession imaging - Google Patents
Enhanced spectral selectivity for steady-state free precession imaging Download PDFInfo
- Publication number
- US20080272778A1 US20080272778A1 US11/800,419 US80041907A US2008272778A1 US 20080272778 A1 US20080272778 A1 US 20080272778A1 US 80041907 A US80041907 A US 80041907A US 2008272778 A1 US2008272778 A1 US 2008272778A1
- Authority
- US
- United States
- Prior art keywords
- ssfp
- recited
- phase
- images
- image
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Images
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
- G01R33/5613—Generating steady state signals, e.g. low flip angle sequences [FLASH]
- G01R33/5614—Generating steady state signals, e.g. low flip angle sequences [FLASH] using a fully balanced steady-state free precession [bSSFP] pulse sequence, e.g. trueFISP
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/4828—Resolving the MR signals of different chemical species, e.g. water-fat imaging
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
- G01R33/5613—Generating steady state signals, e.g. low flip angle sequences [FLASH]
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/5607—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reducing the NMR signal of a particular spin species, e.g. of a chemical species for fat suppression, or of a moving spin species for black-blood imaging
Definitions
- This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to MRI using steady-state free precession (SSFP) with image artifact reduction.
- MRI magnetic resonance imaging
- SSFP steady-state free precession
- Magnetic resonance imaging is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation.
- nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field.
- the radio-frequency signals resulting from the precession of these spins are received using pickup coils.
- an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
- Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space.
- each nuclear spin responds to four different effects--precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation.
- MR Magnetic resonance
- SSFP Balanced steady-state free precession sequences have gained popularity in magnetic resonance imaging (MRI) as they can yield high signal-to-noise ratios (SNR) within very short scan times. It is necessary to suppress the fat signal in applications where the tissue of interest has a comparable or smaller signal than fat, including coronary artery imaging, cartilage imaging and flow-independent angiography.
- MRI magnetic resonance imaging
- SNR signal-to-noise ratios
- FEMR equilibrium magnetic resonance
- LC-SSFP linear combination SSFP
- binomial excitation patterns periodic flip angle variations
- FS-ATR fat suppressing alternating TR
- a drawback of these methods is the wedge shape of the stop-bands.
- the relatively broad stop-bands fail to yield suppression over certain ranges of frequencies, leading to a residual fat signal comparable to the water signal. Consequently, moderate to large resonant frequency variations will compromise the robustness of fat suppression.
- a method of collecting image data with selective spectral suppression for at least two species is provided.
- a sequence of RF excitation pulses with a certain phase progression is applied at a repetition rate to give an SSFP image, in which a spectrally dependent steady-state magnetization is established. Magnetic gradients are applied between said RF pulses.
- the sequence of RF excitation pulses is repeated with different phase progressions to give a plurality of SSFP images, in which a different spectrally dependent steady-state magnetization is established for each SSFP image.
- the plurality of SSFP images is combined using a weighted combination in which the weights depend on a control parameter that adjusts a trade-off between selective spectral suppression and signal-to-noise ratio (SNR).
- SNR signal-to-noise ratio
- a method that repeatedly applies a sequence of RF excitation pulses with a phase progression at a repetition rate, which provides an SSFP image in which a spectrally dependent steady-state magnetization is established. Magnetic gradients are applied between said RF excitation pulses. A plurality of steady-state free procession SSFP images is acquired with different RF phase increments of an object to be imaged. The plurality of SSFP images is combined using a weighted combination using a weighting according to the equations
- Y 1 is an image for a first species and Y 2 is an image for a second species
- D 0-0 is a data set based on consecutive RF pulses that are all in phase
- D 0-180 is a data set based on consecutive RF pulses that are all out of phase
- p is a real number in the range ( ⁇ 1, 0), where p is a control parameter.
- an apparatus comprising a magnet system and a controller electrically connected to the magnet system.
- the controller comprises a display, at least one processor, and computer readable media.
- the computer readable media comprises computer readable code for acquiring a plurality of SSFP images with different RF phase increments of an object to be imaged and computer readable code for combining the plurality of SSFP images using a weighted combination in which the weights depend on a control parameter that adjusts a trade-off between selective spectral suppression and signal-to-noise ratio (SNR).
- SNR signal-to-noise ratio
- FIGS. 1A , 1 B illustrate a SSFP phase sequence.
- FIG. 2 illustrates SSFP transverse magnetization and phase as a function of off-resonance frequency for a SSFP signal profile.
- FIG. 3 is a high level flow chart of an embodiment of the invention.
- FIG. 4 is a schematic top view of a magnetic resonance imaging (MRI) system that may be used in an embodiment of the invention.
- MRI magnetic resonance imaging
- FIGS. 5A and 5B illustrate a computer system that may be used in an embodiment of the invention.
- FIGS. 6A-F show magnitude and phase spectra SSFP datasets.
- FIGS. 7A-B show a 3 D SSFP acquisition of a water bottle accompanied with a linear shim gradient in the readout (vertical) direction to create bands.
- FIG. 8 shows a ratio computed for a range of flip angles and parameter p.
- FIGS. 9A-D show the signal at the center of the pass-band for a range of T 1 and T 2 values.
- a refocused SSFP sequence includes a single RF excitation which is repeated periodically. All gradients used for slice selection or imaging are fully rewound over each repetitive time, TR. In the steady-state, the magnetization at points a and d are the same.
- the steady-state magnetization for SSFP is a function of the sequence parameters flip angle ( ⁇ ), repetition time (TR) and echo time (TE) as well as the tissue parameters T 1 , T 2 , and resonant frequency shift ⁇ f.
- the resultant steady-state MR signal is a strong function of the local resonant frequency, as shown in FIG. 2 , which shows magnetization or signal profiles (i.e., off-resonance spectra).
- SSFP transverse magnetization magnitude (top) and phase (bottom) are shown as a function of off-resonant frequency.
- Each graph shows three different tip angles: the solid line corresponds to a 30° tip angle, the dotted line 60°, and the dashed line 90°.
- TE TR/2 in all cases.
- the periodic nulls in the signal profile, separated by a frequency of 1/TR, are the source of off-resonance banding artifacts.
- the SSFP signal is a function of free precession per TR ( ⁇ ) and the spectrum displays 2 ⁇ -periodic (in ⁇ ) nulls. There are ⁇ radians phase jumps across these nulls. Furthermore, a constant RF phase increment ( ⁇ ) employed from one excitation to the next shifts the SSFP spectrum by ( ⁇ ) in the co-axis. Multiple-acquisition methods shape the SSFP profile by exploiting these characteristics of the SSFP signal.
- the LC-SSFP method produces a stop-band centered at the fat-resonance by combining two phase-cycled SSFP acquisitions. See Vasanawala et al., Magn. Reson. Med. 2000; 43: 82-90.
- the two combined magnetization profiles are out-of-phase in the vicinity of fat-resonance. Consequently, the two profiles are subtracted from each other. Since the magnitudes of the subtracted profiles are not the same for all frequencies, there is residual stop-band signal in the final image.
- the performance of the LC-SSFP method degrades at higher flip angles and when the tissue sample has a relatively low T 1 /T 2 ratio.
- a novel SSFP combination method for improved fat suppression is provided.
- Weighting SSFP datasets by a negative power (between ⁇ 1 and 0) of their magnitudes and combining them as in LC-SSFP achieves a drastic improvement in suppression robustness without affecting the pass-band.
- the range of flip angles and T 1 /T 2 ratios for which LC-SSFP works robustly are expanded.
- the level of stop-band suppression can be adjusted through the power control parameter to meet application-specific needs. 2D and 3D fat- or water-suppressed SSFP imaging in the presence of large off-resonant frequency variations and at higher resolutions can be successfully accomplished with this method.
- FIG. 3 is a high level flow chart of an embodiment of the invention. At least two RF excitation pulses are repeatedly applied at a repetition rate, whereby at least two substantially different spectrally selective steady-state magnetizations are established (step 304 ). Magnetic gradients are applied between RF pulses (step 308 ). A plurality of MRI signals is acquired during the time that the magnetic gradients are applied (step 312 ). The MRI signals are combined using a weighted combination in which the weights depend on a control parameter that adjusts selective spectral suppression to SNR (step 316 ). The combined MRI signals are displayed (step 320 ).
- FIG. 4 is a schematic top view of a magnetic resonance imaging (MRI) system 400 that may be used in an embodiment of the invention.
- the MRI system 400 comprises a magnet system 404 , a patient transport table 408 connected to the magnet system, and a controller 412 controllably connected to the magnet system.
- a patient would lie on the patient transport table 408 and the magnet system 404 would pass around the patient.
- the controller 412 would control magnetic fields and radio frequency (RF) signals provided by the magnet system 404 and would receive signals from detectors in the magnet system 404 .
- RF radio frequency
- FIGS. 5A and 5B illustrate a computer system 500 , which is suitable for implementing a controller 412 used in embodiments of the present invention.
- FIG. 5A shows one possible physical form of the computer system. Of course, the computer system may have many physical forms ranging from an integrated circuit, a printed circuit board, and a small handheld device up to a huge super computer.
- Computer system 500 includes a monitor 502 , a display 504 , a housing 506 , a disk drive 508 , a keyboard 510 , and a mouse 512 .
- Disk 514 is a computer-readable medium used to transfer data to and from computer system 500 .
- FIG. 5B is an example of a block diagram for computer system 500 . Attached to system bus 520 are a wide variety of subsystems.
- Processor(s) 522 also referred to as central processing units, or CPUs
- Memory 524 includes random access memory (RAM) and read-only memory (ROM).
- RAM random access memory
- ROM read-only memory
- RAM random access memory
- ROM read-only memory
- RAM random access memory
- ROM read-only memory
- a fixed disk 526 is also coupled bi-directionally to CPU 522 ; it provides additional data storage capacity and may also include any of the computer-readable media described below.
- Fixed disk 526 may be used to store programs, data, and the like and is typically a secondary storage medium (such as a hard disk) that is slower than primary storage. It will be appreciated that the information retained within fixed disk 526 may, in appropriate cases, be incorporated in standard fashion as virtual memory in memory 524 .
- Removable disk 514 may take the form of the computer-readable media described below.
- CPU 522 is also coupled to a variety of input/output devices, such as display 504 , keyboard 510 , mouse 512 , and speakers 530 .
- an input/output device may be any of: video displays, track balls, mice, keyboards, microphones, touch-sensitive displays, transducer card readers, magnetic or paper tape readers, tablets, styluses, voice or handwriting recognizers, biometrics readers, or other computers.
- CPU 522 optionally may be coupled to another computer or telecommunications network using network interface 540 . With such a network interface, it is contemplated that the CPU might receive information from the network, or might output information to the network in the course of performing the above-described method steps.
- method embodiments of the present invention may execute solely upon CPU 522 or may execute over a network such as the Internet in conjunction with a remote CPU that shares a portion of the processing.
- embodiments of the present invention further relate to computer storage products with a computer-readable medium that has computer code thereon for performing various computer-implemented operations.
- the media and computer code may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well known and available to those having skill in the computer software arts.
- Examples of computer-readable media include, but are not limited to: magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROMs and holographic devices; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and execute program code, such as application-specific integrated circuits (ASICs), programmable logic devices (PLDs) and ROM and RAM devices.
- ASICs application-specific integrated circuits
- PLDs programmable logic devices
- Computer code examples include machine code, such as produced by a compiler, and files containing higher level code that are executed by a computer using an interpreter.
- Computer readable media may also be computer code transmitted by a computer data signal embodied in a carrier wave and representing a sequence of instructions that are executable by a processor.
- the controller 412 signals the magnet system 404 to repeatedly apply RF excitation pulses to an object to be imaged in the magnet system 404 to establish at least two substantially different spectrally selective steady-state magnetizations (step 304 ).
- the controller 412 signals the magnetic system 404 to apply magnetic gradient between RF pulses (step 308 ).
- the controller 412 acquires a plurality of MRI signals during the time that the magnetic gradients are applied (step 312 ).
- the controller 412 combines the plurality of MRI signals using a weighted combination in which the weights depend on a control parameter to adjust selective spectral suppression to SNR (step 316 ).
- An embodiment of the invention provides a weighted combination of the complex-valued MRI signals instead of a weighted combination of the magnitude MRI signals, in which the weights are based on the MRI signals themselves raised to a power and in which the power of the weight is a control parameter p.
- the transverse magnetization values in the two different spectra subtracted from each other are not equal due to the shape of the SSFP spectrum and the presence of signal nulls. Hence, the stop-band will display deviations from a perfect null for certain ranges of frequencies. For this reason, the reduced flatness of the SSFP spectrum with higher flip angles and lower T 1 /T 2 ratios decreases the robustness of the fat-water separation in LC-SSFP.
- the range of magnetization amplitudes observed with off-resonance frequency variation are reduced if the SSFP data set is weighted by its magnitude raised to a negative power p between ⁇ 1 and 0 as shown in FIG.'s 6 A,C. If the difference in the magnitude of magnetization between the two data sets is decreased, then the stop-band will get closer to a perfect null.
- the magnitude of each signal can be raised to a power and used as a weighting factor before linearly combining the two.
- the resulting water image Y w and fat image Y f can be expressed as
- p is the parameter adjusting the level of suppression and is to be varied in the range ( ⁇ 1, 0).
- p D), with FIGS. 6A , B: 0-0 ( ⁇ 0) and FIGS.
- the spectrum of the combined datasets corresponding to the water image is shown in FIG. 6E (linear) and FIG. 6F (logarithmic) scales for a range of p values.
- the improvement in the stop-band suppression asp is decreased toward ⁇ 1 can clearly be seen in FIG. 6F .
- FIG. 7A , B which show a 3D SSFP acquisition of a water bottle accompanied with a linear shim gradient in the readout (vertical) direction to create bands.
- the remnant stop-band signal depicted as gray regions in the LC-SSFP image appears dark in the WC-SSFP image due to improved suppression.
- the improvement in stop-band suppression can be quantified by computing the ratio of the average water signal within a pass-band to the average fat signal within a stop-band. This ratio was computed for a range of flip angles and parameter p as shown in FIG. 8 .
- the water-to-fat signal ratio can be computed as a function of the parameter p and the value of p that yields the desired signal ratio can be chosen for reconstruction.
- FIG. 8 is a contour plot of the logarithm of the ratio of the average pass-band ( ⁇ 80 Hz) signal for water to the average stop-band ( ⁇ 220 ⁇ 80 Hz) signal for fat as a function of flip angle and the parameter p.
- TR/TE 2.3/1.15 ms
- the improvement in stop-band suppression asp is decreased can be seen as an increase in the signal ratio.
- FIGS. 9A-D show the contrast of the LC-SSFP and WC-SSFP methods for 30° and 60° flip angles.
- the WC-SSFP method preserves the T2-dominant LC-SSFP contrast for which the contributions of T 1 and T 2 can be adjusted through varying the flip angle. Therefore, the magnitude-weighted combination does not alter the targeted tissue contrast.
- LC-SSFP The contrast variation of LC-SSFP is displayed for flip angles of 30° ( FIG. 9A ) and 60° ( FIG. 9C ).
- LC-SSFP and WC-SSFP have almost identical tissue contrast.
- the corresponding maximum-intensity projections (MIPs) in the R-L direction are shown in FIG. 10C and FIG. 10F , respectively.
- the vessel depiction in the MIP of the WC-SSFP image is clearly superior due to improved fat suppression.
- Fat-water separation comprising a summation of two SSFP data sets where fat and water are in-phase and out-of-phase has been proposed by Huang T Y, Chung H W, Wang F N, Ko C W, Chen C Y in “Fat And Water Separation In Balanced Steady-State Free Precession Using The Dixon Method,” Magn Reson Med 2004 51:243-247.
- the method is hindered by imperfect signal cancellation caused by signal heterogeneity due to SSFP nulls.
- WC-SSFP can be adapted to address this shortcoming. Magnitude-to-a-power weighting of these SSFP signals prior to combination should improve the robustness of fat-water separation and extend the tolerable range of resonant frequency variation.
- WC-SSFP combination can be applied to improve any multiple-acquisition SSFP method that suffers from signal inhomogeneity of the SSFP profile.
Abstract
Description
- The U.S. government has rights in the disclosed invention pursuant to NIH grants to Stanford University including 5R01_HL075803, and 5R01_HL039297.
- This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to MRI using steady-state free precession (SSFP) with image artifact reduction.
- Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
- Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During a MR experiment, each nuclear spin responds to four different effects--precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MR experiments, a combination of these effects occurs periodically.
- Balanced steady-state free precession (SSFP) sequences have gained popularity in magnetic resonance imaging (MRI) as they can yield high signal-to-noise ratios (SNR) within very short scan times. It is necessary to suppress the fat signal in applications where the tissue of interest has a comparable or smaller signal than fat, including coronary artery imaging, cartilage imaging and flow-independent angiography. There are various methods for suppression; one common way of reducing the fat signal in SSFP is to shape the periodic frequency response such that a broad range of frequencies around the resonant frequency of lipid are selectively masked out. Examples of this group of methods include fluctuating equilibrium magnetic resonance (FEMR), linear combination SSFP (LC-SSFP), binomial excitation patterns, periodic flip angle variations, and fat suppressing alternating TR (FS-ATR) SSFP.
- A drawback of these methods is the wedge shape of the stop-bands. The relatively broad stop-bands fail to yield suppression over certain ranges of frequencies, leading to a residual fat signal comparable to the water signal. Consequently, moderate to large resonant frequency variations will compromise the robustness of fat suppression.
- In accordance with the invention, a method of collecting image data with selective spectral suppression for at least two species is provided. A sequence of RF excitation pulses with a certain phase progression is applied at a repetition rate to give an SSFP image, in which a spectrally dependent steady-state magnetization is established. Magnetic gradients are applied between said RF pulses. The sequence of RF excitation pulses is repeated with different phase progressions to give a plurality of SSFP images, in which a different spectrally dependent steady-state magnetization is established for each SSFP image. The plurality of SSFP images is combined using a weighted combination in which the weights depend on a control parameter that adjusts a trade-off between selective spectral suppression and signal-to-noise ratio (SNR).
- In another manifestation of the invention a method is provided that repeatedly applies a sequence of RF excitation pulses with a phase progression at a repetition rate, which provides an SSFP image in which a spectrally dependent steady-state magnetization is established. Magnetic gradients are applied between said RF excitation pulses. A plurality of steady-state free procession SSFP images is acquired with different RF phase increments of an object to be imaged. The plurality of SSFP images is combined using a weighted combination using a weighting according to the equations
-
- wherein Y1 is an image for a first species and Y2 is an image for a second species, D0-0 is a data set based on consecutive RF pulses that are all in phase and D0-180 is a data set based on consecutive RF pulses that are all out of phase and p is a real number in the range (−1, 0), where p is a control parameter. The combined plurality of SSFP images is displayed.
- In another manifestation of the invention an apparatus, comprising a magnet system and a controller electrically connected to the magnet system is provided. The controller comprises a display, at least one processor, and computer readable media. The computer readable media comprises computer readable code for acquiring a plurality of SSFP images with different RF phase increments of an object to be imaged and computer readable code for combining the plurality of SSFP images using a weighted combination in which the weights depend on a control parameter that adjusts a trade-off between selective spectral suppression and signal-to-noise ratio (SNR).
- The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.
-
FIGS. 1A , 1B illustrate a SSFP phase sequence. -
FIG. 2 illustrates SSFP transverse magnetization and phase as a function of off-resonance frequency for a SSFP signal profile. -
FIG. 3 is a high level flow chart of an embodiment of the invention. -
FIG. 4 is a schematic top view of a magnetic resonance imaging (MRI) system that may be used in an embodiment of the invention. -
FIGS. 5A and 5B illustrate a computer system that may be used in an embodiment of the invention. -
FIGS. 6A-F show magnitude and phase spectra SSFP datasets. -
FIGS. 7A-B show a 3D SSFP acquisition of a water bottle accompanied with a linear shim gradient in the readout (vertical) direction to create bands. -
FIG. 8 shows a ratio computed for a range of flip angles and parameter p. -
FIGS. 9A-D show the signal at the center of the pass-band for a range of T1 and T2 values. -
FIGS. 11A-F show 3DFT-SSFP images of a volunteer's knee acquired with the following parameters: TR=2.7 ms, 1 mm isotropic resolution, α=30°, 192×128×128 encoding, 250 kHz readout bandwidth and a total scan time of 1:18. - As illustrated in
FIGS. 1A , 1B, a refocused SSFP sequence includes a single RF excitation which is repeated periodically. All gradients used for slice selection or imaging are fully rewound over each repetitive time, TR. In the steady-state, the magnetization at points a and d are the same. - Magnetization is tipped about a traverse axis through an angle α. Between excitations, the magnetization undergoes a precession by an angle θ=2πΔfTR about the z-axis (direction of B0), where Δf is the tissue off-resonance, and also experiences both T1 and T2 relaxation.
- During the sequence each spin is affected by RF pulses, relaxation and free precession. The steady-state magnetization for SSFP is a function of the sequence parameters flip angle (α), repetition time (TR) and echo time (TE) as well as the tissue parameters T1, T2, and resonant frequency shift Δf.
- Signal readout is performed each period, with low spatial frequency information acquired at an echo time TE typically spaced midway between the RF excitation pulses. The resultant steady-state MR signal is a strong function of the local resonant frequency, as shown in
FIG. 2 , which shows magnetization or signal profiles (i.e., off-resonance spectra). SSFP transverse magnetization magnitude (top) and phase (bottom) are shown as a function of off-resonant frequency. Each graph shows three different tip angles: the solid line corresponds to a 30° tip angle, the dottedline 60°, and the dashedline 90°. Profiles are shown for three different T1/T2 combinations: T1/T2=200/100 ms (left), T1/T2=600/100 ms (middle), and T1/T2=1000/100 ms (right). TE=TR/2 in all cases. The periodic nulls in the signal profile, separated by a frequency of 1/TR, are the source of off-resonance banding artifacts. - The SSFP signal is a function of free precession per TR (ω) and the spectrum displays 2π-periodic (in ω) nulls. There are π radians phase jumps across these nulls. Furthermore, a constant RF phase increment (Δφ) employed from one excitation to the next shifts the SSFP spectrum by (Δφ) in the co-axis. Multiple-acquisition methods shape the SSFP profile by exploiting these characteristics of the SSFP signal.
- The LC-SSFP method produces a stop-band centered at the fat-resonance by combining two phase-cycled SSFP acquisitions. See Vasanawala et al., Magn. Reson. Med. 2000; 43: 82-90. The two combined magnetization profiles are out-of-phase in the vicinity of fat-resonance. Consequently, the two profiles are subtracted from each other. Since the magnitudes of the subtracted profiles are not the same for all frequencies, there is residual stop-band signal in the final image. The performance of the LC-SSFP method degrades at higher flip angles and when the tissue sample has a relatively low T1/T2 ratio.
- A novel SSFP combination method (weighted-combination or WC-SSFP) for improved fat suppression is provided. Weighting SSFP datasets by a negative power (between −1 and 0) of their magnitudes and combining them as in LC-SSFP achieves a drastic improvement in suppression robustness without affecting the pass-band. The range of flip angles and T1/T2 ratios for which LC-SSFP works robustly are expanded. The level of stop-band suppression can be adjusted through the power control parameter to meet application-specific needs. 2D and 3D fat- or water-suppressed SSFP imaging in the presence of large off-resonant frequency variations and at higher resolutions can be successfully accomplished with this method.
-
FIG. 3 is a high level flow chart of an embodiment of the invention. At least two RF excitation pulses are repeatedly applied at a repetition rate, whereby at least two substantially different spectrally selective steady-state magnetizations are established (step 304). Magnetic gradients are applied between RF pulses (step 308). A plurality of MRI signals is acquired during the time that the magnetic gradients are applied (step 312). The MRI signals are combined using a weighted combination in which the weights depend on a control parameter that adjusts selective spectral suppression to SNR (step 316). The combined MRI signals are displayed (step 320). -
FIG. 4 is a schematic top view of a magnetic resonance imaging (MRI)system 400 that may be used in an embodiment of the invention. TheMRI system 400 comprises amagnet system 404, a patient transport table 408 connected to the magnet system, and acontroller 412 controllably connected to the magnet system. In one example, a patient would lie on the patient transport table 408 and themagnet system 404 would pass around the patient. Thecontroller 412 would control magnetic fields and radio frequency (RF) signals provided by themagnet system 404 and would receive signals from detectors in themagnet system 404. -
FIGS. 5A and 5B illustrate acomputer system 500, which is suitable for implementing acontroller 412 used in embodiments of the present invention.FIG. 5A shows one possible physical form of the computer system. Of course, the computer system may have many physical forms ranging from an integrated circuit, a printed circuit board, and a small handheld device up to a huge super computer.Computer system 500 includes amonitor 502, adisplay 504, ahousing 506, adisk drive 508, akeyboard 510, and amouse 512.Disk 514 is a computer-readable medium used to transfer data to and fromcomputer system 500. -
FIG. 5B is an example of a block diagram forcomputer system 500. Attached tosystem bus 520 are a wide variety of subsystems. Processor(s) 522 (also referred to as central processing units, or CPUs) are coupled to storage devices, includingmemory 524.Memory 524 includes random access memory (RAM) and read-only memory (ROM). As is well known in the art, ROM acts to transfer data and instructions uni-directionally to the CPU and RAM is used typically to transfer data and instructions in a bi-directional manner. Both of these types of memories may include any suitable of the computer-readable media described below. A fixeddisk 526 is also coupled bi-directionally toCPU 522; it provides additional data storage capacity and may also include any of the computer-readable media described below.Fixed disk 526 may be used to store programs, data, and the like and is typically a secondary storage medium (such as a hard disk) that is slower than primary storage. It will be appreciated that the information retained within fixeddisk 526 may, in appropriate cases, be incorporated in standard fashion as virtual memory inmemory 524.Removable disk 514 may take the form of the computer-readable media described below. -
CPU 522 is also coupled to a variety of input/output devices, such asdisplay 504,keyboard 510,mouse 512, andspeakers 530. In general, an input/output device may be any of: video displays, track balls, mice, keyboards, microphones, touch-sensitive displays, transducer card readers, magnetic or paper tape readers, tablets, styluses, voice or handwriting recognizers, biometrics readers, or other computers.CPU 522 optionally may be coupled to another computer or telecommunications network usingnetwork interface 540. With such a network interface, it is contemplated that the CPU might receive information from the network, or might output information to the network in the course of performing the above-described method steps. Furthermore, method embodiments of the present invention may execute solely uponCPU 522 or may execute over a network such as the Internet in conjunction with a remote CPU that shares a portion of the processing. - In addition, embodiments of the present invention further relate to computer storage products with a computer-readable medium that has computer code thereon for performing various computer-implemented operations. The media and computer code may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well known and available to those having skill in the computer software arts. Examples of computer-readable media include, but are not limited to: magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROMs and holographic devices; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and execute program code, such as application-specific integrated circuits (ASICs), programmable logic devices (PLDs) and ROM and RAM devices. Examples of computer code include machine code, such as produced by a compiler, and files containing higher level code that are executed by a computer using an interpreter. Computer readable media may also be computer code transmitted by a computer data signal embodied in a carrier wave and representing a sequence of instructions that are executable by a processor.
- In a more specific example, the above apparatus is used. The
controller 412 signals themagnet system 404 to repeatedly apply RF excitation pulses to an object to be imaged in themagnet system 404 to establish at least two substantially different spectrally selective steady-state magnetizations (step 304). Thecontroller 412 signals themagnetic system 404 to apply magnetic gradient between RF pulses (step 308). Thecontroller 412 acquires a plurality of MRI signals during the time that the magnetic gradients are applied (step 312). - The
controller 412 combines the plurality of MRI signals using a weighted combination in which the weights depend on a control parameter to adjust selective spectral suppression to SNR (step 316). An embodiment of the invention provides a weighted combination of the complex-valued MRI signals instead of a weighted combination of the magnitude MRI signals, in which the weights are based on the MRI signals themselves raised to a power and in which the power of the weight is a control parameter p. - The off-resonance dependence of the phase of the SSFP signal makes it feasible to create stop- and pass-bands through the linear combination of two phase-cycled images with different RF phase increments. By proper selection of TR (2.3 ms at 1.5 T), the separation between the centers of these two bands can be adjusted to match the fat/water frequency separation, approximately 217 Hz at 1.5 T. Therefore, fat and water images can be selectively reconstruct by swapping the stop- and pass-band locations.
- In LC-SSFP (equivalently WC-SSFP for p=0), two SSFP datasets D0-0 and D0-180, with RF phase increments Δφ=0° and 180° respectively, are acquired. The phase difference between the data sets is π/2 radians for one-half of the spectral period and −π/2 radians for the other half as displayed in
FIGS. 6A-D . The addition of a π/2 radians phase to the D0-180 dataset makes the negative half of frequencies in-phase and the other half out-of-phase with the D0-0 dataset. Therefore a summation of D0-0 with the phase-shifted version of D0-180 creates stop- and pass-bands. The transverse magnetization values in the two different spectra subtracted from each other are not equal due to the shape of the SSFP spectrum and the presence of signal nulls. Hence, the stop-band will display deviations from a perfect null for certain ranges of frequencies. For this reason, the reduced flatness of the SSFP spectrum with higher flip angles and lower T1/T2 ratios decreases the robustness of the fat-water separation in LC-SSFP. - The range of magnetization amplitudes observed with off-resonance frequency variation are reduced if the SSFP data set is weighted by its magnitude raised to a negative power p between −1 and 0 as shown in FIG.'s 6A,C. If the difference in the magnitude of magnetization between the two data sets is decreased, then the stop-band will get closer to a perfect null. The magnitude of each signal can be raised to a power and used as a weighting factor before linearly combining the two. The resulting water image Yw and fat image Yf can be expressed as
-
- where p is the parameter adjusting the level of suppression and is to be varied in the range (−1, 0). The
-
- power of the combined image restores the original contrast that would be captured with the LC-SSFP method. The method is exactly equivalent to LC-SSFP for p=0. The stop-band suppression improves with decreasing values of the parameter as displayed in
FIGS. 6E , F. However, the non-linearity of the operation increases as p approaches −1, potentially enhancing partial-volume effects. It is important to note that p=−1 is not feasible as it removes all tissue contrast irreversibly.FIGS. 6A-F show magnitude and phase spectra SSFP datasets after magnitude-to-a-power weighting (Dw=|D|pD), withFIGS. 6A , B: 0-0 (Δφ=0) andFIGS. 6C-D : 0-180 (Δφ=π) phase cycling. The spectrum of the combined datasets corresponding to the water image is shown inFIG. 6E (linear) andFIG. 6F (logarithmic) scales for a range of p values. The improvement in the stop-band suppression asp is decreased toward −1 can clearly be seen inFIG. 6F . - The effective stop-band suppression of the WC-SSFP method was demonstrated on a water phantom. A linear shim gradient in the readout direction was employed to simulate the off-resonance spectrum and create alternating pass- and stop-bands along the phantom. The images were acquired with a flip angle of 25° and a T1/T2=1300/900 ms phantom was used. While LC-SSFP images display some residual signal in the stop-bands, the remnant stop-band signal is almost completely suppressed with the WC method (p=−0.5). The results are shown in
FIGS. 7A , B, which show a 3D SSFP acquisition of a water bottle accompanied with a linear shim gradient in the readout (vertical) direction to create bands. The LC-SSFP and WC-SSFP (p=−0.5) images shown inFIG. 7A andFIG. 7B respectively are identically windowed. The remnant stop-band signal depicted as gray regions in the LC-SSFP image appears dark in the WC-SSFP image due to improved suppression. - The improvement in stop-band suppression can be quantified by computing the ratio of the average water signal within a pass-band to the average fat signal within a stop-band. This ratio was computed for a range of flip angles and parameter p as shown in
FIG. 8 . The simulation parameters were: T1/T2=1000/100 ms for the water tissue, T1/T2=270/85 ms for the fat tissue, TR/TE=2.3/1.15 ms. The effective width of the stop-band at TR=2.3 ms is approximately 160 Hz. Therefore the pass-band was chosen to be the interval [−80 Hz, 80 Hz], whereas the stop-band was within [−300 Hz, −140 Hz]. For a given flip angle, the water-to-fat signal ratio can be computed as a function of the parameter p and the value of p that yields the desired signal ratio can be chosen for reconstruction.FIG. 8 is a contour plot of the logarithm of the ratio of the average pass-band (±80 Hz) signal for water to the average stop-band (−220±80 Hz) signal for fat as a function of flip angle and the parameter p. TR/TE=2.3/1.15 ms, T1/T2=1000/100 ms for water and T1/T2=270/85 ms for fat was assumed for the simulations. The improvement in stop-band suppression asp is decreased can be seen as an increase in the signal ratio. - The effect of the magnitude-to-a-power weighting on the tissue contrast can be observed by simulating the SSFP signal at the center of the passband for a range of T1, T2 values.
FIGS. 9A-D show the contrast of the LC-SSFP and WC-SSFP methods for 30° and 60° flip angles. The WC-SSFP method preserves the T2-dominant LC-SSFP contrast for which the contributions of T1 and T2 can be adjusted through varying the flip angle. Therefore, the magnitude-weighted combination does not alter the targeted tissue contrast. InFIGS. 9A-D the signal of spins that precess at the frequency corresponding to the center of the pass-band for TR=2.3 ms for a range of T1 and T2 values are displayed. The contrast variation of LC-SSFP is displayed for flip angles of 30° (FIG. 9A ) and 60° (FIG. 9C ). The contrast of the WC-SSFP (p=−0.5) method is also shown for 30° (FIG. 9B ) and 60° (FIG. 9D ). LC-SSFP and WC-SSFP have almost identical tissue contrast. - The improved stop-band suppression of the WC-SSFP method was also demonstrated in vivo. Two 3D SSFP acquisitions on a 1.5 T GE Signa Excite scanner with CV/i gradients were performed on a volunteer's knee with the following parameters: TR=2.7 ms, 1 mm isotropic resolution, α=300, 192×128×128 encoding, 250 kHz bandwidth and a total scan time of 1:18. The coronal and sagittal slices of the knee are shown in
FIGS. 10A-F . Coronal and sagittal slices are shown for LC-SSFP (FIGS. 10A , B) and WC-SSFP (p=−0.5) (FIGS. 10D , E) methods. The corresponding maximum-intensity projections (MIPs) in the R-L direction are shown inFIG. 10C andFIG. 10F , respectively. The vessel depiction in the MIP of the WC-SSFP image is clearly superior due to improved fat suppression. - There is residual fat signal in the LC-SSFP images, whereas the WC-SSFP method effectively suppresses the fat signal. At TR=2.7 ms the period of the SSFP spectrum is reduced and the separation between the stop- and pass-bands in LC-SSFP becomes smaller. If the center of the pass-band is aligned with the water-resonance, the fat-resonance will not exactly be aligned with the center of the stop-band. As a result the stop-band suppression robustness of LC-SSFP is reduced. On the other hand, WC-SSFP achieves robust suppression throughout the stop-band. The greater stop-band suppression with the WC-SSFP method manifests itself in the MIP with improved depiction of the vasculature.
- It should be noted that the non-linearity of WC-SSFP combination leads to partial-volume effects due to the destructive interference between fat and water spins within the same voxel. This non-linearity becomes more pronounced asp approaches −1. However, for moderate values of |p|(≦0.5), partial-volume effects are not noticeable. Furthermore, partial-volume effects become less of an issue for high-resolution imaging. At lower resolutions the value of |p| should be chosen as small as possible, while still effectively suppressing the signal in the stop-band.
- Fat-water separation comprising a summation of two SSFP data sets where fat and water are in-phase and out-of-phase has been proposed by Huang T Y, Chung H W, Wang F N, Ko C W, Chen C Y in “Fat And Water Separation In Balanced Steady-State Free Precession Using The Dixon Method,” Magn Reson Med 2004 51:243-247. However, the method is hindered by imperfect signal cancellation caused by signal heterogeneity due to SSFP nulls. WC-SSFP can be adapted to address this shortcoming. Magnitude-to-a-power weighting of these SSFP signals prior to combination should improve the robustness of fat-water separation and extend the tolerable range of resonant frequency variation. Similarly, WC-SSFP combination can be applied to improve any multiple-acquisition SSFP method that suffers from signal inhomogeneity of the SSFP profile.
- While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, modifications and various substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, modifications, and various substitute equivalents as fall within the true spirit and scope of the present invention.
Claims (19)
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US11/800,419 US7449884B1 (en) | 2007-05-04 | 2007-05-04 | Enhanced spectral selectivity for steady-state free precession imaging |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US11/800,419 US7449884B1 (en) | 2007-05-04 | 2007-05-04 | Enhanced spectral selectivity for steady-state free precession imaging |
Publications (2)
Publication Number | Publication Date |
---|---|
US20080272778A1 true US20080272778A1 (en) | 2008-11-06 |
US7449884B1 US7449884B1 (en) | 2008-11-11 |
Family
ID=39939108
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US11/800,419 Active 2027-05-11 US7449884B1 (en) | 2007-05-04 | 2007-05-04 | Enhanced spectral selectivity for steady-state free precession imaging |
Country Status (1)
Country | Link |
---|---|
US (1) | US7449884B1 (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20080224699A1 (en) * | 2007-03-12 | 2008-09-18 | Timothy Hughes | Magnetic resonance method and apparatus with nuclear spins type-specific signal suppression |
US20180209472A1 (en) * | 2017-01-24 | 2018-07-26 | Miba Sinter Austria Gmbh | Bearing cover |
Families Citing this family (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP3968353B2 (en) * | 2004-02-18 | 2007-08-29 | ジーイー・メディカル・システムズ・グローバル・テクノロジー・カンパニー・エルエルシー | MRI equipment |
US7518364B1 (en) * | 2008-01-18 | 2009-04-14 | The Board Of Trustees Of The Leland Stanford Junior University | Species separation using selective spectral supression in balanced steady-state free precession imaging |
Citations (10)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5170122A (en) * | 1991-07-25 | 1992-12-08 | General Electric | NMR imaging using flow compensated SSFP pulse sequences |
US6307368B1 (en) * | 1999-05-14 | 2001-10-23 | Board Of Trustees Of The Leland Stanford Junior University | Linear combination steady-state free precession MRI |
US6586933B1 (en) * | 2002-05-15 | 2003-07-01 | General Electric Company | Method and system for MRI with lipid suppression |
US6714807B2 (en) * | 2001-06-29 | 2004-03-30 | Ge Medical Systems Global Technology Co., Llc | Magnetic resonance imaging system |
US6750651B2 (en) * | 2002-07-03 | 2004-06-15 | The Board Of Trustees Of The Leland Stanford Junior University | Fat suppression in MRI using oscillating steady-state free precession |
US6906516B2 (en) * | 2003-08-05 | 2005-06-14 | The Board Of Trustees Of The Leland Stanford Junior University | Artifact reduction in SSFP MRI using weighted sum of combined signals |
US7187170B1 (en) * | 2005-09-13 | 2007-03-06 | The Board Of Trustees Of The Leland Stanford Junior Univeristy | Multiple acquisition phase-sensitive SSFP for species separating in MRI |
US7230423B2 (en) * | 2004-05-24 | 2007-06-12 | Siemens Aktiengesellschaft | Method for improving the image homogeneity of image data from phase-cycled steady state sequences |
US7253620B1 (en) * | 2004-03-08 | 2007-08-07 | United States Of America, As Represented By The Secretary, Department Of Health And Human Services | Spectrally selective suppression with steady-state free precession |
US7332908B2 (en) * | 2006-05-04 | 2008-02-19 | The Board Of Trustees Of The Leland Stanford Junior University | SSFP MRI with increased signal bandwidth |
-
2007
- 2007-05-04 US US11/800,419 patent/US7449884B1/en active Active
Patent Citations (10)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5170122A (en) * | 1991-07-25 | 1992-12-08 | General Electric | NMR imaging using flow compensated SSFP pulse sequences |
US6307368B1 (en) * | 1999-05-14 | 2001-10-23 | Board Of Trustees Of The Leland Stanford Junior University | Linear combination steady-state free precession MRI |
US6714807B2 (en) * | 2001-06-29 | 2004-03-30 | Ge Medical Systems Global Technology Co., Llc | Magnetic resonance imaging system |
US6586933B1 (en) * | 2002-05-15 | 2003-07-01 | General Electric Company | Method and system for MRI with lipid suppression |
US6750651B2 (en) * | 2002-07-03 | 2004-06-15 | The Board Of Trustees Of The Leland Stanford Junior University | Fat suppression in MRI using oscillating steady-state free precession |
US6906516B2 (en) * | 2003-08-05 | 2005-06-14 | The Board Of Trustees Of The Leland Stanford Junior University | Artifact reduction in SSFP MRI using weighted sum of combined signals |
US7253620B1 (en) * | 2004-03-08 | 2007-08-07 | United States Of America, As Represented By The Secretary, Department Of Health And Human Services | Spectrally selective suppression with steady-state free precession |
US7230423B2 (en) * | 2004-05-24 | 2007-06-12 | Siemens Aktiengesellschaft | Method for improving the image homogeneity of image data from phase-cycled steady state sequences |
US7187170B1 (en) * | 2005-09-13 | 2007-03-06 | The Board Of Trustees Of The Leland Stanford Junior Univeristy | Multiple acquisition phase-sensitive SSFP for species separating in MRI |
US7332908B2 (en) * | 2006-05-04 | 2008-02-19 | The Board Of Trustees Of The Leland Stanford Junior University | SSFP MRI with increased signal bandwidth |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20080224699A1 (en) * | 2007-03-12 | 2008-09-18 | Timothy Hughes | Magnetic resonance method and apparatus with nuclear spins type-specific signal suppression |
US7609059B2 (en) * | 2007-03-12 | 2009-10-27 | Siemens Aktiengesellschaft | Magnetic resonance method and apparatus with nuclear spins type-specific signal suppression |
US20180209472A1 (en) * | 2017-01-24 | 2018-07-26 | Miba Sinter Austria Gmbh | Bearing cover |
Also Published As
Publication number | Publication date |
---|---|
US7449884B1 (en) | 2008-11-11 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
US8638096B2 (en) | Method of autocalibrating parallel imaging interpolation from arbitrary K-space sampling with noise correlations weighted to reduce noise of reconstructed images | |
US10317493B1 (en) | System and method for multislice fast magnetic resonance imaging | |
Hargreaves et al. | Variable‐rate selective excitation for rapid MRI sequences | |
US7253620B1 (en) | Spectrally selective suppression with steady-state free precession | |
US7710115B2 (en) | Independent phase modulation for efficient dual-band 3D imaging | |
EP1307757B1 (en) | Magnetic resonance imaging method with sub-sampled acquisition | |
US8538115B2 (en) | Coil compression for three dimensional autocalibrating parallel imaging with cartesian sampling | |
US8723516B2 (en) | B1-robust and T1-robust species suppression in MRI | |
US6445184B1 (en) | Multiple gradient echo type projection reconstruction sequence for MRI especially for diffusion weighted MRI | |
US7683618B2 (en) | Slice-selective tunable-flip adiabatic low peak power excitation | |
JP4049649B2 (en) | Magnetic resonance imaging device | |
US7741842B2 (en) | Calibration maps for parallel imaging free of chemical shift artifact | |
US20080012566A1 (en) | Maximum likelihood estimator in the presence of non-identically distributed noise for decomposition of chemical species in mri | |
US9389294B2 (en) | Distortion-free magnetic resonance imaging near metallic implants | |
WO2012077543A1 (en) | Magnetic resonance imaging device and contrast-enhanced image acquisition method | |
US6452387B1 (en) | Catalyzing the transient response in steady-state MRI sequences | |
US7518364B1 (en) | Species separation using selective spectral supression in balanced steady-state free precession imaging | |
JP6762284B2 (en) | Magnetic resonance imaging device and noise removal method | |
US7449884B1 (en) | Enhanced spectral selectivity for steady-state free precession imaging | |
Callaghan et al. | Correlated susceptibility and diffusion effects in NMR microscopy using both phase-frequency encoding and phase-phase encoding | |
US6586933B1 (en) | Method and system for MRI with lipid suppression | |
Çukur et al. | Enhanced spectral shaping in steady‐state free precession imaging | |
US6906516B2 (en) | Artifact reduction in SSFP MRI using weighted sum of combined signals | |
US8040135B1 (en) | Contrast and resolution enhancement with signal compensation | |
US7956611B2 (en) | Magnetic resonance imaging apparatus and method |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
AS | Assignment |
Owner name: THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIO Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:CUKUR, TOLGA;NISHIMURA, DWIGHT G.;REEL/FRAME:019633/0517 Effective date: 20070709 |
|
STCF | Information on status: patent grant |
Free format text: PATENTED CASE |
|
AS | Assignment |
Owner name: NATIONAL INSTITUTES OF HEALTH (NIH), U.S. DEPT. OF Free format text: CONFIRMATORY LICENSE;ASSIGNOR:STANFORD UNIVERSITY;REEL/FRAME:021879/0508 Effective date: 20070705 |
|
FPAY | Fee payment |
Year of fee payment: 4 |
|
AS | Assignment |
Owner name: NATIONAL INSTITUTES OF HEALTH (NIH), U.S. DEPT. OF Free format text: CONFIRMATORY LICENSE;ASSIGNOR:THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY;REEL/FRAME:030675/0257 Effective date: 20130620 |
|
FPAY | Fee payment |
Year of fee payment: 8 |
|
MAFP | Maintenance fee payment |
Free format text: PAYMENT OF MAINTENANCE FEE, 12TH YEAR, LARGE ENTITY (ORIGINAL EVENT CODE: M1553); ENTITY STATUS OF PATENT OWNER: LARGE ENTITY Year of fee payment: 12 |