|Pulse Sequences: Better, Faster, Stronger|
High Resolution 2D Imaging Without Gradients with
Jonathan C. Sharp1, Scott B. King2, Donghui Yin2, Vyacheslav Volotovskyy2, Boguslaw Tomanek1
1National Research Council Institute for Biodiagnostics, Calgary, Canada; 2National Research Council Institute for Biodiagnostics, Winnipeg, Canada
We present the first experimental 2D imaging results of a new RF imaging method that does not use the frequency gradient hardware. The TRASE method employs a train of refocusing pulses applied to a sequentially-switched array of phase-gradient RF coils to encode k-space at high resolution. By rapid receive coil switching, during acquisition windows, k-space data acquisition can be significantly accelerated. This serial acceleration can be combined with generation of uncollected data points through k-space symmetries for acceleration factors exceeding the size of the coil array. Only a single transmitter and a single receiver channel are required.
Non-Fourier Imaging and Fast B0 Mapping with Linearly
Laura Sacolick1, Douglas L. Rothman1, Robin A. de Graaf1
1Yale University, New Haven, Connecticut, USA
Most MRI applications use Fourier-based image reconstruction. Frequency offsets which are encoded along the time dimension are either indistinguishable from the constant, frequency-encoding gradient or are lost during the regridding of non-Cartesian data. By varying the gradients during acquisition, the additional time-dependence can be used to reconstruct both the spatial and spectral dimensions. Here a method is presented for reconstructing images acquired with arbitrary gradient functions by a vector-space projection similar to matched filtering. Here, applications of the technique to fast single-image B0 mapping and B0 insensitive spatial localization will be demonstrated.
High Speed MR Elastography Using SEA
John Carl Bosshard1, Naresh Yallapragada1, Mary Preston McDougall1, Steven Michael Wright1
1Texas A&M University, College Station, Texas, USA
Magnetic resonance elastography (MRE) is of interest for assessing tissue stiffness non-invasively. While harmonic MRE requires only single frames to acquire stiffness information, transient MRE requires frames obtained at many temporal offsets to observe propagation of a mechanical transient. By eliminating phase encoding, single echo acquisition (SEA) MRI was used to obtain transient and harmonic elastography movies nearly 2 orders of magnitude faster than conventional MRI. Movies of transverse wave propagation in multiple region media were obtained and shear stiffness values were calculated from elastograms obtained from the velocity encoded data. The technique should be well-suited for transient MRE.
Multiple Repetition Time Balanced SSFP for Improved
Tolga Çukur1, Dwight Georger Nishimura1
1Stanford University, Stanford, California , USA
Although balanced steady-state free precession (bSSFP) imaging yields high SNR efficiency, the bright lipid signal is often undesirable. FEMR and alternating repetition time SSFP have been proposed to create a stop-band around the fat-resonance and selectively image water. Nevertheless, the pass-band performance is sacrificed and the level of suppression is limited. A multiple repetition time SSFP scheme drastically improves the level of suppression without sacrificing the pass-band performance.
On the Use of Steady-State Equations to
Estimate Signal Intensities in 2D TrueFISP Imaging
Bram F. Coolen1, Edwin Heijman2, Klaas Nicolay1, Gustav J. Strijkers1
1Eindhoven University of Technology, Eindhoven, Netherlands; 2Philips Research Europe, Eindhoven, Netherlands
The signal intensity in 2D-TrueFISP images is often described by steady-state formulas. However, incorrect estimations may occur, because the slice selective excitation process can not be incorporated in these formulas. Therefore, the validity of using steady-state formulas for 2D-TrueFISP sequences was evaluated. To that end, measurements and Bloch-simulations were performed that give insight in the formation of steady-state slice profiles. These results were not always reproduced by methods using steady-state formulas. More importantly, this study showed under which circumstances the use of steady-state formulas resulted in correct estimations of the steady-state signal intensity.
Analysis of Eddy-Current Artifacts in Interleaved
Jon-Fredrik Nielsen1, Krishna S. Nayak1
1University of Southern California, Los Angeles, California , USA
Many MRI applications, such as magnetization-prepared imaging with centric view-ordering, require large and frequent changes in the gradient waveforms from one TR to the next. In SSFP imaging, this is problematic, since residual eddy-current fields cause waveform-dependent changes in precession angle, causing signal distortions. Waveform “pairing” mitigates distortions for on-resonance spins, but its performance over the whole 1/TR SSFP bandwidth has not been validated experimentally. We investigate the effect of unequal precession angle on the steady-state magnetization in interleaved SSFP, and propose a strategy for mitigating signal distortions within the entire SSFP bandwidth.
Improved Shim Method for Balanced SSFP
Jongho Lee1, 2, Michael Lustig2, Dong-Hyun Kim3, John M. Pauly2
1National Institute of Health, Bethesda, Maryland, USA; 2Stanford University, Stanford, California , USA; 3Yonsei University, Seoul, Republic of Korea
A new shim method that minimizes the maximum off-resonance frequency is proposed for bSSFP. The result shows an improved spatial coverage compared to the least square shim method.
High Spatial Resolution, Volume Selective 3D FSE
Manojkumar Saranathan1, Ken Hwang2, Reed Busse3, Paul Weishaar4, Akira Kawashima4, Christine Lee4
1Global Applied Science Lab, GE Healthcare, Rochester, Minnesota, USA; 2GE Healthcare, Houston, Texas, USA; 3GE Healthcare, Madison, Wisconsin, USA; 4Mayo Clinic, Rochester, Minnesota, USA
Although conventional multi slice 2D FSE is routinely used for tissue characterization of both malignant and benign processes, diagnostic limitations become apparent in small (e.g. prostate) and complex, tortuous (e.g. perianal fistulae) structures. It would be advantageous to use a high spatial resolution 3D acquisition scheme that would permit multiplanar reformatting. Near-isotropic 3D FSE acquisition has been beset by blurring caused by long echo-train lengths. Modulation of the refocusing flip angles in FSE to minimize signal modulation from T2 relaxation has enabled the use of long echo trains. In this study, we developed a novel technique that combines inner-volume (IV) selection and highly selective spatial saturation bands with an eXtended Echo Train Acquisition (XETA) 3D FSE-Cube sequence. This pulse sequence was evaluated for imaging of the prostate and perianal fistulae.
Flexible and Efficient View Ordering for 3D Sequences
with Periodic Signal Modulation
Reed F. Busse1, Anja C S Brau2, Philip J. Beatty2, Ersin Bayram3, Charles R. Michelich3, Anthony Vu3, Richard Kijowski4, Howard A. Rowley4
1GE Healthcare, Madison, Wisconsin, USA; 2GE Healthcare, Menlo Park, California , USA; 3GE Healthcare, Waukesha, Wisconsin, USA; 4University of Wisconsin, Madison, Wisconsin, USA
Many 3D sequences acquire multiple lines of k-space in a train while magnetization in a transient state, making view ordering an important factor in image quality. A new flexible view ordering method is presented that (a) increases sampling efficiency by using elliptical k-space coverage and non-separable 2D auto-calibration, (b) enables echo train lengths to be independent of matrix size, and (c) maps signal modulation into k-space smoothly to avoid artifacts. With 3DFSE applied to brain and knee imaging, the method is shown to reduce scan time or improve resolution by 1/3 while providing increased control of TE.
Towards Artifact-Free MRI Near Metallic Implants
Wenmiao Lu1, Kim Butts Pauly1, Garry Evan Gold1, John Mark Pauly1, Brian Andrew Hargreaves1
1Stanford University, Stanford, California , USA
MRI near metallic implants remains an unmet need due to severe artifacts caused by huge metal-induced field inhomogeneity. In this work we present an imaging technique designed to achieve artifact-free MRI near metallic implants. The proposed technique combines slice excitation profile imaging (SEPI) and view angle tilting (VAT) technique. While the VAT completely eliminates in-plane distortion, the SEPI resolves the locations of excited spins with an additional phase encoding along the slice direction. When the spins subject to the metal-induced field inhomogeneity are resolved to their actual slice locations, summing the spins at the same location eliminates the through-slice distortion.