Ultrafast magnetic resonance imaging. A new window on brain research.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
D A Feinberg
Publications and source records attributed to D A Feinberg.
VET imaging: magnetic resonance imaging with variable encoding time.
A methodology of MRI data acquisition is introduced which involves lengthening the duration of signal readout period with complementary shortening of the phase encode pulses, and vice versa, in sequences where gradient encoding time is limited by RF pulse spacing. This variable encoding time (VET) methodology can be used to increase spatial resolution or reduce data acquisition time in 2D and 3D FT MRI based on CPMG and gradient echo sequences. Advantages of higher image SNR and different spatial frequency distributions in k-space are discussed and evaluated in preliminary experiments.
Aged↗
A comparison of phase encoding ordering schemes in T2-weighted GRASE imaging.
Gradient and spin echo (GRASE) imaging is an echo train imaging sequence that combines gradient and RF refocusing. This combination introduces phase modulations into the echo train. If the phase encoding order is linear with echo time, these modulations cause severe ghosting artifacts. Changing the order of phase encoding can greatly reduce these artifacts. Several phase encoding orders for T2-weighted sequences are compared in this paper, linear, partially randomized, standard GRASE ordering, and k-banded (kb) GRASE ordering. Different possible implementations of GRASE and kbGRASE are also considered. Computer simulation is used to compare resolution and artifact levels. Phantom and volunteer images are presented. The linear order is most sensitive to ghosting artifacts associated with chemical shift, susceptibility differences and static field inhomogeneities. The standard GRASE order is least sensitive to these but most vulnerable to artifacts associated with short T2 signals, kb-GRASE is a good intermediate between linear and standard GRASE and generally shows the lowest artifact levels. The partially randomized order gives the most diffuse artifacts. Computer simulations show that spatial resolution and contrast with all phase encoding orders are similar.
Artifacts↗
GRASE improves spatial resolution in single shot imaging.
In single shot echo train imaging all the data required for a two dimensional image is acquired from a series of echoes generated following a single RF excitation pulse. Spatial resolution is limited because all samples must be acquired before the signal decays. In this paper we show theoretically that more echoes and hence better spatial resolution can be obtained with single shot GRASE imaging than with either echo planar imaging or single shot RARE imaging. This conclusion holds for both conventional imaging hardware and specialized gradient hardware designed for EPI. High quality single shot GRASE images support the theoretical conclusions.
Brain↗
Increased flexibility in GRASE imaging by k space-banded phase encoding.
GRASE (GRadient and spin Echo) is an echo train imaging technique that combines gradient and RF refocusing. Although overall signal decay is with T2 and field inhomogeneity phase errors do not accumulate, the small residual phase errors are periodic with echo number. The echo order described previously eliminates the phase error periodicity in k space but instead creates periodicity in the T2 modulation function that can also cause artifacts. In addition, with this order, the effective TE must be half the echo train time, and asymmetric Fourier sampling is difficult to implement. A new method is described that greatly reduces artifacts due to T2 decay, permits greater control of T2 contrast, and lends itself to asymmetric Fourier sampling. Different time segments of the echo train are encoded with different bands of spatial frequency in k space (hence "k banding"). Both computer simulations and experimental results demonstrate improvements in GRASE images acquired by this method.
Abdomen↗
A methodology for co-registering abdominal MR images over multiple breath-holds.
Previous studies have demonstrated that the SNR of abdominal MR images can be increased by averaging images obtained in different breath-hold acquisitions. In this note, the authors present a simple new methodology for ensuring that images acquired in multiple breath-hold periods are accurately co-registered. Within each breath-hold, a quick coronal scout scan is followed by a longer axial scan. The scout is used to position the axial slices in a fixed position relative to the organ under examination. This MR technique can, in principle, be automated so as to add less than 1 s to the imaging time of the axial scan. The method can be used to increase SNR by signal averaging or to co-register images acquired during, for example, uptake of contrast agents. SNR improvement with negligible blurring is demonstrated in liver images acquired by this method from healthy volunteers.
Abdomen↗
Multiple breath-hold averaging (MBA) method for increased SNR in abdominal MRI.
Breath-holding during MR imaging eliminates respiratory motion artifacts but places a major time constraint on data acquisition. This constraint limits image signal-to-noise ratio and hence spatial resolution. A new method, multiple breath-hold averaging, is presented that overcomes these time limitations. Several images are acquired in sequential breath-hold periods, separated by periods of normal breathing, and averaged. This averaged image shows the expected increase in SNR with surprisingly little blurring due to misregistration. SNR improvements can be traded for increased spatial resolution. The MBA methodology can also be applied to 3D data acquisitions, dynamic contrast acquisitions, and image subtractions.
Abdomen↗
Functional magnetic resonance imaging. Application to degenerative brain disease and hydrocephalus.
Magnetic resonance (MR) imaging, in addition to its excellent depiction of neuroanatomy, is being developed as a major technique for functional imaging of cerebrospinal fluid motion and for measurement of velocity, strain, and diffusional processes within the brain parenchyma. Functional MR imaging studies are revealing basic physiology of blood flow interactions with CSF motion and dynamic processes of brain parenchyma. Normal pressure hydrocephalus and degenerative brain disease are current focuses of functional MR imaging studies.
Brain↗
Dual contrast GRASE (gradient-spin echo) imaging using mixed bandwidth.
Equal time spacing of RF pulses in the CPMG sequence imposes a constraint of equal signal read periods in spin-echo train imaging. GRASE imaging differs by using multiple read gradients in each pi-pi time interval, which are not constrained to be equal in number or duration. This additional degree of freedom is developed in dual contrast imaging. Closely spaced read periods are used for the PDW image to reduce T2 decay effects, while fewer low-bandwidth read periods in each of several pi-pi intervals are used to raise the signal-to-noise ratio and avoid signal averaging in the T2-weighted image.
Electron Spin Resonance Spectroscopy↗
Phase errors in multi-shot echo planar imaging.
Field inhomogeneity related phase errors in multi-shot echo planar imaging (EPI) are directly visualized and analyzed in the spatial frequency domain data or 'k-space'. The echo time shift (ETS) technique incrementally moves the position of the echo train and improves the phase error function by redistributing phase discontinuities away from the center of k-space.
Artifacts↗
GRASE imaging provides image quality and speed.
Explore the source record for details and available documents.
Equipment Design↗
Single-shot GRASE imaging without fast gradients.
Based on the CPMG sequence, gradient- and spin-echo (GRASE) echo train length is limited by T2 decay rather than the T2* decay and phase error in echo-planar techniques, permitting a longer image acquisition period. An ultrafast GRASE sequence, utilizing a single excitation, generates a 128 x 56 true T2-weighted image in 200 ms on an unmodified commercial scanner without fast gradient switching, extreme field homogeneity, or fat signal suppression.
Abdomen↗
Modern concepts of brain motion and cerebrospinal fluid flow.
Explore the source record for details and available documents.
Biomechanical Phenomena↗
GRASE (Gradient- and spin-echo) imaging: a novel fast MRI technique.
A fast multi-section MR imaging technique is described. Gradient- and spin-echo (GRASE) imaging utilizes the speed advantages of gradient refocusing while overcoming the image artifacts arising from static field inhomogeneity and chemical shift. Image contrast is determined by the T2 contrast in the Hahn spin echoes. A novel k-space trajectory temporally modulates signals and demodulates artifacts.
Brain↗
Real-time magnetic resonance imaging of laser heat deposition in tissue.
We applied diffusion-sensitive echo planar (Instascan) imaging to study thermal changes caused by a Nd:YAG laser. Images of phantom materials and normal rabbit brain tissue in vivo, acquired in 150 ms, every 2s, clearly showed the dynamics of temperature-related signal intensity changes in the regions irradiated by the laser.
Animals↗
GRASE (gradient- and spin-echo) MR imaging: a new fast clinical imaging technique.
A novel technique of magnetic resonance (MR) imaging, which combines gradient-echo and spin-echo (GRASE) technique, accomplishes T2-weighted multisection imaging in drastically reduced imaging time, currently 24 times faster than spin-echo imaging. The GRASE technique maintains contrast mechanisms, high spatial resolution, and image quality of spin-echo imaging and is compatible with clinical whole-body MR systems without modification of gradient hardware. Image acquisition time is 18 seconds for 11 multisection body images (2,000/80 [repetition time msec/echo time msec]) and 36 seconds for 22 brain images (4,000/104). With a combination of multiple Hahn spin echoes and short gradient-echo trains, the GRASE technique overcomes several potential problems of echo-planar imaging, including large chemical shift, image distortions, and signal loss from field inhomogeneity. Advantages of GRASE over the RARE (rapid acquisition with relaxation enhancement) technique include faster acquisition times and lower deposition of radio-frequency power in the body. Breath holding during 18-second GRASE imaging of the upper abdomen eliminates respiratory-motion artifacts in T2-weighted images. A major improvement in T2-weighted abdominal imaging is suggested.
Abdomen↗
Echo-planar imaging with asymmetric gradient modulation and inner-volume excitation.
Single-shot echo-planar imaging is notoriously vulnerable to image artifacts, arising from the necessity of alternate echo time reversal during image reconstruction and from static field inhomogeneity. A technique for overcoming these problems, which further permits imaging on systems with relatively poor gradient waveforms, when data are collected always with the same gradient polarity, is presented. Subsectional and 3D volume imaging are presented as well as a novel phase-correction method for Hermitian symmetry in "half-Fourier" echo-planar imaging.
Humans↗
Tissue perfusion in humans studied by Fourier velocity distribution, line scan, and echo-planar imaging.
In tissue perfusion studies, FT velocity distribution imaging (VDI) intrinsically distinguishes signals from moving blood and volume-averaged tissue. Results in human thyroid gland, in vivo, using VDI line scan technique demonstrated separation of moving blood signal from glandular tissue, while VDI inner-volume echo-planar imaging of brain showed only CSF velocity above the image noise level. New alternating polarity gradient sequences which permit separation of diffusion and slow velocity are discussed. A novel method of 3D FT imaging (two spatial and one velocity dimension) combining inner-volume imaging and echo-planar imaging with velocity resolution of 0.15 mm/s per pixel is demonstrated. A novel graphical method of calculation and display of diffusion dependence in pulsed gradient sequences is presented.
Blood Flow Velocity↗