Echo-planar pediatric imager

The rapid development of high speed, resolution and precision in fMRI

MRI pulse sequences designed to increase the speed and spatial resolution of fMRI have always been a hot topic. Here, we review and chronicle the history behind some of the pulse sequence ideas that have contributed not only to the enhancement of fMRI acquisition but also to diffusion imaging. (i) Partial Fourier EPI allows lengthening echo trains for higher spatial resolution while maintaining optimal TE and BOLD sensitivity. (ii) Inner-volume EPI renamed zoomed-EPI, achieves extremely high spatial resolution and has been applied to fMRI at 7Tesla to resolve cortical layer activity and columnar level fMRI. (iii) An early non-BOLD approach while unsuccessful for fMRI created a diffusion sequence of bipolar pulses called 'twice refocused spin echo' now widely used for high-resolution DTI and HARDI neuronal fiber track imaging. (iv) Multiplexed EPI shortens TR to a few hundred milliseconds, increasing sampling rates and statistical power in fMRI.

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.

A novel fast T1-mapping method

The authors present a novel fast T1-mapping technique that allows a T1 map to be reconstructed from data acquired in less than 3 seconds. Data were acquired by using two modified TurboFLASH (fast low-angle shot) sequences and were processed with a combination of one-dimensional Fourier transforms and a parameter-fitting routine, instead of a standard two-dimensional Fourier transform. Apparent T1 (T1*) maps were obtained, from which T1 maps were calculated. Comparisons of T1 values obtained in phantoms and the human brain by using this technique with those obtained with multipoint inversion-recovery T1 mapping showed that the new method yielded accurate T1 values. Optimization of the method will further improve speed and accuracy. The general approach of this T1-mapping technique is believed to be also applicable to other problems, such as T2 and T2* mapping.

Analysis of hybrid imaging techniques

Hybrid imaging techniques have been proposed as a means of decreasing imaging time without the cost and technical constraints of echo planar imaging. With this technique phase encoding measurements acquired at different echo times in more than one multiecho experiment are used to form a single image. This study analyzes the trade-offs which occur in the selection of this technique over more conventional imaging in terms of signal/noise, contrast, resolution, imaging time, and efficiency. Hybrid imaging is shown to be advantageous when raw speed is essential such as in abdominal or pediatric imaging. When coverage rather than time becomes the important factor, hybrid imaging does not offer a significant advantage over conventional methods. T2 decay will also serve as a roll-off filter which will reduce the spatial resolution, but not the noise, for short TE hybrid imaging.

Image reconstruction for echo planar imaging with nonequidistant k-space sampling

Echo planar imaging is characterized by scanning the 2D k-space after a single excitation. Different sampling patterns have been proposed. A technically feasible method uses a sinusoidal readout gradient resulting is measured data that does not sample k-space in an equidistant manner. In order to employ a conventional 2D-FFT image reconstruction, the data have to be converted to a cartesian grid. This can be done either by interpolation or alternatively by a generalized transformation. Filtering methods are described to minimize ghosting artifact that is typical in echo planar imaging. Results both from computer simulation and from experiments will be presented. Experimental images were obtained using a 2-T whole-body research system.

Echo-planar imaging: magnetic resonance imaging in a fraction of a second

Progress has recently been made in implementing magnetic resonance imaging (MRI) techniques that can be used to obtain images in a fraction of a second rather than in minutes. Echo-planar imaging (EPI) uses only one nuclear spin excitation per image and lends itself to a variety of critical medical and scientific applications. Among these are evaluation of cardiac function in real time, mapping of water diffusion and temperature in tissue, mapping of organ blood pool and perfusion, functional imaging of the central nervous system, depiction of blood and cerebrospinal fluid flow dynamics, and movie imaging of the mobile fetus in utero. Through shortened patient examination times, higher patient throughput, and lower cost per MRI examination, EPI may become a powerful tool for early diagnosis of some common and potentially treatable diseases such as ischemic heart disease, stroke, and cancer.

Cartesian echo planar hybrid scanning with two to eight echoes

A method of fast nuclear magnetic resonance (NMR) imaging which uses only square wave gradient shapes and also collects data on Cartesian coordinates in a way similar to the blipped echo planar imaging method (BEPI) is described. The method is a heuristic attempt at finding an optimal data collection strategy for NMR imaging. Its advantages and disadvantages compared to other methods are carefully analyzed. Clinical size anatomical (head and body) images are shown. It is found that the interecho phase-angle discrepancies that are caused bv the B(0) inhomogeneity and by the gradient wave form distortion can largely be eliminated by using the half Fourier method with full Fourier phase map correction, but one phase map is required for each echo. GRASS/FLASH spoiling techniques are incorporated into the method in order to allow multiecho speed increases for scans which would run under 10 s using a single-echo method. The technique can be applied directly to GRASS/FLASH itself, as is demonstrated by a two-echo GRASS scan. The fastest image was 1.9 s.