Method for spatially interleaving two images to halve EPI readout times: two reduced acquisitions interleaved (TRAIL)

Efficient correction of inhomogeneous static magnetic field-induced distortion in Echo Planar Imaging

Single-shot Echo Planar Imaging (EPI) is one of the most efficient magnetic resonance imaging (MRI) acquisition schemes, producing relatively high-definition images in 100 ms or less. These qualities make it desirable for Diffusion Tensor Imaging (DTI), functional MRI (fMRI), and Dynamic Susceptibility Contrast MRI (DSC-MRI). However, EPI suffers from severe spatial and intensity distortion due to B(0) field inhomogeneity induced by magnetic susceptibility variations. Anatomically accurate, undistorted images are essential for relating DTI and fMRI images with anatomical MRI scans, and for spatial registration with other modalities. We present here a fast, robust, and accurate procedure for correcting EPI images from such spatial and intensity distortions. The method involves acquisition of scans with opposite phase encoding polarities, resulting in opposite spatial distortion patterns, and alignment of the resulting images using a fast nonlinear registration procedure. We show that this method, requiring minimal additional scan time, provides superior accuracy relative to the more commonly used, and more time consuming, field mapping approach. This method is also highly computationally efficient, allowing for direct "real-time" implementation on the MRI scanner. We further demonstrate that the proposed method can be used to recover dropouts in gradient echo (BOLD and DSC-MRI) EPI images.

Doubling the resolution of echo-planar brain imaging by acquisition of two k-space lines per gradient reversal using TRAIL

Single-shot echo-planar imaging (EPI) is an important method for MRI of the brain. A method has been developed to double the resolution of EPI in the phase-encode direction, without requiring increases in the maximum gradient amplitude or slew rate. The new approach is based on an EPI implementation of the TRAIL (two reduced acquisitions interleaved) method, in which two images, acquired in rapid succession, are spatially interleaved. In addition, two lines of k-space are acquired for each reversal of the readout gradient. Two full-length readouts are needed, therefore power deposition is increased and the total acquisition time is doubled compared with conventional EPI. However, the individual readouts do not increase in length, so there is no increase in image blurring, and distortion is halved as a result of the closer temporal spacing of the acquired k-space lines. A correction method is also presented to remove additional potential Nyquist ghosting. The new method is demonstrated in vivo at 4.7 T and could in principle be combined with existing approaches for increasing resolution, such as partial Fourier or parallel imaging.

EPI distortion correction from a simultaneously acquired distortion map using TRAIL

PURPOSE

To develop a method for shot-by-shot distortion correction of single-shot echo-planar imaging (EPI) that is capable of correcting each image individually using a distortion measurement performed during acquisition of the image itself.

MATERIALS AND METHODS

The recently-introduced method known as two reduced acquisitions interleaved (TRAIL) was extended to measure the distribution of the main magnetic field B0 with each shot. This corresponded to a map of distortion, and allowed distortion to be corrected in the acquired images.

RESULTS

Distortion-corrected images were demonstrated in the human brain. The distortion field could be directly visualized using the "stripe" distribution imposed by the TRAIL pulse sequence. This confirmed the success of the correction. Over a time-course measurement of 10 images, variance was reduced by using shot-by-shot distortion correction compared to correction with a constant field map.

CONCLUSION

Shot-by-shot distortion correction may be performed for EPI images acquired using an extension of the TRAIL technique, ensuring that the correction reflects the actual distortion pattern and not merely a previously measured, but possibly no longer valid, distortion field. This avoids errors due to changes in the distortion field or misregistration of a previously measured distortion map resulting from subject motion.

Single-shot echo-planar functional magnetic resonance imaging of representations of the fore- and hindpaws in the somatosensory cortex of rats using an 11.7T microimager

Most of functional magnetic resonance imaging (fMRI) experiments have been performed on horizontal bore magnets. Here, we present practical aspects of fMRI based on single-shot, spin-echo echo-planar imaging (EPI) using a widely available, cost effective 89 mm bore vertical 11.7 T microimager. It was demonstrated that reproducible, high-quality fMRI data can be obtained from alpha-chloralose anesthetized adult rat brain. Both coronal and the more extended horizontal EPI images were acquired to measure blood oxygenation level dependent (BOLD) responses to electrical stimulation of fore- and hindpaws. The BOLD patterns observed match the known representations of fore- and hindpaws in the somatosensory cortex in rats. Preliminary results on BOLD signal enhancement using aminophylline are also presented.

Common SENSE (sensitivity encoding using hardware common to all MR scanners): a new method for single-shot segmented echo planar imaging

A new method is presented that enables image acquisition to be segmented into two readouts. This is achieved using a new pulse sequence that creates two components of magnetization with different spatial profiles. Each component of the magnetization is measured in one of the readouts. This produces two images with complimentary "sensitivity profiles" and near identical contrast. The images can be acquired with a reduced data matrix that corresponds to shorter periods of data acquisition. The reduced matrix images are then combined to produce a full matrix image using reconstruction methods previously applied to images from multiple RF coils in the sensitivity encoding (SENSE) technique. The most promising application for this technique is in improving the performance of echo planar imaging (EPI) at high field. In this application, common SENSE obtains two segments of data in a single excitation of the magnetization (i.e., two readouts are performed per shot). The combination of these segments in image space avoids the difficulties normally associated with segmented EPI methods, namely, increased ghosting from discontinuities in the k-space data. The main advantages are a reduction in distortion and blurring. Common SENSE is compatible with parallel imaging and partial Fourier methods.

Single-shot compensation of image distortions and BOLD contrast optimization using multi-echo EPI for real-time fMRI

Functional magnetic resonance imaging (fMRI) is most commonly based on echo-planar imaging (EPI). With higher field strengths, gradient performance, and computational power, real-time fMRI has become feasible; that is, brain activation can be monitored during the ongoing scan. However, EPI suffers from geometric distortions due to inhomogeneities of the magnetic field, especially close to air-tissue interfaces. Thus, functional activations might be mislocalized and assigned to the wrong anatomical structures. Several techniques have been reported which reduce geometric distortions, for example, mapping of the static magnetic field B(0) or the point spread function for all voxels. Yet these techniques require additional reference scans and in some cases extensive computational time. Moreover, only static field inhomogeneities can be corrected, because the correction is based on a static reference scan. We present an approach which allows for simultaneous acquisition and distortion correction of a functional image without a reference scan. The technique is based on a modified multi-echo EPI data acquisition scheme using a phase-encoding (PE) gradient with alternating polarity. The images exhibit opposite distortions due to the inverted PE gradient. After adjusting the contrast of the images acquired at different echo times, this information is used for the distortion correction. We present the theory, implementation, and applications of this single-shot distortion correction. Significant reduction in geometric distortion is shown both for phantom images and human fMRI data. Moreover, sensitivity to the blood oxygen level-dependent (BOLD) effect is increased by weighted summation of the undistorted images.