Simultaneous multislice imaging with slice-multiplexed RF pulses.

A method for simultaneous multislice imaging is presented that uses a multislice RF pulse that imparts a different linear phase profile to each slice. During readout, slices are unaliased by using extra slice-select gradient lobes, which rephase and dephase individual slices one at a time. Compared to other simultaneous slice methods, this method avoids distortion by slice-select gradients, and does not require extra views or additional hardware. However, because one echo per slice is required, the method requires a longer read period. This can cause non-ideal rephasing of the individual slices due to susceptibility gradients, which manifests itself as crosstalk between slices. There is also a concomitant increase in the minimum TR of the sequence. The method is demonstrated with phantom and in vivo images using gradient-echo and spin-echo versions.     

Multiband accelerated spin‐echo echo planar imaging with reduced peak RF power using time‐shifted RF pulses

Purpose:

To evaluate an alternative method for generating multibanded radiofrequency (RF) pulses for use in multiband slice‐accelerated imaging with slice‐GRAPPA unaliasing, substantially reducing the required peak power without bandwidth compromises. This allows much higher accelerations for spin‐echo methods such as SE‐fMRI and diffusion‐weighted MRI where multibanded slice acceleration has been limited by available peak power.

Theory and Methods:

Multibanded “time‐shifted” RF pulses were generated by inserting temporal shifts between the applications of RF energy for individual bands, avoiding worst‐case constructive interferences. Slice profiles and images in phantoms and human subjects were acquired at 3 T.

Results:

For typical sinc pulses, time‐shifted multibanded RF pulses were generated with little increase in required peak power compared to single‐banded pulses. Slice profile quality was improved by allowing for higher pulse bandwidths, and image quality was improved by allowing for optimum flip angles to be achieved.

Conclusion:

A simple approach has been demonstrated that significantly alleviates the restrictions imposed on achievable slice acceleration factors in multiband spin‐echo imaging due to the power requirements of multibanded RF pulses. This solution will allow for increased accelerations in diffusion‐weighted MRI applications where data acquisition times are normally very long and the ability to accelerate is extremely valuable. Magn Reson Med 69:1261–1267, 2013 Wiley Periodicals, Inc.     

Blipped‐controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g‐factor penalty

Simultaneous multislice Echo Planar Imaging (EPI) acquisition using parallel imaging can decrease the acquisition time for diffusion imaging and allow full‐brain, high‐resolution functional MRI (fMRI) acquisitions at a reduced repetition time (TR). However, the unaliasing of simultaneously acquired, closely spaced slices can be difficult, leading to a high g‐factor penalty. We introduce a method to create interslice image shifts in the phase encoding direction to increase the distance between aliasing pixels. The shift between the slices is induced using sign‐ and amplitude‐modulated slice‐select gradient blips simultaneous with the EPI phase encoding blips. This achieves the desired shifts but avoids an undesired “tilted voxel” blurring artifact associated with previous methods. We validate the method in 3× slice‐accelerated spin‐echo and gradient‐echo EPI at 3 T and 7 T using 32‐channel radio frequency (RF) coil brain arrays. The Monte‐Carlo simulated average g‐factor penalty of the 3‐fold slice‐accelerated acquisition with interslice shifts is <1% at 3 T (compared with 32% without slice shift). Combining 3× slice acceleration with 2× inplane acceleration, the g‐factor penalty becomes 19% at 3 T and 10% at 7 T (compared with 41% and 23% without slice shift). We demonstrate the potential of the method for accelerating diffusion imaging by comparing the fiber orientation uncertainty, where the 3‐fold faster acquisition showed no noticeable degradation. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Homogeneous preparation encoding (HoPE) in multislice imaging.

Fast magnetization preparation techniques acquire a series of echoes after a single magnetization preparation. If these echoes are acquired from different slices using a multislice technique the change in the preparation state of the echoes due to relaxation effects leads to different contrast modification for each slice. Encoding different preparation states along the phase-encoding direction of each slice instead of acquiring each slice in a different preparation state is introduced as a general concept to obtain images of identical contrast and point-spread function. This can be realized either by cycling the slice excitation order several times over the total number of repetitions or by moving the point of time at which the preparation is applied within each repetition. One possible application of this method is chemical shift selective fat saturation imaging. A homogeneous fat suppression across a multislice volume could be achieved using a FLASH sequence at a repetition time of TR = 145 ms, including a single fat saturation preparation. Conventional fat saturated spin-echo imaging at any TR can be accelerated significantly by reducing the number of applied preparations per repetition. A further application of the homogeneous preparation encoding (HoPE) method is described that encodes the spatial self-saturation of the multislice excitation order homogeneously in all slices. Only a reduced number of slices of the total volume are excited in each repetition and the slice excitation order is continuously moved along the imaging volume. This method is applied for time of flight (TOF) imaging. Using a TONE-like series of flip angles for the slice excitations of each repetition homogeneous TOF images can be obtained on the basis of a multislice acquisition.     

Multiplex RARE: A simultaneous multislice spin‐echo sequence that fulfils CPMG conditions

This work presents a new imaging sequence in which multiple slices are simultaneously excited and refocused in a spin‐echo train. The multiple spin‐echo trains are interleaved in such a manner that (i) the Carr‐Purcell‐Meiboom‐Gill conditions are fulfilled at all times, and (ii) the signals from slices can be separated, preventing aliasing. This paper also demonstrates how the sequence may be used in a novel fat‐water Dixon method that enables fast volume coverage. The technique is demonstrated in phantoms and in vivo. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.