Dual-echo interleaved echo-planar imaging of the brain

Quantitative cerebral perfusion using the PRESTO acquisition scheme

PURPOSE

To evaluate the feasibility of using the rapid principles of echo shifting with a train of observations (PRESTO) sequence for measurements of cerebral hemodynamic parameters based on first pass of a contrast agent.

MATERIALS AND METHODS

Simulations were performed to investigate potential resolution loss due to relaxation effects. Experimental evaluation was conducted in healthy monkey brains using PRESTO and echo-planar imaging (EPI).

RESULTS

For short echo trains, an insignificant contribution of the longitudinal and transversal relaxation rates to the signal amplitude in white matter and gray matter was found, whereas a contribution as large as 40% was found in large vessels. Simulations of the point spread function demonstrated that PRESTO, despite its shorter readout trains, only has a small advantage in terms of maintenance of image resolution during bolus passage compared to EPI as long as the EPI echo train can be kept similar to the T2* value at the top of the bolus. Experimental studies revealed that the PRESTO and EPI gray matter to white matter ratio were similar with respect to cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT).

CONCLUSION

The study showed that PRESTO and EPI led to comparable quantitative perfusion parameters.

Image-based ghost correction for interleaved EPI

A new image-based ghost correction technique is described for general interleaved EPI. This technique works reliably with both even- and odd-number interleaved EPI sequences. It estimates the phase distortions causing the ghosts as a general function of (x,y) even in the presence of a significant overlap of parent image and ghosts. If the phase distortion is assumed to be a function of x (frequency-encode direction) only, the new technique is similar to a recently published correction method for general interleaved EPI (Hennel. J Magn Reson 1998;134:206-213). However, that study concluded that the method for general interleaved EPI did not work. It showed that the SNRs of the edge pixels were too low to accurately estimate the phase distortion. The new technique utilizes not one, but many pixels in the image for estimating the phase distortion. The technique requires a user-defined ROI to outline the parent image and identify so-called eligible pixels for estimating the phase distortion. When all eligible pixels are used, the SNR of the phase distortion estimate is comparable to that obtained in single-shot EPI. This study provides the first examples of high-quality, image-based correction of multiple ghosts in general interleaved EPI. Magn Reson Med 45:96-108, 2001.

High spatial resolution EPI using an odd number of interleaves

Ghost artifacts in echoplanar imaging (EPI) arise from phase errors caused by differences in eddy currents and gradient ramping during left-to-right traversal of kx(forward echo) versus right-to-left traversal of kx (reverse echo). Reference scans do not always reduce the artifact and may make image quality worse. To eliminate the need for reference scans, a ghost artifact reduction technique based on image phase correction was developed, in which phase errors are directly estimated from images reconstructed separately using only the forward or only the reverse echos. In practice, this technique is applicable only to single-shot EPI that produces only one ghost (shifted 1/2 the field of view from the parent image), because the technique requires that the ghosts do not completely overlap the parent image. For higher spatial resolution, typically an even number of separate k-space traversals (interleaves) are combined to produce one large data set. In this paper, we show that data obtained from an even number of interleaves cannot be combined to produce only one ghost, and image phase correction cannot be applied. We then show that data obtained from an odd number of interleaves can be combined to produce only one ghost, and image phase correction can be applied to reduce ghost intensity significantly. This "odd-number interleaf EPI" provides spatial and temporal resolution tradeoffs that are complementary to, or can replace, those of even-number interleaf EPI. Odd-number interleaf EPI may be particularly useful for MR systems in which reference scans have been unreliable.

New technical developments in magnetic resonance imaging of epilepsy

Within the last several years a number of technical developments have been made in magnetic resonance imaging (MRI) that can potentially impact clinical and research MR imaging application in epilepsy. These include developments in instrumentation and in pulse sequences. Advances in instrumentation include higher capacity gradient systems and multiple receiver coils as directed to brain imaging. Advances in pulse sequence include use of fast or turbo-spin-echo techniques, variants of echo-planar imaging, and sequences such as fluid-attenuation inversion recovery (FLAIR) targeted to specific applications of brain imaging. The purpose of this paper is to review several of these developments.