An optimized pulse sequence for isotropically weighted diffusion imaging

Cortical activation to voluntary movement in amyotrophic lateral sclerosis is related to corticospinal damage: electrophysiological evidence

OBJECTIVES

The time course of mu and beta sensorimotor rhythms, with event-related desynchronisation (ERD) to preparation and execution of voluntary movement followed by synchronisation (ERS) after movement, is considered to indicate cortical activation and idling, respectively. We investigated ERD and ERS in amyotrophic lateral sclerosis (ALS) patients and the relationship with anatomical and neurophysiological measures of corticospinal tract damage.

METHODS

Pre-movement mu and beta ERD, and post-movement beta ERS were analysed in 16 ALS patients and 15 healthy controls performing self-paced brisk right thumb extensions. Apparent diffusion coefficient (ADC) of corticospinal tract was measured with magnetic resonance imaging (MRI). Motor-evoked potentials (MEPs) to the right abductor pollicis brevis were obtained using transcranial magnetic stimulation (TMS).

RESULTS

Movement-related electromyographic activity was similar in the two groups. Post-movement ERS was significantly reduced in ALS group and negatively correlated with the amount of corticospinal damage as from MRI and TMS measures. ERD did not significantly differ between groups.

CONCLUSIONS

Alterations of cortical activity in ALS patients were limited to the post-movement phase, as indicated by reduced ERS, and could be linked to reduced cortical inhibition rather than to generalised hyperexcitability.

SIGNIFICANCE

The correlation between ERS and corticospinal damage severity might be interpreted as a functional compensation or dysfunction of inhibitory systems paralleling corticospinal damage.

Quantifying diffusion in mucosal systems by pulsed-gradient spin-echo NMR

Mucus, a thick and slimy secretion produced by submucosal cells, covers many epithelial surfaces in mammalian organs and prevents foreign particles that enter the body from accessing cells. However, the mucus layer also represents a potential barrier to the efficient delivery of nano-sized drug delivery systems (polyplexes, lipoplexes, particles) to the underlying mucosal epithelium. Many studies have considered the ability of nano-sized particles and polymers to diffuse within the mucosal network using a range of different techniques, including multiple-particle tracking (MPT), diffusion chamber studies and fluorescence recovery after photobleaching (FRAP). This review highlights the current understanding of the interaction of the diffusion of nano-sized structures within mucosal networks. Moreover, this article presents an introduction to pulsed-gradient spin-echo NMR (PGSE-NMR), a potential new tool to investigate the mobility of molecular species through mucosal networks and related biological gels.

Basics of Magnetic Resonance Imaging and Magnetic Resonance Spectroscopy

In this chapter, the basic principles of magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) (Sects. 2.2, 2.3, and 2.4), the technical components of the MRI scanner (Sect. 2.5), and the basics of contrast agents and the application thereof (Sect. 2.6) are described. Furthermore, flow phenomena and MR angiography (Sect. 2.7) as well as diffusion and tensor imaging (Sect. 2.7) are elucidated.

Automatic gradient preemphasis adjustment: a 15-minute journey to improved diffusion-weighted echo-planar imaging

Image distortion caused by gradient eddy currents is a major problem in the use of diffusion tensor imaging (DTI), as using the uncorrected images for calculation of apparent diffusion coefficient (ADC) and diffusion anisotropy will result in areas of artificially increased anisotropy and ADC at the edge of the images, as well as decreased spatial resolution and accuracy in ADC computations overall. This distortion may be substantially reduced by careful adjustment of the gradient preemphasis unit. A completely automatic method of adjusting the preemphasis unit is proposed which finds the optimal settings for all three gradient directions in approximately 15 min by estimating the magnitudes of the eddy currents at various delay times after a test gradient. The pixel shifts in a 64 x 128 echo-planar diffusion-weighted image with a diffusion gradient strength of 30 mT/m were reduced to less than 0.2.

On the use of the FLAIR technique to improve the correction of eddy current induced artefacts in MR diffusion tensor imaging

Eddy current induced geometric distortions can only be accurately corrected in brain diffusion-weighted echo-planar (DW-EP) images for b-values less than approximately 300 s mm(-2) using the iterative cross-correlation (ICC) algorithm. This is due to the difference in signal intensity of the cerebrospinal fluid (CSF) compartment in the diffusion-weighted and baseline T(2)-weighted echo-planar (T2W-EP) images. At larger values of b, image misalignment artefacts can, however, be removed by directly correlating CSF-suppressed T2W-EP images with non-CSF-suppressed and CSF-suppressed DW-EP images. Separate phantom experiments can also be performed to provide eddy current calibration data. Here the ability of these methods to remove eddy current induced artefacts from DW-EP images collected in volunteer diffusion tensor imaging (DTI) experiments is investigated. Monte Carlo simulations show that in order for the ICC algorithm to produce accurate estimates of the eddy current induced distortions at b-values greater than 1000 s mm(-2), the degree of CSF suppression should be greater than approximately 80%. This condition is typically met for FLAIR inversion times between 0.5 and 0.8 of the spin-lattice relaxation time of CSF. In volunteer studies the most complete image realignment was provided by direct correlation of CSF-suppressed T2W-EP and DW-EP images acquired in the FLAIR DTI experiment. These results indicate that although calibration data obtained from brain or phantom images can significantly reduce eddy current induced distortions, the optimum image realignment achievable using post-processing methods is likely to be that obtained by direct image warping techniques.

Weekly diffusion-weighted imaging of normal-appearing white matter in MS

To elucidate the dynamics and the nature of normal-appearing white matter (NAWM) changes preceding new lesion formation in MS, the authors obtained weekly diffusion-weighted images for 12 weeks from 6 patients. NAWM areas subsequently involved by enhancement had a significant increase in mean diffusivity values starting 6 weeks before the appearance of enhancement (p values ranging between < 0.0001 and 0.04). This suggests that focal edema and demyelination play a part in the NAWM changes preceding new lesion formation in MS.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of NMR in biomedicine. Each bibliography is divided into 9 sections: 1 Books, Reviews ' Symposia; 2 General; 3 Technology; 4 Brain and Nerves; 5 Neuropathology; 6 Cancer; 7 Cardiac, Vascular and Respiratory Systems; 8 Liver, Kidney and Other Organs; 9 Muscle and Orthopaedic. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted.

On the use of water phantom images to calibrate and correct eddy current induced artefacts in MR diffusion tensor imaging

The accurate determination of absolute measures of diffusion anisotropy in vivo using single-shot, echo-planar imaging techniques requires the acquisition of a set of high signal-to-noise ratio, diffusion-weighted images that are free from eddy current induced image distortions. Such geometric distortions can be characterized and corrected in brain imaging data using magnification (M), translation (T), and shear (S) distortion parameters derived from separate water phantom calibration experiments. Here we examine the practicalities of using separate phantom calibration data to correct high b-value diffusion tensor imaging data by investigating the stability of these distortion parameters, and hence the eddy currents, with time. It is found that M, T, and S vary only slowly with time (i.e., on the order of weeks), so that calibration scans need not be performed after every patient examination. This not only minimises the scan time required to collect the calibration data, but also the computational time needed to characterize these eddy current induced distortions. Examples of how measurements of diffusion anisotropy are improved using this post-processing scheme are also presented.