A comparative study of magnetic resonance venography techniques for the evaluation of the internal jugular veins in multiple sclerosis patients

Background and Purpose

The use of magnetic resonance imaging (MRI) to assess the vascular nature of diseases such as multiple sclerosis (MS) is a growing field of research. This work reports on the application of MR angiographic (MRA) and venographic (MRV) techniques in assessing the extracranial vasculature in MS patients.

Materials and Methods

A standardized MRI protocol containing 2D TOF-MRV and dynamic 3D contrast-enhanced (CE) MRAV was run for 170 MS patients and 40 healthy controls (HC). The cross-sectional area (CSA) of the internal jugular veins (IJVs) was measured at three neck levels in all subjects for both MRV techniques to determine the presence of venous stenoses. All data were analyzed retrospectively.

Results

For the values where both methods showed signal, the 3D method showed larger CSA measurement values compared to 2D methods in both IJVs, in both MS and HC subjects which was confirmed with student paired t-tests. Of the 170 MS patients, 93 (55%) in CE-MRAV and 103 (61%) in TOF-MRV showed stenosis in at least one IJV. The corresponding numbers for the 40 HC subjects were 2 (5%) and 4 (10%), respectively. Carotid ectasias with IJV stenosis were seen in 26 cases (15%) with 3D CE-MRAV and were not observable with 2D TOF-MRV. Carotid ectasias were not seen in the HC group. In the 2D TOF-MRV data, banding of the IJVs related to slow flow was seen in 58 (34%) MS cases and in no HC cases. MS patients showed lower average CSAs than the HC subjects.

Conclusion

The 3D CE MRAV depicted the vascular anatomy more completely than the 2D TOF-MRV. However, the 3D CE MRAV does not provide any information about the flow characteristics which are indirectly available in the 2D TOF-MRV in those cases where there is slow flow.     

The microscopic investigation of structures of moving flux lines by neutron and muon techniques

We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, when the flux lines in low T _c type II superconductors are caused to move by a transport current. Using small-angle neutron scattering by the flux line lattice (FLL), we are able to demonstrate directly the alignment by motion of the nearest-neighbor FLL direction. This tends to be parallel to the direction of flux line motion, as had been suspected from two-dimensional simulations. We also see the destruction of the ordered FLL by plastic flow and the bending of flux lines. Another technique that our collaboration has employed is the direct measurement of flux line motion, using the ultra-high-resolution spectroscopy of the neutron spin-echo technique to observe the energy change of neutrons diffracted by moving flux lines. The muon spin rotation (μSR) technique gives the distribution of values of magnetic field within the FLL. We have recently succeeded in performing μSR measurements while the FLL is moving. Such measurements give complementary information about the local speed and orientation of the FLL motion. We conclude by discussing the possible application of this technique to thin film superconductors.     

Femtosecond dynamics of secondary radiation formation from quantum well excitons

The time-resolved secondary emission of resonantly created excitons in GaAs quantum wells is studied using femtosecond up-conversion spectroscopy. The behaviour of the rise and decay of the secondary emission and reflectivity in quantum wells is strongly dependent upon the disorder at the interfaces, the exciton density and the temperature. In the case of low densities and temperatures the emission is independent of the exciton density and rises quadratically in time, in excellent agreement with recent theory for Rayleigh scattering from two-dimensional excitons subjected to disorder. These rise times are compared directly with times measured by time-integrated four-wave mixing (FWM). The comparison of the dynamics displayed in time-resolved secondary radiation and time-integrated FWM provide a clear understanding of the coherence properties of QW excitons in the first few picoseconds after excitation. High-contrast oscillations that are due to quantum beats between the heavy- and light-hole 1s-states are seen. The visibility decay at very low densities is long ps and is related to the action of potential fluctuations on the scattering of heavy-hole and light-hole excitons.     

An iterative reconstruction technique for geometric distortion-corrected segmented echo-planar imaging

In this article, we present a modified interleaved segmented echo-planar imaging (SEPI) sequence with a center-out k-space trajectory that is especially designed for susceptibility-weighted imaging applications. We introduce a simple and efficient technique to phase correct the acquired SEPI data in the presence of moderate field inhomogeneities. This phase correction reduces the distortion in the phase-encoding direction without requiring an extra reference scan. With the use of a center-out k-space trajectory and a low-spatial-frequency phase map, phase discontinuities between segments can be eliminated, in principle, iteratively using a fast Fourier transform from the center segment to the outermost segment in k-space. With an extra echo added in front of the echo train, neither phase unwrapping nor an extra reference scan is required to obtain a low-spatial-frequency phase map. The method is shown to remove blurring and reduce geometric distortion caused by phase changes from echo to echo in both phantom and human data. The method is most useful for high-resolution imaging applications and moderate factors of speed improvement.     

The role of voxel aspect ratio in determining apparent vascular phase behavior in susceptibility weighted imaging

Susceptibility weighted imaging (SWI) uses apparent phase contrast to enhance the contrast-to-noise ratio (CNR) in the magnitude image. In theory, the apparent phase will depend on the aspect ratio when both venous blood and parenchyma occupy the same voxel. To demonstrate the maximal expected effect of the external field from a vein, we model the vein as an infinitely long cylinder perpendicular to the main magnetic field. The results show that the apparent phase of a voxel in the image is a function of resolution, vessel size and, to a lesser degree, vessel center within the voxel. The simulations explain why a negative-phase mask has worked in SWI processing of high-resolution images collected in the transverse direction, despite the expected positive-phase behavior for vessels perpendicular to the main field. The predicted phase behavior from the simulations is in good agreement with that observed from human brain datasets.