Magnetization transfer ratio: a quantitative imaging biomarker for 5q spinal muscular atrophy.

BACKGROUND AND PURPOSE: We quantified peripheral nerve lesions in adults with 5q-linked spinal muscular atrophy (SMA) type 3 by analysing the magnetization transfer ratio (MTR) of the sciatic nerve, and tested its potential as a novel biomarker for macromolecular changes. METHODS: Eighteen adults with SMA 3 (50% SMA 3a, 50% SMA 3b) and 18 age-/sex-matched healthy controls prospectively underwent magnetization transfer contrast imaging in a 3-Tesla magnetic resonance scanner. Two axial three-dimensional gradient echo sequences, with and without an off-resonance saturation rapid frequency pulse, were performed at the right distal thigh. Sciatic nerve regions of interest were manually traced on 10 consecutive axial slices in the images generated without off-resonance saturation, and then transferred to corresponding slices generated by the sequence with the off-resonance saturation pulse. Subsequently, MTR and cross-sectional areas (CSAs) of the sciatic nerve were analysed. In addition, detailed neurologic, physiotherapeutic and electrophysiologic examinations were conducted in all patients. RESULTS: Sciatic nerve MTR and CSA reliably differentiated between healthy controls and SMA 3, 3a or 3b. MTR was lower in the SMA 3 (P < 0.0001), SMA 3a (P < 0.0001) and SMA 3b groups (P = 0.0020) than in respective controls. In patients with SMA 3, MTR correlated with all clinical scores, and arm nerve compound motor action potentials (CMAPs). CSA was lower in the SMA 3 (P < 0.0001), SMA 3a (P < 0.0001) and SMA 3b groups (P = 0.0006) than in controls, but did not correlate with clinical scores or electrophysiologic results. CONCLUSIONS: Magnetization transfer ratio is a novel imaging marker that quantifies macromolecular nerve changes in SMA 3, and positively correlates with clinical scores and CMAPs.

Subharmonic instability of a self-organized granular jet.

Downhill flows of granular matter colliding in the lowest point of a valley, may induce a self-organized jet. By means of a quasi two-dimensional experiment where fine grained sand flows in a vertically sinusoidally agitated cylinder, we show that the emergent jet, that is, a sheet of ejecta, does not follow the frequency of agitation but reveals subharmonic response. The order of the subharmonics is a complex function of the parameters of driving.

MR Neurography: Diagnostic Imaging in the PNS.

The diagnostic work-up of peripheral neuropathies is often challenging and is mainly based on a combination of clinical and electrophysiological examinations. One of the most important difficulties is the accurate determination of the lesion site (lesion localization), lesion extension, and spatial lesion dispersion, which all represent essential diagnostic information crucial for finding the correct diagnosis and hence an adequate therapeutic approach. A typical pitfall in the conventional diagnostic reasoning is the differentiation between a distal, complete cross-sectional nerve lesion and a more proximally located, fascicular nerve lesion. Magnetic resonance neurography (MRN) has been proven to be capable of improving the diagnostic accuracy by providing direct, noninvasive visualization of nerve injury with high structural resolution even reaching the anatomical level of single nerve fascicles (fascicular imaging) and at the same time with large anatomical coverage. It is also feasible to detect structural nerve damage earlier and with higher sensitivity than gold-standard nerve conduction studies. The purpose of this study is to review the literature for current developments and advances in MRN for the precise spatial detection of nerve lesions in focal and non-focal disorders of the peripheral nervous system.

Movers and shakers: granular damping in microgravity.

The response of an oscillating granular damper to an initial perturbation is studied using experiments performed in microgravity and granular dynamics simulations. High-speed video and image processing techniques are used to extract experimental data. An inelastic hard sphere model is developed to perform simulations and the results are in excellent agreement with the experiments. In line with previous work, a linear decay of the amplitude is observed. Although this behavior is typical for a friction-damped oscillator, through simulation it is shown that this effect is still present even when friction forces are absent. A simple expression is developed which predicts the optimal damping conditions for a given amplitude and is independent of the oscillation frequency and particle inelasticities.