Kinematic and dynamic aspects of chimpanzee knuckle walking: finger flexors likely do not buffer ground impact forces.

Chimpanzees are knuckle walkers, with forelimbs contacting the ground by the dorsum of the finger's middle phalanges. As these muscular apes are given to high-velocity motions, the question arises of how the ground reaction forces are buffered so that no damage ensues in the load-bearing fingers. In the literature, it was hypothesized that the finger flexors help buffer impacts because in knuckle stance the metacarpophalangeal joints (MCPJs) are strongly hyperextended, which would elongate the finger flexors. This stretching of the finger flexor muscle-tendon units would absorb impact energy. However, EMG studies did not report significant finger flexor activity in knuckle walking. Although these data by themselves question the finger flexor impact buffering hypothesis, the present study aimed to critically investigate the hypothesis from a biomechanical point of view. Therefore, various aspects of knuckle walking were modeled and the finger flexor tendon displacements in the load-bearing fingers were measured in a chimpanzee cadaver hand, of which also an MRI was taken in knuckle stance. The biomechanics do not support the finger flexor impact buffering hypothesis. In knuckle walking, the finger flexors are not elongated to lengths where passive strain forces would become important. Impact buffering by large flexion moments at the MCPJs from active finger flexors would result in impacts at the knuckles themselves, which is dysfunctional for various biomechanical reasons and does not occur in real knuckle walking. In conclusion, the current biomechanical analysis in accumulation of previous EMG findings suggests that finger flexors play no role in impact buffering in knuckle walking.

Multi-center reproducibility of structural, diffusion tensor, and resting state functional magnetic resonance imaging measures.

PURPOSE: The aim of this study is to assess multi-center reproducibility and longitudinal consistency of MRI imaging measurements, as part of a phase III longitudinal multi-center study comparing the neurotoxic effect following prophylactic cranial irradiation with hippocampal avoidance (HA-PCI), in comparison with conventional PCI in patients with small-cell lung cancer. METHODS: Harmonized MRI acquisition protocols from six participating sites and two different vendors were compared using both physical and human phantoms. We assessed variability across sites and time points by evaluating various phantoms and data including hippocampal volume, diffusion metrics, and resting-state fMRI, from two healthy volunteers. RESULTS: We report average coefficients of variation (CV) below 5% for intrascanner, intravendor, and intervendor reproducibility for both structural and diffusion imaging metrics, except for diffusion metrics obtained from tractography with average CVs ranging up to 7.8%. Additionally, resting-state fMRI showed stable temporal SNR and reliable generation of subjects DMN across vendors and time points. CONCLUSION: These findings indicate that the presented multi-site MRI acquisition protocol can be used in a longitudinal study design and that pooling of the acquired data as part of the phase III longitudinal HA-PCI project is possible with careful monitoring of the results of the half-yearly QA assessment to follow-up on potential scanner-related longitudinal changes in image quality.

Optimal short-time acquisition schemes in high angular resolution diffusion-weighted imaging.

This work investigates the possibilities of applying high-angular-resolution-diffusion-imaging- (HARDI-) based methods in a clinical setting by investigating the performance of non-Gaussian diffusion probability density function (PDF) estimation for a range of b-values and diffusion gradient direction tables. It does so at realistic SNR levels achievable in limited time on a high-performance 3T system for the whole human brain in vivo. We use both computational simulations and in vivo brain scans to quantify the angular resolution of two selected reconstruction methods: Q-ball imaging and the diffusion orientation transform. We propose a new analytical solution to the ODF derived from the DOT. Both techniques are analytical decomposition approaches that require identical acquisition and modest postprocessing times and, given the proposed modifications of the DOT, can be analyzed in a similar fashion. We find that an optimal HARDI protocol given a stringent time constraint (<10 min) combines a moderate b-value (around 2000 s/mm(2)) with a relatively low number of acquired directions (>48). Our findings generalize to other methods and additional improvements in MR acquisition techniques.