Cortical Mechanics and Meiosis II Completion in Mammalian Oocytes Are Mediated by Myosin-II and Ezrin-Radixin-Moesin (ERM) Proteins

Analysis of mouse oocyte mechanics shows that effective tension drops 6-fold from prophase I to metaphase II; the metaphase II egg has a 2.5-fold tension differential between the cortex over the spindle and the opposite cortex. Manipulation of actin, myosin-II, or ERMs alters tension levels and induces spindle abnormalities during meiosis II

Accurate Measurement of Magnetic Resonance Imaging Gradient Characteristics

Recently, gradient performance and fidelity has become of increasing interest, as the fidelity of the magnetic resonance (MR) image is somewhat dependent on the fidelity of the gradient system. In particular, for high fidelity non-Cartesian imaging, due to non-fidelity of the gradient system, it becomes necessary to know the actual k-space trajectory as opposed to the requested trajectory. In this work we show that, by considering the gradient system as a linear time-invariant system, the gradient impulse response function (GIRF) can be reliably measured to a relatively high degree of accuracy with a simple setup, using a small phantom and a series of simple experiments. It is shown experimentally that the resulting GIRF is able to predict actual gradient performance with a high degree of accuracy. The method captures not only the frequency response but also gradient timing errors and artifacts due to mechanical vibrations of the gradient system. Some discussion is provided comparing the method presented here with other analogous methods, along with limitations of these methods

Accurate Measurement of Magnetic Resonance Imaging Gradient Characteristics

Recently, gradient performance and fidelity has become of increasing interest, as the fidelity of the magnetic resonance (MR) image is somewhat dependent on the fidelity of the gradient system. In particular, for high fidelity non-Cartesian imaging, due to non-fidelity of the gradient system, it becomes necessary to know the actual k-space trajectory as opposed to the requested trajectory. In this work we show that, by considering the gradient system as a linear time-invariant system, the gradient impulse response function (GIRF) can be reliably measured to a relatively high degree of accuracy with a simple setup, using a small phantom and a series of simple experiments. It is shown experimentally that the resulting GIRF is able to predict actual gradient performance with a high degree of accuracy. The method captures not only the frequency response but also gradient timing errors and artifacts due to mechanical vibrations of the gradient system. Some discussion is provided comparing the method presented here with other analogous methods, along with limitations of these methods

Spectral Simulations Incorporating Gradient Coherence Selection

Computer-aided methods can considerably simplify the use of the product operator formalism for theoretical analysis of NMR phenomena, which otherwise becomes unwieldy for anything but simple spin systems and pulse sequences. In this report, two previously available programming approaches using symbolic algebra (J. Shriver, Concepts Magn. Reson. 4, 1–33, 1992) and numerical simulation using object-oriented programming (S. A. Smith, T. O. Levante, B. H. Meier, and R. R. Ernst, J. Magn. Reson. A 106, 75–105, 1994) have been extended to include the use of gradient operators for simulation of spatially localized NMR spectroscopy and gradient coherence selection. These methods are demonstrated using an analysis of the response of an AX3 spin system to the STEAM pulse sequence and verified with experimental measurements on lactate

Measurement of chemical shifts and coupling constants for glutamate and glutamine

Proton chemical shifts and coupling constants were obtained for glutamate and glutamine in water (D2O) at pH = 6.6. Initial chemical shift and coupling constant values obtained from experimental spectra were refined using a spectral simulation and optimization program to get a complete set of values that could not otherwise be measured directly from the experimental spectra due to strong spin‐spin couplings. These values are essential for automated spectral fitting procedures that require a priori information

3D phase encoding 1H spectroscopic imaging of human brain

A three-dimensional (3D) phase-encoding proton spectroscopic imaging method is presented for a whole body MRI MRS system. Metabolite images at 2 T of choline, creatine, and N-acetyl aspartate (NAA) of normal brain were obtained with a spatial resolution of 1.5 cc. With PRESS volume preselection and outer volume suppression pulses, brain regions close to the skull could be studied without significant contamination by lipid and water signals

Comparison of k‐space sampling schemes for multidimensional MR spectroscopic imaging

For clinical 31P MR spectroscopic imaging (MRSI) studies, where signal averaging is necessary, some improvement of sensitivity and spatial response function may be achieved by acquiring data over a spherical k‐space volume and varying the number of averages acquired in proportion to the desired spatial filter. Eight different k‐space sampling schemes are compared through simulations that provide graphs of the spatial response functions (SRF), and tabulations of voxel volumes, relative signal‐to‐noise ratios (SNR), and relative data collection efficiencies (SNR per unit volume over the same time). All schemes were based on practical experiments, each of which could be implemented in the same length of time. The results show that in comparison with cubic k‐space sampling with the same number of signal averages at each point, spherical and acquisition‐weighted k‐space sampling can be used to achieve reduced Gibbs ringing along the principal axes directions, and thus reduced contamination from adjacent tissue in these directions, without degradation of voxel volume or SNR