Sampling time effects on signal-to-noise and contrast-to-noise ratios in spin-echo MRI

This work discusses the effect of sampling time on noise, signal-to-noise and contrast-to-noise ratios in magnetic resonance imaging. A simple imaging experiment is performed to demonstrate the effect of sampling time on noise, confirming theoretical expectations that doubling the sampling time while decreasing the read gradient strength by a factor of two reduces statistical noise to 1/square root 2 of its original level. This result suggests that sampling time should be maximized within the constraints of the pulse sequence: namely, that sampling time should be increased and read gradient strength decreased as TE is increased. Revised expressions for signal-to-noise and contrast-to-noise ratios are presented based on the assumption that sampling time increases linearly with echo delay time above a certain minimum TE value. The revised expressions are then used to derive new predictions of the interpulse delay times that maximize signal-to-noise and contrast-to-noise ratios in spin-echo imaging. It is demonstrated that sampling times are critical in determining whether T1-weighted or T2-weighted sequences produce superior tissue contrast in spin-echo imaging.     

Free-space optical interconnection: A technology comparison of vertical-cavity surface-emitting lasers and multiple-quantum-well modulators

We evaluate current vertical-cavity surface-emitting laser (VCSEL) and multiple-quantum-well (MQW) modulator technologies for low-fanout system applications. Si-CMOS transimpedance receiver circuits are used in the comparison. The aggregate bandwidth per unit area is calculated from the bandwidth and the total on-chip power dissipation. The results indicate that the electrical power dissipation in the receiver circuits dominates at low operating frequencies. At high operating frequencies the receiver gain drops significantly, thus more electrical power is dissipated in the transmitter to provide more optical power to the receiver to satisfy the voltage requirement at the receiver output. In VCSEL based systems, the optical power is generated entirely on-chip. Thus, only VCSELs with low threshold current and low series resistance would be able to provide the same aggregate bandwidth as the MQW modulator based systems.     

The parametric transition of strange matter rings to a black hole

It is shown numerically that strange matter rings permit a continuous transition to the extreme Kerr black hole. The multipoles as defined by Geroch and Hansen are studied and suggest a universal behaviour for bodies approaching the extreme Kerr solution parametrically. The appearance of a ‘throat region’, a distinctive feature of the extreme Kerr spacetime, is observed. With regard to stability, we verify for a large class of rings, that a particle sitting on the surface of the ring never has enough energy to escape to infinity along a geodesic.     

微波消解ICP-MS测定唐古特乌头中的多种微量元素

采用HNO3-H2O2混合酸体系微波消解及电感耦合等离子体-质谱法,消解唐古特乌头样品,测定其中Mn、Fe、Co、As、Ni、Se、Cu、Zn、V和Cr共10种微量元素。探讨了样品消解的条件,通过加标回收实验,回收率为94.08%—103.50%,验证了分析数据的可靠性。结果表明,该法具有快速、简便、准确度高、能同时分析多元素的优点,结果令人满意。