Body MRI artifacts in clinical practice: a physicist\u27s and radiologist\u27s perspective.

The high information content of MRI exams brings with it unintended effects, which we call artifacts. The purpose of this review is to promote understanding of these artifacts, so they can be prevented or properly interpreted to optimize diagnostic effectiveness. We begin by addressing static magnetic field uniformity, which is essential for many techniques, such as fat saturation. Eddy currents, resulting from imperfect gradient pulses, are especially problematic for new techniques that depend on high performance gradient switching. Nonuniformity of the transmit radiofrequency system constitutes another source of artifacts, which are increasingly important as magnetic field strength increases. Defects in the receive portion of the radiofrequency system have become a more complex source of problems as the number of radiofrequency coils, and the sophistication of the analysis of their received signals, has increased. Unwanted signals and noise spikes have many causes, often manifesting as zipper or banding artifacts. These image alterations become particularly severe and complex when they are combined with aliasing effects. Aliasing is one of several phenomena addressed in our final section, on artifacts that derive from encoding the MR signals to produce images, also including those related to parallel imaging, chemical shift, motion, and image subtraction

The rationality and reliability of expectations reported by British households: micro evidence from the British household panel survey

This paper assesses the accuracy of individuals' expectations of their financial circumstances, as reported in the British Household Panel Survey, as predictors of outcomes and identifies what factors influence their reliability. As the data are qualitative bivariate ordered probit models, appropriately identified, are estimated to draw out the differential effect of information on expectations and realisations. Rationality is then tested and we seek to explain deviations of realisations from expectations at a micro-economic level, possibly with reference to macroeconomic shocks. A bivariate regime-switching ordered probit model, distinguishing between states of rationality and irrationality, is then estimated to identify whether individual characteristics affect the probability of an individual using some alternative model to rationality to form their expectations. --household behaviour,expectation formation

Formation of Supermassive Black Holes by Direct Collapse in Pregalactic Halos

We describe a mechanism by which supermassive black holes can form directly in the nuclei of protogalaxies, without the need for seed black holes left over from early star formation. Self-gravitating gas in dark matter halos can lose angular momentum rapidly via runaway, global dynamical instabilities, the so-called "bars within bars" mechanism. This leads to the rapid buildup of a dense, self-gravitating core supported by gas pressure - surrounded by a radiation pressure-dominated envelope - which gradually contracts and is compressed further by subsequent infall. These conditions lead to such high temperatures in the central region that the gas cools catastrophically by thermal neutrino emission, leading to the formation and rapid growth of a central black hole. We estimate the initial mass and growth rate of the black hole for typical conditions in metal-free halos with T_vir ~ 10^4 K, which are the most likely to be susceptible to runaway infall. The initial black hole should have a mass of <~20 solar masses, but in principle could grow at a super-Eddington rate until it reaches ~ 10^4-10^6 solar masses. Rapid growth may be limited by feedback from the accretion process and/or disruption of the mass supply by star formation or halo mergers. Even if super-Eddington growth stops at \~10^3-10^4 solar masses, this process would give black holes ample time to attain quasar-size masses by a redshift of 6, and could also provide the seeds for all supermassive black holes seen in the present universe.Comment: 11 pages, 2 figures, Monthly Notices of the Royal Astronomical Society, in press. Minor revision

Efficient Aggregation of Panel Qualitative Survey Data

Qualitative business survey data are used widely to provide indicators of economic activity ahead of the publication of official data. Traditional indicators exploit only aggregate survey information, namely the proportions of respondents who report “up” and “down”. This paper examines disaggregate or firm-level survey responses. It considers how the responses of the individual firms should be quantified and combined if the aim is to produce an early indication of official output data. Having linked firms’ categorical responses to official data using ordered discrete choice models, the paper proposes a statistically efficient means of combining the disparate estimates of aggregate output growth which can be constructed from the responses of individual firms. An application to firm-level survey data from the Confederation of British Industry shows that the proposed indicator can provide early estimates of output growth more accurately than traditional indicators.Survey Data; Indicators; Quantification; Forecasting; Forecast Combination

Panel summary of recommendations

The following Space Station internal contamination topics were addressed: past flight experience (Skylab and Spacelab missions); present flight activities (Spacelabs and Soviet Space Station Mir); future activities (materials science and life science experiments); Space Station capabilities (PPMS, FMS, ECLSS, and U.S. Laboratory overview); manned systems/crew safety; internal contamination detection; contamination control - stowage and handling; and contamination control - waste gas processing. Space Station design assumptions are discussed. Issues and concerns are discussed as they relate to (1) policy and management, (2) subsystem design, (3) experiment design, and (4) internal contamination detection and control. The recommendations generated are summarized