Frequency drift in MR spectroscopy at 3T

Purpose: Heating of gradient coils and passive shim components is a common cause of instability in the B-0 field, especially when gradient intensive sequences are used. The aim of the study was to set a benchmark for typical drift encountered during MR spectroscopy (MRS) to assess the need for real-time field-frequency locking on MRI scanners by comparing field drift data from a large number of sites.Method: A standardized protocol was developed for 80 participating sites using 99 3T MR scanners from 3 major vendors. Phantom water signals were acquired before and after an EPI sequence. The protocol consisted of: minimal preparatory imaging; a short pre-fMRI PRESS; a ten-minute fMRI acquisition; and a long post-fMRI PRESS acquisition. Both pre- and post-fMRI PRESS were non-water suppressed. Real-time frequency stabilization/adjustment was switched off when appropriate. Sixty scanners repeated the protocol for a second dataset. In addition, a three-hour post-fMRI MRS acquisition was performed at one site to observe change of gradient temperature and drift rate. Spectral analysis was performed using MATLAB. Frequency drift in pre-fMRI PRESS data were compared with the first 5:20 minutes and the full 30:00 minutes of data after fMRI. Median (interquartile range) drifts were measured and showed in violin plot. Paired t-tests were performed to compare frequency drift pre- and post-fMRI. A simulated in vivo spectrum was generated using FID-A to visualize the effect of the observed frequency drifts. The simulated spectrum was convolved with the frequency trace for the most extreme cases. Impacts of frequency drifts on NAA and GABA were also simulated as a function of linear drift. Data from the repeated protocol were compared with the corresponding first dataset using Pearson's and intraclass correlation coefficients (ICC).Results: Of the data collected from 99 scanners, 4 were excluded due to various reasons. Thus, data from 95 scanners were ultimately analyzed. For the first 5:20 min (64 transients), median (interquartile range) drift was 0.44 (1.29) Hz before fMRI and 0.83 (1.29) Hz after. This increased to 3.15 (4.02) Hz for the full 30 min (360 transients) run. Average drift rates were 0.29 Hz/min before fMRI and 0.43 Hz/min after. Paired t-tests indicated that drift increased after fMRI, as expected (p &lt; 0.05). Simulated spectra convolved with the frequency drift showed that the intensity of the NAA singlet was reduced by up to 26%, 44 % and 18% for GE, Philips and Siemens scanners after fMRI, respectively. ICCs indicated good agreement between datasets acquired on separate days. The single site long acquisition showed drift rate was reduced to 0.03 Hz/min approximately three hours after fMRI.Discussion: This study analyzed frequency drift data from 95 3T MRI scanners. Median levels of drift were relatively low (5-min average under 1 Hz), but the most extreme cases suffered from higher levels of drift. The extent of drift varied across scanners which both linear and nonlinear drifts were observed.</p

Fishing Production and Fishing Changes in Hong Kong after the Ground Trawl Ban of 31st December 2012: A Geospatial Evaluation

From data published by the Hong Kong SAR (HKSAR) government in their two sole fisheries surveys of 2006 and 2016/2017, the current authors produced regional maps using spatial interpolation to more accurately describe and estimate the geographic coverage of changes in fishing production in Hong Kong waters since the ground trawl ban of 2012 December 31st. These suggest the fishing industry has adapted to smaller craft, and that fishing production increased in several areas in the period after the ground trawl ban came into effect. In addition, the maps enable a smoother assessment to be made of the geospatial changes in fishing production which have occurred since the ground trawl ban and suggest a ‘workaround’ by fishermen. In particular, small fishing craft known as sampans are able to take advantage by being more suitable vessels for areas such as narrower or shallower bays. Marine plastics pollution is also a proxy indicator of these activities, as evidenced by discarded fishing gear that includes plastic nets, floats, and other fishing boat equipment

Laboratory testing of a growth hypothesis for juvenile squid Loligo pealeii (Cephalopoda: Loliginidae)

Growth modeling in squid has been hampered by a paucity of raw growth data on live individuals. We reared wild juvenile squid Loligo pealeii, for up to 97 days post capture, to determine the form of growth and to test the hypothesis that a 5°C difference in temperature would significantly affect growth rates. Precapture growth rates (the instantaneous relative growth rate or percent increase in body mass per day (IRGR)) of 8-11% were estimated using statolith age data. Laboratory growth rates over a maximum of 97 experimental days fell into two phases in which most L. pealeii grew exponentially, albeit at a slower rate in phase 2. In both phases, the values of IRGR were significantly higher for L. pealeii reared at 20°C than for those reared at 15°C, being respectively, 4.36 and 2.69 in phase 1 and 2.57 and 1.63 in phase 2. This study provides strong evidence of phase-specific temperature sensitivity in squid growth. The IRGR values obtained were used to simulate the growth of squid hatched in nature from May to September in a simple predictive model. The growth simulations indicated that, by the end of phase-1 growth, squid hatched in June and July were two and three times the weight, respectively, at the same age, as squid hatched in May, owing to their exposure to warmer temperatures

0730-7268/02 $9.00 �.00 BETTER BOOTSTRAP ESTIMATION OF HAZARDOUS CONCENTRATION THRESHOLDS FOR AQUATIC ASSEMBLAGES

Abstract—The introduction of species sensitivity distribution (SSD) approaches to ecological risk assessment offers the potential for a more transparent scientific basis for the derivation of predicted no-effect concentrations. However, conventional SSD methodologies have relied on standard distributions (e.g., log logistic, log normal) that are not necessarily based on sound ecological or statistical grounds. More recently, bootstrap resampling techniques that do not rely on distributional assumptions have been applied to the problem. Here we describe how a more advanced bootstrap methodology may be applied to derive better point estimates and confidence intervals for SSD estimates of safe environmental concentrations. Motivated by the fact that the true SSD may not fit any standard model category, we go on to consider a hybrid bootstrap regression approach. This can yield a substantially different estimate for the SSD when compared with both the basic bootstrap and the more frequently used parametric curve approaches. With increasing use of SSDs in ecological risk assessment, it is now imperative that the scientific community develops agreement over appropriate methods for their derivation. Keywords—Risk assessment Species sensitivity distribution Bootstrap regressio

A Bayesian Mixture Model for Estimating Intergeneration Chronic Toxicity

Understanding toxic effects on biological populations across generations is crucial for determining the long-term consequences of chemical pollution in aquatic environments. As a consequence, there is considerable demand for suitable statistical methods to analyze complex multigeneration experimental data. We demonstrate the application of a Bayesian mixture model (with random-effects) to assess the effect of intergeneration copper (Cu) exposure on the reproductive output of the copepod, Tigriopus japonicus, using experimental data across three generations. The model allowed us to appropriately specify the nonstandard statistical distribution of the data and account for correlations among data points. The approach ensured more robust inferences than standard statistical methods and, because of the model's mechanistic formulation, enabled us to test more subtle hypotheses. We demonstrate intergeneration Cu exposure effects on both components of reproductive output (1) the ovisac maturation rate, and (2) the number of nauplii per ovisac. Current and parent generation Cu exposures negatively affected current generation reproductive output. However, in terms of reproductive output, there was also some evidence for adaptation to parental Cu exposures, but with an associated cost under Cu concentrations different from the parental exposure. Bayesian mixture and random-effects models present a robust framework for analyzing data of this kind and for better understanding chemical toxicity

Stage-specificity and the synchronisation of life-cycles to periodic environmental variations

Previous work has shown that life-histories consisting of a contiguous series of stages all with density independent development rates exhibiting the same dependence on time cannot synchronise to a periodic environmental variation. This work also examined models representing both dormancy and quiescence at specific points in the life cycle and showed that both could produce strong synchronising effects. In this paper we examine a very general strategic model of an organism with a two-stage life-cycle each stage having a density independent development rate with a characteristic (periodic) time-dependence. We develop a compact representation of this model in terms of a circle map composed from two simple rotations and the ldquointerphase maprdquo representing the relationship between the physiological times for the two life-history stages. We derive a series of analytic results relating the behaviour of systems whose interphase maps are interrelated and give analytic conditions for a broad class of two-stage circle maps to have a fixed point (that is for the systems they describe to reach the critical life-history stage at the same point in each environmental cycle). Finally we report the results of a numerical investigation of the relationship between the biological characteristics of the development functions and the fine-scale details of the locking behaviour of the systems they define