Diffusion-weighted multiple shot echo planar imaging of humans without navigation

A new method is proposed for reducing the artifacts produced in diffusion-weighted imaging. When data are acquired using multiple shot echo planar acquisitions, conventional reconstruction methods produce artifactual images as a consequence of diffusion weighting and small amounts of bulk motion of the subject. If the amount of motion can be determined, it is possible to correct the data before reconstruction, which removes the artifact. A method for estimating the motion from the acquired data has been developed and evaluated. This method assumes that ghost image effects will be minimized when motion has been correctly compensated. By considering the amount of signal in the background of the image, appropriate corrections to the data can be made, and the accuracy of the motion compensation may be estimated. This technique has been evaluated by computer simulation, and its performance has been demonstrated in a phantom and humans with both two- and four-shot echo planar acquisitions and using both “mosaic” and “interleaved” sampling schemes.     

A comparison of fast MR scan techniques for cerebral activation studies at 1.5 Tesla

To evaluate the sensitivity of fast, gradient-echo MR scan techniques in their ability to detect blood oxygenation level dependent (BOLD) signal changes in task activation studies, three dedicated fast scan techniques, each with whole-brain coverage, were compared during a 3-min finger tapping paradigm on nine normal volunteers on a clinical 1.5 T scanner. Multislice (2D) single-shot spiral, 3D spiral, and multislice (2D) single-shot EPI scan techniques were done with similar temporal and spatial resolutions on each of the volunteers in random order. After image registration and statistical analysis, the sensitivity to detect activation was evaluated for the techniques by calculating t scores and number of activated voxels in predetermined regions of interest, including the contralateral primary sensorimotor cortex, the premotor region, the parietal region, the supplementary motor area, and the ipsilateral cerebellum. Baseline images acquired with the three techniques were qualitatively comparable and had a similar effective spatial resolution of around 5 × 5 × 5 mm³, as determined from autocorrelation analysis. The anatomical coverage was somewhat reduced (4 less slices per volume) with EPI at the identical temporal resolution of 1.76 s for all techniques. The use of multislice 2D spiral scan for motor cortex fMRI experiments provided for a superior overall temporal stability, and an increased sensitivity compared with multislice 2D EPI, and 3D spiral scan. The difference in sensitivity between multislice 2D spiral and EPI scans was small, in particular in the case of a ramp-sampled version of EPI. The difference in performance is attributed mainly to the difference in scan-to-scan stability.     

Calculation of signal intensities in hybrid sequences for fast NMR imaging

A number of techniques that recently have been used for fast NMR-imaging are based on a hybrid sequence of echo planar imaging (EPI) and FLASH imaging: after each NMR excitation several k-space lines are measured. The complete k-space is covered by performance of several excitations. It has been observed that there is usually an optimal hybrid sequence that maximizes the signal-to-noise ratio. In this work, a method is presented that allows a determination of the optimal sequence as a function of the relaxation times T₁ and T₂^*.     

Active Acoustic Screening: Reduction of Noise in Gradient Coils by Lorentz Force Balancing

We have designed and constructed a quiet gradient set with restricted access for the combined purposes of evaluating the principles of active acoustic screening, recently introduced by Mansfield, Glover, and Bowtell, and for EPI studies of the head at 3.0 T. The design utilizes the return paths of the conductors in a closed arc loop arrangement to eliminate net Lorentz forces thereby attenuating acoustic noise especially at low frequency. This design should significantly reduce the dangers to patients of high noise levels, especially in high field magnetic resonance imaging systems.     

Proton imaging of siloxanes to map tissue oxygenation levels (PISTOL): a tool for quantitative tissue oximetry

Hexamethyldisiloxane (HMDSO) has been identified as a sensitive proton NMR indicator of tissue oxygenation (pO(2)) based on spectroscopic spin-lattice relaxometry. A rapid MRI approach has now been designed, implemented, and tested. The technique, proton imaging of siloxanes to map tissue oxygenation levels (PISTOL), utilizes frequency-selective excitation of the HMDSO resonance and chemical-shift selective suppression of residual water signal to effectively eliminate water and fat signals and pulse-burst saturation recovery (1)H echo planar imaging to map T(1) of HMDSO and hence pO(2). PISTOL was used here to obtain maps of pO(2) in rat thigh muscle and Dunning prostate R3327 MAT-Lu tumor-implanted rats. Measurements were repeated to assess baseline stability and response to breathing of hyperoxic gas. Each pO(2) map was obtained in 3(1/2) min, facilitating dynamic measurements of response to oxygen intervention. Altering the inhaled gas to oxygen produced a significant increase in mean pO(2) from 55 Torr to 238 Torr in thigh muscle and a smaller, but significant, increase in mean pO(2) from 17 Torr to 78 Torr in MAT-Lu tumors. Thus, PISTOL enabled mapping of tissue pO(2) at multiple locations and dynamic changes in pO(2) in response to intervention. This new method offers a potentially valuable new tool to image pO(2) in vivo for any healthy or diseased state by (1)H MRI. Copyright (c) 2008 John Wiley & Sons, Ltd.     

呼和浩特城区新生儿乙肝疫苗计划免疫效果评价

本文对呼和浩特城区新生儿乙肝疫苗计划免疫效果作了评价。到１９９４年底，城区新生儿乙肝疫苗５年平均报告接种率达９３．７６％；１９９５年采用“批质量保证抽样法”调查，期望接种率达９０％；血清学调查表明，新生儿乙肝疫苗免疫成功率为９６．７７％；１～５岁组儿童（目标人群）抗－ＨＢｓ阳性率达９２．０１％，较计划免疫前５．６７％明显升高（χ２＝３２６．３６，Ｐ＜０．０１）；接种儿童较未接种的同年龄组儿童ＨＢｓＡｇ阳性率下降了５３．９９％，抗－ＨＢｃ阳性率下降了７５．３２％；流行病学效果评价表明，实施计划免疫后的目标人群乙肝发病率（８．３９／１０万）较实施前的同年龄组儿童乙肝发病率（７６．９８／１０万）下降了８９．１０％。     

Assortative mating for IQ and personality due to propinquity and personal preference

The role of personal preference as an active process in mate selection is contrasted with the more passive results of limitations of available mates due to social, educational, and geographical propinquity. The role of personal preference estimated after removing the effects of variables representing propinquity was still significant for IQ and Eysenck's extraversion-introversion and inconsistency (lie) scales, even though small.     

The proliferating field of neural crest stem cells.

Neural crest stem cells were first isolated from early embryonic neural crest in the early 1990s, but in the past 5 years, there has been a burst of discoveries of neural crest-derived stem cells from diverse locations. Here, we summarize these data, highlighting the characteristics of each stem cell type. These cells vary widely in the markers they express and the variety of cell types they appear to generate. They occupy diverse locations, but in some cases multiple stem cell types apparently occupy physically proximate niches. To date, few molecular similarities can be identified between these stem cells, although a systematic comparison is required. We note other issues worthy of attention, including aspects of the in vivo behavior of these stem cells, their niches, and their lineage relationships. Together, analysis of these issues will clarify this expanding, but still young, field and contribute to exploration of the important therapeutic potential of these cells.