Experimental demonstration of diffusion limitations on resolution and SNR in MR microscopy

Magnetic resonance microscopy images at cellular resolution (< 10 microns) are limited by diffusion. SNR and spatial resolution suffer from the dephasing of transverse magnetization caused by diffusion of spins in strong gradients. Such effects may be reduced by using phase encoding instead of frequency encoding readout gradients. Demonstration of the benefits of phase encoding are lacking, and the conditions in which it is preferred are not clearly established. We quantify when phase encoding outperforms a readout gradient with emphasis on the detrimental effects of diffusion on SNR and resolution. A 15.2T MRI scanner, with 1 T/m gradients, and micro solenoid RF coils < 1 mm in diameter, were used to quantify diffusion effects on resolution and SNR of frequency and phase encoded acquisitions. Frequency and phase encoding resolution and SNR per square root time were calculated and measured for images at the diffusion limited resolution. The point-spread-function was measured for phase and frequency encoding using additional constant time gradients with voxels 3-15 microns. The effect of diffusion during the readout gradient on SNR was experimentally demonstrated. The achieved resolutions of frequency and phase encoded acquisitions were measured via the point-spread-function. SNR per square root time and actual resolution were calculated for a wide range of gradient amplitudes, diffusion coefficients, and relaxation properties. The results provide a practical guide on how to choose between phase and frequency encoding. Images of excised rat spinal cord at 10 x 10 microns in-plane demonstrate benefits of phase encoding in the form of higher measured resolution and SNR vs the same image acquired with a conventional readout. We demonstrate the extent to which phase encoding outperforms readout gradients in SNR and resolution over a wide range of voxel sizes, sample, and hardware properties.Comment: 36 pages, 9 figures, 1 table, and 4 supplemental figures. Submitted to Journal of Magnetic Resonance; cleaned up metadata, fixed heading typ

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

Stochastic Income and Conditional Generosity

We study how other-regarding behavior extends to environments with uncertain income and conditional commitments. Should fundraisers ask a banker to donate "if he earns a bonus" or wait and ask after the bonus is known? Standard EU theory predicts these are equivalent; loss-aversion and signaling models both predict a larger commitment before the bonus is known; theories of affect predict the reverse. In field and lab experiments, we allow people to donate from lottery winnings, varying whether they decide before or after learning the lottery's outcome. Males are more generous when making conditional donations before knowing the outcome, while females' donations are unaffected. Males also commit more in treatments where income is certain but the donation's collection is uncertain. This supports a signaling explanation: it is cheaper to commit to donate before the uncertainty is unresolved, thus a larger donation is required to maintain a positive image. This has implications for experimental methodology, for fundraisers, and for our understanding of pro-social behavior

Prevalence of genetic differences in phosphorylcholine expression between nontypeable <it>Haemophilus influenzae </it>and <it>Haemophilus haemolyticus</it>

<p>Abstract</p> <p>Background</p> <p>Although non-typeable (NT) <it>Haemophilus influenzae </it>and <it>Haemophilus haemolyticus </it>are closely related human commensals, <it>H. haemolyticus </it>is non-pathogenic while NT <it>H. influenzae </it>is an important cause of respiratory tract infections. Phase-variable phosphorylcholine (ChoP) modification of lipooligosaccharide (LOS) is a NT <it>H. influenzae </it>virulence factor that, paradoxically, may also promote complement activation by binding C-reactive protein (CRP). CRP is known to bind more to ChoP positioned distally than proximally in LOS, and the position of ChoP within LOS is dictated by specific <it>licD </it>alleles (designated here as <it>licD_I</it>, <it>licD_III</it>, and <it>licD_IV</it>) that are present in a <it>lic1 </it>locus. The <it>lic1 </it>locus contains the <it>licA</it>-<it>licD </it>genes, and ChoP-host interactions may also be influenced by a second <it>lic1 </it>locus that allows for dual ChoP substitutions in the same strain, or by the number of <it>licA </it>gene tetranucleotide repeats (5'-CAAT-3') that reflect phase-variation mutation rates.</p> <p>Results</p> <p>Using dot-blot hybridization, 92% of 88 NT <it>H. influenzae </it>and 42.6% of 109 <it>H. haemolyticus </it>strains possessed a <it>lic1 </it>locus. Eight percent of NT <it>H. influenzae </it>and none of the <it>H. haemolyticus </it>strains possessed dual copies of <it>lic1</it>. The <it>licD_III</it>and <it>licD_IV</it>gene alleles were distributed similarly (18-22%) among the NT <it>H. influenzae </it>and <it>H. haemolyticus </it>strains while <it>licD_I</it>alleles were present in 45.5% of NT <it>H. influenzae </it>but in less than 1% of <it>H. haemolyticus </it>strains (<it>P </it>< .0001). NT <it>H. influenzae </it>had an average of 26.8 tetranucleotide repeats in <it>licA </it>compared to14.8 repeats in <it>H. haemolyticus </it>(<it>P </it>< .05). In addition, NT <it>H. influenzae </it>strains that possessed a <it>licD_III</it>allele had increased numbers of repeats compared to NT <it>H. influenzae </it>with other <it>licD </it>alleles (<it>P </it>< .05).</p> <p>Conclusions</p> <p>These data demonstrate that genetic similarities and differences of ChoP expression exist between NT <it>H. influenzae </it>and <it>H. haemolyticus </it>and strengthen the hypothesis that, at the population level, these differences may, in part, provide an advantage in the virulence of NT <it>H. influenzae</it>.</p