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3He spin exchange cells for magnetic resonance imaging (2002)

Jacob, R. E. ; Morgan, S. W. ; Saam, B.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 92 (2002), S. 1588-1597

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ISSN: 1089-7550

Source: AIP Digital Archive

Topics: Physics

Notes: We present a protocol for the consistent fabrication of glass cells to provide hyperpolarized (HP) 3He for pulmonary magnetic resonance imaging. The method for producing HP 3He is spin-exchange optical pumping. The valved cells must hold of order 1 atm⋅L of gas at up to 15 atm pressure. Because characteristic spin-exchange times are several hours, the longitudinal nuclear relaxation time T1 for 3He must be several tens of hours and robust with respect to repeated refilling and repolarization. Collisions with the cell wall are a significant and often dominant cause of relaxation. Consistent control of wall relaxation through cell fabrication procedures has historically proven difficult. With the help of the discovery of an important mechanism for wall relaxation that involves magnetic surface sites in the glass, and with the further confirmation of the importance of Rb metal to long wall-relaxation times, we have developed a successful protocol for fabrication of 3He spin exchange cells from inexpensive and easily worked borosilicate (Pyrex) glass. The cells are prepared under vacuum using a high-vacuum oil-free turbomolecular pumping station, and they are sealed off under vacuum after ≥100 mg of distilled Rb metal is driven in. Filling of cells with the requisite 3He–N2 mixture is done on an entirely separate gas-handling system. Our cells can be refilled and the gas repolarized indefinitely with no significant change in their wall properties. Relaxation data are presented for about 30 cells; the majority of these reach a "40/40" benchmark: T1〉40 h, and 3He polarizations reach or exceed 40%. Typical polarization times range from 12 to 20 h; 20% polarization can be achieved in 3–5 h. © 2002 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1487438

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Lateral spike conduction velocity in the visual cortex affects spatial range of synchronization and receptive field size without visual experience: a learning model with spiking neurons (2000)

Saam, Mirko ; Eckhorn, Reinhard

Springer

Biological cybernetics 83 (2000), S. L1

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ISSN: 1432-0770

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Computer Science , Physics

Notes: Abstract. Classical receptive fields (cRF) increase in size from the retina to higher visual centers. The present work shows how temporal properties, in particular lateral spike velocity and spike input correlation, can affect cRF size and position without visual experience. We demonstrate how these properties are related to the spatial range of cortical synchronization if Hebbian learning dominates early development. For this, a largely reduced model of two successive levels of the visual cortex is developed (e.g., areas V1 and V2). It consists of retinotopic networks of spiking neurons with constant spike velocity in lateral connections. Feedforward connections between level 1 and 2 are additive and determine cRF size and shape, while lateral connections within level 1 are modulatory and affect the cortical range of synchronization. Input during development is mimicked by spike trains with spatially homogeneous properties and a confined temporal correlation width. During learning, the homogeneous lateral coupling shrinks to limited coupling structures defining synchronization and related association fields (AF). The size of level-1 synchronization fields determines the lateral coupling range of developing level-1-to-2 connections and, thus, the size of level-2 cRFs, even if the feedforward connections have distance-independent delays. AFs and cRFs increase with spike velocity in the lateral network and temporal correlation width of the input. Our results suggest that AF size of V1 and cRF size of V2 neurons are confined during learning by the temporal width of input correlations and the spike velocity in lateral connections without the need of visual experience. During learning from visual experience, a similar influence of AF size on the cRF size may be operative at successive levels of processing, including other parts of the visual system.