1,721,119 research outputs found
Quantitative magnetization transfer by trains of radio frequency pulses in human brain: extension of a free evolution model to continuous-wave-like conditions
No full textA theoretical model of free evolution between repeated magnetic transfer (MT) pulses was extended to continuous-wave (CW)-like conditions showing that only the repetitive "direct" saturation of bulk water changes the transient and stationary behavior. The influence of the pulse repetition period (PR) on progressive saturation was studied in cortical gray matter (GM) and central white matter (WM) under conditions of short periods of free evolution and strong macromolecular saturation. Interpulse delays of 3 ms were achieved in vivo on a 1.5-T MR system with bell-shaped MT pulses of 12-ms duration and nominal flip angles of up to 1440 degrees and single-shot readout by a stimulated echo acquisition mode localization sequence. The frequency offset was chosen between 1 and 3 kHz to avoid excessive direct saturation. The stationary MT ratio (MTR) followed an inverse linear PR dependence, showing a consistent partial saturation of about 90% at zero PR for both WM and GM. Comparison to a relaxation-matched liquid indicated the presence of MT, but not necessarily of direct saturation. The transient behavior indicated considerable direct saturation, but this could also be explained by MT. These inconsistencies showed that the intervals of time evolution in our experiments were too long to be modeled by CW-like conditions. Free evolution takes place during the whole PR rather than during the interpulse delay only. Quantification using the rates of free evolution theory yielded the saturations and rate constants necessary to explain the observed behavior. The theory of rapid CW-like pulsing provides an upper limit for the rate of progressive saturation. This limit is approached at PR below an estimated value of 5 ms. The phenomenological PR dependence of the steady-state MTR may indicate that MT exceeded the direct saturation. Unlike to an idealized CW experiment, the extrapolated value at zero PR is subject to direct effects and not a physically meaningful constant. (c) 2005 Elsevier Inc. All rights reserved
Simultaneous measurement of saturation and relaxation in human brain by repetitive magnetization transfer pulses
No full textMagnetization transfer (MT) by equidistant pulse trains can be described as being analogous to progressive partial saturation, where 'direct' saturation of water is amplified by MT contributions that are dependent on macromolecular content and differential saturation. This concept was applied to study the transition to steady state in the human brain using similar MT-pulses as in imaging. Up to 41 bell-shaped MT-pulses of 12 ms duration were applied at frequency offsets between 0.5 and 15 kHz with flip angles between 1080 and 1440degrees. Central white and parietal gray matter was studied in human subjects using STEAM for localized read-out (TE = 30 ms, TM = 13.7 ms). The apparent degree of saturation, delta(app), and the longitudinal relaxation of the water pool during the pulse repetition period (PR) were fitted to the transient behavior after signal correction for cerebro-spinal fluid. PR was varied between 15 and 100 ms to assess the PR-dependence of the fitted parameters. The MT-term in delta(app) exceeded the direct saturation and attained its maximum at PR greater than or equal to 100 ms. The macromolecular pool was only partially saturated by a single MT-pulse. The offset may be increased to 2.5 kHz to reduce direct saturation without sacrificing NIT in white matter. The estimated relaxation rates (1.04 +/- 0.11 s(-1) in WM; 0.76 +/- 0.13 s(-1) in GM) were faster than are commonly observed at 1.5 T. The apparent saturation is a measure for MT that is not confounded by relaxation. To maximize MT in brain tissue, MT-pulses should be applied at PR 100 ms or longer. At shorter PR, a larger steady state satuation is obtained at the cost of increased contributions from direct saturation. Since this accelerates the convergence, PR should be decreased to reach the steady state within a specified time. A faster transition can always be achieved at a reduced frequency offset via increased direct saturation. Copyright (C) 2004 John Wiley Sons, Ltd
The systematic status of Plagiochila sects. Bidentes Carl and Caducilobae Inoue (Hepaticae) inferred from nrDNA ITS sequences
No full textPlagiochila section Bidentes Carl was erected for some of the tiniest species of the genus that often possess caducous or fragmenting leaves. Current discussions focus on whether P sect. Bidentes represents a natural species group or should be broken up into two lineages, P sect. Bidentes s.str. and P sect. Caducilobae Inoue. Phylogenetic analyses of nrITS sequences of 28 species of Plagiochila produced several independent lineages that correspond with morphologically and phytochemically defined sections of Plagiochila (i.e., P. sects. Arrectae, Fuscoluteae, Glaucescentes, Hylacoetes, Plagiochila, Rutilantes, and Vagae). Plagiochila bidens, the type of P sect. Bidentes, is placed in a clade with several members of P sect. Arrectae Carl; therefore, P sect. Bidentes is treated as a synonym of P sect. Arrectae. The type of P sect. Caducilobae, P caduciloba H.L. Blomq., as well as several other members of the "Bidentes/Caducilobae-complex", cluster with members of P sect. Rutilantes Carl, a group to which can therefore be assigned the majority of species currently placed in P sect. Bidentes, i.e., excluding the type. Morphologically, members of P sect. Rutilantes are characterized by a "free" perianth. In contrast, the members of P sect. Arrectae usually possess perianths that are covered by bracts at least in the basal half. Plagiochila loriloba Herzog ex Carl [syn.: P cuneata var. loriloba (Herzog ex Carl) Herzog] is recognized at the rank of species
Pulsed saturation of the standard two-pool model for magnetization transfer. Part II: The transition to steady state
No full textThe transition of the tissue signal to steady state under periodic selective saturation of macromolecular magnetization can be observed by single-shot echo-planar imaging. The general solution for a two-pool system with linear exchange kinetics contains two transient components. The rapid minor transient causes an initial delay of the transition for fast pulse repetition (PR) and weak saturation. The slow major transient combines progressive direct saturation and transferred saturation. Its PR-dependence provides similar information as the steady state, but is less sensitive to direct saturation and fitting errors. Sampling at different PR allows to quantify all system parameters
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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