PubMed · 14680903
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Ugo Salvolini, Tommaso Scarabino. 2003. High field MRI.. https://doi.org/10.1016/j.ejrad.2003.08.011
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Hypertension is a significant risk factor for cognitive impairment (CI), yet the corresponding neural network abnormalities remain underexplored. In this study, we examined the associations among global and domain-specific cognitive dysfunction, neuroimaging measures, and blood pressure in a subgroup of hypertensive patients with CI (N = 41) from a randomized controlled trial who underwent ultra-high-field 7 T MRI. Structural atrophy related to CI was localized to regions overlapping the attention networks. Both whole-brain and within-network dysfunction of the attention networks were associated with worse global cognitive performance. Notably, hyperconnectivity within key attention network hubs, including the right anterior insula and posterior intraparietal sulcus, was associated with declined processing speed in hypertensive patients, mediating the association between pulse pressure and processing speed. These findings provide new insights into the neural pathophysiology of hypertension-related CI and suggest potential network-based targets for intervention.
The signal formation and properties of steady-state free precession (SSFP) in combination with alternating RF pulse phases or alternating spin precession is analyzed. Simulations and experiments demonstrate that the amplitudes of SSFP echo paths are significantly influenced by application of alternating phases either via the exciting RF pulse or via some external mechanism producing alternating spin precession. The influence of alternating phases on echo amplitudes is different for different echo paths. The primary SSFP echo paths F(0) (-) and F(0) (+) exhibit a signal reduction whereas higher-order echoes F(-1) (-) and F(1) (+) show a signal increase upon application of oscillating phases. This behavior can be described using a simple perturbation theory applied to the frequency response profile of balanced SSFP combined with a final signal integration over one balanced SSFP band. The high sensitivity of SSFP echo amplitudes to alternating RF pulse phases or precession is exemplarily used to detect and visualize propagating transverse acoustic shear waves. Detection of flow or alternating currents are further possibilities to apply this unique feature of SSFP.
In this work, an analytical evaluation of the MR signal is presented using the principle of reciprocity for any transmit and receive probe(s) with general geometrical structure(s). It is shown that the excitation of the spins cannot be explained using the principle of reciprocity in its standard form due to the asymmetry in the permeability tensor of the transverse magnetization. In the reception problem, the standard form of the principle of reciprocity is then utilized to calculate the NMR signal. As expected, the analysis shows that the strength of the NMR signal is a function of the receive coil's (fictitious) circularly polarized component of the field that poses an opposite sense of rotation to that corresponding with the excitation field, produced by the same coil, if it were to be used for excitation.