In the paper, the results of a fast gradient-echo implementation of the singular value decomposition (SVD) encoding technique for dynamic imaging are presented. The method used is an adaptation with several critical modifications of a keyholetype approach previously proposed but not implemented. The method was tested by imaging the events following injection of a contrast agent into a phantom, producing a series of dynamic image updates. It is demonstrated that, for this type of application, the SVD encoding technique adequately follows dynamic changes with even a small number of encodes. The result is compared qualitatively to that obtained by standard Fourier-based keyhole imaging and is shown to provide improved spatial resolution of dynamic events when updating with the same number of encodes.

The nature of the coupling between neuronal activity and the hemodynamic response is the subject of intensive research. As a means to simultaneously measure parametric changes of T* 2 , initial intensity (I 0 ) and perfusion with high temporal resolution, a multi-image EPI technique with slice-selective inversion recovery (ssIR) for arterial spin labeling was developed and implemented. Comparative measurements with and without the preceding slice-selective inversion pulse were performed. I 0 and R* 2 changes induced by primary visual stimulation were separated. For ssIR-multi-image EPI the average change of I 0 over all 12 subjects was 3.4%, corresponding to a perfusion change of 40 ml/min/100 g, whereas only minor I 0 changes were observed without inversion. On average, the R* 2 of the activated pixels changed by -0. Most functional magnetic resonance imaging (fMRI) studies are based on the blood oxygenation level-dependent (BOLD) contrast, which relies on changes in the local deoxyhemoglobin concentration (1,2). The mechanisms leading to the BOLD contrast, i.e., the exact relation between the observed T* 2 /T 2 change and the neuronal activity, are still under discussion since the change in T* 2 /T 2 depends on the complex interaction of several physiological parameters (cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen consumption (CMRO 2 )). Conventional fMRI using BOLD contrast cannot separate the different effects leading to activation-induced signal changes such as T* 2 or inflow increase. Furthermore, BOLD-based fMRI techniques depend upon the nonlinear coupling between CBF and CMRO 2 during cortical activation. It cannot necessarily be assumed that this coupling is present under all circumstances and in all regions of the brain (3,4). On the other hand, it is generally accepted that changes in regional CBF are more directly related to neuronal activity than the BOLD contrast. Thus, perfusion-based fMRI (5-8) provides a potentially better spatial localization of activation sites; however, it is less sensitive and has a lower temporal resolution than BOLD-fMRI. A measurement of both BOLD and regional CBF responses in fMRI offers the possibility of separating and comparing these two phenomena, and thus can yield further insight into the underlying mechanisms of the BOLD effect. The nonreproducibility of neuronal activity or hemodynamic changes, subject motion, and system instabilities, however, impedes an accurate comparison of the different effects when performing separate fMRI experiments.Recently, combined measurements of both image contrasts, obtained by interleaving different pulse sequences during a single fMRI session, have been proposed using two different approaches. BOLD and CBF have been measured consecutively during the same task, for example, by alternated acquisition of FAIR-and T* 2 -weighted EPI images (9 -11). The pulsed arterial spin labeling (ASL) techniques used mostly depend on CBF changes, but the individual IR images are inevitably T 2 /T *...

Time-resolved electron paramagnetic resonance (EPR) is used to study the transient spin polarization behavior of the light-induced radical pair state in the photosynthetic model system 2-tetraalkylphenylenediamine-Zn porphyrin-Znaphthoquinone. The time evolution of the transverse magnetization is monitored at various static magnetic fields. This implies a two-dimensional variation of the signal intensity with respect to both the magnetic field and time axis. Zero quantum coherence between two of the four eigenstates of the radical pair is observed at early times after the laser pulse. The beat frequencies vary significantly with the static magnetic field and extend up to 70 MHz. This model system is the first example where quantum beats have been observed of a correlated radical pair with protonated constituents.

A comparison of dynamic results of a multi‐echo contrast‐enhanced perfusion study obtained from a keyhole imaging experiment and the results from low‐resolution updates is presented. If, for each dynamic state, a separate reference image exists, high spatial resolution in the dynamic results can be preserved through keyhole imaging. If only one reference image can be used, the dynamic keyhole results still offer high spatial frequency content due to spatial phase discontinuities in the images. These often exist at the outline of organs and result from the fat in connective tissues. If the basic assumption of keyhole imaging, namely, that the relevant information is centered in k‐space, is violated, as in T *2‐weighted gradient‐echo images, keyhole imaging can lead to erroneous results even though the update images themselves seem to be free of any artifacts. J. Magn. Reson. Imaging 2000;11:312–323. © 2000 Wiley‐Liss, Inc.

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