To permit rapid optical control of brain activity, we have engineered multiple lines of transgenic mice that express the lightactivated cation channel Channelrhodopsin-2 (ChR2) in subsets of neurons. Illumination of ChR2-positive neurons in brain slices produced photocurrents that generated action potentials within milliseconds and with precisely timed latencies. The number of light-evoked action potentials could be controlled by varying either the amplitude or duration of illumination. Furthermore, the frequency of light-evoked action potentials could be precisely controlled up to 30 Hz. Photostimulation also could evoke synaptic transmission between neurons, and, by scanning with a small laser light spot, we were able to map the spatial distribution of synaptic circuits connecting neurons within living cerebral cortex. We conclude that ChR2 is a genetically based photostimulation technology that permits analysis of neural circuits with high spatial and temporal resolution in transgenic mammals.brain networks ͉ cortical circuitry ͉ synaptic transmission

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The difference between the displacement of the center-ofrotation (mechanical shift) and the electronic centering misalignment (electronic shift) in cone beam SPECT is evaluated. A method is proposed to determine both mechanical shift (MS) and electronic shift ( E S ) using the coordinate of the centroid of a projected point .source sampled over 3600 and the Marquardt nonlinear fitting algorithm. Both shifts are characterized by two orthogonal components.This method is verified using Monte Carlo simulated point source data with different combinations of mechanical and electronic shifts. Both shifts are able to be determined precisely. This method is also applied to a CB-SPECT system. This study suggests that the calibration of the system is needed on a periodic basis. I. bJTRODUC"I0NIn cone beam (CB) SPECT, there are two important kinds of misalignments. One is the mechanical shift (MS) which is the displacement of the mechanical center-of-rotation (COR) off the midline of the cone beam geometry. Mechanical shift happens in object space, and the translations of the projections caused by this shift are thus depth-dependent. The other misalignment is the electronic shift (ES) or the electronic centering misalignment. The ES causes the projections to have a collective translation and this translation generally does not depe.nd on the object or the detection angle. Due to the different characteristics of these two kinds of shifts in CB SPECT system, it is important to distinguish them and make appropriate corrections for each misalignment separately [ 11.In this paper a method is proposed to determine both mechanical shift and electronic shift using the coordinate of the centroid of a projected point source sampled over 3600 and the Marquardt nonlinear fitting algorithm [2, 31. This method provides nine fitting parameters. These parameters include. twodirnensional MS; two-dimensional ES; the focal length of the collimator; the distance between the focal point and the COR and the spatial coordinate of the point. This proposed method is verified using Monte Carlo simulated point source data [4] with different combhations of MS and ES. Both shifts are able to be detennined precisely and accurately using this method. Experimental point source data acquired at different times and different positions are also used to verify this method. The determined E3 parameters are used to correct the projections and MS parameters are incorporated into the reconsauction algorithm [ll. Images are reconshucted with and without the corrections and the image resolutions are measured. These results demonstrate that the lTbiS work is supported by PHS Grant ROl-CA33S41 awarded by the National Cancer Institute and in part by Grant DE-FGOS-91ER60894 awarded by the Department of Energy.image resolutions are improved after shift corrections. The results also indicate that the shift parameters determined in the same experimental setup with point sources located at different places are consistent but change from t i m e to time, suggesting that the ...

An object model based on combinations of object primitives is proposed for Monte Carlo simulated emission and transmission tomographic imaging systems. The primitives include ellipsoids, elliptic cylinders, tapered elliptic cylinders, rectangular solids, and their subsets: half, quarter, and eighth. The probability of a photon surviving interactions with the phantom medium is used as a weight for variance reduction. Calculation of the probability can be computationally intensive without properly organizing the inclusion of subregions within larger regions. A tree data structure is introduced to organize this inclusion relationship and used as the basis for two computationally efficient schemes for determining the intersection locations of a photon path with primitives and for identifying the attenuation coefficients for adjacent intersections for the survival probability computation. The approach has been validated by emission as well as transmission simulations. A thorax phantom containing overlapped ellipsoids and a heart composed of twelve overlapped quarter ellipsoids are employed to demonstrate the capability of the model.