To evaluate the effect of the positive-indefinite matrix on the diffusion tensor-derived parameters, a modified algorithm is proposed for calculating these parameters. Magnetic resonance （MR） diffusion tensor images of five healthy volunteers are collected. The diffusion sensitive gradient magnetic fields are applied along 25 directions and the diffusion weighting value is 1 000 s/mm^2. Many positive-indefinite diffusion tensors can be found in the white matter area, such as the genu and the splenium of corpus callosum. Due to the positive-indefinite matrix, the mean diffusivity （MD） and the fractional anisotropy （FA） are under-estimated and over-estimated by using the conventional algorithm. Thus, the conventional algorithm is modified by using the absolute values of all eigenvalues. Results show that both the robustness and the reliability for deriving these parameters are improved by the modified algorithm.
Many studies have shown that the magnetic resonance signal decay with an extend range of diffusion weighting(b-factor)is a bi-exponential attenuation in the diffusion-weighted magnetic resonance imaging experiments in vivo.Based on this feature and the two-compartmental model,we propose a twice-linear-fitting(TLF)algorithm to estimate the apparent diffusion coefficient(ADC)of the water molecules instead of the commonly used iterative Levenberg–Marquardt(LM)method.The TLF algorithm consists of two liner fitting steps to estimate the fast and the slow apparent diffusion coefficients and their sizes,respectively.It is unnecessary to guess the initial values in the whole fitting process.The time consumption of the TLF algorithm is much less than that of the iterative LM method.Moreover,the TLF algorithm may avoid the extraneous solutions,which often deteriorates the results of the LM method.Compared with the iterative fitting method,the TLF algorithm is a reliable and timeefficient approach to estimate the ADC of water molecules in vivo in magnetic resonance diffusion-weighted imaging experiments.
Song GaoFei WangDongqi HeShouyu ZhangShanglian Bao
Visually-induced erotic arousal evoked by pornographic visual stimuli, such as films or photographs, is a common occurrence in human behavior. The brain activation associated with visual erotic stimuli in heterosexual right handed females is studied. Functional magnetic resonance imaging is used to investigate 15 female partici- panterotic arousal induced by visual stimuli in film and picture forms, respectively, performing three or more times during their menstrual cycle on a 3.0T magnetic resonance imaging scanner. There is activation of a set of bilateral brain areas, including the inferior lateral occipital cortex, the anterior supramarginal gyrus, the parietal operculum cortex, the superior parietal lobules, the right inferior frontal gyrus, the cerebellum, the hypothalamus, the thalamus, the hippocampus, and the mid-brain. From different regions, the brain activation is observed and the inferior frontal gyrus has found to be task-independent. Furthermore, the right inferior frontal gyrus has more activation than the left inferior frontal gyrus. The result shows that the right inferior frontal gyrus plays an important role in pornographic information processing rather than being activated stimuli property specific. It is presented for the first time that the functional laterization of the inferior frontal gyrus is bi-directional rather than single （left） directional.
