tailieunhanh - Essentials of Neuroimaging for Clinical Practice - part 8

Các dữ liệu fMRI được so sánh với một số loại chức năng tham chiếu thời gian để xác định các vùng não cường độ tín hiệu fMRI có liên quan với một tập hợp các chức năng tham khảo. Hầu hết các ứng cử viên tham khảo các chức năng thu được từ thiết kế thí nghiệm. | Functional Magnetic Resonance Imaging 101 fMRI data is the general linear model. The fMRI data are compared with some kind of reference temporal function to determine in which brain regions the fMRI signal intensity is highly correlated with a collection of reference functions. Most candidate reference functions are obtained from the experimental design. For example because the brain s hemodynamic response assumes a fairly consistent profile delayed in onset and longer lasting relative to the inciting stimulus a boxcar function defining the experimental paradigm is often convolved with an estimated hemodynamic response function to yield the reference function. The resulting reference function is smoother than a boxcar and better takes into account the shape of the hemo dynamic response generally resulting in better correlation between the MR signal time courses and the regressor time course. Often a single canonical hemodynamic response function is used across the entire brain and across subjects despite the fact that evidence exists for variation in hemodynamic response shape across subjects and brain regions. Some software packages make provisions for this variation allowing for independent modeling of the hemodynamic response function on a voxelwise basis. Figure 4-2 shows brain activation related to a workingmemory task as seen by the hemodynamic response in the dorsolateral prefrontal cortex of a subject with schizophrenia. Figure 4-2. Statistical map showing bilateral dorsolateral prefrontal cortex activation in an unmedicated subject with schizophrenia during performance of the Sternberg Item Recognition Paradigm a task that requires working memory to function to obtain better-than-chance performance. The t-test statistical map was generated by comparing the images acquired during the five-target 5t condition with those acquired during the Arrows A baseline condition. The task paradigm is depicted graphically below the time course of signal intensity changes