MRI空间编码k空间与重建.ppt

Similarities and Differences Between Frequency and Phase Encodings Both utilizes gradient fields for encoding Phase encoding is done by pre-frequency encoding the signal for a short time interval * * 2D Spatial Encoding – Frequency and Phase Encoding Gx Phase encoding Phase Shift Frequency ? X-Position Phase ? Y-Position 0 cos?0t cos?0t cos?0t 2cos?0t 0 -2 cos?0t 0 cos?0t 0 cos(?0t+? ) cos(?0t+? ) cos?0t 2cos?0t 0 -2 cos(?0t-?) 0 cos(?0t-?) 加相位编码梯度场 0 cos(?0t+? ) cos(?2t+? ) cos?1t 2cos?0t 0 -2 cos(?1t-?) 0 cos(?2t-?) 加频率编码梯度场 A Typical Imaging Sequence （SE） Gz Gy Gx k-Space Frequency-Encoded Signals Gx t kx 0 kx 0 kx 0 180 k-Space Phase-Encoded Signals t Gy ky kx FID vs Echo？ For 2D imaging ky kx Pulse Sequence ky kx ??? FT-1 k空间数据是数字化的MRI原始数据（raw data），为复数 k空间数据时间上是图像的二维FT变换 k空间的数据经FT-1可以得到图像，也为复数，临床图像通常为模图像。 k-空间 ky kx 主要内容 层选 频率编码 相位编码 K空间 1D/2D/3D 采集 2D 采集 - Slice by Slice, Fourier Imaging 3D 采集 2D、3D的区别： RF脉冲是非选择性的 选层梯度场 ? 相位编码 3D FT 扫描时间增加(因为相位编码步数增加) 为节省时间，可降低Z方向的空间分辨力 作业 图像重建作业 headraw.mat 傅里叶逆变换重建图像， 将数据第1到64行数据置0，然后重建 将数据1到64行与193到256行置0，… 将数据奇数行置0， … 将数据奇数行置0， … 将数据放到512*512大小的矩阵中心，其他位置处为0， … * How we make the gradient field: Two coils of wire, one on each end, are supplied with current which generates magnetic fields. The current is supplied in opposite direction, so one coil “adds to” and the other coil “subtracts from” the main magnetic field. At the point midway between the two coils, the magnetic field created by the gradient coils cancel each other, causing the net magnetic field to be equal to B0. The gradient coils are positioned so that this point is at the center of the magnet (isocenter). * Note that the direction of the magnetic field is the same; only the strength is different. CAUTION Signa and Vectra calls X and Y axis opposite! H-F direction is Z axis for both Signa and Vectra L-R direction is X axis for Signa, Y axis for Vectra A-P direction is Y axis for Signa, X axis for Vectra Gaussian 矩形脉冲 Si