MRI Physics from India3课件.pptVIP

Basic Principles - MRI The Origin of the MR Signal The Nucleus of a Hydrogen Atom is a Charged Particle The Nucleus of a Hydrogen Atom has a Nuclear Spin A Spinning Charged Particle, e.g., the Hydrogen Nucleus, will Produce a Magnetic Dipole Magnetic Dipole Bar Magnet Alignment Random in absence of applied field Sum of All Vectors is Zero; no signal can be measured Nuclear Orientations in B0 With B0 = Spin-up = Parallel Alignment Against B0 = Spin-down = Anti-parallel Alignment Alignment Ratio Number of nuclei in low energy state is slightly greater (about one per million spins) than the number in the high energy state Only the excess nuclei in the parallel orientation contribute to the MR signal. The Effect of Field Strength Net Magnetization Vector Vector Sums Magnet and Shim System Basic Pulse Sequences k-space K-space – Trajectories, Nyquist criterion Trajectories … how k-space covered over time Nyquist criterion… reciprocal of spread in one domain is sampling interval in other domain Dkx,y = 1/FOVx,y Dx, Dy = 1/(kx,y Max - kx,y Min) Reconstruction - Fourier Transform The process of converting time domain data to the frequency domain data is called the Fourier transformation. Since spins at different positions resonate at different frequencies, this creates a spatial distribution of Mxy, the image. msec +/-Hz FFT Image Reconstruction – 2D Fourier Transform 2D FFT produces the image. Two single FFT’s performed sequentially will also produce the image. Time Phase/Frequency Frequency Phase/Frequency Image An Image: Fully Processed, 2 FFT’s Raw Data before FFT Summary Hydrogen atom nuclei have positive charge and spin, hence magnetic moment Magnetic moments precess when they interact with external magnetic fields There is a preferential alignment with the field, an effective non-zero magnetization is formed This “longitudinal” magnetization can be tipped by a “transverse” RF magnetic field pulse Once the RF pulse is turned off, magnetic moments continue p