mri5 - 13.0 Magnetic Resonance Imaging We have previously...

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Unformatted text preview: 13.0 Magnetic Resonance Imaging We have previously covered the basic principles of the formation of a samples magnetization when placed in a magnetic field. This is the way NMR was done since the 1940s. In the early 1970s, Paul Lauterbur had the idea to spatially encode the NMR signal to make images. Now we explore the instrumentation and schemes devised since then to make images and what the important parameters are that affect image quality. Instrumentation magnet gradient coils RF coils spectrometer for phase sensitive detection MRI Data Acquisition encoding spatial position: gradients, slice selection frequency encoding, k-space diagrams phase encoding spin-echo pulse sequence MRI Image Reconstruction MR Image Quality, Sampling Requirements, SNR Important Points from MRI Lecture 4 The function of gradient magnetic fields were reviewed.Gradients provide a way to make the frequency of the transverse magnetization, M xy , depend on the position of an isochromat. Slice selection was used to show how the combination of an RF pulse that excites only a band of frequencies and a gradient that spreads out frequencies so that only those spins in the right frequency range will be excited. A good example for slice selection is a sinc pulse combined with a gradient G: Since the sinc pulse has to be truncated (eg, 4cycles) the slice profile is rippled During slice selection, the gradient has an unintended consequence that dephases the spins. The solution is a refocusing gradient that reverses this effect. This allows us to draw our first simple pulse sequence Important Points from MRI Lecture 4 The MR signal was described in the context of the MRI coil performing a volume integral over all the M xy slice selection period period for slice refocus, phase encode, and dephase to prepare for frequency encoding frequency encode period spoiler gradient (no encoding gradients) (x,t)=(x)*t=Gxt ) , ( 2 ) , ( ) , ( y x j e y x f y x f Important Points from MRI Lecture 4 180 RF pulse reverses phase angles and hence - so e j e-j this holds true regardless of how the phase accumulated (ie, G x , G y or both!). y x z - Since we have (u,v) generated by both gradients, (u,v) (-u,-v) Spin echo add a 180 pulse and a slice selective gradient to go with it reverse the dephaser Fat-water Frequency Difference Fat and water differ in frequency by 3.5ppm. At 1.5T this is a difference of f=3.5*1.5T*42.57MHz/T/10-6 =224.1 Hz. So the fat image will be shifted in the frequency direction by 224 Hz....
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This note was uploaded on 10/30/2010 for the course MP 230 taught by Professor Macfall during the Fall '10 term at Duke.

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mri5 - 13.0 Magnetic Resonance Imaging We have previously...

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