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mri4 - 13.0 Magnetic Resonance Imaging We have previously...

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13.0 Magnetic Resonance Imaging We have previously covered the basic principles of the formation of a sample’s magnetization when placed in a magnetic field. This is the way NMR was done since the 1940’s. In the early 1970’s, 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
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Important Points from MRI Lecture 2 We discussed the Bloch Equations in their x,y,z form and their xy, z form and simple situations after a single RF pulse to evaluate the transverse and longitudinal signal evolution We saw how an FID for two different frequencies would result in two peaks in the Fourier spectrum if we plot the real part of the FT. This is the basis of spectroscopy, the frequency differences can come from chemical shifts due to different chemical/electronic environments that allow a sample’s structure to be studied. The MRI instrument was diagrammed and reviewed with emphasis on how a gradient in the magnetic field is created (x, y, and z gradients) and how linearly and circularly polarized RF fields (B 1 ) are created. Note that a linearly oscillating RF field can be thought of as a combination of two rotating fields, one at f L and the other at f L . In the rotating frame the one at f L
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  • Fall '10
  • MacFall
  • Magnetic resonance imaging, Nuclear magnetic resonance, maximum gradient strength, Paul Lauterbur, •Typical MRI gradient, gradient rise time

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