mri1 - 12.0 Physics of Magnetic Resonance MRI produces...

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12.0 Physics of Magnetic Resonance MRI produces high-resolution, high-contrast cross-sectional images throughout the head and body. Like ultrasound, it is non-invasive, limited mainly by power deposition. MRI is also limited by the fact that the signal is generated by the nuclei in the tissue and the only way to increase this signal is to increase the magnetic field, which is very expensive. Beyond imaging, MRI has functional aspects such as chemical species sensitivity, microscopic blood flow sensitivity that makes brain neuronal activity accessible and diffusional sensitivity to evaluate tissue microstructure. This section will cover the basics, without delving into imaging schemes. Later sections will cover imaging methods. •Microscopic Magnetization •Macroscopic Magnetization •Precession and Larmor Frequency •Transverse and Longitudinal Magnetization •RF Excitation •Relaxation •Bloch Equations •Spin Echoes •Contrast Mechanisms
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Magnetic Resonance Imaging Built upon a long history of success in nuclear magnetic resonance spectroscopy (physicists and chemists) Discovered in 1946 by Felix Bloch (Stanford), Edward Purcell (Harvard). Nobel Prize in 1952 to Bloch and Purcell Physicists were interested in spin, Chemists developed the whole field because they found out how to use NMR for chemical structure analysis First images in 1973, Paul Lauterbur First human body images, 1977, Raymond Damadian Commercial installations at 0.3T in late 1970’s First high field 1.5T commercial scanner at Duke in 1982 2003 Nobel Prize to Lauterbur and Mansfield
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(magnetic moment) “intrinsic” non-zero
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(its not really spinning! If so it would have surface velocities > c! The existence of magnetic properties implies some internal angular momentum that must be considered “intrinsic”) (magnetic moment)
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proton concentration in water is 110Molar, P31 is a few millimolar
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Recall: Angular Momentum J = g =g v r g v r J if g also has a charge “q” then we really have a current I=q/T=qv/2πr where T is the period. This current loop acts like a magnetic dipole with dipole moment: m=IA where A is the area of the loop m=qJ/2g In a magnetic field, the energy of a magnetic dipole is: U=-mBcosθ where θ is the angle between m and B ΔU=2mB (parallel to antiparallel to B) dipole field of a current loop source: wikipedia
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Stern-Gerlach Experiment source: http://www.kent.ac.uk/physical-sciences/local/undergrad/physics_notes/Ph503/STERN%20GERL_SPIN%20HYP.pdf
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Stern-Gerlach Experiment: Electron Spin and Magnetic Moment The Stern–Gerlach experiment was performed in Frankfurt, Germany in 1922 by Otto Stern and Walther Gerlach. It showed that electrons had internal angular momentum and that it was quantized, ie, had 2 values of +1/2 and -1/2. The internal angular momentum leads to a magnetic moment.
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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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mri1 - 12.0 Physics of Magnetic Resonance MRI produces...

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