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mri2 final - 12.0 Physics of Magnetic Resonance MRI...

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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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Important Points from MRI Lecture 1 •Electrons in atomic orbits and nucleons in nuclear “shells” have internal angular momentum, S, and orbital angular momentum, L, giving total angular momentum J=L+S. Electrons, protons and neutrons are Fermions with S=1/2 The total angular momentum, either electronic or nuclear is determined by the total angular momentum of unpaired electrons, neutrons or protons. Nuclei with Even #p-Even #n have J=0, Even #p-Odd #n nuclei or Odd #p-Even #n nuclei have J=1/2, 3/2,… (half -integer), while Odd #p- Odd #n nuclei have J=1,2,…(integer). Angular momentum of charged particles or internal charges (neutron) gives rise to magnetic properties such that electrons in atomic orbits or nuclei have magnetic moments (magnetization) m (or μ )= γħJ for nuclei and m= -m B J/ ħ for electrons . •γ is the gyromagnetic ratio in units of rad/sec-Tesla. For H 1 , γ =42.57MHz/T In general, a particle has a magnetic moment given by m=gm P J where g is the g factor that depends on the particle and situation, m P is the Bohr magneton for the electron (m B =eħ/2m e ) and m P is the nuclear magneton for the nucleon (m N =eħ/2m N ). When placed in a magnetic field, B, magnetic moments of a spin=1/2 system can have two orientations (up or down) that have an energy difference of Δ E= γħB. This is like ΔE=ħν where the frequency, ν = γ B In a magnetic field nuclear magnetic moments precess at a Larmor frequency f L = γ B Since m e ~m N /1000, the resonant frequency associated with electrons is nearly 1000 times higher than for nuclei (ie, GHz vs MHz)
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Important Points from MRI Lecture 1 (continued!) A collection of nuclear magnetic moments in a field B 0 are only weakly aligned with the field because of constant exchange of thermal energy from the surroundings. About 1 in
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