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mri3 - 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 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 Important Points from MRI Lecture 2 • Concept of relaxation time • Spin-Lattice relaxation time: T1. Describes recovery time to 66% of equilibrium mangnetization • T2*: time for signal right after RF pulse to drop to 33% of its initial value. Describes the signal loss in a free-induction-decay (FID—the signal after a single RF pulse of some tip angle, α ) • Discussed the remarkable formation of a spin-echo at a time TE (echo-time) after a the initial FID if we place a 180 RF pulse at TE/2 after the initial RF pulse that caused the FID. The amplitude of the spin echo rises in a time T2* to a peak value and then drops off in a time T2*. • Spin-echoes formation can reverse certain signal losses due to magnetic field inhomogeneity, Δ B • Spin-spin relaxation time: T2. Describes time for the spin-echo peak amplitude to drop to 33% of initial value (irreversible loss) • We discussed the relation between T2*, T2 and Δ B : 1/T2*=1/T2+ γΔ B /2 • The reversible loss mechanisms, including inhomogeneities, are collected into a common term written as 1/T2’= γΔ B /2 + other terms and we then have 1/T2*=1/T2+1/T2’. If Δ B=0 (T2’ Æ ∞ ) then T2*=T2. Spin Echo Parameters • TR is the time between 90 ° pulses • τ is the time between the 90 ° pulse and the 180 ° pulse. It is also the time between the 180 ° and the echo • Thus, TE, the time between the 90 ° pulse and the echo is equal to 2 τ • M xy = M(TR, T1) e-(TE/T2) Important Points from MRI Lecture 2 •After an RF pulse, the spin equilibrium is disturbed. T1 is related to how much thermal (lattice or surroundings) energy is available at the Larmor frequency to cause transitions to reestablish equilibrium. An important source of such energy is dipole- dipole interaction modulated by molecular tumbling or correlation time, t m . • If t m is too long (slow tumbling) or too short (fast tumbling) there wont be much energy at the Larmor frequency, in which case T1 is long. This is why T1 for water is long....
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