mri 2 - BE101 Foundations of Biomedical Imaging Fall 2009...

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UCSD BE101 Fa06 Lecture 8 Slide 1 BE101 Foundations of Biomedical Imaging Fall 2009 David Hall, Ph.D. Department of Bioengineering University of California–San Diego Lecture: MRI II
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UCSD BE101 Fa06 Lecture 8 Slide 2 Signal Detection Basic Pulse Sequences Three Dimensional Imaging Suetens’s: Chapter 6 (pages 108-120) Today’s Outline
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UCSD BE101 Fa06 Lecture 8 Slide 3 Recap MRI 1:
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UCSD BE101 Fa06 Lecture 8 Slide 4 Spinning Protons Act like Little Magnets:
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UCSD BE101 Fa06 Lecture 8 Slide 5 They Align with an External Magnetic Field, B 0 :
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UCSD BE101 Fa06 Lecture 8 Slide 6 When is a Proton just like a Dreidle? When it wobbles (aka “precesses”): Frequency of the precession is directly proportional to the strength of the magnetic field and is defined by the Larmor Equation:
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UCSD BE101 Fa06 Lecture 8 Slide 7 Apply an RF pulse on Resonance Observer in Stationary plane sees M 0 spiral down to the XY plane (even to –z axis) Observer in Rotating Reference Frame sees M 0 appear to tip α towards the y’ axis
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UCSD BE101 Fa06 Lecture 8 Slide 8 Cannot Measure M Z Directly B is large in z-direction so how do we measure M z ? In x-y plane apply RF pulse (flip on to x-y plane) measure signal in x-y plane: FID apply additional RF pulses
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UCSD BE101 Fa06 Lecture 8 Slide 9 How to measure T1? RF 90º M XY (t) TR 90º 90º TR M O M XY (TR) z M z M xy xy plane B /1 () (1 ) TR T XY Mt e ∝−
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UCSD BE101 Fa06 Lecture 8 Slide 10 Turn off the Transmitter what happens? Relaxation RF energy is retransmitted - This is the "NMR" signal - At the resonance frequency - Signal proportional to Proton Density • Mz begins to recover - Exponential recovery of Mz - Time constant is called T1 - Longitudinal or Spin-Lattice Relaxation • Spins (Mxy) begin to dephase - Exponential decay of signal - Time constant is called T2 - or Transverse or Spin-Spin Relaxation
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UCSD BE101 Fa06 Lecture 8 Slide 11 Free Induction Decay (FID): An NMR signal in the absence of any magnetic gradients After the RF transmitter is turned off, the protons immediately begin to re-radiate the absorbed energy. If nothing is affecting the homogeneity of the magnetic field all of the protons will be spinning at the same resonance frequency. The initial amplitude of the signal is determined by the portion of the magnetization vector (Mø) that has been tipped onto the XY plane. This, in turn, is determined by the sine of the flip angle, a. The maximum signal is obtained when the flip angle is 90°. The signal unaffected by any gradient is known as a Free Induction Decay (FID). The time constant that determines the rate of decay is called T2. An FID has no positional information.
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UCSD BE101 Fa06 Lecture 8 Slide 12 Signal Detection and Detector 90 degree RF pulse. After disturbance, rotates clockwise in stationary frame and induces a current in antenna (coil).
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This note was uploaded on 04/30/2010 for the course BENG 101 taught by Professor Silva,g during the Fall '08 term at UCSD.

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mri 2 - BE101 Foundations of Biomedical Imaging Fall 2009...

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