hst583 NMR

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Basic Principles of Magnetic Resonance
Jorge Jovicich
jovicich@mit.edu
Contents:
I) Historical Background
II) An MR experiment
- Overview
- Can we scan the subject?
- The subject goes into the magnet
- Brief RF pulses are applied
- The MR signal
- Summary
Overview of a MRI procedure
Subject
Safety screening
Tissue protons align
with magnetic field
(equilibrium state)
Magnetic field
RF pulses
Protons absorb
RF energy
(excited state)
Spatial encoding
using magnetic
field gradients
Relaxation
processes
Relaxation
processes
Protons emit RF energy
(return to equilibrium state)
NMR signal
detection
Repeat
RAW DATA MATRIX
Fourier transform
IMAGE
J. Jovicich, MIT
Can we scan the subject?
Safety Issues
•
Not everybody can undergo a MR examination
• Screening is mandatory to ensure safety / quality
during the MR examination
J. Jovicich, MIT
A typical MRI scanner
The main magnetic field:
•
is VERY strong
( 3T ~
30,000 times the earth’s magnetic field)
•
is
A L W A Y S O N !!
(superconducting currents)
•
it’s strength decays ~ 1 / r
3
(r is the distance from the center)
J. Jovicich, MIT
Can we scan the subject?
Safety Issues
• Risks:
- Ferromagnetic materials (unsafe)
- will be subject to force / torque in the main magnetic field
- risks associated with motion / displacements
- active biomedical implants may not function properly
From: www.symplyphysics.com/flying_objects.html
J. Jovicich, MIT
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