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Magnetic Resonance Imaging
I. Basic Concepts
Lorenz Mitschang Physikalisch-Technische
Bundesanstalt, www.ptb.de 23rd February 2009
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Literature H. Morneburg (Ed.) Bildgebende
Systeme für die medizinische Diagnostik Siemens
AG P. T. Callaghan Principles of Nuclear
Magnetic Resonance Microscopy Oxford University
Press R. A. de Graaf In vivo NMR
Spectroscopy J. Wiley Sons Radiology
books Material partly courtesy of R. Brühl,
F. Schubert, F. Seifert
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Morphology 3D Structure
magnetization density distribution
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Morphology 2D Slices
multiple sclerosis patient
healthy test person
gray matter
white matter
lesion
T1 weighted tissue contrast T1 white matter lt T1
gray matter intensity white matter gt intensity
gray matter
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Volume-selective in-vivo Spectroscopy
multiple sclerosis patient
metabolite identification by chemical
shift quantification of metabolite concentration
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3D Angiography
contrast agent distribution
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Brain Function and Behavior functional MRI
visual stimulation activates visual cortex
contrast by relaxation through enhanced blood flow
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Motion and Flow
fast imaging enables motion detection
blood flow velocity distribution
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Imaging Paradigm
Application 3D, 2D morphology, lesions in-vivo
spectroscopy, temperature Angiography, cancer
cells, metabolism fMRI diffusion, flow,
perfusion much more
Parameter spin density,T1, T2 chemical
shift contrast agent concentration (Gd, SPIO,
13C-labelling) T2 (stimulation) spin echo
formation much more
Effect tissue contrast metabolites,
shifts tissue contrast, temporal
evolution BOLD-effect signal attenuation much
more
local variation
bio-medical problem
MR quantity
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MR Imaging localized determination of MR
parameters
signal out
RF in
I. Do we get sufficient signal from single voxel
? Yes, sometimes signal-to-noise (next
lecture)
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MRI wave-like imaging
attenuation in human tissue
damaging high quality (hard tissue) resolution A
harmless high quality (soft tissue) resolution m
attenuation in human tissue
harmless low quality resolution mm
II. Can we get around the resolution limit ?
Yes, we can localization (next lecture)
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III. How do we obtain the image from the
individual voxel signals ? image
reconstruction algorithms, k-space (part of
answer II.)
IV. What spin manipulations are required for
image formation ? pulse sequences (abound
in the lectures)
V. Are humans, animals, organisms well-doing in
MRI ? lets see now
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MR Patient Treatment

• noninvasive
• nonionizing
• homogeneous static fields are totally safe

• limited time for investigation animal
  (anesthetized) 3 h
• test person 1 h
• sick person 15 min
• noisy 100 decibel
• motion in inhomogeneous static field induces
  currents
• conductivity of biological tissue causes
  absorption of radiation energy as heat
• specific absorption rate

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MR Safety at 3T
wavelength lt object, multi array coils
simulation
1 kW transmitted
experiment
counter rotating
hot spot
cold spot
Norm IEC 60601-2-33 "Particular requirements for
the safety of magnetic resonance equipment for
medical diagnosis local SAR lt 10 W/kg
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MR Safety at 7T
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Next Lectures

• basic signal-to-noise and resolution in MRI
• basic localization methods
• (including reconstruction)
• basic pulse sequences
• (2D, 3D morphology in-vivo spectroscopy)
• specific applications
• visit of MRI scanner ???