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Magnetic Resonance Imaging
Lorenz Mitschang
Physikalisch-Technische Bundesanstalt, www.ptb.de
23rd February 2009
I. Basic Concepts
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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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rotating-head.mov
Morphology: 3D Structure
magnetization density distribution
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Morphology: 2D Slices
healthy test person multiple sclerosis patient
white matter
gray matter
lesion
T1 weighted tissue contrast
T1 white matter < T1 gray matter intensity white matter > intensity gray matter
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Volume-selective in-vivo Spectroscopy
metabolite identification by chemical shift
quantification of metabolite concentration
multiple sclerosis patient
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3D Angiography
contrast agent distribution
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Brain Function and Behavior: functional MRI
contrast by relaxation through enhanced blood flow
visual stimulation activates visual cortex
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Motion and Flow
velocity-encoded-cardiac-MRI.mov
blood flow velocity distribution
fast imaging enables motion detection
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Imaging Paradigm
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
Application
3D, 2D morphology, lesions
in-vivo spectroscopy, temperature
Angiography, cancer cells,metabolism
fMRI
diffusion, flow, perfusion
much more
local variation bio-medical problemMR quantity
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I. Do we get sufficient signal from single voxel ?
Yes, sometimes: signal-to-noise (next lecture)
RF insignal out
MR Imaging = localized determination of MR parameters
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damaginghigh quality(hard tissue)resolution ~A
harmlesshigh quality(soft tissue)resolution ~m
harmlesslow qualityresolution ~mm
II. Can we get around the resolution limit ?
Yes, we can: localization (next lecture)
attenuation in human tissue
attenuation in human tissue
MRI = wave-like imaging
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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 ?
let’s see now …
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MR Patient Treatment
limited time for investigation : animal (anesthetized) ~ 3 htest person ~ 1 hsick 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”
noninvasive
nonionizing
homogeneous static fields are totally safe
kg/W4 dtB BSART
0
21
20
2
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MR Safety at 3T
wavelength < object, multi array coils
Norm IEC 60601-2-33 "Particular requirements for the safety of magnetic resonance equipment for medical diagnosis“
local SAR < 10 W/kg
experiment simulation counter rotating
hot spot
cold spot
1 kW transmitted
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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 ???