Magnetic Resonance Imaging

Magnetic Resonance Imaging

Lorenz Mitschang

Physikalisch-Technische Bundesanstalt, www.ptb.de

23rd February 2009

I. Basic Concepts

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

rotating-head.mov

Morphology: 3D Structure

magnetization density distribution

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

Volume-selective in-vivo Spectroscopy

metabolite identification by chemical shift

quantification of metabolite concentration

multiple sclerosis patient

3D Angiography

contrast agent distribution

Brain Function and Behavior: functional MRI

contrast by relaxation through enhanced blood flow

visual stimulation activates visual cortex

Motion and Flow

velocity-encoded-cardiac-MRI.mov

blood flow velocity distribution

fast imaging enables motion detection

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

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

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

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 …

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

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

MR Safety at 7T

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 ???