Prof. Dr. Lothar Schad - uni-heidelberg.de · Prof. Dr. Lothar Schad 12/9/2008 | Page 1 Physics of...

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RUPRECHT-KARLS-UNIVERSITY HEIDELBERG

Computer Assisted Clinical MedicineProf. Dr. Lothar Schad

12/9/2008 |

Physics of Imaging Systems

Basic Principles of Magnetic Resonance Imaging I

Prof. Dr. Lothar Schad

Master‘s Program in Medical Physics

Chair in Computer Assisted Clinical MedicineFaculty of Medicine Mannheim University of HeidelbergTheodor-Kutzer-Ufer 1-3D-68167 Mannheim, GermanyLothar.Schad@MedMa.Uni-Heidelberg.dewww.ma.uni-heidelberg.de/inst/cbtm/ckm/



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Introduction

Introduction

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(Nuclear)

Magnetic

Resonance

(Kernspin)

Magnet

Resonanz

N S

Notation: NMR & MRI

(Imaging) Tomographie



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Isidor Rabi

19381938

Germany -Columbia

• rebuilt a molecular beam apparatus (Otto Stern)• detected nuclear resonance in a stream of Lithium Chloride molecules

19461946

Harvard

• applied radar technology in investigating magnetic resonance• achieved the first resonance in a practical sample, a block of paraffin

Edward Purcell, Torrey and Pound

E

B

E

B

ν

B

• Nobel prize for physics in 1944

-1/2

+1/2

-1/2

+1/2

© Yves De Deene. University of Gent, Belgium

NMR History: Discovery

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19461946

Leipzig -Stanford

Felix Bloch

• achieved the same in a sample of water• provided the mathematical characterization of the nuclear magnetic resonance phenomenon

• Nobel Prize for physics (Bloch & Purcell) in 1952

the Bloch equations

dMdt

= γ. (M x B)M

B

M x B

L

- G

L x G

L = I.ω

NMR History: Theory




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19491949

Erwin Hahn

• discovered a “second” nuclear resonance signal, the spin echo

• achieved T1 and T2 weighting

excitation pulse

refocusingpulse

TE/2 TE/2

The first observed spin echo by E. Hahn (1950)

Illinois

NMR History: Spin Echo


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19481948

Harvard Nicolaas Bloembergen

Robert Pound

Edward Purcell

• characterized the relaxation times of the nuclear response signal in detail

excitation pulse

refocusingpulse

excitation pulse

refocusingpulse

NMR History: Relaxation Times




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19721972

Downstate MedicalCenterBrooklyn

Raymond V. Damadian

• the first scanner for clinical purposes

The method envisioned scanning with a focused “sweet spot” similar to the scanning rasteron a television.

Either the “sweet spot” would move, or the patient would move across the “sweet spot”,thereby collecting one tissue data point at a time.

United States Patent 3,789,832

NMR History: Imaging I


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19731973

Paul Lauterbur

• second scanner: collecting many points at once.• the improved method was based on the principle of back projection.• magnetic field gradients were used to realize the projections.

Nature 1973;242:190-191

Richard R. Ernst

• 2D Fourier transform MRI19741974

Zurich

NMR History: Imaging II




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The first MR scanners ...

… and the most recent

mobile MRI unitopen MRI unitinterventional MRI unit

NMR History: Scanner


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• routine method in diagnostic of diseases since 1985

• worldwide more than 60 million examinations

• worldwide about 22 000 MRI-scanners for clinical routine

• world market volume of imaging systems (X-ray, CT, MRI, PET, US) about 10 Billion EUR

• European market for medical 3D imaging systems:2001: 386 Million US-Dollar 2008: 733 Million US-Dollar

source: Frost and Sullivan http://www4.medica.de/cipp/md_medica/custom/pub/content,lang,1/ticket,g_a_s_t/oid,479, 12.06.2002

Facts 2002



12/9/2008 | Page 12Imaging Examples: MR & CT

CT

MRI

patient: astrocytoma II

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12/9/2008 | Page 13Why MRI ?

1. best soft tissue contrast2. no radiation exposure

CTWMS: 1025 HuGMS: 1035 Hu } Δ = 1%CSF: 1000 Hu

T2 T1 MRIWMS: 90 ms 550 msGMS: 100 ms 1000 ms } Δ = 100%CSF: >1000 ms 2000 ms

ρ T2 T1

CT

example: patient astrocytoma II



12/9/2008 | Page 14Imaging Examples: Contrast

MRI properties:

+ best soft tissue contrast

+ different contrast

+ arbitrary slice orientation

+ morphology and function

+ no radiation

- only protons visible (no bones)

- no electron density

T1w T2w

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40 slices in about 4 sec

Fast Imaging Technique (EPI)



12/9/2008 | Page 16Nobel Prizes NMR

1944 Nobel prize in physicsIsidor Rabispin of nuclei (1939)

1952 Nobel prize in physicsFelix Bloch and Edward Purcell discovery of NMR (1946)

1991 Nobel prize in chemistryRichard ErnstFourier transformation, MRS (1966)

2002 Nobel prize in chemistryKurt Wüthrich3D structure of proteins, MRS (1982)

2003 Nobel prize in medicinePaul Lauterbur and Peter MansfieldMR imaging, MRI (1973)

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brain activation using finger tapping:- primary motor and sensory

cortex (M1/S1)- supplementary motor cortex

area (SMA)- Cingulum- secondary motor cortex (SII)

activation pattern: 15 s finger-tapping followed by 15 s silent.

Posse et al. Hum Brain Mapp 2001

Functional MRI



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courtesy of Robitaille. Center for Advanced Biomedical Imaging Department of Radiology, Ohio State University, USA

Ultra-High-Field MRI: 8.0 Tesla

Bammer et al. Eur J Radiol 2003

morphology diffusion tensor imaging (DTI)

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12/9/2008 | Page 19Safety and Risk I



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TLZ 01.08.2001

Safety and Risk II

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• tomographic imaging technique(gr. tomos (τομοσ) - slice)

• MR-scanner provides multi-dimensional data array (image) of spatial distribution of physical quantities

- 2D images with arbitrary orientation- 3D volume data- 4D images (spatial/temporal distributions)

• MR-signals originate directly from the human bodyno “Emission”-Tomography; see PET, SPECTno radioactive substance necessary!notice: CT = “Transmission-Tomography”

What means MRI ?



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M0

Comparison: CT - MRI

CT = transmission tomography MRI = “direct” tomography

hν

X-ray tube

detectordetectorelectronics

high voltage projection data

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• MRI works in the radiofrequency domain (e.g. 40 –300 MHz)- no ionizing radiation

• MRI image gives an abundance of information, imagepixel grey value (signal intensity) dependent of:- proton density ρ- spin-lattice-relaxation time T1- spin-spin-relaxation time T2- molecular motion (e.g. flow, diffusion, perfusion)- susceptibility (e.g. hemoglobin concentration)- chemical shift (e.g. fat)

What do we measure with MRI ?



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Frequency[Hz]

Wave Length[m]

Photon Energy[eV]

Radiation Molecular Impact

1026

1024

1022

1020

1018

1016

1014

1012

1010

10 8

10 6

10 4

10 2

10 0

10-18

10-16

10-14

10-12

10-10

10 -8

10 -6

10 -4

10 -2

10 0

10 2

10 4

10 6

1012

1010

10 8

10 6

10 4

10 2

10 0

10 -2

10 -4

10 -6

10 -8

10 -10

10 -12

10 -14

x- and γ-ray DNA break

UV-radiationvisible lightIR-radiation

e--excitation (orbital)oscillationrotation

UKW

KWMW

LW

MRI

source: Lissner and Seiderer. “Klinische Kernspintomographie” 1987

Electromagnetic Spectrum

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• strong magnet producing a homogeneous static magnetic field (0,1 - 8,0 Tesla) (for comparison: earth magnetic field 30 µT - 60 µT)

• radiofrequency unit creating a periodical magnetic field used for spin excitation and signal detection

• gradient coils producing a linear magnetic field gradient for spatial encoding

• receiver coils for signal detection

• computer for controlling the MRI scanner

• input/ouput panel for data-flow and -evaluation

MRI Components



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static magnetic field B0

field strength 1.5 – 3.0 Teslahomogeneity < 1.0 ppm

cryostatcooling liquidHe, (N2)

super contacting coilNbTi, Nb3Sn

M0

Magnetic Field B0

copper wires withniobium-titanium-fibers

nitrogen 77 Khelium 4.2 Kvacuum

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magnet

input/output-panel

computerRF-unit (transmitter)

RF-unit (receiver)

gradient system

MRI Components: Schema



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1 magnet with cryotank and cryoshield2 shim coils3 gradient coils4 RF-resonator5 patient couch

6 transmit/receive duplexer7 preamplifier8 low/high-pass filter 9 ESB-module10 RF leak proof connections

MRI Components: Network

console

massstorage

computer image & video

storage

RF low signal processing

central pulse

sequence

centralclock

synthe-sizer

RF pulse generator

amplifier shim current supply

magnet current supply

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transmitter

receiver

gradient

shimim

age

proc

esso

r350 MHz

control panel

computer

350 MHz

radio- gradients Gxyz static field B0frequency RF shim coils

MRI Components: Physical Parameters

static field B0 M0

radiofreq. RF signal

gradients Gxyz image

technicalcomponent

physicalparameter



12/9/2008 | Page 30MRI Systems

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12/9/2008 | Page 31MRI Hosting



12/9/2008 | Page 32MRI Installation

Prof. Dr. Lothar Schad - uni-heidelberg.de · Prof. Dr. Lothar Schad 12/9/2008 | Page 1 Physics of...

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