Master s Program in Medical Physics. Physics of Imaging Systems Basic Principles of Magnetic Resonance Imaging I. Prof. Dr. Lothar Schad.

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1 1 12/9/2008 Page 1 Master s Program in Medical Physics Physics of Imaging Systems Basic Principles of Magnetic Resonance Imaging I Chair in Faculty of Medicine Mannheim University of Heidelberg Theodor-Kutzer-Ufer 1-3 D Mannheim, Germany Lothar.Schad@MedMa.Uni-Heidelberg.de 12/9/2008 Page 2 Introduction Introduction Seite 1

2 2 12/9/2008 Page 3 Notation: NMR & MRI (Nuclear) (Kernspin) Magnetic N S Magnet Resonance Resonanz (Imaging) Tomographie 12/9/2008 Page 4 NMR History: Discovery Germany - Columbia 1938 Isidor Rabi rebuilt a molecular beam apparatus (Otto Stern) detected nuclear resonance in a stream of Lithium Chloride molecules E -1/2 +1/2 Nobel prize for physics in 1944 B Harvard 1946 Edward Purcell, Torrey and Pound applied radar technology in investigating magnetic resonance achieved the first resonance in a practical sample, a block of paraffin E -1/2 ν +1/2 B Yves De Deene. University of Gent, Belgium B Seite 2

3 3 12/9/2008 Page 5 Leipzig - Stanford 1946 NMR History: Theory 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 B M x B M dm dt = γ. (M x B) -G L L x G L = I.ω Yves De Deene. University of Gent, Belgium 12/9/2008 Page 6 Illinois 1949 NMR History: Spin Echo Erwin Hahn discovered a second nuclear resonance signal, the spin echo achieved T1 and T2 weighting excitation pulse refocusing pulse The first observed spin echo by E. Hahn (1950) TE/2 TE/2 Yves De Deene. University of Gent, Belgium Seite 3

4 4 12/9/2008 Page 7 NMR History: Relaxation Times Harvard Nicolaas Bloembergen excitation pulse refocusing pulse 1948 Robert Pound excitation pulse refocusing pulse Edward Purcell characterized the relaxation times of the nuclear response signal in detail Yves De Deene. University of Gent, Belgium 12/9/2008 Page 8 Downstate Medical Center Brooklyn NMR History: Imaging I Raymond V. Damadian the first scanner for clinical purposes 1972 United States Patent 3,789,832 The method envisioned scanning with a focused sweet spot similar to the scanning raster on 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. Yves De Deene. University of Gent, Belgium Seite 4

5 5 12/9/2008 Page NMR History: Imaging II 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: Zurich 1974 Richard R. Ernst 2D Fourier transform MRI Yves De Deene. University of Gent, Belgium 12/9/2008 Page 10 NMR History: Scanner The first MR scanners... interventional MRI unit open MRI unit mobile MRI unit Yves De Deene. University of Gent, Belgium and the most recent Seite 5

6 6 12/9/2008 Page 11 Facts 2002 routine method in diagnostic of diseases since 1985 worldwide more than 60 million examinations worldwide about 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 /9/2008 Page 12 Imaging Examples: MR & CT patient: astrocytoma II CT MRI Seite 6

7 7 12/9/2008 Page 13 Why MRI? CT ρ T2 T1 CT WMS: 1025 Hu GMS: 1035 Hu } Δ = 1% CSF: 1000 Hu T2 T1 MRI WMS: 90 ms 550 ms GMS: 100 ms 1000 ms } Δ = 100% CSF: >1000 ms 2000 ms example: patient astrocytoma II 1. best soft tissue contrast 2. no radiation exposure 12/9/2008 Page 14 Imaging Examples: Contrast T1 w T2 w MRI properties: + best soft tissue contrast + different contrast + arbitrary slice orientation + morphology and function + no radiation - only protons visible (no bones) - no electron density Seite 7

8 8 12/9/2008 Page 15 Fast Imaging Technique (EPI) 40 slices in about 4 sec 12/9/2008 Page 16 Nobel Prizes NMR 1944 Nobel prize in physics Isidor Rabi spin of nuclei (1939) 1952 Nobel prize in physics Felix Bloch and Edward Purcell discovery of NMR (1946) 1991 Nobel prize in chemistry Richard Ernst Fourier transformation, MRS (1966) 2002 Nobel prize in chemistry Kurt Wüthrich 3D structure of proteins, MRS (1982) 2003 Nobel prize in medicine Paul Lauterbur and Peter Mansfield MR imaging, MRI (1973) Seite 8

9 9 12/9/2008 Page 17 Functional MRI 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 /9/2008 Page 18 Ultra-High-Field MRI: 8.0 Tesla morphology diffusion tensor imaging (DTI) Bammer et al. Eur J Radiol 2003 courtesy of Robitaille. Center for Advanced Biomedical Imaging Department of Radiology, Ohio State University, USA Seite 9

10 10 12/9/2008 Page 19 Safety and Risk I 12/9/2008 Page 20 Safety and Risk II TLZ Seite 10

11 11 12/9/2008 Page 21 What means MRI? 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 body no Emission -Tomography; see PET, SPECT no radioactive substance necessary! notice: CT = Transmission-Tomography 12/9/2008 Page 22 Comparison: CT - MRI CT = transmission tomography MRI = direct tomography hν X-ray tube detector detector electronics M 0 high voltage projection data Seite 11

12 12 12/9/2008 Page 23 What do we measure with MRI? MRI works in the radiofrequency domain (e.g MHz) - no ionizing radiation MRI image gives an abundance of information, image pixel 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) 12/9/2008 Page 24 Electromagnetic Spectrum Frequency [Hz] Wave Length [m] Photon Energy [ev] Radiation Molecular Impact x- and γ-ray DNA break UV-radiation visible light IR-radiation UKW KW MW LW MRI e - -excitation (orbital) oscillation rotation source: Lissner and Seiderer. Klinische Kernspintomographie 1987 Seite 12

13 13 12/9/2008 Page 25 MRI Components 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 12/9/2008 Page 26 Magnetic Field B 0 static magnetic field B 0 field strength homogeneity Tesla < 1.0 ppm nitrogen 77 K helium 4.2 K vacuum M 0 copper wires with niobium-titanium-fibers super contacting coil NbTi, Nb 3 Sn cryostat cooling liquid He, (N 2 ) Seite 13

14 14 12/9/2008 Page 27 MRI Components: Schema magnet RF-unit (receiver) input/output-panel gradient system RF-unit (transmitter) computer 12/9/2008 Page 28 MRI Components: Network console mass storage computer image & video storage RF low signal processing central clock synthesizer central pulse sequence RF pulse generator amplifier shim current supply magnet current supply 1 magnet with cryotank and cryoshield 2 shim coils 3 gradient coils 4 RF-resonator 5 patient couch 6 transmit/receive duplexer 7 preamplifier 8 low/high-pass filter 9 ESB-module 10 RF leak proof connections Seite 14

15 15 12/9/2008 Page 29 MRI Components: Physical Parameters radio- gradients G xyz static field B 0 frequency RF shim coils gradient shim transmitter receiver technical component physical parameter static field B 0 M 0 radiofreq. RF signal control panel computer 350 MHz 350 MHz image processor gradients G xyz image 12/9/2008 Page 30 MRI Systems Seite 15

16 16 12/9/2008 Page 31 MRI Hosting 12/9/2008 Page 32 MRI Installation Seite 16