Ari Borthakur, PhD Associate Director, Center for Magnetic Resonance & Optical Imaging Department of Radiology Perelman school of Medicine, University of Pennsylvania Principles of MRI

slide 2 A brief history… Isidor Rabi wins Nobel prize in Physics (1944) for for his resonance method for recording the magnetic properties of atomic nuclei. Felix Bloch and Edward Purcell share Nobel Prize in Physics (1952) for discovery of magnetic resonance phenomenon. Richard Ernst wins Nobel Prize in Chemistry (1991) for 2D Fourier Transform NMR. His post-doc, Kurt Wuthrich awarded a Nobel Prize in Chemistry (2002) for 3D structure of macromolecules. Nobel Prize in Physiology & Medicine awarded to Sir Peter Mansfield and Paul Lauterbur (2003) for MRI. Who’s next? Seiji Ogawa for BOLD fMRI?

slide 3 Outline Slide # 3 Hardware:  MRI scanner  RF coil  Gradients Image contrast Software:  Pulse sequences  Fourier Transform

slide 4 How does it work?

slide 5 Original Concept* 1. Rotating Gradient 2. Filtered Back-projection *Lauterbur, Nature (1973)

slide 6 Fourier Imaging* 1. Three orthogonal gradients:  Slice Selection  Phase Encoding  Frequency Encoding 2. k-space  Representation of encoded signal *Kumar, et al. J. Magn. Reson. (1975)

slide 7 MRI scanners 19 Tesla4.7 Tesla1.5 Tesla To generate B 0 field and create polarization (M 0 )

slide 8 MRI coils Head coil Mouse MRI platform Torso coil To transmit RF field (B 1 ) and receive signal

slide 9 Gradients To spatially encode the MRI signal

slide 10 Image Contrast Contrast in MRI based on differences in: Relaxation times (T 1, T 2, T 2 *, T 1    Local environment Magnetization = spin density  Concentration of water, sodium, phosphorous etc. Macromolecular interactions  Chemical-exchange, dipole-dipole, quadrupolar interactions Contrast agents  Gadolium-based, iron-oxide, manganese

slide 11 Magnetization Bitar et al. RadioGraphics (2006) “spin”“spin ensemble”spin-up or spin-down = Boltzmann distribution Net Magnetic Moment

slide 12 Energy levels Larmor Equation Energy gap N + /N - = exp (-  E/k B T) Boltzmann distribution

slide 13 MR experiment M 0 aligned along B 0 M z =M 0 RF pulse applied in x-y “transverse” plane M “nutates” down to x-y plane M z =M 0 cos  and M xy =M 0 sin  RF off Detect M xy signal in the same RF coil

slide 14 Equation of motion Bloch Equation (simple form)

slide 15 Effect of RF pulse The 2 nd B field Choosing a frame of reference rotating at  rf, makes B 1 appear static:

slide 16 Nutation Lab frame of reference e.g. B 1 oscillating in x-y plane Rotating frame of reference e.g. B 1 along y-axis ~brian/mri-movies/

slide 17 After RF pulse-Relaxation Spin-Lattice relaxation time=return to thermal equilibrium M 0 Spin-spin relaxation with reversible dephasing Spin-spin relaxation without “reversible” dephasing T 1 >T 2 >T 2 *

slide 18 Signal detection ~brian/mri-movies/

slide 19 How does MRI work? Water Fat 4.7 ppm 1.2 ppm NMR signal ?=?= MR Image 1) Spatial encoding gradients 2) Fourier transform

slide 20 Pulse Sequence (timing diagram) Less MRI time/low image quality RF phase slice freq 90  TE Echo planar imaging Less MRI time/low image quality RF phase slice freq 90  TE Spin-echo TR 180  RF phase slice freq  (<90  ) TE Gradient-echo TR RF phase slice freq 90  Fast spin-echo TR 180 

slide 21 MRI B0B0 M0M0 B1B1 GzGz GxGx GyGy

slide 22 RF phase slice freq  TE Gradient-echo TR Slice Selection* *Mansfield, et al. J. Magn. Reson. (1976) *Hoult, J. Magn. Reson. (1977)

slide 23 MzMz Slice Selection B0B0 MzMz MzMz M xy GzzGzz RF BW RF =  G z  z  z{

slide 24 Frequency Encoding* RF phase slice freq  TE Gradient-echo TR *Kumar, et al. J. Magn. Reson. (1975) a.k.a “readout”

slide 25 Frequency Encoding MzMz GxxGxx BW read =  G xFOV x FOV x

slide 26 RF phase slice freq  TE Gradient-echo TR Phase Encoding* *Kumar, et al. J. Magn. Reson. (1975) *Edelstein, et al. Phys. Med. Biol. (1980)  pe

slide 27 Phase Encoding  G y2 y 1/  pe =  G yFOV y FOV y  G y1 y  G y0 y Each PE step imparts a different phase twist to the magnetization along y

slide 28 Traversing k-space Spin-warp imaging* RF phase slice freq  TE Gradient-echo TR kyky kxkx *Edelstein, et al. Phys. Med. Biol. (1980)

slide 29 FT k-space freq phase image Fourier Transform

slide 30 SpiralRadial Echo-planar phase freq Echo planar freq Radial freq Spiral Traversing k-space

slide 31 Fourier Transform

slide 32 Typical FT pairs

slide 33 K-space The signal a coil receives is from the whole object: The image is a 2D FT of the k-space signal*: *Kumar, et al. J. Magn. Reson. (1975) replace : K-space signal :

slide 34 K-space weighting High-frequency data=edges Low-frequency data=contrast/signal

slide 35 MRI Examples

slide 36 3D Neuro MRI

slide 37 Susceptibility-Weighted Imaging (SWI)

slide 38 BOLD fMRI

slide 39 Phase Contrast MR Angiography

slide 40 TSE Fat Saturated TSE TE = 25 ms Resolution:200x200µm 2

slide 41 7T Project: Hyper-polarized 3 He MRI K Emami, et al., Mag Reson Med 2009

slide 42 MRI MRI “head coil” Williams et al., PNAS T Project: ASL-Perfusion MRI Arterial Spin Labeling (ASL) technique TAG Image Control

slide 43 ASL MRI & PET of Alzheimer’s Disease ASL MRI Scan 18- FDG PET Scan CONTROL AD Detre et al., Neuroimage 2009

slide 44 Take home… Slide # 44 Magnet  Create B 0  Produce M 0 RF coil  Transmit B 1 field  Detect Signal Image contrast  Relaxation, concentration, interactions etc. Gradients  Spatial encoding  Signal in k-space Fourier Transform  From k-space to image space Pulse sequences  Traverse k-space  Image Artifacts