Part III Physics: Medical Physics Option Magnetic Resonance Imaging
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Part III Physics: Medical Physics Option Magnetic Resonance Imaging Dr T A Carpenter http://www.wbic.cam.ac.uk/~tac12 Part III Physics: Medical Physics Magnetic Resonance Imaging 1999 Lecture Content Lecture I – Overview of Nuclear Magnetic Resonance – Excitation and Signal detection – One pulse and Two pulse experiments – Hardware Part III Physics: Medical Physics Option Magnetic Resonance Imaging Lecture Content Lecture II – How does NMR become MRI – Effects of Magnetic Field Gradients – Imaging pulse sequences – contrast – examples Part III Physics: Medical Physics Option Magnetic Resonance Imaging Lecture Content Lecture III – functional MRI – Diffusion MRI – interventional MRI – examples Part III Physics: Medical Physics Option Magnetic Resonance Imaging Useful Web Sites Rochester Institute: http://www.cis.rit.edu/htbooks/mri/mri-main.htm UCLA Brain Mapping Centre: http://brainmapping.loni.ucla.edu/BMD_HTML/SharedCode/Shared.htm Part III Physics: Medical Physics Option Magnetic Resonance Imaging Part III Physics: Medical Physics Option Magnetic Resonance Imaging Part III Physics: Medical Physics Option Magnetic Resonance Imaging Part III Physics: Medical Physics Option Magnetic Resonance Imaging Part III Physics: Medical Physics Option Magnetic Resonance Imaging Part III Physics: Medical Physics Option Magnetic Resonance Imaging NMR History 1921: Compton: electron spin 1924: Pauli: Proposes nuclear spin 1946: Stanford/Harvard group detect first NMR signal mid -50 to mid 70’s NMR become powerful tool for structural analysis mid-70 first superconducting magnets Part III Physics: Medical Physics Option Magnetic Resonance Imaging NMR History 1976: Lauterbur: First NMR image of sample tubes in a chemical spectrometer 1981: First commercial scanners <0.2T 1985: 1.5T scanner 1986: Rapid developments in SNR, resolution etc 1998: Whole body 8T at OSU Part III Physics: Medical Physics Option Magnetic Resonance Imaging Nuclear Zeeman Effect Application of strong magnetic field B0 lifts degeneracy of nuclear spin levels DE For spin 1/2: DE = g h B0 g Gyromagnetic ratio (constant of nucleus) For hydrogen g = 42.5 Mhz/T Part III Physics: Medical Physics Option Magnetic Resonance Imaging Population Difference Given by Boltzman Statistics: na nb = exp( -ghBo/kT ) population difference is small <1 in 106 NMR is very insensitive Part III Physics: Medical Physics Option Magnetic Resonance Imaging Semi-Classical Model Gyroscopic motion of magnetic moment about B0 B0 m Use classical mechanics(Larmor) w0 = - g B0 Part III Physics: Medical Physics Option Magnetic Resonance Imaging Ensemble Average M Part III Physics: Medical Physics Option Magnetic Resonance Imaging Rotating Frame Consider precessing moment in a frame of reference rotating at the larmor frequency around B0 w = gBo x y Part III Physics: Medical Physics Option Magnetic Resonance Imaging X’ Y’ Rotating Frame Classical treatment of M Effect of RF in laboratory Frame: Y X Part III Physics: Medical Physics Option Magnetic Resonance Imaging Equivalent to sinusoidal Brf Rotating Frame Classical treatment of M B0 Effect of RF in rotating Frame: Y X Brf Part III Physics: Medical Physics Option Magnetic Resonance Imaging X’ Y’ Rotating Frame Classical treatment of M B0 Effect of RF in rotating Frame: Y X Brf Part III Physics: Medical Physics Option Magnetic Resonance Imaging X’ Y’ Rotating Frame Classical treatment of M B0 Effect of RF in rotating Frame: Y X Part III Physics: Medical Physics Option Magnetic Resonance Imaging Brf X’ Y’ Rotating Frame Classical treatment of M B0 Effect of RF in rotating Frame: Y X Part III Physics: Medical Physics Option Magnetic Resonance Imaging Brf X’ Y’ Rotating Frame Classical treatment of M B0 Effect of RF in rotating Frame: Y X Part III Physics: Medical Physics Option Magnetic Resonance Imaging Brf X’ Y’ Rotating Frame Classical treatment of M B0 Effect of RF in rotating Frame: Y X Part III Physics: Medical Physics Option Magnetic Resonance Imaging Brf X’ Y’ Rotating Frame Classical treatment of M B0 Effect of RF in rotating Frame: Y X Part III Physics: Medical Physics Option Magnetic Resonance Imaging Brf X’ Y’ Signal Detection rotating Frame: B0 X’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Y’ X Y Fourier Transformation FT Sampling frequency = 2 expected frequency spread (Nyquist) Part III Physics: Medical Physics Option Magnetic Resonance Imaging Effect of RF pulses: B0 z z 90o degree pulse x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 28 Effect of RF pulses: B0 z 90o degre e pulse z x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 29 Effect of RF pulses: B0 z z 180o pulse (invertin g pulse) x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 30 Effect of RF pulses: B0 z z 180o pulse (invertin g pulse) x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 31 o Effect of 180 RF pulses: B0 z 180o degre e pulse z x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 32 o Effect of 180 RF pulses: B0 z 180o degre e pulse z x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 33 o Effect of 180 RF pulses: B0 z 180o degre e pulse z x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 34 o Effect of 180 RF pulses: B0 z 180o degre e pulse z x’ x’ B1 (rf) y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging y’ Basic Spin Echo Imaging 35 o Effect of 180 RF pulses: x’ x’ B1 (rf) y’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Basic Spin Echo Imaging 36 o Effect of 180 RF pulses: x’ x’ B1 (rf) y’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Basic Spin Echo Imaging 37 o Effect of 180 RF pulses: x’ x’ B1 (rf) y’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Basic Spin Echo Imaging 38 o Effect of 180 RF pulses: x’ x’ B1 (rf) y’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Basic Spin Echo Imaging 39 o Effect of 180 RF pulses: x’ y’ x’ B1 (rf) y’ x’ x’ y’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Basic Spin Echo Imaging 40 o Effect of 180 RF pulses: x’ y’ x’ B1 (rf) y’ x’ x’ y’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Basic Spin Echo Imaging 41 Two Pulse sequences (I) Two Pulse sequences (I) 90—— ——90 180—— ——90 Saturation recovery Inversion recovery 1 2 3 4 5 6 1 2 3 4 5 6 T1 T1 Part III Physics: Medical Physics Option Magnetic Resonance Imaging T1 Spin Lattice Relaxation Time Describes the return to equilibrium for spins from the excited state Spins loose heat to the rest of the world Requires fluctuating magnetic field near the Larmor frequency for an effective transfer of energy from a spin to surrounding lattice Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ e -t/ T2* e -t/ T2 y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ e -t/ T2* e -t/ T2 y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Two Pulse sequences (II) 90—— ——180 —— —— Spin Echo sequence x’ y’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging T2 and T2 e H -t/ H * e T * 2 -t/ T2 H O O H Part III Physics: Medical Physics Option Magnetic Resonance Imaging Basic Spin Echo Imaging 54 Spin-Spin Relaxation Time Static inhomogeneities refocussed by 180 pulse Time varying imhomogeneity are not T2 changes in disease give rise to diagnostic value of MRI Part III Physics: Medical Physics Option Magnetic Resonance Imaging Superconducting Magnet Helium vessel containing super-con coil Vacuum Part III Physics: Medical Physics Option Magnetic Resonance Imaging Superconducting Magnet Bore 100cm 80cm Part III Physics: Medical Physics Option Magnetic Resonance Imaging B0 0 4T 0 8T Shimming Part III Physics: Medical Physics Option Magnetic Resonance Imaging Other Magnet Types Permanent magnet, e.g. light weight rare earth magnets, <0.3T Part III Physics: Medical Physics Option Magnetic Resonance Imaging Other Magnet Types Part III Physics: Medical Physics Option Magnetic Resonance Imaging Other Magnet Types Electromagnet <0.3T Part III Physics: Medical Physics Option Magnetic Resonance Imaging Special Superconducting Magnets Active Shielding – Extra coils reduce stray field – Improves siting 12 4 0.5T wholebody Part III Physics: Medical Physics Option Magnetic Resonance Imaging 10 5mT contour 2 3T AS wholebody RF Coils Remember Brf must be B0 Field is subject, can use solenoid. Part III Physics: Medical Physics Option Magnetic Resonance Imaging RF Coils Remember Brf must be B0 Saddle coil, Brf is coil access. Efficiency is low, and homogeneity is poor Field is subject, cannot use solenoid. Part III Physics: Medical Physics Option Magnetic Resonance Imaging Part III Physics: Medical Physics Option Magnetic Resonance Imaging How to Make Images Impose (separately): dBz dx dBz dy dBz dz X gradient Gx Y gradient Gy Z gradient Gz Typical values are 10-100 mT/m Part III Physics: Medical Physics Option Magnetic Resonance Imaging How to make images For a Z gradient wz = -g(B0 + Gz.z) -hz Part III Physics: Medical Physics Option Magnetic Resonance Imaging +hz How to make images Part III Physics: Medical Physics Option Magnetic Resonance Imaging Imaging Gradients Special coils (together with power supplies) provide linear variation in B0 in X, Y and Z directions Z B0 Z Part III Physics: Medical Physics Option Magnetic Resonance Imaging Imaging Gradients Special coils (together with power supplies) provide linear variation in B0 in X, Y and Z directions X,Y Part III Physics: Medical Physics Option Magnetic Resonance Imaging Selection of Slice Use Fourier relationship: RF Amplitude (volts) Part III Physics: Medical Physics Option Magnetic Resonance Imaging Selection of slice Slice thickness adjusted by changeimg gradient strength or slice bandwith (longer pulse has narrower frequency spread) Slice position adjusted by changing the centre frequency of the pulse Part III Physics: Medical Physics Option Magnetic Resonance Imaging k-space k-space is the raw data space before fourier transformation into the image 2D image will be represented by a 2D array of data points spread throughout k-space Differing the k-space trajectory will alter image contrast Part III Physics: Medical Physics Option Magnetic Resonance Imaging Image vs k-space (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging k(t)= g/2G(t)dt S(k) Image vs k-space (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging k(t)= g/2G(t)dt S(k) Image vs k-space (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging k(t)= g/2G(t)dt S(k) Image vs k-space (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging k(t)= g/2G(t)dt S(k) Image vs k-space FT (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging k(t)= g/2G(t)dt S(k) GE k-space trajectory RF GS GR GP S(t) (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging k(t)= g/2G(t)dt S(k) GE k-space trajectory RF GS GR GP S(t) (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory RF GS GR GP S(t) (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr SE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr SE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr SE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr SE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr SE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr SE k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr Definitions TR RF GS GR GP S(t) Part III Physics: Medical Physics Option Magnetic Resonance Imaging Definitions TE RF GS GR GP S(t) Part III Physics: Medical Physics Option Magnetic Resonance Imaging Controlling contrast 1 2 3 4 5 6 1 2 3 4 5 6 T1 T2 Part III Physics: Medical Physics Option Magnetic Resonance Imaging Proton Density TR TE 1 2 3 4 5 6 1 2 3 4 5 6 T1 T2 Part III Physics: Medical Physics Option Magnetic Resonance Imaging T2 Contrast TR TE 1 2 3 4 5 6 1 2 3 4 5 6 T1 T2 Part III Physics: Medical Physics Option Magnetic Resonance Imaging 30ms Part III Physics: Medical Physics Option Magnetic Resonance Imaging 90ms 0.5T Multislice Multiecho TR2000/30..90 T1 Contrast TE TR 1 2 3 4 5 6 1 2 3 4 5 6 T1 T2 Part III Physics: Medical Physics Option Magnetic Resonance Imaging Part III Physics: Medical Physics Option Magnetic Resonance Imaging Effect of Flip angle a B0 Brf X’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Y’ Effect of Flip angle a 90o pulse B0 Brf X’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Maximum signal but have to wait 5T1 for recovery Y’ Effect of Flip angle a B0 Brf X’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging Y’ Effect of Flip angle a Flip angle 30o: B0 Brf X’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging detect M0sin a = 0.5 M0 remaining M0cos a = 0.87 M0 Y’ 41/9/15 41/9/60 41/9/90 Contrast versus a 500/9/15 500/9/90 TR/TE/a Part III Physics: Medical Physics Option Magnetic Resonance Imaging Contrast versus TR Why ? freeze involuntary patient motion visualization of dynamic process – fast imaging: minutes – turbo imaging: seconds More complex MRI experiments – obtain multiple images vary some parameter e.g. TI reduce patient examination time Part III Physics: Medical Physics Option Magnetic Resonance Imaging Why does MRI take so long Answer – Only one phase encode line acquired per excitation – Spin Echo: 256*3s for T2, 256*0.6s for T1 – Gradient Echo: 256*35ms (but have to do 3D Solution – get more phase encode lines per excitation Part III Physics: Medical Physics Option Magnetic Resonance Imaging Echo Planar Imaging Fastest imaging method Typical AQ time is 30-100ms Low RF deposition Very fast gradient switching Highly demanding on MRI hardware – B0 homogeneity – gradient switching Part III Physics: Medical Physics Option Magnetic Resonance Imaging GE-PEI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE EPI k-space trajectory +kp RF GS GR GP S(t) -kp (r) Part III Physics: Medical Physics Option Magnetic Resonance Imaging -kr k(t)= g/2G(t)dt S(k) +kr GE vs EP Imaging T E T R AQ BW Gread Switch ms ms ms khz GE 10 35 10 25 EPI 50 0.5 250 mT/m ms 2.5 25 500 100 Assume FOV 25cm AQ = 10ms Matrix 256 time/sample = 10-2/256 Bandwidth = 25kHz Gread = 25 x 103/0.25 = 100 000Hz/m Part III Physics: Medical Physics Option Magnetic Resonance Imaging = ~ 2.5 mT/m GE vs EP Imaging T E T R AQ BW Gread Switch ms ms ms khz GE 10 35 10 25 EPI 50 0.5 250 mT/m ms 2.5 25 500 100 Assume FOV 25cm AQ = 0.5ms Matrix 128 time/sample = 5x10-4/128 Bandwidth = 250kHz Gread = 250 x 103/0.25 = 1 000 000Hz/m Part III Physics: Medical Physics Option Magnetic Resonance Imaging = ~ 25 mT/m MRI at 3T 128x128 single shot, GE echo planar. X,Y,Z shim only (~30s) No template or navigator correction Straight FFT after row reversal Part III Physics: Medical Physics Option Magnetic Resonance Imaging fMRI (functional MRI) Monitor T2 or T2* contrast during cognitive task eg acquire 20-30 slices every 4 seconds Design experiment to have alternating blocks of task and control condition Look for statistically significant signal intenisty changes correlated with task blocks Part III Physics: Medical Physics Option Magnetic Resonance Imaging Echo-Planar fMRI GE-images with EPI response Part III Physics: Medical Physics Option Magnetic Resonance Imaging stimulus fMRI correlation maps Signal response averaged over region Resting O2 & glucose oxyhaemoglobin deoxyhaemoglobin Part III Physics: Medical Physics Option Magnetic Resonance Imaging Activated ATP ADP O2 & glucose Blood flow ‘over-compensation’ Part III Physics: Medical Physics Option Magnetic Resonance Imaging %O2 BOLD signal Effect of Intravascular Oxygenation level deoxy oxy Blood vessel Paramagnetic Part III Physics: Medical Physics Option Magnetic Resonance Imaging Diamagnetic T2 (and T2*) reduced because of diffusion through field gradients T2* curves activated and rest resting TE activated Signal difference ~ 1-5 % signal oxyhaemoglobin activated deoxyhaemoglobin rest Part III Physics: Medical Physics Option Magnetic Resonance Imaging time (ms) Unilateral Finger Opposition (high res) Part III Physics: Medical Physics Option Magnetic Resonance Imaging Definitions Diffusion relates to the microscopic Brownian thermal motion of molecules Perfusion, classically is defined as that process that results in the delivery of nutrients to cells, normally expressed as ml/min/100g wet weight of tissue Part III Physics: Medical Physics Option Magnetic Resonance Imaging Effect of Diffusion on NMR Rms. of an ensemble is zero For a single molecule diffusion results in a gaussian distribution of displacements r Part III Physics: Medical Physics Option Magnetic Resonance Imaging Diffusion and Spin echoes d d D Part III Physics: Medical Physics Option Magnetic Resonance Imaging Diffusion and Spin echoes I/I0 = e -bD b = g2g2d2(D-d/3) Part III Physics: Medical Physics Option Magnetic Resonance Imaging D and ADC I/I0 = e -bD 10 b = g2g2d2(D-d/3) Log (I/I0) 8 6 water 4 DMSO 2 0 0 500 1000 b 1500 H2O = 2.1 x 10 -3 mm2s-1 DMSO = 0.55 x 10 -3 mm2s-1 normal = 0.71 x 10 -3 mm2s-1 ischaemic = 0.55 x 10 -3 mm2s-1 Part III Physics: Medical Physics Option Magnetic Resonance Imaging Diffusion Weighted Imaging RF Gs Gr Gp Part III Physics: Medical Physics Option Magnetic Resonance Imaging Diffusion Weighted Imaging RF Gs Gr Gp Gdiffusion Part III Physics: Medical Physics Option Magnetic Resonance Imaging d d D Typical Values: d = 20, D = 50 10 Log (I/I0) 8 6 water 4 DMSO 2 0 0 500 1000 b Part III Physics: Medical Physics Option Magnetic Resonance Imaging 1500 Gmax 0.5 1 5 b 31 124 3104 10 12418 Practical Problems in Human DWI Gross Motion – Head motion – breathing Pulsitility – CSF/brain pulsation Anisotropy – D is direction dependant, especially white matter Part III Physics: Medical Physics Option Magnetic Resonance Imaging Practical Problems in Human DWI Gross Motion – Echo Planar Imaging – navigator echoes Pulsitility – gating plus navigator echoes Anisotropy – Measure trace, Dxx + Dyy + Dzz – Measure full tensor (all matrix elements) Part III Physics: Medical Physics Option Magnetic Resonance Imaging Diffusion Weighted EPI (b=1570 s/mm2) READ Part III Physics: Medical Physics Option Magnetic Resonance Imaging PHASE SLICE FOV 25cm, TE 118ms TY DW-EPI 128x128 interpolated to 256x256 Partial k-acquisition (62.5%) 4 interleaves, d = 28ms ; D = 66 ms ADC trace Diffusion Weighted EPI (b=1570 s/mm2) Cambridge Part III Physics: Medical Physics Option Magnetic Resonance Imaging NIH van Zijl Anistropy Index Diffusion Weighted EPI (b=1570 s/mm2) Part III Physics: Medical Physics Option Magnetic Resonance Imaging 15 MRI and O water PET Part III Physics: Medical Physics Option Magnetic Resonance Imaging Gadolinium blous experiment in rat brain 6 Relaxation rate change (s-1) 5 4 3 2 1 0 -1 -20 -10 0 10 Part III Physics: Medical Physics Option Magnetic Resonance Imaging Image number 20 30 40 50 (relative to blous injection) 60 Effect of Intravascular Gd Tissue Blood vessel Tissue Part III Physics: Medical Physics Option Magnetic Resonance Imaging Effect of Intravascular Gd Tissue Blood vessel Tissue Part III Physics: Medical Physics Option Magnetic Resonance Imaging T2 (and T2*) reduced because of difussion through field gradients Gadolinium blous experiment in rat brain 6 Relaxation rate change (s-1) 5 4 3 2 1 0 -1 -20 -10 0 10 Part III Physics: Medical Physics Option Magnetic Resonance Imaging Image number 20 30 40 50 (relative to blous injection) 60 Data Analysis Fit first pass of the bolus (avoid recirculation) Gamma variate, or (better) Monte Carlo Estimate arterial input function from large vessel signal rrCBV, rrCBF but absolute MTT Part III Physics: Medical Physics Option Magnetic Resonance Imaging T2 weighted FSE images (3555/80/4) rrCBV-map map of the bolus delay (MTT image) Perfusion weighted MRI of a patient with a high grade stenosis (>90%) of the right internal carotid artery leading to a terminal supply zone infarction in the region of the middle cerebral artery, from http://www.picker.com/mr/acr/perfusn/perfusn.htm Part III Physics: Medical Physics Option Magnetic Resonance Imaging Caution Numbers obtained are not for true perfusion (as measured by PET) Similar to dynamic CT, DSC measures micro-capillary flow However good correlation between PET and DSC (in pigs), in humans?? Part III Physics: Medical Physics Option Magnetic Resonance Imaging True Perfusion by MRI Arterial spin labeling – EPISTAR, ASL, QUIPS – label arterial blood on the way into brain – subtract images with and without labelling – difference is due to arterial water that has entered tissue, i.e. perfusion Part III Physics: Medical Physics Option Magnetic Resonance Imaging Scanner Overview Part III Physics: Medical Physics Option Magnetic Resonance Imaging
