HST.583 Functional Magnetic Resonance Imaging: Data Acquisition and Analysis Fall 2008

Transcription

1 MIT OpenCourseWare HST.583 Functional Magnetic Resonance Imaging: Data Acquisition and Analsis Fall 2008 For information about citing these materials or our Terms of Use, visit:

2 HST.583: Functional Magnetic Resonance Imaging: Data Acquisition and Analsis, Fall 2008 Harvard-MIT Division of Health Sciences and Technolog Course Director: Dr. Rand Gollub. MR phsics and safet for fmri Lawrence L. Wald, Ph.D. Massachusetts General Hospital Athinoula A. Martinos Center

3 Outline: Wed. Sept 24 (LLW): MR signal, Gradient and spin echo Basic image contrast Mon. Sept 29 (LLW): Encoding the image Wed Oct 1 (LLW): Fast imaging for fmri, artifacts fmri BOLD

4 What is NMR? NUCLEAR MAGNETIC RESONANCE A magnet, a glass of water, and a radio wave source and detector.

5 What is NMR? Nuclear magnetism M =

6 E B ΔE= hυ Earth s Field protons N (N N )/N TOT = 1 ep(-δe/kt) 10-4 W E compass S

7 Compass needles υ N Earth s Field North Main Field Bo W E S Freq = γ B MH/T

8 Groscopic motion Main Field Bo North M Proton has magnetic moment Proton has spin (angular momentum) >>groscopic precession Larmor precession freq. = MH/T υ = γ B o

9 EXCITATION : Displacing the spins from Equilibrium (North) Problem: It must be moving for us to detect it. Solution: knock out of equilibrium so it oscillates How? 1) Tilt the magnet or compass suddenl 2) Drive the magnetiation (compass needle) with a periodic magnetic field

10 Ecitation: Resonance Wh does onl one frequenc efficientl tip protons? Resonant driving force. It s like pushing a child on a swing in time with the natural oscillating frequenc.

11 RF Field (B 1 ) applies a torque to the spins is "longitudinal" direction - is "transverse" plane Static Field, B0 Applied RF Field (B 1 ) Mo The RF pulse rotates Mo the about applied field

12 "Eciting" the Magnetiation: tip angle Static Field, B

13 Detecting the Magnetiation: Farada s Law 90 A moving bar magnet induces a Voltage in a coil of wire. (a generator ) The RF coil design is the #1 determinant of the sstem SNR υ o V(t)= -dφ/dt Φ = n B spins A

14 Detecting the NMR: the noise 90 Noise comes from electrical losses in the resistance of the coil or electrical losses in the tissue. For a resistor: Pnoise = 4kTRB υ o V(t) Noise is white. >>Noise power α bandwidth Noise is spatiall uniform. R is dominated b the tissue. >> big coil is bad.

15 The NMR Signal RF time Voltage (Signal) time υ o υ B o Mo 90 υ o V(t)

16 Signal to Noise Ratio in MRI Most important piece of hardware is the RF coil. SNR α voel volume (# of spins) SNR α SQRT( total time of data collection) SNR depends on the amount of signal ou throw awa to better visualie the brain (gain image contrast)

17 Phsical Foundations of MRI NMR: 60 ear old phenomena that generates the signal from water that we detect. MRI: using NMR signal to generate an image Three magnetic fields (generated b 3 coils) 1) static magnetic field Bo 2) RF field that ecites the spins B1 3) gradient fields that encode spatial info G, G, G

18 Three Steps in MR: 0) Equilibrium (magnetiation points along Bo) 1) RF Ecitation (tip magn. awa from equil.) 2) Precession induces signal, dephasing (timescale = T2, T2*). 3) Return to equilibrium (timescale = T1).

19 Magnetiation vector during MR RF Voltage (Signal) encode time M M

20 Three places in process to make a measurement (image) 0) Equilibrium (magnetiation points along Bo) 1) RF Ecitation (tip magn. awa from equil.) 2) Precession induces signal, allow to dephase for time TE. 3) Return to equilibrium (timescale =T1). proton densit weighting T2 or T2* weighting T1 Weighting

21 Contrast in MRI: proton densit Form image immediatel after ecitation (creation of signal). Tissue with more protons per cc give more signal and is thus brighter on the image. No chance to dephase, thus no differences due to different tissue T2 values. Magnetiation starts full relaed (full M), thus no T1 weighting.

22 T2*-Dephasing Wait time TE after ecitation before measuring M. Shorter T2* spins have dephased vector sum initiall at t= TE

23 T2* Dephasing Just the tips of the vectors

24 1.0 T2* deca graphs Transverse Magnetiation T2* = 200 Tissue #1 T2* = 60 Tissue # Time (milliseconds)

25 T2* Weighting Phantoms with four different T2* deca rates... There is no contrast difference immediatel after ecitation, must wait (but not too long!). Choose TE for ma. inten. difference.

26 Gradient Echo (T2* contrast) Dephasing is entirel from a spatial difference in the applied static fields. 90 Bo + G t = 0 t = T Red arrows processes faster due to its higher local field

27 Gradient Echo (T2* contrast) Dephasing is entirel from a spatial difference in the applied static fields. 90 Bo + G t = 0 t = T Bo + G t = T t = 2T

28 Gradient Echo RF ecitation G t S t t Boring!

29 7T 32ch MGH arra 2D FLASH, TR/TE= 500/ mm 3 (48nl) 8min acq Courtes of Dr. Christopher J. Wiggins. Used with permission. Wald, RSNA 2007 A.A. Martinos Center, MGH Radiolog

30 Wald, RSNA 2007 A.A. Martinos Center, MGH Radiolog 7T 32ch MGH arra Courtes of Dr. Christopher J. Wiggins. Used with permission. G. Wiggins, C. Wiggins, Martinos Center MGH 2D FLASH, TR/TE= 500/ mm 3 (48nl) 8min acq

31 7 Tesla 230um 2D FLASH , 1.5mm 3 8min acq Courtes of Dr. Christopher J. Wiggins. Wald, Munich, 2008 Used with permission.

32 7 Tesla 230um 2D FLASH, mm 3 8min acq Wald, Munich, 2008 Courtes of Dr. Christopher J. Wiggins. Used with permission.

33 7 T, 32ch 200um 200um 1mm an 2D T2* weighted 200um 200um 1mm ( Wald, Munich, matri) 2008 U-fiber?? Courtes of Dr. Christopher J. Wiggins. Used with permission.

34 Spin Echo (T2 contrast) Some dephasing can be refocused because its due to static fields Echo! t = 0 t = T t = T (+) t = 2T Blue arrows precesses faster due to local field inhomogeneit than red arrow

35 Spin Echo 180 pulse onl helps cancel static inhomogeneit The runners can have static speed distribution. If a runner trips, he will not make it back in phase with the others.

36 T2 weighed spin echo image NMR Signal white gra Time to Echo, TE (ms)

37 Other contrast for MRI In brain: (gra/white/csf/fat) Proton densit differ ~ 20% T1 relaation differ ~ 2000% How to eploit for imaging? Var repetition rate - TR

38 RF T1 weighting in MRI (w/ 90 o ecite) TR encode encode encode Voltage (Signal) M gre matter (long T1) white matter (short T1) time

39 T1-Weighting white matter T1 = 600 gre matter T1 = 1000 Signal CSF T1 = TR (milliseconds) 3000

40 RF encode T1 weighting in MRI (w/ 30 o ecite) TR encode encode Voltage (Signal) M white matter (short T1) time

41 Image contrast summar: TR, TE TR Long Short Proton Densit T1 T2 poor! Short TE Long

42 Source of T1 and T2 contrast in brain: Melin content Laer 1: no cell bodies, moderate melination Image removed due to copright restrictions. Diagram showing the arrangement of nerve cells and fibers in laers and sublaers parallel to the surface in a vertical section through the human striate area or visual cortical center. Nissel stain: cell bodies Weigert stain: fibers Determine functional Laer 4b: thick region boundaries with melination based (line on of Gennari) MR strucure alone White matter: heav melination Melin differences are the primar source of T1 and T2 contrast of gra/white matter.

43 Cortical laers in Monke at 7T MPRAGE 250um 250um 750um (4 hours) Intensit along line perpendicular To V1