Wald, fMRI MR Physics Massachusetts General Hospital Athinoula A. Martinos Center MR physics and safety for fMRI Lawrence L. Wald, Ph.D.

Wald, fMRI MR Physics Outline: Monday Oct 18 (LLW): MR signal, Gradient and spin echo Basic image contrast Monday Oct 25 (LLW): Fast imaging for fMRI, artifacts Wed. Oct 20 (JJ): Encoding the image Wed. Oct 25 (LLW): fMRI BOLD contrast Mon. Nov. 1 (JJ): Review, plus other contrast mechanisms for fMRI (CBV, CBF)

Wald, fMRI MR Physics Why, introduction How: Review of k-space trajectories Different techniques (EPI, Spiral) Problems from B0 Susceptibility artifacts Fast MR Imaging Techniques

Wald, fMRI MR Physics Why fast imaging Capture time course, (e.g. hemodynamic) eliminate artifact from motion (during encode.)

Wald, fMRI MR Physics Magnetization vector durning MR RF Voltage (Signal) time Mz time encode

Wald, fMRI MR Physics Review of Image encoding, journey through kspace Two questions: 1) What does blipping on a gradient do to the water magnetization. 2) Why does measuring the signal amplitude after a blip tell you info about the spatial frequency composition of the image (k-space).

Wald, fMRI MR Physics Aside: Magnetic field gradient Uniform magnet Field from gradient coils Total field B o G x x B o + G x x x z

Wald, fMRI MR Physics Step two: encode spatial info. in-plane x y x B Field (w/ x gradient) BoBo B TOT = B o + G z x Freq. Signal B o along z oo  with gradient without gradient “Frequency encoding” v =  B TOT =  (B o + G x x)

Wald, fMRI MR Physics How does blipping on a grad. encode spatial info? y B Field (w/ z gradient) BoBo BoBo GyGy  y1y1 y2y2 z y y1y1 y2y2 all y locs process at same freq. all y locs process at same freq. spins in forehead precess faster...  y  =  B TOT =  B o  y G y  y  =  y  =  B o  y (G y 

Wald, fMRI MR Physics How does blipping on a grad. encode spatial info? z y y1y1 y2y2 yxyx z x y z oo 90° yx after RFAfter the blipped y gradient... position y 1 position 0position y 2 z z BoBo  y  =  y  =  B o  y (G y 

Wald, fMRI MR Physics How does blipping on a grad. encode spatial info? The magnetization vector in the xy plane is wound into a helix directed along y axis. Phases are ‘locked in’ once the blip is over. y

Wald, fMRI MR Physics  y  =  y  =  B o  y (G y  The bigger the gradient blip area, the tighter the helix y small blip medium blip large blip GyGy

Wald, fMRI MR Physics What have you measured? Consider 2 samples: uniform water 1 cm no signal observed signal is as big as if no gradient

Wald, fMRI MR Physics 10 mm kxkx kyky 1/10 mm 1/5mm 1/2.5mm 1/1.2mm = 1/Resolution Measurement intensity at a spatial frequency...

Wald, fMRI MR Physics kxkx kyky 1 / Res x 1 / FOV x FOV x = matrix * Res x Fourier transform

Wald, fMRI MR Physics Sample 3 points in kspace More efficient! t kxkx 1/10 mm 1/5mm 1/2.5mm 1/1.2mm = 1/Resolution t GyGy GyGy Frequency and phase encoding are the same principle!

Wald, fMRI MR Physics Conventional “Spin-warp” encoding “slice select” “phase enc” “freq. enc” (read-out) RF t S(t) t GzGz t GyGy GxGx t t kxkx kyky one excitation, one line of kspace... a1a1 a2a2

Wald, fMRI MR Physics Image encoding, “Journey through kspace” The Movie…

Wald, fMRI MR Physics kxkx kyky 1 / Res x 1 / FOV x FOV x = matrix * Res x Fourier transform

Wald, fMRI MR Physics Conventional “Spin-warp” encoding “slice select” “phase enc” “freq. enc” (read-out) RF t S(t) t GzGz t GyGy GxGx t t kxkx kyky one excitation, one line of kspace... a1a1 a2a2

Wald, fMRI MR Physics “Echo-planar” encoding RF t S(t) (no grads) t GzGz t GyGy GxGx t kxkx kyky one excitation, many lines of kspace... etc... T2*

Wald, fMRI MR Physics Bandwidth is asymmetric in EPI kxkx kyky Adjacent points in k x have short  t = 5 us (high bandwidth) Adjacent points along k y are taken with long  t (= 500us). (low bandwidth) The phase error (and thus distortions) are in the phase encode direction.

Wald, fMRI MR Physics RF t GzGz t GyGy GxGx t esp = 500 us for whole body grads, readout length = 32 ms esp = 270us for head gradients, readout length = 17 ms Characterization of EPI performance length of readout train for given resolution or echo spacing (esp) or freq of readout… ‘echo spacing’ (esp)

Wald, fMRI MR Physics What is important in EPI performance? Short image encoding time. Parameters related to total encoding time: 1) echo spacing. 2) frequency of readout waveform. Key specs for achieving short encode times: 1) gradient slew rate. 2) gradient strength. 3) ability to ramp sample. Good shimming (second order shims)

Wald, fMRI MR Physics Susceptibility in MR The good. The bad. The ugly.

Wald, fMRI MR Physics Enemy #1 of EPI: local susceptibility gradients B o field maps in the head

Wald, fMRI MR Physics Enemy #1 of EPI: local susceptibility gradients B o field maps in the head

Wald, fMRI MR Physics What do we mean by “susceptibility”? In physics, it refers to a material’s tendency to magnetize when placed in an external field. In MR, it refers to the effects of magnetized material on the image through its local distortion of the static magnetic field B o.

Wald, fMRI MR Physics Susceptibility effects occur near magnetically dis-similar materials Field disturbance around air surrounded by water (e.g. sinuses) Field map (coronal image) 1.5T BoBo Ping-pong ball in water…

Wald, fMRI MR Physics B o map in head: it’s the air tissue interface… Sagittal Bo field maps at 3T

Wald, fMRI MR Physics Susceptibility field (in Gauss) increases w/ B o 1.5T 3T 7T Ping-pong ball in H 2 0: Field maps (  TE = 5ms), black lines spaced by 0.024G (0.8ppm at 3T)

Wald, fMRI MR Physics Other Sources of Susceptibility You Should Be Aware of… Those fillings might be a problem…

Wald, fMRI MR Physics Local susceptibility gradients: 2 effects 1)Local dephasing of the signal (signal loss) within a voxel, mainly from thru- plane gradients 2)Local geometric distortions, (voxel location improperly reconstructed) mainly from local in-plane gradients.

Wald, fMRI MR Physics 1) Non-uniform Local Field Causes Local Dephasing Sagittal B o field map at 3T 5 water protons in different parts of the voxel… z slowest fastest x y z 90° T = 0 T = TE

Wald, fMRI MR Physics Local susceptibility gradients: thru-plane dephasing in grad echo EPI Bad for thick slice above frontal sinus… 3T

Wald, fMRI MR Physics Thru-plane dephasing gets worse at longer TE 3T, TE = 21, 30, 40, 50, 60ms

Wald, fMRI MR Physics Problem #2 Susceptibility Causes Image Distortion in EPI Field near sinus To encode the image, we control phase evolution as a function of position with applied gradients. Local suscept. Gradient causes unwanted phase evolution. The phase encode error builds up with time.  =  B local  t y y

Wald, fMRI MR Physics Susceptibility Causes Image Distortion Field near sinus y y Conventional grad. echo,   encode time  1/BW

Wald, fMRI MR Physics Susceptibility in EPI can give either a compression or expansion Altering the direction kspace is transversed causes either local compression or expansion. choose your poison… 3T whole body gradients

Wald, fMRI MR Physics Susceptibility Causes Image Distortion Field near sinus z Echoplanar Image,   encode time  1/BW Encode time = 34, 26, 22, 17ms 3T head gradients

Wald, fMRI MR Physics EPI and Spirals kxkx kyky GxGx GyGy kxkx kyky GxGx GyGy

Wald, fMRI MR Physics EPISpirals Susceptibility:distortion,blurring,dephasing Eddy currents:ghostsblurring k = 0 is sampled:1/2 through1st Corners of kspace:yesno Gradient demands:very highpretty high

EPI and Spirals EPI at 3T Spirals at 3T (courtesy Stanford group)

Wald, fMRI MR Physics B0B0 Nasal Sinus

Wald, fMRI MR Physics B0B0 Nasal Sinus + mouth shim

Wald, fMRI MR Physics Effect of Ear & Mouth Shim on EPI From P. Jezzard, Oxford B0B0

Wald, fMRI MR Physics With fast gradients, add parallel imaging Acquisition: SMASH SENSE Reconstruction: Folded datasets + Coil sensitivity maps Reduced k-space sampling { Folded images in each receiver channel

Wald, fMRI MR Physics (iPAT) GRAPPA for EPI susceptibility Fast gradients are the foundation, but EPI still suffers distortion 3T Trio, MRI Devices Inc. 8 channel array b=1000 DWI images iPAT (GRAPPA) = 0, 2x, 3x

Wald, fMRI MR Physics 4 fold acceleration of single shot sub- millimeter SE-EPI: 23 channel array 23 Channel array at 1.5T With and without 4x Accel. Single shot EPI, 256x256, 230mm FOV TE = 78ms Encoding with RF…

Extending the phased array to more channels: 23 channel “Bucky” array for 1.5T

Wald, fMRI MR Physics 9 Fold GRAPPA acceleration 3D FLASH 23 Channel array at 1.5T Can speed up encoding by an order of magnitude! 9 minute scan down to 1 minute… 3D Flash, 1mm x 1mm x 1.5mm, 256x256x128

Wald, fMRI MR Physics Whys stop at 32? 96 Channels on its way… Receiver array: Andreas Potthast Helmut array: Graham Wiggins

Wald, fMRI MR Physics Questions, comments to: Larry Wald