High Resolution 3D Diffusion Pulse Sequence Dept. of Radiology Medical Imaging Research Lab. University of Utah Eun-Kee Jeong, Ph.D. Ph.D. Seong-Eun Kim, Ph.D. M.D. Gregory Katzman, M.D. Dennis L. Parker, Ph.D.

Diffusion MRI BASICS

G G echo TE At the echo time TE, NMR signal is decayed by, - T2 decay (spin-spin diffusion) - diffusive motion For any set of diff. gradient pulses Signal loss : by intra-voxel phase dispersion

Conventional T 2 WI DW-EPI Diffusion Imaging : Detection of Acute Stroke

  Diffusion gradients sensitize MR Image to motion of extra-cellular water   Higher diffusive motion  lower signal intensity Tissue Sample A Tissue Sample B Freely Diffusing Water = Dark Freely Diffusing Water = Dark Larger D Larger D Restricted Diffusion = Bright Restricted Diffusion = Bright Smaller D Smaller D Diffusion Imaging: Principles CELL EXTRA-CELLULAR SPACE FREELY DIFFUSING WATER IN EXTRA-CELLULAR SPACE

X Diffusion-Weighting Y Diffusion-Weighting Z Diffusion-Weighting G FE G PE G SS RF Diff. Grad. along different axis SS PE FE

Diffusion Imaging : Diffusion Imaging : TRACTOGRAPHY arc. fasciculus unc. fasciculus

. DWI probes micro motion of H 2 O in tissue.. Largest diffusion in body (CSF): -drift velocity v d = ~0.1mm/sec (ADC = ~2.6x10 -3 mm 2 /sec)   x = ~10  m/100ms TE  Requires large gradient!!   x = ~10  m/100ms TE  Requires large gradient!!. Any physiological motion - velocity v > ~10 mm/sec -> too huge for diff. gradient - motion induced artifact on Multi-shot DWI -One-shot EPI-DWI freezes physiological motion.. Susceptibility artifact Diffusion EPI: GOOD & BAD

single-shot DW-EPI 8 shots DW-EPI CSF motion artifact Off-line correction needed! PE Single-/Multi-shot EPI-DWI

Geometric Distortion in DW-EPI  non-EPI DW MRI.  non-EPI DW MRI. DW EPI DW Propeller (2D FSE based + Motion correection) A patient with post-op symptoms, aneurysm clip caused artifacts in EPI. b=0 b=1000

PURPOSE To develop a high-res., non-EPI Diff., and multi-shot pulse sequence.

METHODS Multi-shot, 3D pulse sequence for higher SNR. - - Multi-shot  Motion-induced phase must be corrected. Motion correction or motion insensitive pulse sequence - - Navigator echo technique is not used. - Preparation - Diffusion Preparation technique Used! - - Small motion may not degrade the resultant images. High resolution: Imaging matrix  256 read-out - - EPI: 128 Read-out  256 RO will generate more geometric distortion.

METHODS Started from GE’s 3D SSFP (FIESTA) Segmentation of 3D SSFP - - Gradient balancing is interrupted.  some loss of steady state. - - Multi-shot 3DFSE-like (CPMG) pulse sequence. - - Each RF pulse  tips some longitudinal magnetization to transverse plane. Segmented 3D SSFP-DW - - Diffusion is encoded as Prep. Pulses in between two segmentations. - - Any phase error caused during DW Prep will be lost by 90 -x  amplitude modulated 3D Seg.SSFP   +x  +y  -y  -x RF G D

3D Seg. SSFP  Segmented 3D SSFP TR  RF G FE G PE G SE   3D SSFP: maintain high steady state (longitudinal & transverse) Diff. Prep.

Acquired signal  Diff. Prep. Magnetization + Re-grown Magnetization l l MR signal is mixture of: - - Diffusion prepared magnetization - - Re-grown magnetization - - Will be significant for spins with short T 1. : T 1 (brain tissues) = ~800 ms  long enough! - - More for non-centric ordering of phase-/slice-encoding - - Centric slice ordering was used to reduce the contribution of re-grown spins.

Longitudinal Magnetization for n th  pulse Typical imaging parameters M ss : Longitudinal steady-state magnetization N : Number of echoes : 32, 48, 64  t : effective echo spacing : ~ 4 ms T 1 : spin-lattice relaxation time : ~ 800 ms  : flip angle : 15 ~ 45 o b tt bb TR To be minimized!!

CENTRIC View-Ordering: 3D Seg.SSFP-DW 3D Seg.SSFP-DW Significant re-grown magnetization Mostly DW magnetization echo number  slice encoding Grad. centric non-centric b = 500

RESULTS

3ddw: SRSS: dog heart vs. EPI DW (fresh in ethanol 70% + water 30 %) (fresh in ethanol 70% + water 30 %) 256x192x48 etl:48TE:66ms FOV: 16cm/2.5 mm tone_factor = 0.4 rf10_on = 0 opflip = 45 o SpSat: Default S/I EPI(256x128) 3D SSSFP b = 0 b = 500

3D Seg.SSFP-DW: Human Volunteer b: 0 (S/I) ADC map b: 0 (S/I) ADC map 256x256x64 etl: x0.86x1.5 mm 3 RCVR BW: 62.5kHz NEX: 2  = 48 o optic nerve

CONCLUSION Segmentation of 3D SSFP: successful Segmentation of 3D SSFP: successful - Chemical fat saturation - Spatial saturation - Diffusion gradients 3D Segmented SSFP-DW 3D Segmented SSFP-DW - High Resolution 3D DWI is acquired. - Almost No susceptibility artifact - Anisotropy is observed. Problem: table vibration  ????

G  G  (a) (b) t=0 t 1 t 1 +  t 2 t 2 +  Gradient RF Phase: stationary & moving spins x z x z x z4 y x z t 1 t 1 +  t 2 t 2 +  x 4 x 1 x 2 (=0) x 3 G

3D Seg.SSFP-DW Sptial SAT, Chem Sat, DW Prep ON  ChemSat Diff.Prep SpSat  ’ -  Echo Train … … -  -  G FE G PE G SE RF Diffusion Prep.

3D Seg.SSFP-DW: ph-/sl- encoding order? 256x160x32, FOV = 16cm,  z = 1.5mm Diffusion grad. along R/L. b = Excised dog heart preserved in formalin  T 1 : ~200 ms

Apodization w/ zero-Filling: 512 ZIP2, ZIP x256 No apodization gaussian apodization Trianglular Apod.  SS 512x256x96(True ACQ.) 256x256x64:phFOV 0.75 etl:48 FOV: 25.6cm/1.0 mm(isotropic) tone_factor = 0.4 rf10_on = 0 opflip = 45 o SpSat: OFF encode_mode=1 (Y-centric), TR/TE: ~250/~60 ms Scan time: ~1:00 min.

3D Seg.SSFP-DW : along different direction A/P R/L S/I A/P R/L S/I 256x256x64 etl: x0.86x1.5 mm 3 RCVR BW: 62.5kHz  = 48 o NEX: 1

3ddw: dog heart (1 yr old) D = 0.5x mm 2 /sec

Signal Intensity vs. b value b = 0 (S/I) b = 0 (S/I) D = 1.48x10 -3 mm 2 /sec ADC Map Single exponential decay Signal from re-grown Mag.: negligible

3D Seg.SSFP-DW : Fresh Celery b = 0 b = 750 A/P R/L S/I A/P R/L S/I 256x160x64 etl:64 NEX: 2 FOV: 16 cm Sl. thickness: 3.0mm  = 40 o ADC maps

3D Seg.SSFP-DW : SpSat, DTIPrep 256x192x32   = 45 oTR:min FOV = 16cm sl.thick.=2.0mm b = 500  S/I  1/ON, 0/OFF D Seg.SSFP   +x  +y  -y  -x RF G D non-centric centric

3ddw: SRSS ( Square root of Sum of squares ) S/I R/L A/P S/I R/L A/P R/L+A/P A/P+S/I S/I+R/L 256x192x48 etl:48TE:66ms FOV: 16cm/1.5 mm SpSat: Default S/I tone_factor = 0.4 Head coil opflip = 45 o rf10_on = 0 tipup_f=0

3D Seg.SSFP-DW vs. EPI-DW 3D SSSFP EPI(256x128) b=750 b=750 b=750 b=750 S/I A/P