Artefact Correction in DTI

Transcription

1 Artefact Correction in DTI (ACID) Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London Siawoosh Mohammadi

2 Motivation High-end DTI: tractography Potential problems in DTI z y x Lazar, NMR Biomed., 21 Mohammadi et al., MRM, accepted

3 Overview Diffusion Tensor Imaging (DTI) in brief Example application in DTI Three artefacts in DTI Eddy Current (EC) distortions Local Perturbation Fields (LPFs) Signal-dropout due to mechanical vibration Take home message

4 Diffusion Tensor Imaging (DTI) in brief n DW images Diffusion tensor DT represented by ellipsoid m b= images

5 Overview Diffusion Tensor Imaging (DTI) in brief Example application in DTI Three artefacts in DTI Eddy Current (EC) distortions Local Perturbation Fields (LPFs) Signal-dropout due to mechanical vibration Take home message

6 Patients (TLE) and Control Keller et al., Journal of Neuroimaging, accepted

7 7T high resolution DTI Heidemann et al., MRM, 21

8 Grey matter DTI Amygdala parcellation Variability in grey matter diffusion Bach et al., J Neurosci., 211 Nagy et al., ISMRM, 211 Cortical radial and tangential diffusivity MacNab et al., ISMRM, 211

9 High angular resolution diffusion imaging (HARDI) ODF - Orientation Distribution Function Aganj et al., MRM, 21

10 Overview Diffusion Tensor Imaging (DTI) in brief Example application in DTI Three artefacts in DTI Eddy Current (EC) distortions Local Perturbation Fields (LPFs) Signal-dropout due to mechanical vibration Take home message

11 EC distortion artefact Stejskal & Tanner, JCP, 1965 Reese et al., MRM, 23

12 EC and imaging gradients EC G z y z y Skare S., thesis, 22 EC G y EC G x x y x y

13 Whole-brain eddy current distortions original image z y y z y x x y distorted image translation in-plane shearing scaling through-plane shearing eddy current field components EC B EC G x EC G y EC G z Mohammadi et al., MRM, 21

14 Eddy currents: bright edges / blurring Without eddy current and motion correction With eddy current and motion correction

15 Less blurring leads to higher sensitivity in FA group comparison Relevance Better tensor estimates towards the cortex improves GM DTI specificity Keller et al., JON, accepted Nagy et al., ISMRM, 211 Better image quality in high resolution DTI and HARDI, where ST pulse is necessary Heidemann et al., MRM, 21 Aganj et al., MRM, 21

16 Overview Diffusion Tensor Imaging (DTI) in brief Example application in DTI Three artefacts in DTI Eddy Current (EC) distortions Local Perturbation Fields (LPFs) Signal-dropout due to mechanical vibration Take home message

17 Problem: effective gradient, e.g., due to ECs diffusion weighting period readout period expected gradients effective gradients FA original Error in B matrix FA inhomogeneity EC distortion

18 SM2 How to measure the LPFs? Mohammadi et al., Neuroimage, under review

19 Folie 18 SM2 cite zoltan Siawoosh Mohammadi;

20 Measuring LPFs on different MR systems (a) DTI1 (b) DTI2 (c) DTI3 ε11 ε22 ε11 ε ε ε22 ε 33 ε 12 ε 33 ε 12 ε 33 ε ε 13 ε ε 13 ε ε 13 ε = B δb Mohammadi et al., Neuroimage, under review B * with δ B = 2 Σ B and Σ = ε ε ε ε ε ε ε ε ε

21 LPF correction: repositioning experiment.1 z DTI3,2 = 53±3 z DTI3,1 = 41±3 number of voxel tr( δb) Measured MD MD meas DTI3,1 MD meas DTI3, MD [1-3 mm s 2 ] cor2 MD DTI3,1 number of voxel 5 cor2 MD DTI3,2 Corrected MD MD cor2 DTI3,1 MD cor2 DTI3, MD [1-3 mm s 2 ] Mohammadi et al., Neuroimage, under review

22 Relevance Improved sensitivity of group comparison of MD due to repositioning effect Keller et al., JON, accepted Better grey matter DTI due to reduced FA contrast inhomogeneity MacNab et al., ISMRM, 211

23 Overview Diffusion Tensor Imaging (DTI) in brief Example application in DTI Three artefacts in DTI Eddy Current (EC) distortions Local Perturbation Fields (LPFs) Signal-dropout due to mechanical vibration Take home message

24 Vibration artefacts in blip up and blip down DTI data sets Gallichan et al., HBM, 21

25 Problem: signal-dropout due to axial rotation Unshifted echo (blip-up PE) [arbitrary units] 1 k-space coverage echo k min k= k max k y /PE Shifted echo (blip-up PE) k y eff m1 Ω ( r) z [arbitrary units] 1 k } y Mohammadi et al., MRM, accepted k min k= k max k y /PE

26 Recover signal using phase encoding reversal Blip up Blip down Mohammadi et al., MRM, accepted

27 d Mohammadi et al., MRM, accepted Correction of vibration artefacts in DTI using phase-encoding reversal (COVIPER)

28 Relevance Robust data, e.g., avoiding false positives in FA group studies Keller et al., JON, accepted Better data quality in grey matter Less signal-dropout artefacts in HARDI MacNab et al., ISMRM, 211 Aganj et al., MRM, 21

29 Take home message Retrospective artefact correction is possible Sensitivity and robustness of DTI can be improved Three artefacts related to the diffusion weighting gradients were presented We are not finished yet

30 Acknowledgements MR physics group in WTCN, London Nikolaus Weiskopf (my supervisor and head of MR physics at the WTCN) Zoltan Nagy Oliver Josephs Chloe Hutton (special thanks for the acronym ) Antoine Lutti External collaborators Michael Deppe (University of Münster) Harald Möller (Max Plank Institute Leipzig) Dirk Müller (University of Münster) Mark Symms (Department of Clinical and Experimental Epilepsy, UCL, London) David Carmichael (Imaging and Biophysics, UCL, London) This work was supported by the Wellcome Trust.