Artefact Correction in DTI (ACID) (ACID) Wellcome Trust Centre for - - PowerPoint PPT Presentation

Artefact Correction in DTI (ACID) (ACID)

Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London Siawoosh Mohammadi

Motivation

High-end DTI: tractography Potential problems in DTI Lazar, NMR Biomed., 2010

x y z

Mohammadi et al., MRM, accepted

Overview

• Diffusion Tensor Imaging (DTI) in brief
• Example application in DTI
• Three artefacts in DTI
• Three artefacts in DTI

– Eddy Current (EC) distortions – Local Perturbation Fields (LPFs) – Signal-dropout due to mechanical vibration

• Take home message

Diffusion Tensor Imaging (DTI) in brief

n DW images

DT represented by ellipsoid Diffusion tensor

+

m b=0 images

Overview

• Diffusion Tensor Imaging (DTI) in brief
• Example application in DTI
• Three artefacts in DTI
• Three artefacts in DTI

– Eddy Current (EC) distortions – Local Perturbation Fields (LPFs) – Signal-dropout due to mechanical vibration

• Take home message

Patients (TLE) and Control

Keller et al., Journal of Neuroimaging, accepted

7T – high resolution DTI

Heidemann et al., MRM, 2010

Grey matter DTI

Amygdala parcellation Variability in grey matter diffusion

Bach et al., J Neurosci., 2011 MacNab et al., ISMRM, 2011

Cortical radial and tangential diffusivity

Nagy et al., ISMRM, 2011

High angular resolution diffusion imaging (HARDI)

Aganj et al., MRM, 2010 ODF - Orientation Distribution Function

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

EC distortion artefact

Stejskal & Tanner, JCP, 1965 Reese et al., MRM, 2003

EC and imaging gradients

         

EC z

G

y z y

Skare S., thesis, 2002          

EC y

G          

EC x

G

x y x y

Whole-brain eddy current distortions

x y y z x y

• riginal

image

y z

EC

B

         

EC z

G          

EC y

G

in-plane shearing through-plane shearing scaling

eddy current field components distorted image

         

EC x

G

Mohammadi et al., MRM, 2010

translation

Eddy currents: bright edges / blurring

Without eddy current and motion correction With eddy current and motion correction

Relevance

• Better tensor estimates

Keller et al., JON, accepted

• Less blurring leads to

higher sensitivity in FA group comparison

Nagy et al., ISMRM, 2011

• Better tensor estimates

towards the cortex improves GM DTI specificity

Heidemann et al., MRM, 2010 Aganj et al., MRM, 2010

• Better image quality in high

resolution DTI and HARDI, where ST pulse is necessary

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

Problem: effective gradient, e.g., due to ECs

expected gradients effective gradients

diffusion weighting period readout period

FA original Error in B matrix EC distortion FA inhomogeneity

How to measure the LPFs?

Mohammadi et al., Neuroimage, under review SM2

Folie 18 SM2 cite zoltan

Siawoosh Mohammadi; 08.11.2011

Measuring LPFs on different MR systems

(c) DTI3 (b) DTI2 (a) DTI1

11

ε ε ε ε

22

ε ε ε ε

11

ε ε ε ε

22

ε ε ε ε

11

ε ε ε ε

22

ε ε ε ε

0.1

• 0.1

• 0.1

10 20 30 40 50 60

0.02

• 0.02

33

ε ε ε ε

+ + + +

ε ε ε ε12

+ + + +

ε ε ε ε13

+ + + +

ε ε ε ε23

33

ε ε ε ε

+ + + +

ε ε ε ε12

+ + + +

ε ε ε ε13

+ + + +

ε ε ε ε23

33

ε ε ε ε

+ + + +

ε ε ε ε12

+ + + +

ε ε ε ε13

+ + + +

ε ε ε ε23

Mohammadi et al., Neuroimage, under review

B B B δ + =

*

          = Σ Σ =

+ + + + + + + + 33 23 13 23 22 12 13 12 11

2 ε ε ε ε ε ε ε ε ε δ and with B B

) ( B δ tr

cor2 DTI3,1

MD ∆

cor2 DTI3,2

MD ∆

zDTI3,2 = 53±3 zDTI3,1 = 41±3

−0.1 −0.05 0.05 0.1

0.1

• 0.1

LPF correction: repositioning experiment

0.5 1 1.5 5000

MDmeas

DTI3,1

MDmeas

DTI3,2

0.5 1 1.5 5000

MDcor2

DTI3,1

MDcor2

DTI3,2

] [10 MD

2 3

• s

mm ] [10 MD

2 3

• s

mm

number of voxel number of voxel

Measured MD Corrected MD Mohammadi et al., Neuroimage, under review

Relevance

Keller et al., JON, accepted

• Improved sensitivity of

group comparison of MD due to repositioning effect

MacNab et al., ISMRM, 2011

• Better grey matter DTI

due to reduced FA contrast inhomogeneity

JON, accepted

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

Vibration artefacts in blip up and blip down DTI data sets

Gallichan et al., HBM, 2010

Problem: signal-dropout due to axial rotation

1

Unshifted echo (blip-up PE)

[arbitrary units]

k-space coverage echo

1

Shifted echo (blip-up PE)

kmin kmax kmin kmax

ky/PE ky/PE [arbitrary units]

k=0 k=0

) (

1

r

eff z y

m k Ω − ∝ ∆

}

y

k ∆

Mohammadi et al., MRM, accepted

Recover signal using phase encoding reversal

Blip up Blip down

Mohammadi et al., MRM, accepted

Correction of vibration artefacts in DTI using phase-encoding reversal (COVIPER)

ed Mohammadi et al., MRM, accepted

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, 2011 Aganj et al., MRM, 2010

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
• Three artefacts related to the diffusion weighting gradients were

presented

• We are not finished yet

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
• 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.