Diffusion MRI: Basic Principles and Clinical Applications

Transcription

1 Diffusion MRI: Basic Principles and Clinical Applications Computerunterstützte Klinische Medizin Medizinische Fakultät Mannheim Universität Heidelberg Theodor-Kutzer-Ufer 1-3 D Mannheim Content Principle of Diffusion Apparent Diffusion Coefficient (ADC) Discussion Diffusion Tensor Imaging (DTI) Discussion Intravoxel Incoherent Motion (IVIM) Discussion Diffusion MRI I Slide 2/18 I

2 Principle of Diffusion Diffusion: Random movement of molecules/atoms due to Brownian motion (temperature) Diffusion coefficient D: Measure for amount of movement D water = 10-3 mm²/s 9 µm wandering in 40 ms Diffusion in tissue is complex and holds information about tissue structure and function Diagnostic value 30 µm Diffusion MRI I Slide 3/18 I Apparent Diffusion Coefficient (ADC) Model Gradients G make rotation frequency dependend on spatial position If spins change position (diffusion) first and second gradient have different effects Signal attenuation/loss x Diffusion MRI I Slide 4/18 I

3 Apparent Diffusion Coefficient (ADC) Model b-value = b(g, δ, ) determines impact of signal loss due to diffusion Assuming equal diffusion in all directions (isotropic) Gradients G make rotation frequency dependend on spatial position If spins change position (diffusion) first and second gradient have different effects Signal attenuation/loss / with Apparent Diffusion Coefficient ADC Diffusion MRI I Slide 5/18 I Apparent Diffusion Coefficient (ADC) Model / Diffusion MRI I Slide 6/18 I

4 Clinical Application of ADC Model Stroke Diffusion MRI I Slide 7/18 I ADC Model - Discussion Possible issues with ADC model approach? Diffusion MRI I Slide 8/18 I

5 Anisotropic Diffusion White Matter Consists of highly directional neurion fibres (anisotropic) Mosby's Medical Dictionary, 8th edition. 2009, Elsevier. Diffusion MRI I Slide 9/18 I Anisotropic Diffusion White Matter Diffusion along fibres easier than across myelin sheath 1 µm axon 0,5 µm Diffusion MRI I Slide 10/18 I Beaulieu. NMR Biomed

6 Diffusion Tensor Imaging (DTI) Water molecule Diffusion ellipsoid Diffusion tensor Diffusion MRI I Slide 11/18 I Diffusion Tensor Imaging (DTI) ADC = Diffusion Tensor Fractional Anisotropy (FA): Amount of anisotropy Main direction 0 " #$ " 1 Diffusion MRI I Slide 12/18 I

7 DTI Fibretracking Investigation of main diffusion axis of neighbouring voxels Similar directions can indicate fibre structures Diffusion MRI I Slide 13/18 I DTI - Discussion Possible issues with DTI approach? Diffusion MRI I Slide 14/18 I

8 Intravoxel Incoherent Motion (IVIM) Model Water moves inside capillary bed (perfusion) Randomly oriented capillaries Same effect on the MRI signal as diffusion in the tissue Two compartments: inside and outside vascular system (both free diffusion) Resulting motion referred to as pseudo-diffusion (D* > ADC) [image from: Le Bihan et al., Radiology, 1988, 168, ] Diffusion MRI I Slide 15/18 I Intravoxel Incoherent Motion (IVIM) Model Water moves inside capillary bed (perfusion) Randomly oriented capillaries Same effect on the MRI signal as diffusion in the tissue Two compartments: inside and outside vascular system (both free diffusion) Resulting motion referred to as pseudo-diffusion (D* > ADC) IVIM Range Effect primarily present for low b-values ADC Range Diffusion MRI I Slide 16/18 I

9 IVIM Application Stroke Cancer: Glioblastoma [Federau, C., Kieran O Brien, et al. Neuroradiology 56.8 (2014): ] [Federau, Christian and Kieran O'Brien. NMR in Biomedicine 28.1 (2015): 9-16.] Diffusion MRI I Slide 17/18 I IVIM - Discussion Possible issues with IVIM approach? Which b-values do I choose? Diffusion MRI I Slide 18/18 I

10 Errors due to bad choice of b-values b-values too low: Curve fits well to data, but only because of extra degree of freedom No reasonable signal course Fit parameters strongly deviate from ground truth SNR for high b-values not sufficient: No reasonable signal course (noise corrupts fit) Fit parameters strongly deviate from ground truth Diffusion MRI I Slide 19/18 I Thank you for your attention! Diffusion MRI I Slide 20/18 I