Diffusion Weighted MRI. Zanqi Liang & Hendrik Poernama

Diffusion Weighted MRI Zanqi Liang & Hendrik Poernama 1

Outline MRI Quick Review What is Diffusion MRI? Detecting Diffusion Stroke and Tumor Detection Presenting Diffusion Anisotropy and Diffusion Tensor Cyst Detection Fiber Tractography Tumor Surgery and Schizophrenia Future Research Challenges 2

1. MRI Procedure Magnetic field Tissue protons align with magnetic field (equilibrium state) RF RF pulses 1.Magnetic Field Protons absorb Relaxation Spatial encoding RF RF energy 2.Radio-Frequency processes using magnetic (excited Pulse field state) gradients 3.Relaxation NMR signal detection Protons emit RF RF energy (return to to equilibrium state) Relaxation processes Repeat RAW DATA MATRIX Fourier transform IMAGE 4

What is the D in DMRI? Diffusion The spontaneous spreading of matter (particle), heat or momentum Rate of concentration change proportional to diffusion coefficient Water makes up 60 80% of our body weight. 6

Diffusion and MRI When the patient enters the large tunnel of a static magnetic field, nuclear spins (small magnets inside each proton nucleus) are lined up along the direction of the big magnet Magnetic field gradients of certain duration will then add a smaller magnetic field to spins located in different regions within the tissue. (Magic ink) 7

Conventional MRI and DMRI Conventional MRI Measures the phase changed due to local magnetic properties of the surrounding tissue Can distinguish different types of tissue. Eg: liver, fat, muscle and water... Diffusion MRI Measures the phase changed due to the changed position of individual spins More sensitive to cellular changes than conventional MRI sequences 8

Detecting Diffusion Particles move around randomly (Brownian Motion) Average displacement determined by diffusion coefficient Given observation time and displacement, the diffusion coefficient could be calculated Movement of particles attenuates signal in addition to T2 and T1 relaxations in a magnetic field gradient Angular frequency depends on magnetic field Movement of particles decreases phase coherence much like T2 relaxation 10

Diffusion Represents Molecular Events 12

Magnetic Field Gradient Field inhomogeneities exist in traditional MRI, interfering with T2 signal Applying stronger field gradient amplifies diffusion effect Gradient timing Constant gradient Pulsed gradient S = S 0 e ( b ADC Diffusion weighted image (Also called trace image)measure s S ADC image weighted image measures ADC ) 13

Figure 1.1 A typical pulse sequence for diffusion imaging.the shaded areas represent field gradient pulses.dw diffusion weighted,te time evolution 14

Apparent Diffusion Coefficient (ADC) Water self diffusion is constant Water movement is restricted by tissues Since brain cerebrospinal fluid (CSF) contains water that can move around freely, its ADC value is much higher than that of other brain tissues (either gray matter or white matter) S = S 0 e ( b ADC Low ADC gives strong echo, High ADC gives low echo ) 15

Acute Cerebral Ischemia Ischemia: Lack of blood supply Ischemic tissues have lower ADC Causes: Blood clot Shock (excessive bleeding, heart failure) Consequences: Stroke symptoms Permanent CNS damage (CNS does not regenerate) High risk of death Treatments must be rapid to prevent permanent damage 17

Importance of Diffusion weighted MRI Required to decide whether risky treatments are necessary Localization of Attack Measuring Severity Distinguish hemorrhage from Ischemia CT and T2 wighted MRI shows damage 5 6 hours after attack Diffusion weighted MRI detects within minutes 18

Acute Ischemia 19

Edema Stroke can also be due to edema. Edema means water molecule transfer from extracellula to intracellula. More water molecule is restricted leads to decreasing ADC 20

Edema Stroke due to Edema Lesioned neurons: Decreased ADC Increased Trace T2 and CT does not show the damage 21

Tumor 22

Diffusion Anisotropy Cell membranes decreases diffusion rate Diffusion rate at a point is direction dependent Axons act like pipes, myelin amplifies this effect Consequences: Diffusion must be scanned in all 3 axes Diffusion rate is represented as vector (modeled as ellipsoid) 24

Diffusion Tensor Imaging Scanning in 3 axes is not sufficient Head alignment causes inconsistent data Solution: Use a tensor matrix and scan from 6 directions Calculate eigenvalues D = D D D xx yx zx D D D xy yy zy D D D xz yz zz 25

Tensor Calculations ( D λi) X = 0 D xx λ D xy D xz D yx D yy λ D yz = 0 D zx D zy D zz λ 27

Diffusion Tensors & Anisotropy DTI allows researchers to quantify the diffusion of water in brain tissue Diffusion for each image voxel is described by 3 perpendicular vectors λ1 λ1 λ2 λ3 Anisotropic diffusion occurs when water movement is restricted to one primary direction (e.g., myelinated fibers) λ2 λ3 Isotropic diffusion occurs when there is no restriction to water movement (e.g., ventricles, CSF) 28

Diffusion Ellipsoids Diffusion ellipsoids reconstructed from real DTI data. 29

Mean Diffusivity & Fractional Anisotropy Mean Diffusivity (ADC) Fractional Anisotropy λ λ = 1 + λ3 λ2 + 3 FA = 3 2 2 2 ( λ λ ) + ( λ λ ) + ( λ λ ) 1 2 1 2 λ + λ + λ 2 2 2 3 3 2 Addition of eigenvalues Difference in eigenvalues Overall diffusion Directional diffusion 30

Color Coded Direction Color map used to indicate dominant diffusion direction 31

Left: Conventional T2W image does not show white matter fiber tracts in the brain. Middle: Anisotropy map highlights the white matter bundles in the brain. Right: The z-map high intensity regions correspond to large out of plane diffusion. 32

Interpreting Diffusivity and FA Diffusivity and FA help determine the number, size and myelination of fibers, whereas only FA gives information about directionality. Number of fibers Myelination of fibers High Diffusivity Low FA High FA Low Diffusivity High Diffusivity Low FA High FA Low Diffusivity Size of fibers Directionality of Fibers High Diffusivity Low FA High FA Low Diffusivity Low FA Same Diffusivity High FA Same Diffusivity 33

Tumor 35

Cyst 36

Fiber Tracking 38

Whole Brain Tractography 39

Application of Fiber Tracking White matter research White matter disruption due to tumor Diseases related to fiber dysfunction 42

Other applications: Neurosurgery, Brain Tumors 43

White matter fiber tracts and Schizophrenia There are widespread gray matter deficits reported in MRI structural studies, but fewer reports of white matter abnormalities. Functional abnormalities are reported in different brain regions and different systems using fmri and PET. Several theories link schizophrenia with disconnection between different brain regions. 44

Cingulum Bundle The most prominent connection between limbic structures. Consolidates information by interconnecting thalamus, prefrontal, parietal, temporal lobes (including amygdala, hippocampus and parahippocampal gyrus) with cingulate gyrus. 45

Schizophrenia related symptoms most frequently linked with cingulate dysfunction Thought disorder Disorganized behavior Hallucinations Flattening of affect Delusions Lack of attention 46

Coronal sections of diffusion tensor maps show cingulate fasciculi (out of plane diffusion component- coded in orange)(arrows), above the corpus callosum (in plane component- coded in blue). Patient with schizophrenia on the left, comparison subject on the right. Note the difference in area of the bundle. 47

550 500 normal controls schizophrenics 450 RA 400 350 300 right left Diffusion anisotropy within the left cingulum bundle in schizophrenia group was 7.4 % lower than in normal comparison subjects (mean of the percentage difference for all eight slices), while diffusion anisotropy on the right side within the CB in schizophrenics was only 2 % lower than in normal comparisons. 48

Research Challenges Image Resolution. (Limited by gradient strength) White Matter Segmentation. Statistical Analysis. Current Region Of Interests: Superior Temporal Gyrus. Uncinate Fasciculus. Cingulate Bundle. Arcuate Fasciculus. 50

Fiber Clustering Automated tools separate fibers on the basis of their shape and projections 51

Conclusion The concept behind Diffusion MRI is relatively simple, yet there are many different applications utilizing this technology. 52

References Gillard, Jonathan, Adam Waldman, and Peter Barker. Clinical MR Neuroimaging. Cambridge: Cambridge University Press, 2005. http://www.uchsc.edu/neuroimaging/diffus/diffus.htm http://www.jubileum.lu.se/mr_physics/molecular%20motion.htm Yoshiura T,Wu O, Sorensen AG (1999) Advanced MR techniques: Diffusion MR imaging,perfusion MR imaging, and Spectroscopy. Neuroimaging Clin N Am 9:439 453 Chun T, Filippi CG, Zimmerman RD, Ulug AM (2000) Diffusion changes in the Edemic human brain. Am J Neuroradi-ol 21:1078 1083 Engelter ST, Provenzale JM, Petrella JR, DeLong DM,Mac-Fall JR (2000) The effect of stroke on the apparent diffusion coefficient of normal-appearing white matter. Am JRoentgenol 175:425 430 Helenius J, Soinne L, Perkio J (2002) Diffusion-weighted MR imaging in normal human brains in various age groups. Am J Neuroradiol 23:194 199 Gideon P, Thomsen C, Henriksen O (1994) Increased self-diffusion of brain water. J Magn Reson Imaging 4:185 188 53