EXCITE Afternoon Hands-On MRI Sessions : fMRI & DTI

EXCITE Afternoon Hands-On MRI Sessions: fMRI & DTI

Contrast in MRI - Relevant Parameters Relaxation times: • T1 Spin-lattice relaxation time (longitudinal relaxation time) Return of spin system to equilibrium state • T2 Spin-spin relaxation time (transverse relaxation time) Loss of phase coherence due to fluctuations of interacting spins (‘phase memory time’) • T2* Decay time of free induction decay Signal loss due to magnetic field inhomogeneity (difference in magnetic susceptibility) • ADC Apparent diffusion coefficient Signal loss due to diffusion of water molecules in an inhomogeneous magnetic field • k water exchange rate Exchange of water between macromolecule bound fraction and bulk (free) water

Relations and Limitations Sensitivity: Signal-to-Noise Ratio (SNR) Spatial resolution Temporal resolution • Signal: magnetization (number of spins, magnetic field strength, …. ) • Noise: thermal noise of receiver system, physiological noise, …

MRI Contrast IBT

Relaxation times • MRI delivers good soft tissue contrast • Tissue specific magnetic parameters for contrast generation • T2 / T2*: how fast is signal lost after excitation • T1: how fast is magnetization gained back after excitation for next experiments • Sequence parameters and sequence type determine contrast

exp(-t/T2*) The NMR signal Mz +1.0 My Mx 1-exp(-t/T1) Relaxation Relaxation Mi(t)/Meq 0.0 -1.0 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 time (s) time (s) time (s)

T1 weighting • Relevant parameters: • Repetition time (TR) = time between two excitations • Flip angle -> how much magnetization is left for next excitation • Strong T1 weighting for large flip angle and short TR Mz MzA T1 Relaxation during TR MzB θ Mxy

T1 weighting: Example • Two metabolites with T1=500ms (blue) and T1=250ms (red) • Flip angle: 60° • Signal proportional to DMz • TR=3000ms Mz time IBT

T2 / T2* weighting • Relevant parameter: • Echo time (TE) = time between excitations and data acquisition • Strong T2 weighting for long TE Mxy t / ms TEshort TEmedium TElong

Proton density weighting • Intensity scales with number of signal generating nuclei per volume element • Keep influence of relaxation times small: • Short TE -> small effect of T2 / T2* on signal • Long TR -> small effect of T1

Functional MRI (fMRI) IBT

Functional MRI (fMRI) • Uses echo planar imaging (EPI) for fast acquisition of T2*-weighted images. • Spatial resolution: • 3 mm (standard 1.5 T scanner) • < 200 μm (high-field systems) • Sampling speed: • 1 slice: 50-100 ms • Problems: • distortion and signal dropouts in certain regions • sensitive to head motion of subjects during scanning • Requires spatial pre-processing and statistical analysis. EPI (T2*) dropout T1 But what is it that makes T2* weighted images “functional”? IBT

The BOLD contrast REST • neural activity   blood flow   oxyhemoglobin   T2*  MR signal ACTIVITY Source: Jorge Jovicich, fMRIB Brief Introduction to fMRI

Peak Brief Stimulus Undershoot Initial Undershoot The temporal properties of the BOLD signal • sometimes shows initial undershoot • peaks after 4-6 secs • back to baseline after approx. 30 secs • can vary between regions and subjects

MRI and Diffusion IBT

Brownian motion • Molecules or atoms in fluids and gases move freely • Collisions with other particles causes trembling movement • Brownian motion: microscopic random walk of particles in fluids of gases (R. Brown 1827) • Brownian motion depends on thermal energy, particle properties and fluid density

Diffusion • Diffusion: irreversible automatic mixing of fluids (or gases) due to Brownian motion • Root mean square displacement depends on diffusion coefficient D and time t: (A. Einstein) • Diffusion coefficient D affected by cell membranes, organelles, macromolecules (Le Bihan 1995)

Anisotropy • Restrictions on water diffusion usually without spherical symmetry  anisotropic diffusion in biological tissue • Diffusion tensor (=3x3-matrix) instead of diffusion coefficient accounts for anisotropic diffusion in 3D • Principal diffusion direction: direction with largest diffusion coefficient r1 r2 r3 Restricted Diffusion Free Diffusion

Example: nerve fibre • Diffusion perpendicular to fibre restricted • Water diffusion indicates white matter organization

Diffusion and MRI • Diffusion leads to signal loss in MRI

Diffusion gradients • Signal attenuation depends on diffusion coefficient and gradient waveforms • GE: sum of diffusion weighting gradients zero • SE: diffusion weighting gradients have equal area • Single shot techniques freeze out physical motion 180° diffusion gradient diffusion gradient 90° EPI readout TE

Diffusion weighted imaging DWI • b-value (=b-factor) describes diffusion weighting analogous to TE in T2 weighted sequences • b-value determined by diffusion weighting gradients (i.e. gradient form, strength, distance) signal b-factor [s/mm2] 200 400 600 800 1000 0 S0: signal without diffusion weighting; D: diffusion coefficient in direction of gradient

ADC PS MS l1 MP S M P l2 FA l3 3D ellipsoid DWIs + Reference Color-coded FA DTI • Ellipsoid represents diffusion tensor • Fibre structure via map of diffusion anisotropy: calculate fractional anisotropy (or relative anisotropy or volume ratio)

Principal diffusion coefficient and vector: longest axis of diffusion tensor

Brain structures via analysis of principle diffusion vectors Superior longitudinal fasciculus Pons Corpus callosum Tapetum Medulla Corticospinal tract Optic radiation Middle cerebellar peduncle Superior cerebellar peduncle Medulla

MR Angiography IBT

MR Angiography Image Slice Image Slice Blood flow Gradient echo imaging: Don’t wait for gradient echo  Bright signal from unsaturated spins in slice Saturation: apply 90°slice-selective pulse saturation imaging Stationary spins Mz Inflowing spins time IBT

EXCITE Afternoon Hands-On MRI Sessions : fMRI & DTI