Contrast Mechanisms in MRI. Michael Jay Schillaci

Transcription

1 Contrast Mechanisms in MRI Michael Jay Schillaci

2 Overview Image Acquisition Basic Pulse Sequences Unwrapping K-Space Image Optimization Contrast Mechanisms Static and Motion Contrasts T1 & T2 Weighting, Field Strength, T2*, Dephasing Endogenous Contrasts BOLD Imaging Motion Contrasts Time of Flight, Diffusion, Perfusion

3 Basic Pulse Sequences

4 Image Formation Integrate magnetization to get MRI signal Select a z slice and form image of XY plane variations S z ( t) = Area M XY ( x, y, t) e iγ t 0 [ xg ( τ ) + yg ( τ )] X Y dt dxdy Contrast from difference in magnetization in different tissues Image at several times to get average Horizontal density Vertical density

5 Basic MRI Scan Terminology Orientation: Coronal Sagittal Axial Coronal Sagittal Axial Matrix Size: # of Voxels in dimension Field of view (FOV): Spatial extent of dimension Resolution: FOV/Matrix size. Axial Orientation 64x64 Matrix 192x192mm FOV 3x3mm Resolution Sagittal Orientation 256x256 Matrix 256x256mm FOV 1x1mm Resolution

6 Image Creation The scanning process 1. Protocol sets Gradients and Encodes K-Space Weights 2. Signal is Determined with Fourier Transform 3. Image Created with Inverse Transform S z ( t) = Area M XY ( x, y, t) e iγ t 0 [ xg ( τ ) + yg ( τ )] X Y dt dxdy Step 2 k y = 2πγ t 0 G y dt Step 1 Step 3 k x = 2πγ t 0 G dt x N 1 y x ik ( tl ) yl ik ( t ) yl xn n ( l, yn ) s( kx, k ) n y e e l m x l= 0 N 1 n= 0 x n dk x n dk y l

7 Image Acquisition FOV x 1 π = = 2π = 2 Δx γg T γg N T x s x s FOV y 1 = Δy π = 2π 2 γg T γg T y pe x s G y varies in each cycle Data Acquisition (DAQ)

8 Slice Selection Gradient: G sl Gradient Field Ensures Field Greater on Top Larmor Frequency Depends on z Position RF pulse Energizes Matched Slice Field Strength Z Position

9 Frequency Encoding Gradient: G ro Apply transverse gradient when we wish to acquire image. Slice emits signal at Larmor frequency, e.g. lines at higher fields will have higher frequency signals. X Position Field Strength

10 Phase Encoding Gradient: G pe Apply Orthogonal RF pulse Apply before readout Adjusts the phase along the dimension (usually Y) Y Position Field Strength

11 Unwrapping K-Space Field of View: FOV x 1 π = = 2π = 2 Δx γg T γg N T x s x s FOV y 1 = Δy π = 2π 2 γg T γg T y pe x s Choose phase encoding time so that Δx = Δy Pixel Size: Δx = FOV N x x Δy = FOV N y y Image Adapted from Prof. Yao Wang s Medical Imaging course notes at:

12 Image Optimization Adjustment of Flip Angle Parameter Maximum SNR typically between 30 and 60 degrees Long TR sequences (2D) Increase SNR by increasing flip angle Short TR sequences (TOF & 3D) Decrease SNR by increasing flip angle S Maximizing the signal TR T1 1 e = M 0 sinθ e TR T1 1 cosθe gives the: Ernst Angle: Spoil cosθ E = e TR T 1 TE T * 2

13 Gradient Echo Imaging 1. Assume perfect spoiling - transverse magnetization is zero before each excitation: M zb = M za cosθ 2. Spin-Lattice (T1) Relaxation occurs between excitations: M zc TR = T1 M + zbe M0 1 e TR T 1 S Spoil = M 0 1 e sinθ 1 cosθe TR T 1 TR T 1 e TE T * 2 1. Assume steady state is reached during repeat time (TR): 2. Spoiled gradient rephases the FID signal at echo time (TE): S = M sinθe Spoil M = zc M za za TE T * 2

14 Spin Echo Imaging Spin echo sequence applies a 180º refocusing pulse Half way between 90º pulse and DAQ Allows measurement of true T2 time T2 T2*

15 The Refocusing Pulse 1 Actual Signal T2 Signal T2* TE 0.5 TE Spins Rotate at Different Rates Refocusing Pulse Re-Aligns Spins

16 Volume Reconstruction 3D volumes composed of 2D slices Slice thickness. Thicker slices have more hydrogen so more signal (shorter scan time) Thinner slices provide higher resolution (longer scan time) Optional: gap between slices. Reduces RF interference (SNR) SNR V N Fewer slices cover brain 3mm 1mm Gap 2mm Thick

17 Static Contrast Mechanisms

18 T1 and T2 Weighting T1 Contrast Echo at T2 min Repeat at T1 max T2 Contrast Echo at T2 max Repeat at T1 min Net Magnetization is T1 Contrast Weighting TR TE Max T1 Contrast Min T2 Contrast T2 Contrast Weighting TR TE M XY TR TE T T M e e = re cov ery decay Min T1 Contrast Max T2 Contrast

19 Static Contrast Images Examples from the Siemens 3T T1 Weighted Image (T1WI) (Gray Matter White Matter) T2 Weighted Image (T2WI) (Gray Matter CSF Contrast) Anatomical Image Diagnostic Image

20 Flip Angle Variation RF Pulse Magnitude Determines Flip Angle Duration and magnitude are important +z M B 0 θ θ M Z +y +x B C M XY M Z = M cos( θ ) M XY = M sin ( θ ) Adapted from:

21 Field Strength Effects Increased field strength Net magnetization in material is greater Increased contrast means signal is increased Image 1 resolution is better Muscle Tissue 1 MRI adapted from:

22 Tissue Contrast and Dephasing Dephasing of H 2 O and Fat MRI signal is a composite of Fat and H 2 O signals H 2 O and Fat resonate at different frequencies T1 F = 210 ms, T1 W = 2000 ms ( T1 F > T1 W fat is brighter) Relative phase gives TE dependence M F Φ FW M W Parallel ( Φ FW = 0 o TE = ms Anti-Parallel (Φ FW = 180 o TE = ms

23 Endogenous Contrast

24 BOLD Imaging Blood Oxyenation Level Dependent Contrast dhb is paramagnetic, Hb is less Susceptibility of blood increases linearly with oxygenation BOLD subject to T2* criteria Oxygen is extracted from capillaries Arteries are fully oxygenated Venous blood has increased proportion of dhb Difference between Hb and dhb is greater for veins Therefore BOLD is result of venous blood changes

25 Sources of the BOLD Signal BOLD is a very indirect measure of activity Blood flow Neuronal activity Metabolism [dhb] BOLD signal Blood volume

26 Neuronal Origins of BOLD BOLD response predicted by dendritic activity (LFPs) Increased neuronal activity results in increased MR (T2*) signal LFP=Local Field Potential; MUA=Multi-Unit Activity; SDF=Spike-Density Function Adapted from Logothetis et al. (2002)

27 The BOLD Signal BASELINE ACTIVE

28 BOLD Imaging Blood Oxyenation Level Dependent Contrast Susceptibility of blood changes with oxygenation Blood flow correlated with task performance Differential activations can be mapped BASELINE ACTIVE

29 Static Contrast - T2* Relaxation T2* accounts for magnetic defects and effects T2 T2 = + + * T2 T2M MS T2 is relaxation due to spin-spin interaction of nuclei T2 M is relaxation induced by inhomogeneities of main magnet T2 MS is relaxation induced by magnetic susceptibility of material M B 0 M χ m

30 BOLD artifacts fmri is a T2* image we will have all the artifacts that a spinecho sequence attempts to remove. Dephasing near air-tissue boundaries (e.g., sinuses) results in signal dropout. Non-BOLD BOLD

31 Motion Contrast

32 Flow Weighting Time-of-Flight Contrast Saturation Excitation Acquisition No Flow Medium Flow High Flow No Signal Medium Signal High Signal Vessel Vessel Vessel

33 ADC Diffusion Tensor Imaging Anisotropy Diffusion Coefficients Magnitude (ADC) Maps Proton pools Direction (Anisotropy) Maps Velocity Reconstruct Fiber Tracks with Clustering l = 2Dt S = S e o D γ G T 3 3

34 Indices of Diffusion Anisotropy Relative anisotropy: ( ) ( ) ( ) RA = λ λ + λ λ + λ λ λ Fractional anisotropy: FA = 3 ( λ1 λ) + ( λ2 λ) + ( λ3 λ) 2( λ1 + λ2 + λ3 ) MD FA Vector

35 DTI in Stroke Research Examine integrity of fiber tracts Tractography - trace white matter paths in gray matter Assess neglect as a disconnection syndrome Stroke Healthy

36 Arterial Spin Labeling Perfusion Flow of fluid into vessels to supply nutrients/oxygen The amount and direction of flow matters

37 Pulsed Labeling Imaging Plane Alternating Inversion Alternating Inversion FAIR Flow-sensitive Alternating IR EPISTAR EPI Signal Targeting with Alternating Radiofrequency

38 ASL Pulse Sequences RF EPI Signal Targeting with Alternating Radiofrequency 180 o 90 o 180 o FAIR EPISTAR Gx Gy Gz RF Gx Gy Gz Odd Scan 180 o Alternating Proximal Inversion Alternating opposite Distal Inversion Even Scan Odd Scan Even Scan Flow-sensitive Alternating IR 90 o 180 o Image Image