Transcript Slide 1
Functional MRI: Image Contrast and Acquisition Karla L. Miller FMRIB Centre, Oxford University Functional MRI Acquisition Basics of FMRI FMRI Contrast: The BOLD Effect Standard FMRI Acqusition Acquisition Confounds and Limitations Beyond the Basics New Frontiers in FMRI What Else Can We Measure? The BOLD Effect BOLD: Blood Oxygenation Level Dependent Deoxyhemoglobin (dHb) has different resonance frequency than water dHb acts as endogenous contrast agent dHb in blood vessel creates frequency offset in surrounding tissue (approx as dipole pattern) The BOLD Effect Frequency spread causes signal loss over time BOLD contrast: Amount of signal loss reflects [dHb] Contrast increases with delay (TE = echo time) Vascular Response to Activation neuron capillary HbO2 dHb HbO2 HbO2 HbO2 HbO2 dHb 2 HbO dHb 2HbO HbO HbO dHb dHb 2 2 HbO2 HbO2 HbO2 HbO2 HbO2 dHb HbO dHb2 HbO2 dHb HbO2 HbO HbO dHb 2 2 HbO2 HbO2 HbO HbO2 2 HbO HbO2 2 HbO2 O2 metabolism blood flow blood volume HbO2 = oxyhemoglobin dHb = deoxyhemoglobin [dHb] Sources of BOLD Signal Blood flow Neuronal activity Metabolism [dHb] BOLD signal Blood volume Very indirect measure of activity (via hemodynamic response to neural activity)! Complicated dynamics lead to reduction in [dHb] during activation (active research area) BOLD Contrast vs. TE 1–5% change • BOLD effect is approximately an exponential decay: S(TE) = S0 e–TE R2* S(TE) TE R2* • R2* encapsulates all sources of signal dephasing, including sources of artifact (also Functional MRI Acquisition Basics of FMRI FMRI Contrast: The BOLD Effect Standard FMRI Acquisition Confounds and Limitations Beyond the Basics New Frontiers in FMRI What Else Can We Measure? The Canonical FMRI Experiment on on on on Stimulus pattern off off off off off Predicted BOLD signal time • Subject is given sensory stimulation or task, interleaved with control or rest condition • Acquire timeseries of BOLD-sensitive images during stimulation • Analyse image timeseries to determine where signal changed in response to stimulation What is required of the scanner? image 1 2 3 … • Must resolve temporal dynamics of stimulus (typically, stimulus lasts 1-30 s) • Requires rapid imaging: one image every few seconds (typically, 2–4 s) • Anatomical images take minutes to acquire! • Acquire images in single shot (or a small number of shots) Review: Image Formation Fourier transform ky kx k-space image space • Data gathered in k-space (Fourier domain of image) • Gradients change position in k-space during data acquisition (location in k-space is integral of gradients) BOLD Signal Dropout Non-BOLD BOLD Dephasing near air-tissue boundaries (e.g., sinuses) BOLD contrast coupled to signal loss (“black holes”) DTI Basics – Water Diffusion (DTI – Diffusion Tensor Imaging) Einstein on Brownian Motion 1905 five important papers Why USE DTI MRI : Detection of Acute Stroke “Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center Conventional T2 WI DW-EPI Sodium ion pumps fail - water goes in cells and can not diffuse – DW image gets bright (note – much later cells burst and stroke area gets very dark) Why USE DTI MRI Tumor T2 (bright water) T2 (bright water) DWI (x direction) (T2 (bright water)+(diffusion)) Contrast (T1 + Gadolinium) Why DTI MRI (more recently): Fiber Tracking Diffusion Weighted Image X direction Higher diffusion in X direction lower signal Artifact or Abnormality David Porter - November 2000 T2 + diffusion T2 Sequence Time RF Gx - Gy T2 Image Gz Excite (gradient strength) Measure diffusion Regular T2 image Measuring Diffusion in other directions (examples) single-shot EPI diffusion-weighted (DW) images with b = 1000s/mm2 and diffusion gradients applied along three orthogonal directions Higher diffusion lower signal Dxx Dzz Dyy courtesy of Dr Sorensen, MGH, Boston David Porter - November 2000 How can we track white matter fibers using DTI • Measures water diffusion in at least 6 directions • Echo-planar imaging (fast acquisition) • Collecting small voxels (1.8 x 1.8 x 3mm), scanning takes about 10 minutes Higher diffusion lower signal water Diffusion ellipsoid Diffusion ellipsoid White matter fibers • Useful for following white matter tracts in healthy brain Higher diffusion lower signal White matter fibers Isotropic Anisotropic Adapted from: Beaulieu (2002). NMR in Biomed; 15:435-455 DTI ellipsoid measure 6 directions to describe z no diffusion y x Ellipsoid represents magnitude of diffusion in all directions by distance from center of ellipsoid to its surface. Ellipsoid Image Information available through DTI Tract Pierpaoli and Basser, Toward a Quantitative Assessment of Diffusion Anisotropy, Magn. Reson. Med, 36, 893-906 (1996) Tractography Superior view color fiber maps Lateral view color fiber maps Zhang & Laidlaw: http://csdl.computer.org/comp/proceedings/vis/2004/8788/00/87880028p.pdf. axial cor sag Diffusion Tensor Imaging data for cortical spinal tract on right side blue = superior – inferior fibers green = anterior – posterior fibers red = right – left fibers Note tumor is darker mass on left side of axial slice MRISC FA + color (largest diffusion direction) red = right – left green = anterior – posterior blue = superior - inferior MRS – Magnetic Resonance Spectroscopy • Proton spectroscopy (also can do C, O, Ph,.. Nuclei) • Looking at protons in other molecules ( not water) (ie NAA, Choline, Creatine, …….) • Need > mmol/l of substances high gyromagnetic ratio ( ) • Just like spectroscopy used by chemist but includes spatial localization Just looking at Proton Spectroscopy • Just excite small volume • Do water suppression so giant peak disappears • Compare remaining peaks precession Frequency Frequency MRS – Magnetic Resonance Spectroscopy NAA = N-acetyl aspartate, Cr = Creatine, Cho = Choline amplitude NAA Cr Cho Frequency of precession Multi – Voxel Spectroscopy (aka Chemical Shift Imaging – CSI) • Do many voxels at once • Can be some disadvantages with signal to noise (S/N) and “voxel bleeding” Evaluate Health of Neurons (NAA level) Normalize with Creatine (fairly constant in brain) Red means High NAA/CR levels Epilepsy Seizures (effects metabolite levels) • find location • determine onset time Other Nuclei of interest for Spectroscopy 23Na in Rat Brain (low resolution images are sodium 23 images) (high resolution images are hydrogen images) Common Metabolites used in Proton Spectroscopy Important Concepts • • • • • • • • • What energies are used in each modality? How does the energy interact with the tissue? How is the image produced? What is represented in the image? What are important advantages and disadvantages of the major imaging modalities? What are the fundamental differences between the Xray technologies (2D vs 3D, Radiography vs CT vs Fluoroscopy)? What are the two major types of MRI images (T1, T2), and how are they different? How are Angiograms produced (both Xray and MRI)? Why are the advantages of combining imaging modalities? Important Concepts • What does DTI, diffusion tensor imaging, measure? • What structures that we are interested in effect DTI images? • What does the DTI ellipsoid represent? • How might DTI be useful for clinical application or research? • What are we looking at with proton spectroscopy? • What are the three major metabolites we typically measure? • What do we “need” to be able to do proton spectroscopy? • What might proton spectroscopy be used for?