FMRI.ppt
- 2. 2 An fMRI scan is a functional magnetic resonance imaging scan that measures and maps the brain’s activity. An fMRI scan uses the same technology as an MRI scan. An MRI is a noninvasive test that uses a strong magnetic field and radio waves to create an image of the brain. The image an MRI scan produces is just of organs/tissue, but an fMRI will produce an image showing the blood flow in the brain. By showing the blood flow it will display which parts of the brain are being stimulated. What is an fMRI scan and how does it work?
- 3. Change in oxyhemoglobin concentration during increase blood flow 3
- 4. Difference Between MRI & fMRI 4
- 5. 5 fMRI Vs. MRI The MRI and fMRI differ from each other in a way that the MRI views the anatomical structure while the fMRI views the metabolic function. An MRI studies the water molecule’s hydrogen nuclei whereas an fMRI calculates the levels of oxygen. An MRI’s structural imaging views at a high resolution the differences between tissue types with respect to space. On the other hand, an fMRI’s functional imaging views the tissue differences with respect to time.
- 6. Early Uses of NMR • Most early NMR was used for chemical analysis • No medical applications • 1971 – Damadian publishes and patents idea for using NMR to distinguish healthy and malignant tissues • “Tumor detection by nuclear magnetic resonance”, Science • Proposes using differences in relaxation times • No image formation method proposed • 1973 – Lauterbur describes projection method for creating NMR images • Mansfield (1973) independently describes similar approach 6
- 7. Terms Used for MRI • NMR (Nuclear Magnetic Resonance) MR (Magnetic Resonance) MRI (Magnetic Resonance Imaging) 7
- 8. fMRIacquisition technique •BOLD(BloodOxygenLevel Dependent), the most common, provides a mixed signal dependant on:– bloodFLOW–bloodVOLUME–bloodOXYGENATION. •Variations on the technique can be used to emphasize or de-emphasize one or another of these components:- Pefusion-based fMRI * blood flow-Injections of various compounds * blood volume-Diffusion-based fMRI cell swelling (after excitation) 8
- 9. 9 Figure 2.1 BOLD mechanism of functional MRI (A) Blood-oxygen level-dependent signal mechanism in magnetic timbre imaging (B) oxyhaemoglobin and deoxyhaemoglobin blood flow during rest and activation
- 11. •fMRI-BOLD is best used for studying processes that can be rapidly turned on and off like language, vision, movement, hearing, and memory. •The study of emotion is hampered by its slow and variable onsetand its inability to be quickly reversed. Some have, however, succeed in using this technique to study fear (Whalen et al.)fMRI-BOLD paradigms generally have several periods of rest alternating with several periods of activation. •Images are then compared over the entire activation to the rest periods. Images over the first 3 to 6 seconds of each period are generally discarded due to the delay in hemodynamicresponse. •Alternating paradimsare used because the signal intensity generated by the MRI scanner drifts with time.Design of fMRIExperiments 11
- 13. Clinical applications • Mapping motor and language areas in patients with brain tumors • Neurosurgeon guided by fMRI 13 tumor
- 14. fMRI Experiment Stages: Prep 14 1) Prepare subject • Consent form • Safety screening • Instructions and practice trials if appropriate 2) Shimming • putting body in magnetic field makes it non-uniform • adjust 3 orthogonal weak magnets to make magnetic field as homogenous as possible 3) Sagittals Take images along the midline to use to plan slices In this example, these are the functional slices we want: 12 slices x 6 mm
- 15. fMRI Experiment Stages: Anatomicals 15 4) Take anatomical (T1) images • high-resolution images (e.g., 0.75 x 0.75 x 3.0 mm) • 3D data: 3 spatial dimensions, sampled at one point in time • 64 anatomical slices takes ~4 minutes 64 slices x 3 mm
- 16. Slice Thickness e.g., 6 mm Number of Slices e.g., 10 SAGITTAL SLICE IN-PLANE SLICE Field of View (FOV) e.g., 19.2 cm VOXEL (Volumetric Pixel) 3 mm 3 mm 6 mm Slice Terminology 16 Matrix Size e.g., 64 x 64 In-plane resolution e.g., 192 mm / 64 = 3 mm
- 17. FMRI is… a technique for measuring metabolic correlates of neuronal activity • Uses a standard MRI scanner • Acquires a series of images (numbers) • Measures changes in blood oxygenation • Use non-invasive, non-ionizing radiation • Can be repeated many times; can be used for a wide range of subjects • Combines good spatial and reasonable temporal resolution 17
- 18. Functional Magnetic Resonance Imaging (FMRI) • MRI scanning of brain function (vs. structure) • An indirect measure of increased regional cerebral blood flow during neural activity • During increased brain activity, MRI signal intensity (“brightness”) increases with the increase in oxyhemoglobin concentration • Tells us which brain regions are “working” during task performance 18
- 22. Main Components of a Scanner • Magnetic: Static Magnetic Field Coils • Resonance: Radiofrequency Coil • Imaging: Gradient Field Coils • Shimming Coils • Data transfer and storage computers • Physiological monitoring, stimulus display, and behavioral recording hardware 22
- 23. 1. Magnetic: Static Field Coils The scanner contains large parallel coilings of wires. These generate the main magnetic field (B0), which gives the scanner its field strength (e.g., 3T). 23
- 24. Surface Coil Volume Coil 2. Resonance: Radiofrequency Coils Electronic coils tuned to radio signals send energy into the brain and record an emitted “echo”. 24
- 25. RF (Radiofrequency) Coils • Used to transmit and receive RF energy • Needed to create images 25
- 26. 3. Imaging: Gradient Coils Three gradient coils are used, one in each of the cardinal directions. These allow spatial encoding of the MR signal. 26
- 27. Main coils 27
- 28. RF Exposure Standards • The FDA limits RF exposure to less than a 1 degree C rise in core body temperature 28
- 29. RF Exposure Standards • 4W/Kg whole body for 15 min • 3W/Kg averaged over head for 10 min • 8W/Kg in any gram of tissue in the head or torso for 15 min • 12W/Kg in any gram of tissue in the extremities for 15 min 29
- 30. The scanner is controlled by a pulse sequence. 30
- 33. Force and field directions 33 Right hand rule
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- 49. Tools Necessary for fMRI • High-field MRI (1.5T or greater) scanner • BOLD effect (fMRI signal) increases with field strength • Fast imaging sequence • Echo Planar Imaging (EPI) • Stimulus presentation equipment • Projector to show visual stimuli • Response devices such as button box to record subject’s response • Headphones for auditory stimuli (and hearing protection) 49
- 50. How to perform an fMRI experiment 50
- 51. 51 How does an fMRI map the brain's activity?
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- 53. The big advantage of fMRI is that it doesn't use radiation like X-rays, computed tomography (CT) and positron emission tomography (PET) scans. If done correctly, fMRI has virtually no risks. It can evaluate brain function safely, noninvasively and effectively. fMRI is easy to use, and the images it produces are very high resolution (as detailed as 1 millimeter). Also, compared to the traditional questionnaire methods of psychological evaluation, fMRI is far more objective. Yet fMRI also has its disadvantages. First, it's expensive. Second, it can only capture a clear image if the person being scanned stays completely still. And third, researchers still don't completely understand how it works. 53 What Are the Advantages and Disadvantages of fMRI?
- 54. MRI Safety • Static B0 Field • Projectiles • Implants/other materials in the body • RF Field • tissue heating • Gradient fields • peripheral nerve stimulation • acoustic noise 54
- 55. Forces on Ferrous Objects 55 Crash cart meets a 1.5T magnet
- 56. Projectile Effects: External 56 “Large ferromagnetic objects that were reported as having been drawn into the MR equipment include a defibrillator, a wheelchair, a respirator, ankle weights, an IV pole, a tool box, sand bags containing metal filings, a vacuum cleaner, and mop buckets.” -Chaljub et al., (2001) AJR Chaljub (2001) Chaljub (2001) Schenck (1996) The Scanner is Never Off!
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- 58. Functional Brain Mapping with MRI • Basic concept - changes in neuronal activity produces a measurable change in MR signal • Collect 100-500 MRI scans continuously (1 every 2- 3s each typically cover 30-50 slices) • Experimenter induces changes in activity at known points in time by having subject perform some cognitive or motoric task • Analyses statistically tests for MR signal changes that corresponding to experimental task 58
- 59. Super Bowl Ads • Marco Iacoboni at UCLA used fMRI to examine the brain’s response to different super bowl ads • Ranked ads based on brain responses • Found differences in the ads that stimulated the brain most and those people reported as liking the most 59
- 60. Brain Activity During Disney Ad 60 Mirror Neurons
- 61. Brain Activity During FedEx Ad 61 Fear response in Amygdala during scene where the human is squashed by the dinosaur