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Structural and Functional Imaging This is a Functional MRI Image !?

Structural and Functional Imaging This is a structural MRI image (an “anatomical” image)

Structural and Functional Imaging What you really want is both images co-registered

Structural and Functional Imaging What you really want is both images co-registered But what is this really an image of!?

MRI Image Formation First you need a scanner: – The first MRI scanner

MRI Image Formation Our Scanner

MRI Image Formation Our Scanner

Structural and Functional Imaging 2-dimensional structural and functional images are comprised of pixels Pixels

Structural and Functional Imaging Brain scan images (CAT, PET, MRI, fMRI) are all made up of pixels (stands for picture elements) But of course the brain is 3-D not 2-D Pixels

Structural and Functional Imaging “Slices” are assembled into “volumes” Pixels

Structural and Functional Imaging Volumes are composed of “volume elements” or voxels Voxels

Structural and Functional Imaging Another thing you want: the ability to tell other people where something is – “the activity was centered on voxel #653” will not work in a scientific journal

Structural and Functional Imaging MRI anatomical spaces – Talairach Space: Based on detailed analysis of one elderly woman Talairach & Tournoux (1988) – Montreal Neurological Institute Template (MNI) based on average of 152 different brains, each normalized to Talairach space advantage: gyri and sulci are more representative disadvantage: it’s blurry – MNI “Representative Brain” the one brain from the 152 in the MNI Template set that is most like the average advantage: it’s not blurry disadvantage: it’s still just one person’s brain

Structural and Functional Imaging Reasons for normalizing to standard stereotaxic space (templates)

Structural and Functional Imaging 1.structural and functional volumes may not otherwise be coregistered due to movement distortion 2.results can be described in standard coordinates 3.data across sessions can be averaged These pertain to a single subject’s data

Structural and Functional Imaging 4.Volumes will not match because of variability across individuals 5.data across participants can be averaged These pertain to dealing with multiple subjects

The Talairach Coordinate System AC PC

The Talairach Coordinate System AC - PC line defines y-axis +y -y

The Talairach Coordinate System x-axis perpendicular to interhemispheric plane +y -y -x +x

The Talairach Coordinate System z-axis perpendicular to x- y plane +y -y -x +x +z -z

Structural and Functional Imaging Cortical Flattening – Software such as BrainVoyager can “inflate” the cortex like a balloon so that sulci and gyri are “flattened” – functional data can be transformed with the same complex function – functional and structural data can be overlaid so that distribution on cortical sheet can be visualized

Functional Imaging Oxygenated hemoglobin is diamagnetic - it has no magnetic effects on surrounding molecules Deoxygenated hemoglobin is paramagnetic - it has strong magnetic effects on surrounding molecules! Hemoglobin

Functional Imaging blood flow overshoots baseline after a brain region is activated More oxygenated blood in that region increases MR signal from that region

Functional Imaging It is important to recognize that fMRI “sees” changes in the ratio of oxygenated to deoxygenated blood - nothing more – BOLD: Blood Oxygenation Level Dependant contrast

Functional Imaging How do we create those pretty pictures? We ask the question “When the subject engages in this cognitive task, where does blood oxygenation change significantly?” “where does it change randomly?”

Experimental Design in fMRI Experimental Design is crucial in using fMRI Simplest design is called “Blocked” – alternates between active and “rest” conditions ActiveRestActiveRest 60 sec

Experimental Design in fMRI Experimental Design is crucial in using fMRI Simplest design is called “Blocked” – alternates between active and “rest” conditions ActiveRestActiveRest 60 sec

Experimental Design in fMRI A voxel in tissue insensitive to the task demands shows random signal change ActiveRestActiveRest 60 sec Signal

Experimental Design in fMRI A voxel in tissue that responds to the task shows signal change that matches the timecourse of the stimulus ActiveRestActiveRest 60 sec Signal

Experimental Design in fMRI A real example of fMRI block design done well: – alternate moving, blank and stationary visual input MovingBlankStationaryBlank 40 sec

Experimental Design in fMRI Voxels in Primary cortex tracked all stimuli

Experimental Design in fMRI Voxels in area MT tracked only the onset of motion

Experimental Design in fMRI Voxels in area MT tracked only the onset of motion How did they know to look in area MT?

PET: another way to measure blood Oxygenation Positron Emission Tomography (PET) Injects a radioisotope of oxygen PET scanner detects the concentration of this isotope as it decays

Advantages of fMRI Advantages of MRI: 1.Most hospitals have MRI scanners that can be used for fMRI (PET is rare) 2.Better spatial resolution in fMRI than PET 3.Structural MRI is usually needed anyway 4.No radioactivity in MRI 5.Better temporal resolution in MRI

Advantages of PET Advantages of PET: 1.Quiet 2.A number of different molecules can be labeled and imaged in the body

Limitations of fMRI All techniques have constraints and limitations A good scientist is careful to interpret data within those constraints

Limitations of fMRI Limitations of MRI and PET: 1.BOLD signal change does not necessarily mean a region was specifically engaged in a cognitive operation 2.Poor temporal resolution - depends on slow changes in blood flow 3.expensive