Principles of NMR Protons are like little magnets Radio Frequency pulse will knock protons at an angle relative to the magnetic field once out of alignment, the protons begin to precess protons gradually realign with field (relaxation) protons “echo” back the radio frequency that originally tipped them over That radio “echo” forms the basis of the MRI image

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 recall that the precession frequency depends on the field strength anything that changes the field at one proton will cause it to de-phase The de-phased region will give off less echo

Functional Imaging blood flow overshoots baseline after a brain region is activated Deoxygenated blood in some region causes relatively less signal from that region More oxygenated blood in some region causes relatively more 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?”

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

MRI Image Formation Modern Scanners

MRI Image Formation Our Scanner

MRI Image Formation Our Scanner

MRI Image Formation Our Scanner

MRI Image Formation Our Scanner

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

Experimental Design in fMRI Experimental Design is crucial in using fMRI Simplest design is called “Blocked” Cognitive state alternates between “active” and “rest” conditions Scanner takes a sequence of “volume” images Active Rest Active Rest 60 sec 60 sec 60 sec 60 sec

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

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

Experimental Design in fMRI A real example of fMRI block design done well: alternate moving, blank and stationary visual input Moving Blank Stationary Blank 40 sec 40 sec 40 sec 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?

Structural and Functional Imaging What you really want is an image, not a squiggly line Make a map of a statistic (like t-score or z-score) that describes how well each voxel tracked the cognitive task: Set all “non-significant” voxels to be transparent

Structural and Functional Imaging What you really want is an image, not a squiggly line Make a map of a statistic (like t-score or z-score) that describes how well each voxel tracked the cognitive task: Set all “non-significant” voxels to be transparent

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: Most hospitals have MRI scanners that can be used for fMRI (PET is rare) Better spatial resolution in fMRI than PET Structural MRI is usually needed anyway No radioactivity in MRI Better temporal resolution in MRI

Advantages of PET Advantages of PET: Quiet 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: BOLD signal change does not necessarily mean a region was specifically engaged in a cognitive operation Poor temporal resolution - depends on slow changes in blood flow expensive

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