Measuring Blood Oxygenation in the Brain

Functional Imaging Functional Imaging must provide a spatial depiction of some process that is at least indirectly related to neural activity in most imaging (i.e. PET, fMRI) that process is change in blood oxygenation related to changes in regional cerebral blood flow Why should we measure blood oxygenation?

Functional Imaging Why should we measure blood oxygenation? Onset of a stimulus (or cognitive task) changes local blood oxygenation –first with a decrease –then with an “overshoot”

Functional Imaging Why should we measure blood oxygenation? Onset of a stimulus (or cognitive task) changes local blood oxygenation –first with a decrease –then with an “overshoot” How do we measure changes in blood oxygenation?

Functional Imaging Recall that precessing protons give off a radio “echo” as they realign with the magnetic field

Functional Imaging Recall that precessing protons give off a radio “echo” as they realign with the magnetic field We pick up the combined echo from many protons that are in phase

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

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 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 Heme

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! Thus deoxygenated tissue gives of less MR echo because the protons de-phase quickly

Functional Imaging blood flow overshoots baseline after a brain region is activated More oxygenated blood in that region increases MR signal from that region (other regions de-phase faster)

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 How do we create those pretty pictures?

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 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 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

PET: another way to measure blood Oxygenation Although oxygenation is measured differently, the logic of PET and fMRI are similar: compare active and “rest” conditions

Advantages of fMRI All techniques have certain advantages A good scientist leverages these advantages

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