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. Subtraction methodfunctional neuroimaging research
 Assumption of the subtraction technique is that only a single feature is added from one condition to another, which means that after subtracting one condition from another, any differences between the two images must be due to this single feature.
 A typical use of the subtraction method is a comparison between a reference or baseline condition and an active condition in which a new cognitive process is added to the cognitive processes present in the reference condition.
 The PET and fMRI only examine differences in brain activity relative to a control or baseline
 Changes are usually very small (5-20%)
 Important that only a single feature be added or removed across conditions
 Ultimately this is a very difficult problem

8. fMRI
 Used to monitor changes in blood flow and blood volume in the brain within a couple of hundred milliseconds, better spatial resolution than PET. fMRI offers poor selectivity, and requires a higher signal to noise ratio in order to get a good image. Whole brain areas as well as specific loci can be assessed and displayed within an anatomical framework. Also, non invasive
 Basic Principles of MR imaging
o Imagethe disruption of hydrogen atoms in tissue
o There are difference in distribution of H+ atoms and magnetic properties of different tissues
o Differences also reveal changes in blood flow
o Based on different magnetic properties of oxygenated (arterial) and deoxygenated (venous) blood
o When subjected to certain gradient echo pulse sequences, oxygenated blood provides a stronger MR signal—the difference between hemoglobin in oxygenated vs deoxygenated blood provides the fMRI image
o Cognitive activation-vasodilation-where neurons are more active-oxygenated blood will exceed metabolic need
 +++Physics of fMRI Imaging
o Fixed magnetic field is generated
o varies from 0.3...