Magnetic Resonance Imagining (MRI) Magnetic Fields In an externally applied magnetic field, atomic nuclei with an odd number of nucleons (protons + neutrons) precess = Spin axis wobbles Rate determined both by properties of the atom And by the strength of the magnetic field Hydrogen atoms in a 1.5 Tesla field precess at ~64 MHz = Radio frequency [rf] range

MRI Magnetic Fields Cont’d After a few seconds in a magnetic field, the spin axes of a small fraction of the relevant nuclei align with each other = Axes all wobble in same way Strength of the magnetic field determines how big the fraction is Spin axes are aligned, but not precessing in phase at this point Alignment of spin axis of nuclei causes the whole magnetic field generated by the spinning nuclei to precess around the axis of static magnetic field

Resonance An EM pulse with a narrow range of radio frequencies (rf pulse) is applied to the aligned nuclei in the magnetic field When pulse frequency = precession frequency, nuclei resonate to it Pushes all of the spinning nuclei into phase with one another Amplitude of wobble of the whole magnetic field generated by the spinning nuclei increases (= spin axis is pushed farther out) How far spin axis moves (= flip angle) depends on rf pulse intensity and duration

Why is this useful? Takes characteristic amount of time for nuclei: To get out of phase (=de-phase) And to settle back to original amplitude of the wobble in fixed field Depending on the kinds of molecules the atoms are in As nuclei settle back into alignment with fixed field, they emit measurable EM energy themselves Variations in how long it takes the nuclei to de-phase & to settle back to original wobble in fixed field Can be used to distinguish among different substances

How does this make IMAGING possible? How know where in object EM energy being measured comes from?) Use gradient magnetic fields (Lauterbur Nobel Prize) Generate field with gradation in field strength with only a narrow band at 1.5 Tesla Only hydrogen atoms within that narrow band respond to 64 MHz pulse So, response must come from atoms within 1.5 T portion of field Keep moving position of 1.5 T band to localize source of responses to repeated rf pulses (“slices”) Narrowness of band determines granularity of localization of response

Siemens Allegra 3T Biomedical Imaging Center (BIC)

Fixed field magnet is always on!

Structural MRI Anatomical scans generally measure hydrogen atoms in water Since different kinds of tissue have different proportions of water Typical anatomical scan voxel granularity = 1 x 1 x 7 mm

Functional MRI (fMRI) Substance measured is hemoglobin (iron) in blood Blood flow increases to active brain regions Increases more than is usually needed So ratio of de-oxygenated to oxygenated blood decreases Oxygenated & de-oxygenated hemoglobin respond differently to magnetic field and rf pulses Thus, can detect where blood flow increases during some event Takes 6 - 9 seconds for the response to peak Fastest reliably detectable pre-peak response so far = 2 - 4 sec Signal strength change very small – generally less than 1% change

fMRI Cont’d Spatial resolution: Temporal resolution: Ultimate limit probably spatial specificity of the circulatory system Worse than structural MRI Typical functional scan voxels = 3 x 3 x 7 mm Temporal resolution: IF blood flow is what’s measured, never going to be faster than seconds Working on detecting the brief initial decrease in oxygenated blood preceding increased blood flow Working on imaging other substances

Subtractive Logic Most of the brain is active during most events Try to isolate regions that are specific to some aspect of the event of interest So, construct 2 conditions that you believe have just some crucial interesting difference Treat one as baseline and subtract it from the other, to get rid of all the activity the 2 conditions have in common And analyze (part of) what’s left

Subtractive Logic, Cont’d Similar logic used in comparing conditions in most other kinds of experiments, too But there’s been a very unfortunate tendency in the imaging literature so far, For researchers who don’t have a good understanding of the many ways that different kinds of stimuli and/or tasks and/or situations can differ, To claim that they’ve located “phonological word processing”, or “irregular morphological inflection processing”, or some such aspect of language processing When other confounded differences between conditions are equally good candidates (such as plain old difficulty) for explaining the effects