1. 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

2. 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

3. 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

4. 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

5. 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

6. Siemens Allegra 3T Biomedical Imaging Center (BIC)

7. Fixed field magnet is always on!

8. 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

9. 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 seconds for the response to peak
Fastest reliably detectable pre-peak response so far = sec
Signal strength change very small – generally less than 1% change

12. 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

13. 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

14. 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