1. Structural and Functional Imaging Functional images tend to be lower resolution and fail to convey spatial information Pixels

2. Structural and Functional Imaging Structural images have finer (smaller) pixels Pixels

3. Structural and Functional Imaging Why? What’s wrong with the functional image alone? More subtly: a functional image typically isn’t a picture of the brain at all! It’s a picture of something else – PET, fMRI = oxygenated blood – EEG = electric fields – MEG = magnetic fields

4. Tools for measuring brain function The main story about functional imaging is a trade-off between spatial resolution and temporal resolution

5. Principles of MRI

6. Some terms: – Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency – Magnetic Resonance Imaging (MRI) uses spatial differences in resonance frequencies to form an image basis of anatomical MRI – functional Magnetic Resonance Imaging (fMRI) exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

7. Principles of MRI Some terms: – Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency – Magnetic Resonance Imaging (MRI) uses spatial differences in resonance frequencies to form an image basis of anatomical MRI – functional Magnetic Resonance Imaging (fMRI) exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

8. Principles of MRI Some terms: – Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency – Magnetic Resonance Imaging (MRI) uses spatial differences in resonance frequencies to form an image basis of anatomical MRI – functional Magnetic Resonance Imaging (fMRI) exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

9. Principles of MRI Some terms: – Nuclear Magnetic Resonance (NMR) quantum property of protons energy absorbed when precession frequency matches radio frequency – Magnetic Resonance Imaging (MRI) uses spatial differences in resonance frequencies to form an image basis of anatomical MRI – functional Magnetic Resonance Imaging (fMRI) exploits magnetic properties of hemaglobin to create images changes in cortical blood flow

10. Principles of NMR Protons are like little magnets – they orient in magnetic fields like compass needles – what way do they normally point?

11. Principles of NMR Protons are like little magnets – they orient in magnetic fields like compass needles – what way do they normally point? – normally aligned with Earth’s magnetic field

12. Principles of NMR Protons are like little magnets – they orient in magnetic fields like compass needles – what way do they normally point? – normally aligned with Earth’s magnetic field – NMR uses a big magnet to align all the protons in a sample (e.g. brain tissue)

13. Principles of NMR Protons are like little magnets – Radio Frequency pulse will knock protons at an angle relative to the magnetic field

14. 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

15. 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)

16. 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

17. Principles of NMR Protons are like little magnets – The following simple equation explains MRI image formation

18. 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

19. 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

20. 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

21. 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

22. 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