In-class group activity 12:
Use an on-line simulation from the University
of Colorado PhET group to explore a simplified version of NMR and MRI.
Link to the simulation:
http://phet.colorado.edu/en/simulation/mri
Click "Run Now!" or "Download".
Start with the simplified NMR simulation.
- Familiarize yourself with the interface.
- Adjust the magnetic field to 2 T.
- Power up the radio wave source.
- Find the resonance frequency for hydrogen atoms
and verify the g = B/f (MHz/T) given in the notes and below.
- Find the resonance frequency for sodium atoms
and verify the g (MHz/T) for sodium atoms given in the notes.
- Find g (MHz/T) for sulfur atoms and the unknown
atoms. You may want to increase the field to 3 T.
Estimate g from your results. How might you estimate this more
accurately?
Switch to the MRI simulation.
- Familiarize yourself with the interface.
- Set the main field to 2 T, power up the radio
source, and find the resonance frequency. It should be very
close to the resonance frequency for hydrogen you found above.
Record the resonance frequency.
- Add a tumor. Adjust the resonance
frequency slightly to produce the strongest signal from the
tumor. Record the tumor resonance frequency. Is there a
shift?
- Remove the tumor, but add a horizontal and a
vertical gradient field of 0.06 T. The magnetic field now is no
longer uniform, but is a function of position.
Slowly bring up the frequency from 10 MHz and 110 MHz and observe that
MRI signals are only generated in selected regions of the head.
What are the approximate resonance frequencies for the upper left and
the lower right portion of the head?