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MRI 4A-VECTOR SUMMING & CANCELLATION

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MRI 4A-VECTOR SUMMING & CANCELLATION

1. Each proton’s magnetic field can be represented by a vector in a 3D coordinate system where: Z = direction of B 0 (longitudinal component), X and Y = transverse components.
2. If two vectors are equal in size but opposite in direction, they cancel each other out.
3. Remember that there is a slight preponderance for protons to align spin-up compared to spin-down when placed inside B 0
4. This means that most of the proton magnetic fields will cancel each other out.
5. There will be a few spin-up protons which are not cancelled out.
6. This will be in the Z (longitudinal) direction and gives us a net longitudinal magnetization.
7. We are unable to measure this net magnetization because it is very tiny and in the same direction as B 0 (which is very large).
8. In order to measure this net magnetization (which will give us our signal) we need to “flip” it into the transverse direction => hence the favoured angle of 90 o


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REFERENCES

1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018

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