**
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
}

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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