Exam 3: Magnetic Resonance Imaging (MRI)

What are the main safety considerations with MRI?

The magnet is ALWAYS on!!

What is the fringe field, missile effect, and what are the potential risks?

The fringe field is a strong but invisible magnetic field that surrounds the MRI magnet. It poses risks to those who come too close to it and have metal on them (if loose, a metallic object will fly into the magnet - the “missile” effect), or, if implanted in the body, can be ripped out.

What is the unit of magnetic field strength?

Tesla units (T)

What are the basic ideas behind each of the words in the full name of the method? (N)MRI: What does “N” stand for? What is the role of hydrogen protons?

• Nuclear - studying what goes on in the hydrogen proton nucleus

• What is the role of hydrogen protons? The hydrogen proton has an electric charge; it rotates, and the moving electric charge generates the magnetic field - the hydrogen proton behaves like a tiny bar magnet

What are the basic ideas behind each of the words in the full name of the method? (N)MRI: What does the “M” stand for? How do hydrogen protons behave in a strong magnetic field? What is B₀?

• Magnetic: what happens in the magnetic field

  • How do hydrogen protons behave in a strong magnetic field? When placed in a static magnetic field (B₀), hydrogen protons behave like tiny bar magnets, and they tend to get aligned with the B₀ field, which results in net magnetization (the person in the magnet is slightly magnetized). Hydrogen protons precess (rotates/spin/wobble) around the axis of this static magnetic field at their characteristic frequency 

  • What is B₀? B zero field; the main magnetic field of the scanner

What are the basic ideas behind each of the words in the full name of the method? (N)MRI:

• What does the “R” stand for?

• What is an RF pulse?

• Resonance: at same frequency with them

• Radio frequency pulse - an electromagnetic wave; disturbs the proton alignment with B₀; has to be delivered at the RESONANT frequency (Larmor frequency) because only then can energy be exchanged

  • The RF pulse excites the protons and flips the magnetization vectors of the precessing hydrogen protons into a transverse plane, where they precess in phase with each other, which means that they are synchronized. After the pulse, the protons dephase (get out of phase) and spontaneously get realigned again with the main field (B₀).

What are the basic ideas behind each of the words in the full name of the method? (N)MRI:

• What does the “I” stand for?

• What do the anatomical images reflect?

• Imaging

• What do the anatomical images reflect?

  • T1 relaxation (recovery) rate - differs for different tissue types

    • how quickly protons return to their original state of alignment (this is reflected in the commonly used anatomical images, aka “structural” images)