Lecture 2 MRI Signal Generation Flashcards
What are atoms made of?
Protons, neutrons and electrons
How many protons does a hydrogen atom contain?
1
What property does a single proton have (essential to MRI)?
A spin
What are the 2 effects of spin motion (of protons)?
Magnetic moment
Angular momentum
What is a magnetic moment?
Magnetic moment (μ)- because proton carries positive charge, its spin generates an electrical current creating a small magnetic source and torque (rotational force) when placed within a magnetic field (strength- magnetic moment μ is the maximum torque per unit of magnetic strength)
Why do protons have angular momentum?
Proton has odd-numbered atomic mass (1) so spin results in angular momentum (J)
How can we figure out the direction of magnetic moments and the angular momentum of protons?
The right hand rule
Define a spin and spin system
- Spin- a nucleus with the NMR (nuclear magnetic resonance) property
- Spin system- a collection of such nuclei
How are the spin axes of protons oriented under ‘normal’ circumstances (in absence of a strong external magnetic field)
Randomly
What is necessary to align the spin axes of protons?
A strong magnetic field
In relation to magnetic fields, what is ‘flux’?
The local strength of a magnetic field indicated by the density of field lines
In the presence of a strong magnetic field, as well as aligning with the magnetic field, what also happens to protons?
The spinning protons initiate a motion known as precession (in which axis of spin itself rotates around a central axis like a spinning top!)
What is the Lamour frequency?
The frequency that all protons precess at when experiencing the same external magnetic field strength.
Are protons more likely to precess parallel or antiparallel to a strong magnetic field?
Parallel
(A spinning object responds to an applied force by moving its axis in a direction perpendicular to the applied force)
What are the two states for precessing protons relative to the magnetic field?
- Parallel state- parallel to magnetic field, low energy, there will always be more protons in this state depending on strength of magnetic field
- Antiparallel state- antiparallel to magnetic field, requires more energy, less stable, less protons in this state