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
What is net magnetisation?
The combined magnetisation of all nuclei in some volume of space (M)
Net magnetisation is a vector whose orientation is along the longitudinal direction and magnitude is proportional to the difference between the number of spins in parallel and antiparallel states. Does more spins in parallel state decrease or increase net magnetisation?
Increase
If the external magnetic field is increased, what happens to the net magnetisation?
It increases
How is measuring the net magnetisation of spins in a magnetic field analogous to measuring the weight of an object?
You must perturb the equilibrium state of the spins and then observe how they react to the perturbation
Spins take high energy state or low energy state.
Transitions between states can be triggered by the delivery of energy to the spin system.
In MRI, how is this energy delivered?
In the form of radiofrequency pulses (from radiofrequency coils)
Define excitation (in the context of MRI signal generation)
Excitation: the process of providing radiofrequency energy to atomic nuclei so that some spins change from low to high energy states
What is the resonant frequency for hydrogen that electromagnetic (radiofrequency) coils are adjusted to?
42MHz/T
Explain the excitation of a spin system and signal reception
Prompts
Excitation
Energy delivered in form of radiofrequency pulses
Radiofrequency coils bombard spins in magnetic field with photons
Thus radiofrequency energy changes some spins from a low to high energy state
90* and 180* excitation pulse
When electromagnetic waves (radiofrequency pulses) are turned off, excitation of atomic nuclei stops, excess spins at the higher energy level must return to the lower levels so equilibrium can be restored- when this occurs, the spins emit photons whose energy is equal to the energy difference between the two states
During this reception period, changes in transverse magnetisation can be detected using a radiofrequency coil tuned to the Lamour frequency
The changing current in these detector coils constitutes the MR signal
The MR signal detected through receiver coils does not remain stable forever and changes in 2 ways during signal reception:
Transverse magnetisation quickly loses coherence
Longitudinal magnetisation slowly recovers
What is this process called?
Relaxation
(transverse relaxation/ spin-spin relaxation)
(longitudinal relaxation/ spin-lattice relaxation)
The MR signal detected through receiver coils does not remain stable forever and changes in 2 ways during signal reception:
Transverse magnetisation quickly loses coherence
Longitudinal magnetisation slowly recovers
What is this process called?
Relaxation
(transverse relaxation/ spin-spin relaxation)
(longitudinal relaxation/ spin-lattice relaxation)
Over time, spins lose their coherence and precess at different frequencies (they get out of phase)- leading to an exponential decay in the MR signal, what is this process called?
T2 decay
(spins lose coherence relatively quickly resulting in diminishing net magnetisation in the transverse plane)
(The time constant that describes the decay of the transverse component (i.e., transverse relaxation) of net magnetization due to accumu- lated phase differences caused by spin–spin interactions.)
After excitation, some of energy of spin system is emitted as radiofrequency waves detected by receiver coils as the MR signal
As spin system loses energy, it recovers to same state before excitation- with net magnetisation aligned along the longitudinal axis
Longitudinal recovery is relatively slow
What is this process called?
T1 recovery
(The time constant that describes the recovery of the longitudinal component of net magnetisation over time)
The Larmor frequency for Hydrogen is
A) 12 MHz/T
B) 22 MHz/T
C) 32 MHz/T
D) 42 MHz/T
D) 42 MHz/T
Following a RF pulse (90* to the B0/z plane), the magnetic signals from hydrogen atoms are detected in the
A) longitudinal (B0, z) plane
B) transverse (x, y) plane
C) sagittal (s, g) plane
D dorso-lateral (d, l) plane
