MR 3 Flashcards
What is responsible for MR contrast?
Relaxation
After a 90º RF pulse, longitudinal and transverse magnetisation return back to ______ _________.
Thermal equilibrium
What does longitudinal (M_z) recovery depend on? What is the rate of recovery determined by?
The exchange of spin energy with the lattice.
The longitudinal (spin-lattice) time constant: T₁
What does T₁ represent?
The time to return to thermal equilibrium.
What does transverse (M_x M_y) decay depend on? What is the rate of recovery determined by?
The relative dephasing of spins.
The transverse (spin-spin) time constant: T₂ (or T₂*)
What does T₂ (or T₂*) represent?
The rate of dephasing.
Which equations are used to describe the rate of change of magnetisation?
The Bloch equations
What are the Bloch equations?
Phenomenological equations describing the variation (rate of change) of magnetisation with time.
How are the Bloch equations solved?
By applying the appropriate boundary conditions.
Give the Bloch equation for magnetisation in the z-direction
M = magnetisation
T₁ = longitudinal time constant
Give the Bloch equation for magnetisation in the x-direction
M = magnetisation
T₂ = transverse time constant
γ = Gyromagnetic ratio
B = magnetic field (z-direction)
Give the Bloch equation for magnetisation in the y-direction
M = magnetisation
T₂ = transverse time constant
γ = Gyromagnetic ratio
B = magnetic field (z-direction)
What are the 3 general rules for the values of T₁ and T₂?
- T₂ < T₁ generally (spins dephase faster than they return to equilibrium polarization).
- T₂ ~ T₁ in systems in which the molecules are highly mobile.
- T₂ «_space;T₁ in solids.
What is longitudinal relaxation (T₁) recovery?
The process by which a spin system returns to Boltzmann equilibrium distribution following the absorption of RF energy.
Longitudinal relaxation (T₁) recovery requires the exchange of ____ energy with _______ energy of molecules.
Spin
Thermal
What is correlation time (τ)?
The rate at which nuclei undergo Brownian motion (tumbling).
What is the impact of Brownian motion?
The tumbling of nuclei with spin and a magnetic moment generates a randomly fluctuating magnetic field (B_xy) that varies at the Larmor frequency. This causes transitions in spin states.
When is relaxation most efficient?
When 1/τ (the rate of molecular motion) is closer to the Larmor frequency.
State two properties that T₁ depends on
- Temperature
- Magnetic field strength
How is T₁ recovery measured?
- A 180º pulse is sent to invert the longitudinal magnetisation.
- The longitudinal magnetisation then recovers for a time, TI.
- A 90º pulse is sent to tip the longitudinal magnetisation into the xy-plane.
- This gives rise to an FID (signal), which can then be recorded in order to fit the solution to the Bloch equation.
- This is repeated for different values of TI.
Give the Bloch equation solution used to calculate T₁
TI = recovery time
M = magnetisation
T₁ = longitudinal time constant
Give the equation for the T₁ signal measured from FID
S = signal
TI = recovery time
T₁ = longitudinal time constant
What is transverse (T₂ or T₂*) relaxation?
The process by which spins lose phase coherence due to spins dephasing and spins flipping between different states (T₁).
What are the three processes by which spins lose phase?
- Longitudinal recovery
- Time-varying magnetic field
- Static magnetic field
How does longitudinal recovery cause spins to lose phase?
The cause of spin flipping that destroys phase coherence.
How do time-varying magnetic fields cause spins to lose phase?
Neighbouring spins are in different fields or are shielded by electron clouds so have different Larmor frequencies and dephase.
How do static local fields cause spins to lose phase?
Inhomogeneities in the applied field, differences in magnetic susceptibility, and imaging gradients cause spins to lose phase.
Give the equation for transverse decay
1/T₂* = transverse decay
T₂ = transverse time constant
T₂’ = static constant
How is T₂ measured? (spin echo sequence)
- A 90º pulse is sent to create coherent transverse magnetisation.
- After a time TE/2 (where TE = echo time), a 180º pulse is sent to reverse the transverse field.
- The signal of the resulting FID is recorded and then fit to the solution of the Bloch equation.
- This is repeated for different values of TE.
Give the Bloch equation solution used to calculate T₂
t = TE = echo time
M = magnetisation
T₂ = transverse time constant
Give the equation for the T₂ signal measured from FID
S = signal
TE = echo time
T₂ = transverse time constant
Why doesn’t an MRI signal have a sign before it is corrected analytically?
MRI usually handles magnitude signals so the signal is always positive. Polarity correction can be applied to fix this.
Give 5 applications of T₁ and T₂ measurements
- Monitoring contrast agent measurements
- Monitoring white matter changes for multiple sclerosis
- Monitoring changes in tumours
- Monitoring changes in the contents of the GI tract
- Assessing liver fibrosis
What are contrast agents?
An agent that is injected into the bloodstream to provide controllable additional contrast in tissues.
What information can be found based on the rate of uptake of a contrast agent?
- Blood flow
- Blood volume
- Vessel permeability
Why is Gd-DTPA such a good contrast agent?
It has magnetic moments 1000x larger than nuclear magnetic moments. These enhance relaxation by dipole-dipole interactions.
Give 3 risks of using Gd-DTPA
Anaphylaxis: allergic reaction
Extravasation: tissue trauma due to injection into tissue rather than vessel
Nephrogenic systemic fibrosis: fibrosis of the skin (terminal)
