Origin of signal: Excitation, Relaxation Flashcards
What is the main field?
B0
Along the Z direction
What is found inside the scanner?
- Precession
2. Bulk/net magnetization
What is Larmor frequency?
Every proton/every spin, every nuclear magnet moment rotates about B0
What is Larmor frequency?
Product of general magnetic ratio and B0
What is bulk magnetization?
Creation of magnetization vector M which is the sum of every nuclear magnetic moment
What is parallel to the Z axis, same axis in the equilibrium state?
The M vector
Where can you not measure any signal?
At equilibrium
How can you rotate M vector?
Using Radiofrequency pulse and B1 field
What is perpendicular to Z?
B1 field
What are the same?
Z and B0
Where does B1 field rotate?
About Z axis at Larmor frequency
What is the flip angle?
Angle just after excitation between M vector and Z axis
What does the amplitude depend on?
Scanner you are using e.g. 1.5T or 3T
What is perpendicular to B0?
Magnetisation vector M
What is used to measure signal?
Coil
What is the coil able to measure?
Transverse component of M vector
What can M vector be decomposed into?
Transverse component and Longitudinal component
What is needed to be able to rotate M towards the transverse plane?
- Radio Frequency (RF) field
- Transmit field
- B1 field
What creates a variation in magnetic transverse component through the coil?
M rotates about B0 all the time
How can you avoid having just only one signal?
Measure one M per voxel
What is each M vector?
The sum of every nuclear magnetic moment, every proton within one voxel
What is the total magnetic field?
No gradient (B0) + GzZ gradient
What does B1 field used for excitation have?
Frequency
This frequency is required to excite the protons - rotate the M vector - create a signal
What happens if B1 field is not at the Larmor frequency or frequency of M vector around B0?
It will not be able to rotate M
With a gradient
Frequency of the protons will be different one side of the table than the other because the frequency of protons is proportional to B0?
when will all the protons be excited?
Apply radio frequency pulse at omega 0
What happens if you apply a gradient to the radio frequency pulse at omega 0?
Just a slice will be excited when the gradient is 0
What is omega 0?
Gamma B0
What do you do if you want to select a different slice?
- Change the slope of the gradient (amplitude)
2. Change the bandwidth of the pulse
What do you do if you want to change the width of the slice?
- Change frequency of the pulse
- Change the slope of the gradient (amplitude)
- Change the bandwidth of the pulse
What happens once a detectable signal is generated?
It decays
What wants to be aligned with the strong main field?
Magnetisation vector
What occurs once detectable signal is generated?
T1 Recovery/Relaxation of longitudinal magnetisation
RF pulse at 90 degrees
Transverse magnetisation is at its maximum
There is no longitudinal magnetisation
Relaxation
- Precession (static in rotating frame)
- Return to equilibrium
- Mz regrows
- Mxy decays
What is the definition of T1?
Time taken to reach 63% of the equilibrium value
What does the time taken for T1 depend on?
Ability of the local environment to faciliate relaxation
Depends on the micro structure of the tissue
What decays more rapidly than the longitudinal component
Transverse component
What is T2 decay of transverse component?
- A more rapid process
2. Nuclei alter the field experienced by each other and lead to ‘dephasing’ and signal loss
What is M?
Sum of protons in one voxel
What is the definition of T2?
Time taken to decay to 37% of its original value
What does the time taken for T2 depend in?
How quickly the spins get out of phase
3 T2 - 90milliseconds
No signal left
What is another phenomenon of Transverse component?
Additional T2’ decay
Due to inhomogeneity of the main field B0
T2*
T2 Decay + T2’ Decay
1/T2* = 1/T2 + 1/T2’
T2 Decay
The signal loss cannot be recovered
T2* Decay
The additional signal loss can be recovered
How can we recover the signal lost because of B0 homogeneity?
RF excitation
What is B1 pulse characteristics?
B1 oscillates in x-y plane with frequency 𝜔_𝑟𝑓, and only excites magnetization precessing at 𝜔_0=𝜔_𝑟𝑓. 𝑀⃗ rotates about B1 at rate 𝜔_1. Flip angle: 〖𝜃=𝜔〗_1∙𝑡 𝑀_𝑧=𝑐𝑜𝑠𝜃 𝑀_𝑥𝑦=𝑠𝑖𝑛𝜃
What are the types of RF pulse?
90º excitation
180º excitation
180º inversion
What is present during excitation?
There is only B0 and B1
B0 is around Z and B1 is rotating
What T and Omega 1?
- T= duration of the pulse
2. Omega 1 = the frequency about which M vector rotates about B1
What can you do to change theta?
- Increase time of the pulse (duration)
2. Change Omega 1 - Change B1
What is 180 degree Inversion?
Pulse that inverts the spin magnetisation
What does spin echo sequence include?
slice selective 90-degree pulse followed by one or more 180 degree refocusing pulses
