lecture 7 - MR physics Flashcards
1
Q
MRI atoms
A
- Nuclear Magnetic Resonance Imaging - refers to the nucleus of atoms, not nuclear energy or radiation
- different atom species resonate at different frequencies
- standard MRI uses just hydrogen atoms, but you can also do it with P or Na nuclei
2
Q
MRI-scanner main components
A
- main magnet: creates a strong magnetic field
- radiofrequency coil: transmits and receives radio frequency waves
- gradient coils: create additional magnetic fields whose strength varies along XYZ dimensions - this is important for localizing the signal
- patient table: moves the patient in and out
- computer system - controls the scanner from another room
3
Q
MRI safety issues
A
- if you’re in the way of the moving oxygen tank you get seriously hurt. therefore, an elaborate safety screening is important
- attraction by the main magnet - bringing anything magnetic into the scanner will cause it to become a projectile
- RF transmission and reception heats up conductive materials inside the scanner (just like microwave)
- loud sounds, dizziness, claustrophobia, and peripheral nerve stimulation
4
Q
magnetization
A
- with MR we look at net magnetization of all nuclei in a volume (voxel)
- given a field B0, spins align with this magnetic field, and the net magnetisation (M) can be seen as a vector with two components:
1. longitudinal: parallel to the magnetic field (Z)
2. transverse: perpendicular to the magnetic field (X & Y)
5
Q
radio frequency pulse
A
- the scanner contains RF transmitters that emit an RF pulse at the RF frequency - also called the (transient) B1 field – perpendicular to the main magnetic field B0
- the RF pulse causes the phases of all spins to tip over, and aligns their phases
- the longitudinal M decreases, and a strong transversal M is created
- essentially, we have control over the M’s flip angle (90 degrees here)
6
Q
B0 and B1 fields
A
- B0: primary and constant magnetic field of the scanner – determines the precession frequency of protons
- B1: magnetic field associated with the RF pulse. B1 is not static like B0; it is dynamic and only present during the RF pulse. this temporarily alters the spins and phases of the protons to produce the MRI signal
7
Q
return to equilibrium after RF pulse
A
- Longitudinal Relaxation (T1):
- After the RF pulse, the longitudinal component of the magnetization vector (L), which is along the main magnetic field (B0), will grow back to its equilibrium state.
- This process is characterized by the time constant T1.
–> curves up in the graph
–> at T1, L reaches approximately 63% of its maximum value - Transverse Relaxation (T2):
- Simultaneously, the transverse component (T), which is perpendicular to B0, decays back to zero.
- This decay is described by the time constant T2
–> curves down in the graph
–> at T2, T falls to approximately 37% of its initial value
8
Q
different MR contrasts related to T1 and T2 weighting
A
T1 and T2 are mostly independent properties of tissue
- T1 RECOVERY is faster for white matter, which appears bright on T1-weighted images. Gray matter and CSF appear dark
- T2 DECAY is faster for CSF and gray matter, which appear bright on T2-weighted images. White matter appears dark
- T1 and T2 relaxation times are mostly independent properties of tissues, meaning they provide different and complementary information about tissue properties.
- T2* contrast is mentioned as a variation of T2 contrast. It is influenced by local magnetic field inhomogeneities, which can be caused by a variety of factors, including variations in blood oxygenation levels.
9
Q
echoes
A
- spin echo
- gradient echo
- multi echo
10
Q
Free Induction Decay
A
- after an RF pulse, the signal decays into nothingness – mainly due to T2* decay
- for this reason, we need to create echoes that we can ‘hear’
- two ways of doing this
1. gradient echo
2. spin echo
11
Q
Spin Echo
A
- use a second RF pulse at time TE/2 to ‘flip’ or ‘push’ the spins back, effectively reversing the dephasing
- This second pulse is often a 180-degree pulse - As a result, the spins rephase and create a new signal peak at the time TE, which is the Spin Echo.
- This refocusing of the spins compensates for T2* decay and gives us a signal that is dependent on the true T2 relaxation time of the tissue.
- Echo Time (TE): the time between the first RF pulse and the peak of the echo signal
- high signal-to-noise ratio (SNR)
- more T2 than T2* weighting: The Spin Echo sequence eliminates the T2* dephasing effects. As a result, the signal decay that is observed after the echo is primarily due to true T2 relaxation. Therefore, the image contrast reflects differences in the T2 properties of the tissues rather than the T2* properties, which would be more influenced by magnetic field inhomogeneities.
12
Q
T1 and T2
A
terms that describe the two main types of relaxation processes
- T1 Relaxation: the process by which the net magnetization vector (M) returns, or “relaxes,” back to its equilibrium state in alignment with the external magnetic field (B0), after being tipped into the transverse plane by an RF pulse.
–> longitudinal magnetization - T2 Relaxation: describes the loss of phase coherence among spins in the transverse plane, leading to a reduction in the net transverse magnetization.
–> transverse magnetization
13
Q
Gradient Echo
A
- Instead of an RF pulse (spin echo), the gradient coils are used to produce the echo by ‘tilting the playing field’.
- After the initial RF pulse creates transverse magnetization, the spins begin to dephase due to T2* decay, leading to a loss of signal.
- A dephasing gradient is first applied, which increases the dephasing rate among the spins.
- A rephasing gradient is then applied, which reverses the dephasing effect of the first gradient, causing the spins to rephase and produce an echo signal (= GRADIENT ECHO (GRE)).
- can acquire data faster than spin echo
14
Q
gradient echo is most used for
A
- anatomies
- fMRI
15
Q
spin echo is most used for
A
- anatomies
- diffusion imaging
