MRI basic principles

What are the qualities of spin in subatomic particles?

Protons and neutrons spin in opposite directions

What are MRI active nuclei?

Atoms with net spin due to an odd mass number

What is a common tendency of active nuclei?

Align their axis of rotation to an applied magnetic field

What is the direction of the alignment of low energy nuclei?

Parallel with the magnetic field

What is the net magnetic vector?

Sum of all the magnetic moments of the parallel nuclei

What is precession?

Rotation of nuclei around the axis of the magnetic field field

What is precession frequency?

Speed at which precession occurs

What causes resonance?

When object is exposed to an external force with a similar frequency

What is excitation?

Application of similar radiofrequency pulse causing resonance

What are the results of resonance?

Net magnetic vector moves out of alignment with the magnetic field

What is the flip angle?

Shift from longitudinal plane to the transverse plane

What is in phase?

Magnetic moments of nuclei moving together because of resonance

How is mri signal produced?

Moving transverse magnetisation produces magnetic field fluctuations inside receiver coils, inducing electric voltage

What is relaxation?

Net magnetic vector realigns with axis of magnetic field when signal is turned off

What is image weighting?

Contrast between different hydrogen due to the influence electrical charges of surrounding atoms

What do T1 recovery images measure?

Amount of magnetisation in the longitudinal plane increases by 67%

What do T2 decay images measure?

Amount of magnetisation in the transverse plane decreases by 33%

What is free induction decay?

Loss of signal in the receiver coil due to dephasing of nuclei

What are the two factors determining the relaxation times of different body tissues?

Inherent energy of tissues

How closely molecules are packed

What is repetition time (TR)?

Time between applications of repeated radiofrequency pulses

What is echo time (TE)?

Time from application of radiofrequency pulse to the peak of signal

What is a crucial factor of T1 weighted images and why?

Repetition time controls how far each vector recovers before it is excited again

What MRI sequences does T1 weighted imaging use?

Short repetition time and short echo time

What is a crucial factor of T2 weighted images and why?

Echo time controls amount of t2 decay before signal is received

What MRI sequence does T2 weighted imaging use?

Long repetition time and long echo time

What is the difference in t1 and t2 imaging?

In t1 fat is bright and fluid is dark while in t2 fat is dark and fluid is bright

What is proton density signal?

Amount of signal generated based on the number of protons in different tissues

What MRI sequences does proton density imaging use?

Long repetition time and short echo time to focus on proton concentration instead of relaxation time