MRI

Question 1

What is nuclear spin?

Answer 1

Protons spinning on own axis

Question 2

What is basis for MRI?

Answer 2

• Hydrogen positively charged and has magnetic field (found in body water/fat)
• Magnetic field produced by each one cancels out to produce Null NET Magnetisation

Question 3

What happens when a patient is moved into the MRI scanner?

Answer 3

• Hydrogen atoms subject to EMF (external magnetic field)
• Align along EMF and protons still spinning aka precession
• Atoms spin up = low energy
• Atoms spin down = high energy
• frequency of precession depends on strength of EMF

Question 4

What is Net Magnetisation Vector?

Answer 4

• Paralell to direction of EMF (external magnetic field)

- cannot be measured when aligned with EMF

Question 5

What is resonance defined as?

Answer 5

-Transfer of energy between systems at specific frequency

Question 6

What happens when Larmor frequency (frequency of the precession) matches the radiofrequency pulse from the MRI scanner?

Answer 6

• Hydrogen atoms excited (resonance)
• Protons spin in phase
• atoms have titled away from longitudinal EMF to transverse plane

Question 7

What does the receiver coil do?

Answer 7

Detects change in MF and generates electrical current

-forms NMR signal

Question 8

What happens in relaxation?

Answer 8

• Radio frequency pulse switches off
• Protons release gained energy
• Spin random directions
• NMV relaxes parallel to longitudinal EMF

Question 9

What do gradient coils do?

Answer 9

• Add smaller MF’s
• Separate signals from different parts of body
• Combined signals from around body processed using Fourier transform to locate individuals signal
• Determines brightness of each pixel from aptitude of the wave

Question 10

How is T1+T2 categorised?

Answer 10

• Most tissues have different T1/T2 times
• T1 = fat appears white
• T2 water appears white
• Proton density (combined)

Question 11

What are some uses of MRI?

Answer 11

• Angiography
• Venography
• Spectroscopy
• Perfusion
• Dynamic contrast
• fMRI (functional)

Question 12

What are some advantages of MRI?

Answer 12

• Not use ionising radiation
• Images acquired in multiple planes
• Superior soft tissue contrast to CT/US/RNI
• Images can be weighted
• Accurate anatomical/metabolic/functional information provided
• Angiography can be done with out IV contrast unlike CT
• Non invasive

Question 13

What are some disadvantages of MRI?

Answer 13

• Time consuming
• Less comfortable
• Very expensive
• Not safe for some patients
• Higher safety considerations for staff
• Images susceptible to artefacts due to magnetic sensitivity