MRI - technological principles

Question 1

What are the 4 steps of a MR exam

Answer 1

patient placed in magnet
RF wave turned on
RF wave turned off
patient emits signal which is received and used for reconstruction of the image

Question 2

Talk to me about Hydrogen in the bod

Answer 2

abundant in the body in H2O and other molecules
it is magnetic
nucleus = single positively charged atom, proton, spinning on its own axis - precession
hydrogen nucleus is therefore a continually rotating charge (e.g. current)

Question 3

How are protons normally aligned in the body and what happens when these poor protons are subjected to a strong external magnetic field

Answer 3

normally aligned in a random fashion but when exposed to an external magnetic field the protons align themselves within the external magnetic field - most parallel to the magnetic field (z-axis) but some anti-parallel

Question 4

What is net magnetisation

Answer 4

slightly more protons line up parallel to the external magnetic field, as for every proton facing anti-parallel there is a parallel proton to counterbalance it, therefore overall left with some protons pointing up the z-axis along the external magnetic field

Question 5

What is the longitudinal magnetisation

Answer 5

the net magnetisation as it lies longitudinal to the external magnetic field

Question 6

Why can we not just measure this net magnetisation

Answer 6

because the net magnetisation is very small and virtually impossible to measure alongside the external magnetic field

Question 7

what do we do to this net magnetisation in order to be able to measure

Answer 7

we want to tip it thorough 90 degrees along the x-y plane and so we need to give the protons energy to do this, so we use RF pulses

Question 8

What 2 effects do the RF pulses have the protons

Answer 8

1) lifts protons to a higher energy level so they point down the z-axis
2) causes protons to precess step so they group together

Question 9

What is the result of these effects

Answer 9

1) reduces the longitudinal magnetisation in the z-axis

2) establishes a new magnetisation in the transverse plane and move around the x-y plane with the precessing protons

Question 10

How is an electrical current induced

Answer 10

the newly established transversal magnetisation moves in phase with the precessing protons and is constantly moving, therefore the transversal magnetisation induces an electric current

Question 11

How is an MR signal produced

Answer 11

a moving magnetic field causes an electric current in antenna = MR signal

Question 12

What aspect helps to sort out where the signal is coming from

Answer 12

gradient coils in the x, y and z axis - causes the different cross sections to have a magnetic field of different strengths and therefore the frequency of the signals to differ so the part of the anatomy the signal is coming from can be identified

Question 13

How is the image then displayed on the monitor

Answer 13

MR signal is digitised and then converted into the image and displayed on the monitor

Question 14

What happens when the RF pulses are switched off

Answer 14

whole system relaxes and goes back to its original state

Question 15

Explain T1 restoration

Answer 15

protons are lowered back to their lower energy level pointing back up along the z-axis, returning the longitudinal magnetisation back to its original value

Question 16

Explain T2 restoration

Answer 16

protons get out of phase again and ‘fan’ out causing the transversal magnetisation to decrease and then disappear

Question 17

How can tissues have differing T1 and T2 relaxation times be used

Answer 17

these time differences can be used to generate image contrast, MRI uses T1 or T2 weighted sequences to exploit the differing tissue relaxation times to produce images