MRI

Question 1

What is MRI?

Answer 1

a spectroscopic imaging technique used in medical settings to produce images of the inside of the human body

Question 2

How does MRI work?

Answer 2

absorption + emission of energy of the RF EM Spectrum

Question 3

Adv of MRI (4)

Answer 3

• Utilises non ionizing radition
• ability to image in any plane
• very low incidents of side effects
• ability to diagnose, visualize + evaluate various illnesses

Question 4

Components of MRI (4)

Answer 4

• Magnet - powerful uniform magnetic field
• Gradient magnet (lower in strength)
• RF equipment
• Powerful Computer

Question 5

Field strength of MRI magnet

Answer 5

.5 - 2.0 T

Question 6

Types of magnets used in MRI (4)

Answer 6

• Resistive Magnets
• Permanent Magnets
• Super conducting magnets
• Gradient Magnets (used to create variable field)

Question 7

Proton and neutron spins are known as …

Answer 7

… nuclear spins

Question 8

What is spin?

Answer 8

A small magneti field, either + or - and mathematical value of 1/2

Question 9

An unpaired component has a spin of 1/2 and two particles …

Answer 9

… with opposite spis cancel out

Question 10

In NMR, the magnetic field is produced by …

Answer 10

… the unpaired nuclear spins

Question 11

Why are Protons important to MIR? (4)

Answer 11

• +’ve charge
• spin about central axis
• a moving (spinning) charge creates a magnetic field
• straight arrow (vector) indicates the direction of the magnetic field

Question 12

When placed in a large magnetic field, hydrogen atoms have …

Answer 12

… a strong tendancy to align in the direction of the magnetic field

Question 13

The magnetic field runs down the …

Answer 13

… centre of the tube, so the hydrogen protons will align at either the head or feet

Question 14

Majority of protons will …

Answer 14

… cancel out, but the net number of protons is sufficient to produce an image

Question 15

MRI machine applies a RF pulse that is …

Answer 15

… specific to hydrogen

Question 16

RF pulse are applied through a coil that is …

Answer 16

… specific to the part of the body being scanned

Question 17

The gradient magnets are …

Answer 17

… rapidly turned on and off which alters the magnetic field

Question 18

Larmor Equation

Answer 18

Question 19

What does the Larmor Frequency?

Answer 19

Frequency need to excite transverse magnetization (precession)

Question 20

When the RF pulse is turned off, the hydrogen protons …

Answer 20

… slowly return to their natural alignment within the magnetic field and release their excess stored energy

Question 21

What happens to the released energy? (3)

Answer 21

• Released as heat
• Exchanged and absorbed by other protons
• Released as Radio Waves

Question 22

Measuring the MR Signal (3)

Answer 22

• Moving proton vector induces a signal in the RF antenna
• Signal is picked up by coil and sent to computer
• Computer uses Fourier transform to convert to an image

Question 23

Magnetisation State (2)

Answer 23

A

• Protons spinning in nature, w/o external mag field.
• Directions of spins are random and cancel out each other
• Net magnetisation is 0

B

• Large magnetic field, B_0, spins align themselves either
  
  • against (high energy state)
  • along (low energy state)
• Slight abundance of spins aligned in low energy state

Question 24

Proton Alignment (4)

Answer 24

A - spin of a proton aligned to B0 in the z-axis

B - An external perturbing magnetic field, B1, knocks the vector out its axis, which is now aligned at a new angle w.r.t B0

C - As the pertubing field B1 is removed, the vector gradually starts returning back to its original state

D - begins to wobble

Question 25

Resonance Eqn

Answer 25

v - resonant frequency

gamma - gyromagnetic ratio

B_0 - magnetic field

Question 26

T1 Parameter

Answer 26

the spin-lattic relaxation time-scale for the longitudinal magnetisation to come back to its original value

Question 27

T2 Parameter

Answer 27

the spin-spin relxation time for the transverse magnetisation to come back to its initial value

Question 28

T1 weighted imaging has …

Answer 28

… higher spatial resolution

Question 29

T2 weighted imaging has …

Answer 29

…. higher tissue contrast

Question 30

What does the image represent?

Answer 30

Every tissue layer has a different volume of water thus a different number of hydrogen atoms which produce different images based on the # hydrogen atoms