Lecture Two: Anatomical MRI

Question 1

What wavelength does MRI use?

Answer 1

Radiowaves

Question 2

What normally determines resolution?

Answer 2

The wavelength used in imaging, MRI is the exception as they use fancy tools to improve resolution

Question 3

What can MRI be used for?

Answer 3

• Can modulate contrast to highlight pathology

- Can indicate Anatomy and Function (fMRI)

Question 4

Whats a property of all molecules?

Answer 4

Spin (quantum property)

Dissarrangement (not aligned)

Question 5

Describe in brief terms MRI

Answer 5

Based on NRI, radiowaves excite particles under the presence of a magenetic field where they align in a plane. The decay of excite releases energy that is detected and generates an image

Lots of other factors naturally

Question 6

Describe the use of radiowaves in MRI

Answer 6

Radiowaves at a specific frequency excite particles (ground -> excited) to an excited state.

Under the influence of magentic fields the particles align.

Excited particles ‘decay’ over time and release radiowaves that are detected by coils.

Question 7

What specific particle is typically excited?

Answer 7

H+ as it is abundant in water and used mainly in clinical imaging.

Question 8

What is B0?

Answer 8

Head to foot direction of magnetic field in MRI (Z plane)

Question 9

Describe what happens when H are excited by radiowaves?

Answer 9

Radiowaves excite H particles from their aligned B0, into the X,Y plane. (flip), (flip is typically 90 degrees)

Here they start to rotate around the central axis. Procession = particular frequency they rotate at. (also known as wobbles)

Question 10

What is measured in MRI?

Answer 10

The procession, as these wobbles release radiowaves that are detected

Question 11

Whats the relevant equation for procession?

Answer 11

Resonate Hz of precision =

w = y.B0

Question 12

What is the magentic field measured in?

Answer 12

Teslas (is the unit for B0)

Question 13

How can the relaxation / decay from an excited state be divided into?

Answer 13

T1 and T2, describes how the H+ relaxes

Question 14

What is T1?

Answer 14

The time for the axis plane to return to Z

Question 15

What is T2?

Answer 15

Decay of X,Y measurement

Question 16

Whats faster T2 or T1?

Answer 16

T2 is faster than T1

Process is known as rephasing

Question 17

How is dephasing significant in MRI?

Answer 17

When depahsing occurs the total signal output of molecules decreases and is not detected. (summation is reduced)

Occurs naturally

Question 18

What is a spin echo?

Answer 18

Radio Hz burst over time, this uses gradients to maintain the molecules in an excited state so their alignment produces radio signals that summate and produce an image.

Question 19

Describe what a spin echo does the the molecules;

Answer 19

• Radio Hz alters spin direction / wobble

- Increase in radio Hz is used to rephase

Question 20

Describe the MRI process using echo spin;

Answer 20

T1 = 90 degree phase Hz pulse
- Decay of wobble
T2 = 180 degree phase Hz pulse
- Rephase

signal aquired

Echo time 2x (T2-T1)

This T1,T2 (ACQ) is repeated many times to develop an image

Question 21

How is an image built up?

Answer 21

Repeated rephasing

Question 22

What is Tr?

Answer 22

Time between 90 degree repeats

Question 23

What is Te?

Answer 23

Time between 90 degree Hz and Acq

Question 24

How is contrast altered?

Answer 24

By altering Tr and TE

Question 25

How is MRI generated from NMR?

Answer 25

MRI is generated by gradients from NMR

• Gradients of magnetic fields generated by coils
• 3 axis of gradients

Question 26

What is the gradients function?

Answer 26

Gradients act to change Hz of nuclie rotation.

• These Hz components can be used to spatially locate tissues
• Back projections create images

Question 27

Why is MRI a slow process?

Answer 27

Because of the time it takes for relaxation from the excited state to occur

Question 28

Describe the physical basis of NMR used in MRI;

Answer 28

• NMR ; nuclie with ODD atomic weight have SPIN and magnetic moment
• When placed in a static magnetic field B0 they develop a net alignment with the field
• NMR sensitivity is related to the size of the magnetic field used

Question 29

What are MRI measurements sensitive to?

Answer 29

NMR sensitivity and relative abundance of nuclie being excited

Question 30

Whats the equation for radio Hz used in MRI?

Answer 30

Lamor Frequency

w = y B0

Question 31

When nuclie relax what does the frequency of radio waves released depend on?

Answer 31

Strength of B0
Atomic species
Chemical of physical environment

Question 32

Describe how T1 and T2 relate to tissues

Answer 32

Solids ; Very short T1 and T2 (ms)
Water; Very long T1,T2 (S)
Tissues; T2 usually shorter than T1 (30 vs 800ms)
WM has shorter T1 than GM
Tumors have longer T1
Fat has shorter T1 than water

Question 33

What is usually the strength of the magnetic field?

Answer 33

1.5T

Question 34

How is an image constructed?

Answer 34

Using gradients to spatially locate the signal

Question 35

What are the uses of MRI?

Answer 35

• Proton density imaging
• T1 and T2 weighting
  
  • T1 weighting shows good WM/GM contrast
  • T2 weighting shows bright CSF
• Angiograms
• DWI for early stroke
• BOLD
• In vivo spectroscopy

Question 36

What are the advantages of MRI?

Answer 36

• Signal can be dependant on tissue type due to the individual environment of nuclie
• Ability to change contrast with a wide variety of imaging protocols
• Sensitivity to motion (Angiograms DWI)
• Good resolution (1mm)
• Non-invasive (no ionising radiation)

Question 37

What are the disadvantages of MRI?

Answer 37

• Motion artefacts can destroy BOLD signal
• Contraindications (pacemakers, metal clips, pins etc)
• Claustrophobia due to small bore