Why is MRI so popular?

Question 1

Why MRI?

Answer 1

1. Based on magnetic resonance of water molecules
2. Non-ionising
3. Full 3D imaging technique
4. Has excellent contrast in soft tissue
5. Very versatile

Question 2

What are the MRI Field strength?

Answer 2

1T = 1 Tesla
0.00005T = Average Earth's field
1.5T = Typical clinical MR scanner
3T = becoming standard MR in UK
7T = many in US, Europe, Asia, Australia
10.5T = Highest human MR scanner

Question 3

What does MRI look at?

Answer 3

Molecular level =10-9 m
Atomic level =10-10 m
Nucleus of hydrogen atom=10-13 m

Question 4

Where does spin rotate (or precess)?

Answer 4

About applied magnetic field B0 i.e. along z axis

Question 5

What is the frequency of this precession proportional to?

Answer 5

Applied field

Question 6

What does each water molecule within body have?

Answer 6

Small magnet associated with it

Question 7

What is Larmor equation?

Answer 7

v= γB0

Question 8

What does MRI rely on?

Answer 8

Magnetic properties of hydrogen atom to produce images

Question 9

What is H nucleus composed of?

Answer 9

1. Single proton
2. Spinning charged particle
3. Produces a magnetic field = magnetic moment

Question 10

How are the protons orientated?

Answer 10

Randomly orientated with no applied field

Question 11

What are the MRI components?

Answer 11

1. Primary magnet
2. Gradient magnet
3. Radiofrequency (RF) coils
4. Computer system

Question 12

What does the primary magnetic field refer to?

Answer 12

The strength of the static permanent field

e.g. 1.5 or 3T

Question 13

How does hydrogen atoms align?

Answer 13

parallel or antiparallel to primary field (B0)

known as longitudinal magnetisation

Question 14

How does a greater proportion of the hydrogen protons align?

Answer 14

Direction parallel to the primary magnetic field or low energy state than align anti-parallel to the primary magnetic field (high energy state)

net result = net magnetic vector (M)

Question 15

What is precession?

Answer 15

The protons spin on their axis

protons spin around long axis of primary magnetic field

Question 16

What is the precession rate termed?

Answer 16

Larmor frequency

Question 17

What is in phase?

Answer 17

When protons precess together?

Question 18

What is out of phase?

Answer 18

When protons precess separately

Question 19

What does frequency change with?

Answer 19

In proportion to the magnetic field strength

At 1.5T = 63.9 MHz

Question 20

What do gradient coil generate?

Answer 20

Secondary magnetic field over the primary field
located within the bore of the primary magnet
They are arranged in opposition to each other to produce positive and negative poles

Question 21

Why do gradient coil give MRI the capacity to do?

Answer 21

Image directionally along the z, x and y axis

Question 22

What do gradient magnet alter?

Answer 22

the strength of primary magnetic field changing the precession frequency between slices
this can be used for slice selection and localised in the x, y and z axis called spatial encoding of MR images

Question 23

Where does the z gradient run along?

Answer 23

Long axis to produce axial images

Question 24

Where does the y gradient run along?

Answer 24

Along the vertical axis to produce coronal images

Question 25

Where does the x gradient run along?

Answer 25

Along the horizontal axis to produce sagittal images

Question 26

What is RF coils used for ?

Answer 26

Transmitting radio frequency or RF pulse and receiving signals in MR

Question 27

What are RF coils designed for?

Answer 27

``` Specific body regions e.g. Head Body Knee Shoulder Wrist Ankle produce best diagnostic images ```

Question 28

What is RF coil used to transmit/

Answer 28

Second magnetic field/RF pulse which results in a disturbance of the proton alignment

Question 29

How is longitudinal magnetisation decreased?

Answer 29

Some low energy parallel protons flip to a high energy state

Question 30

When does net magnetization vector turn toward transverse plane?

Answer 30

When protons become synchronised and precess in phase

Question 31

What is the transverse plane?

Answer 31

Right angles to the primary magnetic field = transverse magnetisation

Question 32

What is radio frequency or RF coil for?

Answer 32

Receive signal to create images as protons resume their normal state in primary magnetic field prior to the transmission of the RF pulse = this is called relaxation

Question 33

What is relaxation in the longitudinal axis?

Answer 33

T1 relaxation

Question 34

What is relaxation in the transverse axis?

Answer 34

T2 relaxation

Question 35

What happens after the RF pulse?

Answer 35

Several protons flip back to their low energy state parallel to primary magnetic field z axis giving energy to surroundings the lattice This results in changes to the longitudinal relaxation known as T1/ or spin lattice relaxation

Question 36

What increases with time?

Answer 36

Magnetisation | T1 curve

Question 37

What is spin-spin relaxatin?

Answer 37

After RF pulse, protons that were in phase begin to dephase out of larmor frequency in the tranverse (x-y) plane Reduction in tranverse magnetisation

Question 38

Why does spins dephase much quicker than T2?

Answer 38

Inhomogeneities in the magnetic field (B0)

Question 39

What is T2*?

Answer 39

T2 relaxation + field inhomogeneities

Question 40

What is net magnetic vector?

Answer 40

Sum of longitudinal and transverse magnetisation

Question 41

Where does the net magnetic vector spiral around?

Answer 41

Z axis with net precession

Question 42

What results in free induction decay (FID)?

Answer 42

Changing magnetic moment of net magnetic vector Induces an electrical signal Signal received by the RF coil in transverse plane

Question 43

What is contrast ?

Answer 43

Difference in appearance of different tissues in an image

Question 44

What is X-ray contrast based on?

Answer 44

Transmission

Question 45

What is one of the most important characteristic of MRI?

Answer 45

Range of contrasts available | Can visualise aspect of tissue structure, composition and function

Question 46

What is magnetic susceptibility?

Answer 46

Degree of magnetization of a material in response to applied magnetic field

Question 47

What is magnetisation transfer?

Answer 47

physical process by which macromolecules and their closely associated water molecules (the "bound" pool) cross-relax with protons in the free water pool.

Question 48

What are the advantages of MRI?

Answer 48

* MRI does not use ionizing radiation; * MRI has a much greater range of available soft tissue contrast; * MRI scanning can be performed in any imaging plane; * MRI uses safer contrast agents; * MRI can produce quantitative images that reflect underlying biophysical properties;