Chapter 16 continued

Question 1

What happens to protons in a strong magnetic field?

Answer 1

Most protons line up (N facing S) in a low energy state

The remaining protons line up the other way round, in a high energy state

Question 2

Define precession?

Answer 2

The movement of the axis of a spinning object (proton) around another axis

Question 3

What does ‘lining up’ mean?

Answer 3

It refers to the direction which the protons spin

Question 4

Define Larmor frequency?

Answer 4

The frequency of precession of nuclei in an external magnetic field.

Question 5

The greater the external magnetic field…? (2)

Answer 5

The greater the difference in energy levels

The larger the excess alignment

Question 6

What happens once they are all aligned?

Answer 6

They still spin but are out of phase

Question 7

f0 = ?

Answer 7

Frequency of precession

Question 8

B0 = ?

Answer 8

External magnetic strength

Question 9

γ = ?

Answer 9

Gyromagnetic ratio

Question 10

3 equations for precession?

Answer 10

ω0 = γB0

ω0 = 2πf0

Therefore:

f0 = γB0/2π

Question 11

Define spin?

Answer 11

A quantum property of many atomic and sub atomic particles. It makes the nuclei of certain atoms behave like tiny magnets when placed in a magnetic field.

Question 12

Explain stages up to relaxation of photons?

Answer 12

1) protons processing magnetic field are exposed to RF waves whose freq.=freq. of precession. Tf, due to resonance, each proton absorbs a photon and flips into a higher energy state.
2) RF wave stopped, protons gradually return to lower energy state by emitting RF waves.

Question 13

Explain what T1 and T2 relaxation times are?

Answer 13

T1: spin-lattice relaxation time - energy transferred to surrounding lattice of nearby atoms

T2: spin-spin relaxation time - energy transferred to other spinning nuclei

Question 14

What’s do the relaxation times depend on?

Answer 14

The environment of the nuclei - for biological materials it depends on the water content

Question 15

Times for different tissues? (3)

Answer 15

Water/watery tissues (eg. Cerebrospinal fluid) = few seconds)

Fatty tissues = few hundred milliseconds

Cancerous tissues = somewhere in middle

Question 16

List 5 components of MRI scanners?

Answer 16

Large superconducting magnet
RF coil 1
RF coil 2
Set of gradient coils
Computer

Question 17

Explain role of large superconducting magnet?

Answer 17

Produces external magnetic field to align the protons, up to 2T strong. Has to be cooled to 4.2K using liquid helium

Question 18

Explain the roles of RF coils 1 and 2?

Answer 18

1) transmits RF pulses into body

2) detects signal emitted by relaxing protons

Question 19

Explain role of set of gradient coils?

Answer 19

Produce additional magnetic field that varies across the body. Ensure that Larmor freq. of nuclei varies across the body, tf only a small part of body is at the right field value for resonance. Tf computer can precisely locate source of RF in patients body.

Question 20

Explain role of computer?

Answer 20

Controls gradient coils and RF pulses and builds up image of patients body

Question 21

Basic explanation of MRI? (3 steps)

Answer 21

1) patient lies in bed in electromagnet, field turned on
2) RF transmitted causing protons to flips
3) receiving coils detect RF and image built up on computer

Question 22

6 advantages of MRI?

Answer 22

No ionising radiation
Non invasive
No moving mechanisms
Patient feels nothing and no after effects
MRI gives better soft tissue contrast than CAT
Computer images generated, and can be viewed as 3D or as slices of body

Question 23

3 disadvantages of MRI?

Answer 23

MRI shows bone less clearly than CAT scan

Can’t be used on patients with magnetic objects (eg. Surgical objects)

Can also affect pacemakers