Image Weighting and Instrumentation

Question 1

Imaging requires what pulses to be transmitted

Answer 1

RF pulses

Question 2

All the pulses have to flip

Answer 2

the NMW at some angle with respect to static magnetic field B0

Question 3

T2 time for fat is

Answer 3

shorter than CSF

Question 4

Fat T1 & T2 times are

Answer 4

250 and 80

Question 5

White matter T1 & T2 times are

Answer 5

800 and 90

Question 6

CSF T1 & T2 times are

Answer 6

2400, and 280

Question 7

T2 transverse is always

Answer 7

shorter than longitudinal T1 time

Question 8

Repetition time is

Answer 8

the time from the application of one RF pulse to the release of the next pulse

Question 9

Echo time is

Answer 9

the time from the application of RF pulse to the peal of the signal induced in the coil

Question 10

Echo time determines how

Answer 10

much decay of transverse magnetisation is allowed to occur before signal is read

Question 11

How are images obtained

Answer 11

Obtained thru contrast with T1 recovery; T2 decay and proton/spin density

Question 12

T1 has

Answer 12

short TE & TR times

Question 13

T2 has

Answer 13

long TE & TR times

Question 14

Proton Density image is

Answer 14

one where the contrast depends on the number of protons per unit volume (minimise both T1 & t2 effects)

Question 15

What are the processes required to make a MRI

Answer 15

• Nuclear alignment
• RF excitation of protons
• Spatial encoding
• Image formation

Question 16

What is the hardware to make these processes

Answer 16

Main maget (B0); gradient coils; RF source, and image processer/comp system

Question 17

What is the purpose of the main magnet?

Answer 17

To generate static magnetic field and promote homogenity throughout the device - must also be stable

Question 18

Larmar Equations is

Answer 18

precessional freq = gyromatic ratio x magnetic field strength (B0)

Question 19

Advantages for high field strength

Answer 19

signal from patient increases; faster scan times; probe physiological processes

Question 20

Disadvantages of high field strengths

Answer 20

bioeffects; heating; nerve stimulation; cost; sitting issues; artifacts

Question 21

What are the main categories of magnets

Answer 21

Permament; resistive and superconducting

Question 22

Advantages of main magnet

Answer 22

no power supply; low maintenance; large bore

Question 23

Disadvantages of main magnet

Answer 23

low SNR; long scan times; siting - heavy

Question 24

Resistive magnet

Answer 24

less field strength; current passes thru solenoid,

Question 25

Main advantages of resistive

Answer 25

good uniformity, i.e. generates similar field strengths from one end to the other * light weight compared to permanent magnet * lower capital costs * can be turned off in an emergency

Question 26

Disadvantages for resistive magnets

Answer 26

* requires continuous electric power to produce magnetic field - > cost * produces a significant amount of heat * creates significant stray magnetic fields -> safety * low inherent SNR

Question 27

Supreconducting magnets

Answer 27

Superconducting coils allow electric current to flow without any resistance at temperatures below - 2500C (23 K)

Question 28

Quenching is

Answer 28

where higher temperatures cause loss of superconductivity and resistance - uncontrolled can lead to explosive boiling & damage

Question 29

Computer performs image construction where it

Answer 29

performs Fourier transformations (FT) on the FID before displaying images

Question 30

An image artefact is an

Answer 30

unwanted pattern or structural appearance that does not represent the actual anatomy

Question 31

What are the sources of MRI artefacts

Answer 31

Instrumentation, patient related artefacts, metallic susceptibility, system related artefacts

Question 32

Zipper artefact is caused by the

Answer 32

leakage of the RF signals into scanner room - faults in faraday cage door seal, unshielded equipment in MRI

Question 33

Motion artefact is caused by

Answer 33

random motion during the image sequence which results in the blurring of the image

Question 34

Source of motion can include

Answer 34

anterior abdominal or thoracic wall during respiration, flow related from arteries/veins, eye movements, patient moving

Question 35

Magnetic susceptibility artefact occurs

Answer 35

as a result of the microscopic gradients or variation in the magnetic field strength that occurs near the interfaces of substance of different magnetic susceptibility - metal objects insidebuman body

Question 36

Magic angle artefact is

Answer 36

when the tissues with tightly bound collagen lie at an angle of 55 to the main static magnetic field

Question 37

How does magic affect the area

Answer 37

restricts the molecular motion of water molecules and elongation of T2 times

Question 38

A patient undergoing MRI will see that

Answer 38

both long and trans relaxation processes occur at the same time