MRI

Question 1

MRI details

Answer 1

• no IR
• uses magnetic properties of hydrogen
• structural and functional modality
• great soft tissue imaging
• great image quality in head and neck, pelvic and hip bones
• can view in all planes
• uses NMR

Question 2

what can’t occur

Answer 2

LOCALISATION: as the treatment position is altered due to the magnetic coils

Question 3

what is NMR

Answer 3

the magnetic resonance transition of nuclear spin states when a radio frequency signal is applied in the presence of a magnetic field

Question 4

why is there a net spin

Answer 4

due to an uneven number of protons and neutrons, hydrogen atoms align the precession of the spin occurs at a characteristic frequency

Question 5

what happens to the protons in hydrogen when there is a stronger magnetic field

Answer 5

they will temporarily change direction, the protons will align with the external magnetic field

Question 6

what is the measurement of the protons relaxing

Answer 6

T1 and T2

Question 7

what is T1

Answer 7

time constant for the time of the longitudinal magnetism to return - relaxation phase

Question 8

describe the alignment etc process T1

Answer 8

• changes in the magnetic field causes changes in the electrical field
• the coil creates an electrical field which runs through
• a measurement is taken for the change from longitudinal to transverse
• the energised protons lose energy from the RF pulse relaxing back into the longitudinal arrangement
• transverse component decays
• longitudinal component is re-established
• exponent; effect

Question 9

describe T2

Answer 9

• protons experience interface from another spin magnet nearby which then becomes affected
• which decays the transverse magnetic component
• orientation becomes random
• exponential with time

Question 10

what are the 4 principles in MRI

Answer 10

1. the nucleus
2. magnetism
3. placing the body in a magnetic field
4. introducing a RF pulse

Question 11

describe the nucleus step within the principles

Answer 11

• H has an uneven spin
• velocity is constant
• lamor frequency = number of spins
• the direction of axis spin can change
• proton spin produces an electrical current

Question 12

describe the magnetism stage

Answer 12

• hydrogen has the highest magnetic signal
• the normal state has no magnetisation
• random alignment
• MRI uses the combined effect of a group of protons

Question 13

describe the placing the body in a magnetic field stage

Answer 13

• alignment occurs in one direction
• along the long axis there is a small magnetisation
• the precessing spin magnets emit RF waves
• signal is weak
• temporary effect under the influence of the magnet

Question 14

describe the introduction of the RF pulse phase

Answer 14

• the RF pulse frequency = number of protons in hydrogen
• introducing an RF pulse then changes the orientation
• RF is perpendicular to the long axis of the magnet
• if the Lamar frequency is equal to the hydrogen protons. then they change direction
• spin magnets flip 90

Question 15

what happens when the protons flip 90 back

Answer 15

they reduce net magnetisation to 0 and some magnetisation is established in the transverse direction. They resonate, precess in synchrony, which causes the transverse magnetic signal to become strong

Question 16

what prep is needed for MRI

Answer 16

• coil positioning
• breathing instructions
• contrast

Question 17

what are the limitations

Answer 17

• pregnant women
• cardiac pacemaker
• cochlear implant
• clips for brain aneurysms
• metallic objects
• acoustic noise
• claustrophobia

Question 18

what are the considerations

Answer 18

• patient movement
• 3D planning system
• QA programme
• coils
• dish couches
• mineral filled catheters
• surgical clips as tumour bed markers
• immobilisation and reproducibility
• patient size

Question 19

advantages in RT localisation

Answer 19

• contrast may be altered
• no effects with the artefacts
• BEV for beam
• visualises tumours in difficult areas
• good visualisation against normal tissue
• 3D
• open design so patient friendly

Question 20

disadvantages in RT localisation

Answer 20

• inferior imaging for bony structures
• image registration is difficult
• no electron density
• catheters for marking are larger than in CT
• cost and accessibility
• head and body could
• long scan times
• head MRI compatible accessorises
• distortions especially in large fields

Question 21

describe the use of contrast

Answer 21

• oral or iv
• safer than conventional
• re-allergies
• good kidney function
• high magnetic signal