MRI

Question 1

what are the advantages of MRI compared to CT and X-ray

Answer 1

1. doesn’t give radiation
2. visualises soft tissues - can see target organs and tissues in high contrast

Question 2

what is the Tesla a measurement of?

Answer 2

magnetic flux density (Strength)

Question 3

how strong are the MRI magnetic fields

Answer 3

0.2-3tesla

Question 4

what are the three types of MRI magnets

Answer 4

1. superconducting
2. permanent
3. resistive

Question 5

describe how a superconducting magnet works

Answer 5

1. The wire is wound around the cylinder to produce a coil = charge
2. The coil is “supercooled” using helium liquid – the colder a conductor the stronger the magnetic force.
3. The coil is subsequently immersed in liquid nitrogen.

Question 6

define superconductivity

Answer 6

the property of a material to produce direct current (DC) electricity and strong magnetic fields without energy loss when they are cooled below a critical temperature.
*In MRI this is important as strong magnetic fields can be created with minimal energy.

Question 7

loss of superconductivity results in

Answer 7

electrical discharge that can gasify the liquid helium = asphyxiate the patient.

Question 8

what is the purpose of homogenisation coils or gradient coils in an MRI scanner

Answer 8

they are aligned to the 3 planes of the body, ensuring images are not jumbled and they have spatial coherence.

Question 9

describe how MRI scanners have an influence on the atomic level 1.

Answer 9

1. All protons within the body “spin/rock” which generates a small magnetic charge, when introduced to a strong magnetic field the protons within the hydrogen atoms align with the magnetic field.
2. Striking the realigned protons with a low-frequency RF pulse causes them to spin around by 90o or 180o.
3. When the RF pulse “passes” protons realign to their original state, by doing so they release tiny amounts of RF which is detected by the MRI coils.
   a. The rate at which RF is released is dependent on the tissue type – this is what allows the computer to visualise the body.

Question 10

what is the function of RF coils

Answer 10

transmit and receive RF waves that are similar to the wavelength of the radio waves to disrupt protons influenced within the magnetic field.

Question 11

why is the timing of the RF pulse important?

Answer 11

Is key for differentiating between fat and water.
1. T1 weighted images – RF pulse timing that highlights fat tissue in the body (dark)
2. T2 weighted images – RF pulse timing that highlights fat and water in the body (light)

Question 12

what are t1 weighted images

Answer 12

RF pulse timing that highlights fat tissue in the body (dark)

Question 13

what are t2 weighted images

Answer 13

RF pulse timing that highlights fat and water in the body (light)

Question 14

what is the function of a faraday cage

Answer 14

The faraday cage prevents contamination of radio waves from other sources as well as leaking of radio waves

Question 15

what were the problems associated with creating the MR-linac

Answer 15

1. The high magnetic field of a standard MRI will shut off a linac
2. The RF wave utilised in a Linac can destroy MRI circuitry and ruin images.

Question 16

The electron return effect is a limitation of the MR-LINAC, describe what it is.

Also how much does it increase dose

Answer 16

1. Secondary electrons from photon interactions are influenced by the magnetic field of the MRI – which generates the “electron return effect”- electrons are not going where the planning system thinks. The magnetic field is influencing electron scatter.
2. Increases dose to 30-40% and causes uncertainty in dose distribution.

Question 17

what are the benefits of the MR-linac

Answer 17

1. Have adaptive capabilities – image in real-time.
2. Online radiotherapy – adapt to the patient’s breathing for example.
3. Account for changes in inter-fractional motion and anatomical variations by applying these deformations ‘live’ based on imaging data received.
4. Better tissue visualisation
5. NO MOTHER FUCKING DOSE

Question 18

why does MRI imaging need to be image fused to be interpreted on the planning software?

Answer 18

the planning software is based off a map of electron attenuation whereas MRI is a map of hydrogen or proton density. Coregister the mr and ct data, mark the tumour on the mr, track the mr tumour delineated of mr and put on ct

Question 19

what is an advantage of using MRI guided radiotherapy

Answer 19

1. enables adaptive radiotherapy
2. better tissue visualisation