MRI

Question 1

What are we aiming to do with standard RT?

Answer 1

Deliver a homogenous dose to the CTV
Deliver the same dose to all patients
Keep OAR doses below constraints

Question 2

What are we assuming about tumours and OARs?

Answer 2

(Homogenous dose)
Tumour density is uniformly distributed throughout CTV
Tumour cells react the same to radiation

(Same dose to different patients)
Tumours in different patients need the same dose

(OAR dose below constraint)
All parts of an OAR are equally important for function

Question 3

How accurate are our assumptions?

Answer 3

Not very
Tumours are very heterogenous, large changes in tumour density; different cellular subtypes in same lesion respond to RT differently
Different patients react differently to same dose
OARs are made up of functional subunits which vary between patients and will function to different degrees

Question 4

What are examples of advanced RT?

Answer 4

Dose painting
Personalised dose prescription
Treatment response monitoring
Functional OAR dose sparing

Question 5

What is dose painting and what does it require

Answer 5

Treat sub volumes within GTV of higher tumour density or tumour activity with higher boost dose
Need method of imaging tumour density/activity

Question 6

What is personalised dose prescription and what does it require?

Answer 6

Prescribe different doses to different patients depending on tumour characteristics
Requires method of predicting response of tumour before treatment starts

Question 7

What is treatment response monitoring and what does it require?

Answer 7

Aims to determine treatment response early in treatment course
Requires a method of determining response before seeing anatomical changes

Question 8

What is functional OAR dose sparing and how does it work?

Answer 8

Spare high functioning sub volumes of OAR rather than relative volumes
Requires method of determining map of OAR function

Question 9

What is the motivation for functional imaging?

Answer 9

Tumours and OARs are very heterogeneous, adapting plans to account for this could result in better outcomes

Question 10

What is functional imaging?

Answer 10

Imaging technique that spatially characterises tumour or OAR function
Imaging a quantity that correlates with tumour/OAR characteristic eg tumour cell density or perfusion

Question 11

Requirements for functional imaging

Answer 11

Clinical relevance
Sensitivity/specificity to treatment effects
Reliability
Praciticality

Question 12

Considerations of 1. clinical relevance, 2. sensitivity to treatment effects 3. reliability, 4. practicality

Answer 12

1. Firm biological rational, sufficient spatial resolution, information available in a timely manor
2. Correlated to outcome data
3. Sufficient accuracy and precision; uncertainties understood and quantified; reproducible across scanners/trusts; limited variation on scatter parameters; geometric accuracy; appropriate QA
4. tolerated by patients and clinically feasible to implement in hospitals

Question 13

What are some methods of functional imaging?

Answer 13

DW MR
DCE MR
BOLD MR
PET MR

Question 14

What is diffusion?

Answer 14

Bulk motion of group of molecules in solution due to random motion
In tissue, diffusion is restricted by cellular and sub cellular structures, correlation between cell density and amount of restriction in diffusion

Question 15

How do we measure diffusion?

Answer 15

Spin echo sequence
Add pair of equal gradients each side of 180 pulse
If no diffusion, gradients cancel out, no signal loss
If diffusion, gradients do not cancel out and signal loss
Take measurments with different b values and create ADC map

Question 16

How is DW MR useful for RT and what are issues?

Answer 16

Low ADC –> high tumour cell density

Usually acquired with single shot echo planar imaging which is very susceptible to geometric distortion especially near tissue/ air or tissue/bone interfaces
SNR relatively poor leading to large slice thicknesses and voxel sizes
Also have diffusion outside blood vessels - these create a bi exponential

Question 17

What is DCE MR

Answer 17

Aims to measure perfusion - blood flow to tissue using gadolinium contrast to give signal to blood. Shortens T2 making it bright on T1w images, acquire multiple T1w images and can measure changes as contrast agent moves into capillaries

Question 18

Analysis of DCE MR

Answer 18

Can be used for qualitative analysis on enhancement time curves
Quantitative analysis possible by assuming model of how blood perfuses from blood vessel to tissue. Can be fitted to measurements of concentration of gadolinium as a function of time C(t). C(t) calculated from changes in T1 measured from MR images. Tofts model most common

Question 19

Use of DCE MR in RT

Answer 19

Tumours are highly perfused and high Ktrans can indicate tumour. Heterogeneity of tumour perfusion can also indicate likelihood of tumour response. Hypoxic regions are more resisitant to RT, poor response
Complex, hard to do routinely, high inter observer and inter protocol variation makes repeatable results difficult

Question 20

What is BOLD MR

Answer 20

Blood oxygen level dependent MR
Oxyhaemoglobin and deoxyhaemoglobin have different T2* values
Increased neuronal activity causes initial drop in oxy and increase in deoxy, gollowed by incresased blood flow which increases oxy, 1-5% increase in T2*
BOLD MR acquires images after stimulus or functional task

Question 21

How do you analyse BOLD MR data

Answer 21

Acquire images during stimulus and rest. Subtract stimulus from rest to get a contrast map. Threshold contrast map to produce activation map. Overlay this onto anatomical image in same session to show functional regions of brain

Question 22

Why use BOLD MR and what are the issues

Answer 22

Can map functional areas of brain to spare

Uses EPI sequences which are susceptible to geometric distortion
BOLD signal very small so easily affected by motion artifacts
Determining statistically significant changes is not straight forward

Question 23

What is PET MR and the challenges?

Answer 23

Combined PET and MR scanners allowing for simultaneous acquisition. Useful when registrations between sequential images are difficult due to changes in patient anatomy

Need a method of acquiring accurate images in RT position and method of attenuation correcting PET (generate sCT)