MRI Flashcards
What are we aiming to do with standard RT?
Deliver a homogenous dose to the CTV
Deliver the same dose to all patients
Keep OAR doses below constraints
What are we assuming about tumours and OARs?
(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
How accurate are our assumptions?
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
What are examples of advanced RT?
Dose painting
Personalised dose prescription
Treatment response monitoring
Functional OAR dose sparing
What is dose painting and what does it require
Treat sub volumes within GTV of higher tumour density or tumour activity with higher boost dose
Need method of imaging tumour density/activity
What is personalised dose prescription and what does it require?
Prescribe different doses to different patients depending on tumour characteristics
Requires method of predicting response of tumour before treatment starts
What is treatment response monitoring and what does it require?
Aims to determine treatment response early in treatment course
Requires a method of determining response before seeing anatomical changes
What is functional OAR dose sparing and how does it work?
Spare high functioning sub volumes of OAR rather than relative volumes
Requires method of determining map of OAR function
What is the motivation for functional imaging?
Tumours and OARs are very heterogeneous, adapting plans to account for this could result in better outcomes
What is functional imaging?
Imaging technique that spatially characterises tumour or OAR function
Imaging a quantity that correlates with tumour/OAR characteristic eg tumour cell density or perfusion
Requirements for functional imaging
Clinical relevance
Sensitivity/specificity to treatment effects
Reliability
Praciticality
Considerations of 1. clinical relevance, 2. sensitivity to treatment effects 3. reliability, 4. practicality
- Firm biological rational, sufficient spatial resolution, information available in a timely manor
- Correlated to outcome data
- Sufficient accuracy and precision; uncertainties understood and quantified; reproducible across scanners/trusts; limited variation on scatter parameters; geometric accuracy; appropriate QA
- tolerated by patients and clinically feasible to implement in hospitals
What are some methods of functional imaging?
DW MR
DCE MR
BOLD MR
PET MR
What is diffusion?
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
How do we measure diffusion?
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
How is DW MR useful for RT and what are issues?
Low ADC β> 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
What is DCE MR
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
Analysis of DCE MR
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
Use of DCE MR in RT
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
What is BOLD MR
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
How do you analyse BOLD MR data
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
Why use BOLD MR and what are the issues
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
What is PET MR and the challenges?
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)
What is an artefact?
A signal in an image which obscures the reality of the object being imaged
What can cause a motion related artefact?
Gross patient movement
Flow
Respiratory motion
Cardiac motion
Peristaltic motion
MR data is acquired over a period of ~100ms, comparable timescale for physiological motion, misregistration of position in phase encode direction
What kind of gross patient artefacts could we get and why?
Smearing artefacts as a result of non periodic motion
Discrete ghosts from periodic motion
How can we reduce peristaltis artefacts?
Give the patient an antispasmodic drug to reduce bowel movement
Immediately before exam, short lived
How could we change the direction of an artefact?
Change the phase encoding direction, can manipulate artefact to be less of an issue
What methods can we use to reduce motion without modifying sequences?
Antispasmodics
Communication with patient
Sedation
What sequence modifications can we use to reduce artefacts?
Change phase/frequency encoding directions
Breath hold, respiratory triggering
Increase signal averages
Use saturation bands
Radial acquisitions
Why does radial acquisition help reduce artifacts?
Centre of k space is oversampled, which reduces the appearance of motion artefacts (as image contrast is mainly provided by the centre of k space)
How does chemical shift work?
Complex fat molecules have many electrons which mean nucleus experiences a lower magnetic field and has lower Larmor frequency
Physical shift in location in frequency encoding direction for spins contained within fat
How could we reduce chemical shift artefact?
Increasing the receive bandwidth reduces the number of pixels artefact is spread over (but also reduces SNR). Could use fatsat but this will change image appearence
Why does phase wrap occur?
Nyquist frequency: highest frequency we can sample is equal to half of sample rate
FoV determines highest spatial frequency to be sampled
Gradients/RF also affect anatomy beyond FoV
Spins outside of FoV alias into FoV
How can we reduce phase wrap?
Increase acquired FoV (increase FoV to remove aliasing but then only construct original FoV, takes longer)
Swap phase and frequency directions
Apply fat saturation
How does parallel imaging work?
Usually undersample the acquired data to reduce acquisition time
kmax unchanged, dk doubled
End up with reduced FoV, aliased image then use coil sensitivity information to reconstruct full FoV image
Why would we get parallel imaging related artefacts?
Sensitivity map errors lead to artefacts
Eg patient moving between calibration scan and image acquisition
This is more likely to occur with separate calibration scans, use an integrated scan instead
Ensure calibration scans are repeated if patient moves
How does geometry factor affect SNR?
SNR_ipat = SNR_conventional/
g. R^1/2
g is geometry factor, depends on distinctness of what coil elements see
Little overlap of coils in phase encode direction, gΒ»1
Multiple coil elements seeing same part of FoV, gΒ«1
What equipment failures could occur?
External RF from breakdown in RF cage
Coil connector failures
Calibration failures - error in gradient calibration could impact gross scaling
Why is MRI not widely used in RT?
Image intensity is qualitative as opposed to HU
Achievable resolution can be larger than CT in body
Geometric accuracy is variable (depending on manufacture, patient, imaging sequence)
How does magnet design impact uniformity
Relative length to diameter important, B1 more uniform when length:diameter ratio is high
Ideal is long thin bore
Competing demands: short bore/flared openings give better patient experience/compliance but worse homogeneity
How does inhomogeneous signal create distortion
B0 varies with r, frequency is no longer linearly related to location, get geometric distortion
High B0 homogeneity is a requirement
How is homogeneity quoted?
ppm over a diameter spherical volume (DSV)
Often quoted as VRMS volume root mean square
Typical and guaranteed values
How does B0 magnitude affect homogeneity?
Higher field - > worse homogeneity
How does gradient uniformity impact homogeneity?
G term varies with r
Frequency no longer linearly related to location, geometric distortion
Need high gradient linearity, which gets worse towards end of bore
What creates eddy currents?
Lenzβs law: change in magnetic flux induces EMF, creates current whose magnetic field opposes the change
Switching gradient induces current flow in surrounding structures: magnet or RF coil structure
This alters the gradient waveform and distorts the image
Proportional to dB/dt and decays away exponentially
When do eddy currents alter gradient waveform?
During and after the gradient pulse
How can we minimise the effect of eddy currents?
Preventative method:
Design a coil system with 2 layers: primary and secondary
Primary creates the desired gradient field
Secondary designed to cancel distant field
Compensatory method:
Pre-emphasis
What is typical gradient shielding?
External G field reduced to <1% at scanner bore
Eddy currents reduced to ~1%
Canβt remove eddy currents through coil design only: use eddy coil correction
What is pre emphasis?
Used to null out effect of eddy current
Measure eddy current response and drive the gradient with the precise opposite function to null out effect
How do RF coils work?
Detect signal from patient via magnetic induction
Localise coils minimise r
Array coils have large number of small loops, which have smaller sensitive volume
How does sensitivity of coil change with r?
Falls with 1/r^2 with distance from element
SNR lower at greater depth, need to correct with software
What are implications for RT of signal detection from coils?
Usually want to strap coils to patient to maximise sensitivity but this can physically distort soft tissue and affect body contour
Need dedicated RT system to mount coil on bridge away from patient - no distortion but lower SNR
What is relationship between B, magnetic field strength H and susceptibility constants?
π΅=π_π (1+π_π )π»
Different tissues have different B due to having range of magnetic susceptibility constants
Field gradients between adjacent tissues, so get inhomogeneities in B even in uniform magnet
What causes dielectric effects?
Tissue has high dielectric constant which reduces RF propagation speed and wavelength
Wavelength in tissue at 3T is about 30cm, comparable to size of some patients
Get standing wave effects, and can get image intensity variations, signal voids where the B1 field is at a node on the standing wave
When are you more likely to see dielectric effects?
Higher field:
30cm at 3T
60cm at 1.5T
Unlikely to see effects at 1.5T
What are the basic approaches to counteracting scan imperfections?
Avoiding/minimising distortion in first place
Correcting distortion in post processing
Selection of scan parameters is key factor in setting SNR and distortion
How does rBW affect scan time and SNR?
Low rBW, slow sampling
Longer minimum TE,TR, long sample time
Less noise sampled, higher SNR
High rBW, rapid sampling, shorter TE and TR, but more noise sampled and lower SNR
How does rBW affect distortion?
Ratio of local field variation Ξπ΅_0 (π) to pixel bandwidth Ξπ/π
means distortion in pixels
Use highest possible rBW to limit distortion
How are RF coil profiles corrected for?
Collect reference scan which reveals spatial variation in sensitvity
Inverse of reference scan applied to intensity to flatten profile
What did the FLAME trial look at?
Contouring intraprostatic lesions using DW MR and DCE MR
Give isotoxic boost to lesion
Improved survival
What did PET Plan trial look at?
Using FDG PET to contour CTV
PET only vs PET plus CT
Prescription is then isotoxic
Effectively extreme dose painting: only treating where functional imaging identifies
What is a major cause of radiation resistance in tumours?
Hypoxia
How can oxygen supply and demand be imaged and what could this information be used for?
Measure tumour cell density (oxygen consumption) and tumour perfusion (oxygen supply) simultaneously
Hypoxic regions have high cell density and low perfusion
Could be used to personalise dose prescription or dose paint
How has oxygen supply and demand been investigated?
Cervix patients were imaged with DW MR and DCE MR and the hypoxic fraction was determined
Disease free survival better in group with lower HF
There is no clear region to target with dose painting but clear prediction of treatment outcome, could be used to personalise dose prescription
What artefacts can we get related to sequences as opposed to patients?
Chemical shift artefacts
Phase wrap artefacts
Parallel imaging arte1facts
Equipment failure
What patient related effects can we get in MR imaging?
Magnetic susceptibility
Dielectric effects
