Advanced MRI for Radiotherapy Flashcards
What can cause geometric distortions in MRI?
Scanner limitations: gradient non-linearity - rely on perfectly linear magnetic field
Patient body geometry & composition: susceptibility - distorting magnetic field
What are the advantages of MRI?
Soft tissue contrast = aids contouring
Non-ionising = better for at risk patients (children)
Describe diffusion weighted MRI.
Contrast in images is sensitive to tissue microstructure: high cell density restricts water diffusion.
Individual molecules in solution (or gas) move in a random fashion due to their thermal energy and collisions with other molecules in the solution (“Brownian motion”). A group of molecules is free to diffuse in any direction.
MRI measures the distance the water molecules move in a given time.
Pulsed Gradient Spin Echo sequence used to acquire image. The spins are excited, refocused, then the signal is acquired. Diffusion gradients sensitise signal to properties of water.
ADC maps: gauge effect of therapy – changing cellularity, necrosis.
What is the unrestricted average distance that any molecule will move in time?
= sqrt(6 D t)
where D is the diffusion coefficient.
Describe the imaging sequence used for diffusion weighted imaging.
Pulsed Gradient Spin Echo:
Pair of diffusion-weighting gradient pulses
Positioned either side of the 180 pulse
Equal amplitude
Equal duration
Applied in same direction (but can be any direction)
Provide “diffusion weighting”
What is the ‘b value’ in MRI?
b-value: Describes degree of diffusion weighting applied to a scan.
b = gamma^2 * G^2 * sigma^2 * (Delta – sigma/3)
where:
G – Gradient strength
sigma – Gradient duration
Delta – Gradient separation
Signal intensity decreases as b value increases:
S / S0 = exp(-b * D)
where D = Apparent diffusion coefficient of water (it is not the true diffusion coefficient as diffusion is restricted)
What does the ‘b value’ in MRI depend upon?
G – Gradient strength
sigma – Gradient duration
Delta – Gradient separation
What can be deduced when plotting the log of signal against b value in DWI in MRI?
Measure Apparent DiffusionCoefficient (ADC) from gradient
This is proportional to how quickly the water is diffusing.
Why is the apparent diffusion coefficient used in MRI instead of the diffusion coefficient?
Water in tissue cannot move in any direction; it is restricted diffusion by cellular structure (barriers etc.). Thus water molecules don’t go as far as expect for temperature. Hence called apparent diffusion coefficient – as it is in diffusion restricted environment..
ADC is determined by the tissue microstructure.
What is the equation for the restricted range of motion for a molecule in DWI?
< sqrt(6 D t)
where D is the diffusion coefficient.
What does DWIBS stand for in MRI, and what is it?
Diffusion-Weighted Whole-Body Imaging
Highlights tissue with restricted diffusion such as tumours with high cell density
Use for detection of metastases
“PET-like” appearance (highlight tissue with diffusion restriction (high cell density) – flip signal to look like PET)
Free-breathing acquisition
What is ADC in MRI dependent upon?
The direction of measurement.
Anisotropic tissue structure means anisotropic water diffusion (diffusion is more hindered in some directions).
What is diffusion tensor imaging used for in MRI?
Assess connectivity in the brain = useful for defining tumour boundaries.
Data suggests tumours spread along tracts, so margins can account for this.
(White matter tracts – show connectivity of brain
Useful for neurosurgery/beam orientation
Colour shows directional dependence.)
What contrast agent is used for MRI?
Gadolinium
What is the contrast agent delivered to the tissue proportional to in MRI?
Perfusion rate. (no blood flow = no contrast agent arrival)
