Diffusion Imaging Flashcards
What is diffusion?
Diffusion is the random Brownian motion of molecules due to thermal processes
What is diffusion imaging and how does the signal link to diffusion?
An MRI technique that measures the mobility of molecules
It compares x, y, z diffusion gradients to a baseline T2 weighted image to look for signal loss
If the image shows mild signal loss compared to baseline, the machine assumes normal diffusion and assigns a grey voxel to the area
If all 3 gradients show no or minimal signal loss compared to baseline, then this is consistent with diffusion restriction and the machine assigns a white voxel to that area
(if the protons do not move throughout the pulse sequence i.e., diffusion restriction, the protons will be back in phase following the second gradient pulse giving off strong signal, whereas, if the protons are moving freely they will not all be in phase and therefore produce no signal)
What is the principle behind DWI?
In living tissue, molecules of water move freely through to various tissues in the body
However, in certain pathologic conditions such as the tightly packed cells of a tumour or the local swelling and pressure produced by the lack of blood flow during a stroke in the brain, creates an environment which restricts the movement of these molecules
DWI measures this restriction of diffusion
What is the commonly applied method for producing diffusion-weighted contrast?
Pulsed gradient spin echo
Explain the DWI sequence
Consists of a 90 degree pulse which flips the magnetic field perpendicular to the main field
Diffusion gradient turned on causing the protons to precess at different frequencies and out of phase
Another 180 degree RF pulse
Equal and opposite diffusion gradient pushing the hydrogen atoms back into the 90 degree perpendicular plane and back in phase giving maximum signal
Depending on whether the atom is stationary or freely moving it will change the output signal
Freely moving atoms cause more signal loss they also cause the protons to be out of phase meaning there is less signal and appear dark. Restricted protons precess in phase giving off the strongest signal so appear white
What contributes to the signal?
DWI uses the same sequence as T2 but with the addition of diffusion gradients
The fundamental idea behind diffusion-weighted imaging is the attenuation of T2* signal based on how easily water molecules are able to diffuse in that region. The more easily water can diffuse (i.e. the further a water molecule can move around during the sequence) the less initial T2* signal will remain
How does DW contrast behave?
Like inverse T2 weighting
Watery tissues that have very mobile molecules give lower signal intensity whilst more static and solid tissues give a stronger signal
Why is DWI a T2 weighted image?
Due to the 2 diffusion gradients and the length of time to dephase and rephase the protons, a long TE is needed to allow for this before the signal is recorded
Any image with a long TE is considered a T2 weighted image
What are ADC maps?
To differentiate between artefact and true pathology, we use the apparent diffusion coefficient, they are independent of T2 effects and show only the effects of diffusion
Used to confirm diffusion restriction from DWI images
What do ADC maps represent?
In an ADC map, the computer analyses the diffusion in each of the x y z planes and assigns a grey scale value reflecting the apparent diffusion in each voxel of tissue
Areas of free diffusion show as bright white, whereas areas of diffusion restriction show up as darker
What does the signal attenuation depend on?
In many human tissues, the amount of signal attenuation due to diffusion varies depending on the direction of the applied diffusion gradient (x, y, z)
What are quantitative diffusion images?
As we also acquire a baseline image S(0) with no diffusion gradients, we can calculate the diffusion coefficient (D) in each gradient
These are quantitative diffusion images where each voxel represents a diffusion value and the different images represent diffusion measurements in different directions
We can average the maps across all of the directions to produce a map of the mean diffusivity
What is diffusion restricted by in human tissue?
By the complex microstructural environment
What is the reason for diffusion tensor imaging?
DWI can only tell us whether there is or isn’t any diffusion restriction
It doesn’t give us any information about the direction or magnitude of the molecular movement
What is a diffusion tensor?
In human tissue, diffusion is restricted by the complex microstructural environment, to characterise this environment, the diffusion process must be measured in many directions and is often described mathematically by a diffusion tensor
The diffusion tensor defines an ellipsoid in each voxel that characterises the diffusion properties
Represents the matrix of the xyz coordinates in any given voxel
How are different types of diffusion represented?
Isotropic diffusion = spherical
Anisotropic diffusion = ellipsoid
Briefly describe the mathematics of diffusion tensor imaging
Voxels can be described in the x y and z planes by vectors which each have its own coordinate system in 3D space
This creates a 3x3 matrix of vectors (Txx, Txy, Txz/ Tyx, Tyy, Tyz/ Tzx, Tzy, Tzz) = this represents the diffusion tensor
Now the rows describe the x y and z direction - Txx, Tyy, Tzz represent the axis perpendicular to the plane and therefore, diffusion measurements are made in 6 directions
This matrix can be multiplied to create an overall vector by multiplying the matrix by a vector
The new vector is compared to the first vector to find a scalar value (is the new vector a multiple of the first vector)
If there is a scalar value then the vector is called an eigenvector and the scalar multiple is an eigenvalue
What do the eigenvalues and eigenvectors represent?
The eigenvalues and eigenvectors of the tensor indicate the principal diffusion magnitudes and orientations
