Diffusion weighted imaging measures the motion of spins (specifically in water). The signal is dependent on the diffusion coefficient within the material i.e. how freely the water can diffuse. The more a particle can move in a given amount of time, the higher the diffusion coefficient.
Water diffuses randomly via Brownian motion. In pure water and gel, water can diffuse freely with no impediment or restriction. within soft tissues, water diffusion is impeded by cell membranes and intracellular organelles.
Sequence
A spin-echo sequence is typically used, specifically echo-planar imaging (EPI). EPI minimises the effect of patient motion as it is a very quick sequence. This is important as DWI images the very small motion of water molecules which will be masked by any macroscopic body motion.
Two diffusion gradients are added either side of the 180º RF pulse. The first diffusion gradient dephases the spins. The second diffusion gradient rephases and returns a signal only from the spins that have remained within the area i.e. those that are stationary. Any spins that have moved out of the area aren’t rephased and do not return a signal.
The diffusion gradient is applied in multiple directions. The minimum number of directions is 3 run perpendicular to each other (e.g. x-, y-, and z-axes) but, usually, 6-20 directions are used. Each voxel’s signal is is an average of the signal from all directions.
Then, a standard sequence is run to generate echoes and create the signal.
b-value
The degree of diffusion weighting is represented as the b-value. The more sensitive the DWI sequence is to molecular motion, the higher the b-value.
Higher b-value:
- More sensitive to diffusion
- More noise
- Less signal
Increase the b-value by:
- Larger diffusion gradient (increase the amplitude or the duration)
- Increased time between dephasing and rephasing diffusion gradients
b0 – A DW pulse sequence is first run with the diffusion gradients switched off. This creates a T2*-weighted image that is used for the calculated maps later.
b600-700 – Useful in neonatal brain imaging and body MRI.
b1000 – Strong diffusion weighting. Used to look for cerebral infarcts.
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Apparent diffusion coefficient
As DWI images have T2 weighting. Therefore, a lesion that shows as bright on DWI may be bright because of restricted diffusion or because of inherent high T2 signal. The apparent diffusion coefficient (ADC) map is a calculated image that removes the effects of inherent T2 signal.
The signal of a tissue decreases exponentially with increasing b-values. If we plot the log of the signal against the b-value, the slope will give us the diffusion characteristics without any T2 signal influence i.e. the ADC signal. Tissues with free diffusion will change signal over different b-values much more than those with restricted diffusion. More diffusion = greater change in signal = a steeper slope = a higher ADC value. This is why restricting lesions will appear dark on the ADC map.
Diffusion tensor imaging
If the probability of diffusion is the same in every direction, this is called isotropic diffusion e.g. in CSF. Anisotropic diffusion is when diffusion is not equal in every direction e.g. along nerve bundles and white matter tracts. In standard DWI we remove this effect by averaging out the signal obtained from multiple directions. However, we can use this asymmetry in diffusion tensor imaging. The three main techniques are the fractional anisotropy map, the principal diffusion direction map and fibre-tracking maps.
Fractional anisotropy map
Fractional anisotropy (FA) is a measure, from 0 to 1, of the amount of diffusion asymmetry within a voxel. A sphere, which is isotropic, has an FA of 0. The more asymmetric the diffusion becomes the closer it is to 1. The FA map is gray-scale. The brighter the voxel, the more anisotropic the diffusion.
Principal diffusion direction map
Colours and brightness are assigned to the voxels based on the degree of anisotropy (represented as brightness) and the direction (represented as colours).
Fibre tracking map
The direction of the asymmetry is used to compute fibre trajectories with automated software. A “seed voxel” is selected by the user and the software follows the direction of the adjacent voxels to create an image of the tracts.
Artifacts
T2 shine-through
As the DWI sequence has T2/T2* weighting, high signal on DWI could either be due to restricted diffusion or intrinsic high T2 signal. The ADC map removes the effect of T2 signal. Any region that has low signal on ADC is truly restricting.
T2 dark-through
Just as a lesion with high intrinsic T2 signal will cause T2 shine-through, a lesion with low intrinsic T2 signal will cause low signal on the DWI, called T2-dark through.
Metal artifact
Because of the T2* weighting of DWI the sequence is very susceptible to anything that disrupts the local magnetic field such as metal or blood products. The region of signal loss around metal can be very large. In the presence of haemorrhage, the signal on DWI is less predictable.
Σ Summary
- DWI sequence
- Spin echo, usually echo-planar imaging (EPI). Fast so body motion minimised
- Diffusion gradients either side of 180° pulse
- Stationary spins (i.e. restricted diffusion) return high signal
- Mobile spins (i.e. free diffusion) do not return signal
- Gradients applied in at least 3 different directions
- Signal in voxel averaged from each direction
- b-value
- Higher b-value:
- More sensitive to diffusion
- More noise
- Less signal
- Increase b-value by:
- Larger diffusion gradient (increase amplitude or duration)
- Increase time between dephasing and rephasing diffusion gradients
- b0 – sequence run without diffusion gradients. T2*/T2 weighted
- b600-700 – used in neonatal and body imaging
- b1000 – used for cerebral infarcts
- Higher b-value:
- Apparent diffusion coefficient (ADC)
- Log of DWI signals at different b-values plotted. Slope gives ADC signal
- Removes effect of intrinsic T2 signal
- Diffusion tensor imaging
- Isotropic diffusion = diffusion same in every direction
- Anisotropic diffusion = asymmetric diffusion
- Fractional anisotropy map
- Measure of asymmetry
- 0 = isotropic, 1 = extremely anisotropic
- Grey-scale image
- Principal diffusion direction map
- Measures anisotropy and direction
- Degree of anisotropy = brightness
- Direction = colour
- Fibre tracking map
- Automatic generation of fibre tracks by software
- Artifacts
- T2 shine-through – intrinsic high T2 signal shows as bright on DWI. ADC removes this effect
- T2 dark-through – intrinsic low T2/T2* signal shows as low signal on DWI
- Metal artifact – DWI very susceptible to artifact created by metal and blood products
