Diffusion weighted image
Diffusion weighted imaging (DWI) is currently indispensable in radiology. Diffusion means the random movement of molecules in a substance; the Brownian motion. Diffusion weighted imaging is a very fast technique where the diffusion behavior of hydrogen molecules is determined under different field strengths. The diffusion images obtained are T2 weighted images.
The degree of proton motion depends among other things on (fig. 22):
- Cellularity of the tissue; many vs few cells (in cell-rich tissue there is relatively lower diffusion)
- Integrity of the cellular membrane. In an infarction, the ion pump of the cell membrane will break down and ions & water will stay in the cell (= cytotoxic edema). This will increase intracellular pressure, leading to reduced intracellular diffusion.
- Blockage of fluid; large vs small molecules. Tissues with large molecules have relatively lower diffusion.
Figure 22. Degree of diffusion (protons + arrows) in various situations.
When protons can move freely and therefore diffuse away, signal loss will occur in DWI. This can be seen e.g. in CSF. Background information: in order to obtain a signal, the proton must receive two pulses. If the proton does not receive the second pulse (because the moving proton is now in a different position), signal loss will occur.
In reduced diffusion (= diffusion restriction), there is limited movement of protons, shown as a high signal intensity on DWI. This can been seen in disorders including cytotoxic edema and inflammation. Importantly, DWI is a strong T2 weighted image.
As a reminder: tissues with a high water content have high signal intensity on T2 weighted images. To be sure that tissue diffusion has been reduced, we need to filter the T2 effect out. To this end a quantitative calculation of diffusion is made; the so-called ADC map (apparent diffusion coefficient). The ADC map filters out the T2 effect and produces inverse images. Diffusion is reduced when the tissue has high signal intensity on DWI and low signal intensity on ADC (fig. 23).
When both DWI and ADC have high signal intensity, we have a T2 effect without diffusion component. Better known as the T2 shine-through. An example is (reactive) vasogenic edema. In vasogenic edema there is more free moving water in the extracellular space. This may develop in response to a tumor.
Figure 23. Signal intensity of DWI and ADC in diffusion restriction, increased diffusion and T2 shine-through.
Remember: when evaluating diffusion, also look at the ADC. We do not use the term diffusion restriction until the tissue has high signal intensity on DWI and low signal intensity on ADC.
Click to see overlay
Figure 24. Diffusion restriction secondary to cytotoxic edema in an infarction in the left hemisphere (middle cerebral artery territory). The DWI has high signal intensity and ADC low signal intensity. Note also the (physiologically) increased diffusion of the CSF.
In addition to the above pathology, diffusion restriction may also occur in cell-rich tumors (including epidermoid and lymphoma). It is a good tool to distinguish acute ischemia (= abnormal diffusion) vs chronic ischemia and pus in an abscess (= abnormal diffusion) vs necrosis in a tumor.
Diffusion restriction does not mean that we are always dealing with pathology. For instance, the myelum, testicles/stroma of the ovaries, spleen/lymphatic nodes and red bone marrow will all show diffusion restriction. The reason for reduced movement in these tissues is not entirely clear and may be associated with high cellularity.
In recent years there has been extensive research of new applications to detect/characterize pathology using diffusion weighted imaging (e.g. in prostate carcinoma). It may also be an additional tool to evaluate the effect of therapy on tumors; reduced tumor cellularity after treatment may lead to reduced diffusion restriction.
