Brain Diffusion MRI Series — Part 2: DWI brightness and true diffusion restriction
Updated: 3 minutes ago
In Part 1, we learned the basic pairing: when we see a bright area on DWI, we also look at its appearance on the ADC map. Now let us explore, step by step, why this is necessary. Our aim is to understand whether the brightness we see really indicates reduced water movement.
We have found a bright area on DWI what do we do next
Let us start with this question: is water movement actually reduced in this region? Brightness on DWI may suggest this, but it does not provide the answer on its own. This is because the appearance of DWI is influenced by both water movement and the tissue’s T2 properties. Our next step is therefore to find the same region on the ADC map. [1,2]
We can think of ADC as a map that helps us assess water movement. When water movement decreases, the ADC value falls and the region usually looks darker. If an area that is bright on DWI also shows low signal on the ADC map, this supports true diffusion restriction, provided that the images are technically reliable. [1]
The reasoning is this: DWI draws our attention to an area; ADC helps us assess whether that brightness is related to reduced water movement.
Why can DWI appear bright when water movement is not reduced
Some tissues are very bright on T2-weighted images. Some of this brightness may remain visible on DWI. As a result, we may see a bright area on DWI even when water movement is not substantially reduced. This is called T2 shine-through: the contribution of T2 brightness to the DWI appearance. [2]
Let us consider the two situations side by side. In true restriction, DWI brightness is accompanied by low ADC. In T2 shine-through, DWI is bright, but ADC is not reduced; it may be normal or high. Seeing brightness on DWI is therefore not the same as seeing diffusion restriction. [1,2]
Why does water movement sometimes increase and sometimes decrease in edema
Think of brain tissue as cells with small spaces between them. Water is present both inside the cells and in these spaces between them. When edema develops, what matters is where the excess water accumulates.
In vasogenic edema, the blood vessels become more permeable. Fluid leaves the vessels and accumulates between the cells, widening these spaces. This increases the space available for water movement, allowing water to move more freely. ADC therefore usually rises. However, the increased water content makes the tissue bright on T2-weighted images. This brightness may also appear on DWI. In other words, the brightness we see on DWI may reflect the contribution of T2 brightness rather than reduced water movement. We call this T2 shine-through. Thus, ADC is usually high in vasogenic edema. If high ADC is accompanied by brightness on DWI, this appearance may be explained by T2 shine-through rather than true diffusion restriction. [3]
The process is different in the cytotoxic edema that develops in acute infarction. A cell with reduced blood supply cannot produce enough energy or maintain its internal water balance. Water moves into the cell, causing it to swell. As the cells enlarge, the spaces between them narrow. This change makes it harder for water to move through the tissue. As a result, the affected area is bright on DWI and shows low signal on the ADC map in cytotoxic edema. This represents diffusion restriction. [1,4]
In short, in vasogenic edema, fluid accumulates between the cells and the spaces widen; in cytotoxic edema, the cells swell and the spaces narrow. This difference helps us understand why water movement, and therefore ADC, changes in opposite directions. [1,3]
The same lesion may look different over time
The pseudonormalization phase of subacute infarcts: in ischemic infarction, the initially low ADC rises over time and may reach values close to those of normal brain tissue, usually during the second week. Breakdown of cell membranes and tissue structure contributes to freer water movement. This appearance does not mean that the tissue has recovered. [4] However, DWI remains bright because the tissue’s T2 brightness persists. This should not be confused with true acute restriction; if the images are interpreted without checking the ADC map, a subacute infarct may be mistaken for a new acute infarct. [4]
How do we interpret a dark area on the ADC map in hemorrhage
When blood accumulates, its blood products alter the MRI signal. At certain stages of hemorrhage, this effect is so strong that the region appears markedly dark on DWI. This is called the T2 blackout effect. The darkness does not indicate that water is moving freely; it may result from signal suppression by blood products. [5] The ADC map is also calculated using the signals from these images. If there is very little measurable signal to begin with, the calculation may become unreliable. In such an area, we cannot directly interpret a dark appearance on ADC as “reduced water movement.” We must first consider whether the signal used for the calculation is sufficient. [6]
On the other hand, true diffusion restriction may also occur in hemorrhage. [5] For example:
1. Hemorrhagic transformation of an ischemic infarct: if hemorrhage develops in the setting of an acute ischemic stroke, true cytotoxic edema (movement of water into the cells) is already present in the brain parenchyma. In this situation, both true restriction and signal changes related to blood products may contribute to the dark appearance on the ADC map.
2. The region surrounding an acute or subacute intracerebral hematoma: the mixture of cytotoxic and vasogenic edema immediately around the hematoma may contain areas of true diffusion restriction. [7]
3. Purulent material accompanying hemorrhage (e.g., hemorrhage within an abscess or infected tumor necrosis): if the contents of an intracranial lesion are viscous and rich in cellular debris, true restriction may be seen.
A practical rule for hemorrhage
SWI or T2* is essential for assessing susceptibility artifacts: when DWI and ADC appearances are discordant, or suspicious darkness is seen in an area where hemorrhage is suspected, SWI (susceptibility-weighted imaging) or T2*-weighted gradient-echo sequences must be reviewed. If there is marked blooming (an enlarged area of signal loss or darkening) at the lesion site on SWI, the possible effect of blood products on DWI and ADC assessment should be considered. If there is substantial signal loss in the corresponding region, a definitive interpretation based on ADC should be avoided. [5,6]
Does a brighter appearance mean greater restriction
A very bright area on DWI does not necessarily indicate a greater reduction in water movement. The tissue’s T2 properties and the acquisition settings also contribute to this brightness. We therefore cannot rank two foci as showing “more restriction” based on DWI brightness alone. We must also assess the ADC map. Brightness draws our attention; it does not, on its own, measure the degree of restriction. [1]
Let us read the images together
DWI appearance | ADC appearance | How to interpret it |
Bright | Low | True diffusion restriction is supported. |
Bright | Normal or high | True restriction is not demonstrated. T2 shine-through may be considered. |
Marked signal loss | Dark or unreliable | We do not diagnose restriction from ADC darkness alone. We check the other images. |
Compare the same region with the appropriate tissue
After locating the bright focus on DWI, we must look at that exact focus on the ADC map. It is easy to assess adjacent tissue by mistake, especially with small lesions. For example, if we look at the center of a mass on DWI but its edge on ADC, we are comparing two different regions.
When we say “ADC is low,” we should also consider what it is low relative to. For a focus in brain tissue, we compare it, where possible, with the same type of unaffected tissue on the opposite side. Comparing cortex with white matter can be misleading. If both sides are affected, we cannot automatically assume that the opposite side is normal. [1]
How can we describe this in the report
Example wording for true diffusion restriction:
“The focus shows increased DWI signal with a corresponding reduction in ADC, consistent with diffusion restriction.”
Example wording when the other images also support a T2 effect:
“The increased DWI signal is not accompanied by a definite reduction in ADC, suggesting T2 shine-through.”
If image quality does not allow a reliable assessment, we state this limitation. Demonstrating restriction is also not the same as identifying its cause; to establish the diagnosis, we assess the location, the other images, and the clinical information together. [1,8]
Next part
In Part 3, we will discuss image distortions that can mimic real findings and what to consider when assessing a technically limited diffusion study.
Illustration note: the MRI-like image is an AI-generated educational illustration, not an actual patient image. From left to right, the columns show DWI, ADC, and postcontrast T1-weighted images. The shades shown in the illustration cannot be used in place of quantitative ADC measurements.
Abbreviations: DWI, diffusion-weighted imaging; ADC, apparent diffusion coefficient; MRI, magnetic resonance imaging.
🔎New to the series? Start with Part 1: Understanding DWI and ADC.
References
1. Palombo M, Bodini B, Grussu F, et al. ESR Essentials: diffusion-weighted MRI—practice recommendations by the European Society for Magnetic Resonance in Medicine and Biology. Eur Radiol. 2026;36(3):2198–2208. doi:10.1007/s00330-025-12033-x. View source
2. Burdette JH, Elster AD, Ricci PE. Acute cerebral infarction: quantification of spin-density and T2 shine-through phenomena on diffusion-weighted MR images. Radiology. 1999;212(2):333–339. doi:10.1148/radiology.212.2.r99au36333. View source
3. Provenzale JM, Petrella JR, Cruz LCH Jr, Wong JC, Engelter S, Barboriak DP. Quantitative assessment of diffusion abnormalities in posterior reversible encephalopathy syndrome. AJNR Am J Neuroradiol. 2001;22(8):1455–1461. View source
4. Lansberg MG, Thijs VN, O’Brien MW, et al. Evolution of apparent diffusion coefficient, diffusion-weighted, and T2-weighted signal intensity of acute stroke. AJNR Am J Neuroradiol. 2001;22(4):637–644. View source
5. Silvera S, Oppenheim C, Touzé E, et al. Spontaneous intracerebral hematoma on diffusion-weighted images: influence of T2-shine-through and T2-blackout effects. AJNR Am J Neuroradiol. 2005;26(2):236–241. View source
6. Maldjian JA, Listerud J, Moonis G, Siddiqi F. Computing diffusion rates in T2-dark hematomas and areas of low T2 signal. AJNR Am J Neuroradiol. 2001;22(1):112–118. View source
7. Olivot JM, Mlynash M, Kleinman JT, et al. MRI profile of the perihematomal region in acute intracerebral hemorrhage. Stroke. 2010;41(11):2681–2683. doi:10.1161/STROKEAHA.110.590638. View source
8. Le Bihan D, Poupon C, Amadon A, Lethimonnier F. Artifacts and pitfalls in diffusion MRI. J Magn Reson Imaging. 2006;24(3):478–488. doi:10.1002/jmri.20683. View source
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