Contrast staining of the brain parenchyma is encountered in a variety of situations where there has been disruption of the normal blood-brain barrier resulting in the passage of contrast out of the microcirculation and its accumulation in the extracellular space. It can be difficult to distinguish from hemorrhagic transformation.
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Terminology
A variety of terms have been used to denote this phenomenon including contrast extravasation, metallic hyperdensity, and contrast enhancement 4. These are not well-defined.
When the distinction between contrast staining and hemorrhagic transformation is not possible then a more generic term cerebral intraparenchymal hyperattenuation or postinterventional cerebral hyperdensity has been suggested.
Epidemiology
Contrast staining primarily in the setting of ischemic stroke, most frequently post endovascular clot retrieval but also following intravenous thrombolysis 1-7.
Pathology
As a result of increased permeability of the blood-brain barrier iodine-containing contrast can leave the microcirculation and accumulate in the extracellular space 5.
Importantly if the basal lamina is also disrupted then red blood cells, admixed with contrast, can also escape the circulation leading to haemorrhogic transformation ranging from petechial hemorrhagic infarction of frank hematoma formation 4.
Generally, contrast staining is seen more frequently in the anterior circulation, perhaps due to reduced sensitivity of autoregulation of the posterior circulation 7.
Radiographic features
CT
On CT the affected area will appear hyperdense and often conform to the affected region anatomically, often primarily affecting grey matter, with swelling.
A wide variety of definitions for contrast staining have been used in the literature, many of them at least somewhat contradictory 4. This highlights the difficulty in distinguishing between contrast, contrast mixed with blood and blood only.
In the setting of post-endovascular clot retrieval, contrast staining tends to be less hyperdense (<50 HU) than hemorrhage 3. On the other hand if the density of brain parenchyma is significantly higher than that of blood clot (>90 HU) then the presence of contrast can be inferred 2.
Generally contrast staining will resolve rapidly and no longer be visible within 24 to 48 hours 4,6. Thus, followup imaging is advocated as the best way of distinguishing between constrat staining and hemorrhage 6.
Dual-energy CT
Dual-energy CT is particularly important in distinguishing hemorrhagic transformation from contrast staining as iodine overlay maps will show the presence of contrast and, most importantly, virtual non-contrast images will show an absence of the region of hyperattenuation, indicating that all the visible hyperdensity is due to contrast rather than blood.
MRI
MRI done within the first 72 hours of thrombectomy or thrombolysis should be interpreted with caution as residual intraparenchymal contrast staining may be not visible or be misinterpreted as hemorrhagic transformation 2.
Treatment and prognosis
Contrast staining will typically resolve over a few days, significantly faster than blood 4.
Differential diagnosis
The primary differential diagnosis is that of intraparenchymal hemorrhage. Not only can distinguishing between the two be challenging acutely, they can often co-exist (e.g. hemorrhage following endovascular clot retrieval) 4.
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