This marked dephasing due to concentrated deoxy-Hb spills over into diffusion-weighted images, producing the so-called "T2-blackout effect." In reality, diffusion remains restricted in the center of acute hematomas, just as it does in hyperacute ones. However, the expected high signal on DW images representing restricted diffusion is masked by susceptibility-induced signal loss.
Although T2 and T2* are significantly shortened by the acute hematoma, T1 values are relatively unaffected. This is because the conformation of the globin proteins do not allow water molecules close access to the paramagnetic centers of deoxy-Hb. Outer sphere relaxation mechanisms predominate, resulting in strong effects on T2 with little or no change in T1 values. Thus, as with hyperacute hemorrhages, T1-weighted images remain isointense to brain.
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Ambient tissue oxygen levels affect the evolution of hemorrhage. Either too high or too low tissue oxygen will result in delayed evolution between oxy-Hb and deoxy-Hb.
Some hematomas in contact with healthy capillary networks remain exposed to relatively high levels of oxygen (at least peripherally). Examples include hematomas adjacent to vascular malformations, intramuscular hematomas, and subarachnoid hemorrhage. In these cases high ambient O2 keeps more blood in the oxyhemoglobin state with slower transition to deoxyhemoglobin. Methemoglobin levels are also slow to rise, as oxygen powers the methemoglobin reductase systems driving equilibrium back to deoxyhemoglobin.
Paradoxically, too low levels of ambient oxygen may also result in delayed hematoma evolution. Examples include intratumoral hemorrhage and hemorrhagic infarctions. Here blood may stay in the deoxyhemoglobin state for a longer period of time, because ambient O2 or peroxide is needed as an electron receptor to oxidize heme iron from its ferrous (Fe+2) state in deoxyhemoglobin to the ferric (Fe+3) state in methemoglobin.
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What are the different forms of hemoglobin and why do they have different magnetic properties?
I know what T2-shine-through is, but what is T2-blackout?