martedì 8 dicembre 2009
Right cerebellar encephalomalacia
There is increased signal in the right cerebellar hemisphere on T2 weighted images. On FLAIR images, there is decreased signal arising from the right cerebellar hemisphere secondary to fluid attenuation. There is also apparent diffusion restriction in the left cerebellar hemisphere on DWI and ADC map images. In fact the L side is the normal side.
Diagnosis: Right cerebellar encephalomalacia
Encephalomalacia is usually a consequence of aging and/or brain insult, and in this case, the patient had a prior right cerebellar infarct. The brain parenchyma becomes atrophic and becomes replaced by CSF. Encephalomalacia usually does not cause acute symptoms and was likely not the cause of this patient's symptoms.
Diffusion weighted imaging is the most sensitive MR sequence to detect acute stroke. Changes in diffusion weighted images can be seen as early as 30 minutes after insult. At the cellular level, there is random movement and diffusion of water through cellular membranes. In tissues with greater water mobility, there is increased signal loss on diffusion weighted images. In stroke, there is a disruption of cellular membrane ion pumps, which leads to an influx of water into the intracellular space due to osmosis. Intracellular water does not move as freely as extracellular water and this decreased movement leads to increased signal on diffusion weighted images. An increased signal on diffusion weighted images alone is not sufficient to diagnose acute ischemia however, as tumor, infection and trauma can increase intra- and extra-cellular water leading to increased signal on DWI. The apparent diffusion coefficient map diminishes the increased signal associated with increased extracellular water and takes into account only the mobility of water. As the movement of water decreases in ischemic cells, the water diffusion coefficient decreases. This manifests as signal loss on ADC map images. Extracellular fluid on ADC map images will remain bright. Thus, acute ischemia is identified by increased signal on DWI and associated dark signal in the same region on ADC map. The use of DWI and ADC map images is sensitive (88-100%) and specific (86-100%) for acute stroke.
In this patient, there was a sequence of DWI images that demonstrated increased signal in the left cerebellar hemisphere. However, on ADC map images, there was no associated loss of signal. The adjacent right cerebellar encephalomalacia (and bright signal due to the high extracellular water e.g. CSF), gave the illusion of signal loss in the left cerebellar hemisphere. On T2 and FLAIR images, acute ischemia shows up as increased signal. In this case, there was no increased T2 signal throughout the brain parenchyma. The encephalomalacia in the right cerebellar hemisphere shows up bright on T2 weighted images due to CSF replacement of brain parenchyma.