Here are the two images again.
Abnormal image (81 year old man who sustained a cardiac arrest):
Normal comparison image:
The surface of the brain consists of folds of grey matter at the surface with white matter lying deep to that layer. Slight differences in tissue density show up on the CT images as a slight difference in their relative brightness, with the outer layer of grey matter appearing slightly brighter on the images and the deeper layer of white matter appearing slightly darker.
In a normal brain, you should be able to make out an undulating boundary between the two layers, as shown here:
Here’s a stock photograph of a similar brain slice from a pathology specimen showing the grey and white matter in “real life”:
The abnormal image from the ER patient shows loss of the normal grey-white junction. That’s the critical visual finding. This can happen when there’s internal swelling of the brain tissues.
When the ER patient suffered his cardiac arrest at home, he lost the normal blood flow to the brain for a few minutes too long. This caused his entire brain to subsequently swell within the confined space of the skull, obliterating the normal grey-white junction.
In medical terms, this is called “diffuse cerebral edema” (as Tod correctly surmised), in this case due to “anoxic brain injury”.
Unfortunately, these patients tend to do very poorly, even if their heart function is subsequently restored.
Here’s more information from a neuroradiology textbook (see Figure I on the lower right of the page).