This was difficult.

Finding: bilateral hypodense caudate nuclei (several folks put unilateral, but I think they are fairly dark bilaterally)

Cause: this was by far the most common - bilateral hypoxia/ischemia (which turned out to be global. We had to inquire about the actual history which was “found down with resuscitation”.

Someone actually brought up Carbon Monoxide poisoning, which is a great answer. I believe it classically tends to affect the globus pallidus, but it can affect the caudates as well. So points to you as well.

Nice article on ischemia: http://jnnp.bmj.com/content/70/suppl_1/i7.full.pdf