The detection of narrow spots in a head angiogram may be due to all of the following reasons, except:

The detection of narrow spots in a head angiogram often reflects specific pathophysiological changes within the blood vessels. One of the primary causes of narrowed areas, or stenosis, is the presence of fatty deposits, known as atherosclerosis, which can restrict blood flow. Calcium deposits also play a significant role in this process, as they can lead to hardening of the arteries, contributing to vascular narrowing.

Clots, or thrombi, can form in blood vessels and might temporarily narrow or occlude the lumen, which would be visible in an angiogram. However, watery blood typically does not cause narrowing; instead, it may allow for more effective blood flow-through vessels. While variations in blood viscosity can affect circulation, they do not produce localized narrow spots on an angiogram.

Thus, the presence of watery blood is not a contributing factor to the visibility of narrowed areas in the imaging of head angiograms, distinguishing it from the other conditions that lead to visible lesions or narrowings.