Bugging Diverticulum/Achalasia Barium Swallow Question

[cman06]

So I know this is probably a dumb question, but for some reason I just can't reason this out right now and I've searched to no avail. But, with respect to FA, why is the barium in Zenker Diverticulum black whereas the barium in Achalasia imaging is white the expected white? Aren't they both x-ray fluoroscopy with regular barium solutions? What modality causes more dense material to show black????

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[Lavan]

Fluoroscopy images (real-time x-rays) are displayed inverted (contrast black). But the spot films are normal x-rays (contrast white). Spot films taken during fluoro exams use more radiation, are a still one-frame shot, and look better with crisper anatomical detail. It has nothing to do with the pathology, just whatever images they've chosen to show you.