A fluorescein angiogram involves having dye injected into the vein
with photographs taken through the dilated pupil of the retinal blood
vessels.  Leakage in the retinal blood vessels can be seen in the wet
type of macular degeneration or in diabetic retinopathy.  The vascular
tree felt to be compromised (possibly) in low tension glaucoma (same
as "NTG") is the microscopic short ciliary vessels behind the eye that
run along the optic nerve as it enters the back of the eye.  These
can't be seen by looking through the pupil and they're too tiny to see
with x-ray angiography (with radioactive dye).  That's why this test
is of no use in glaucoma.  Hope that helps.
Rick Cohn, M.D. (notice there's no "e" in that name)
Glaucoma Specialist
Winter Park, FL