Leakage is caused by neovascularization, and since peripheral blood flow is low in such areas, LSFG shows low values.
On the other hand, when leakage is detected by FA (fluorescein angiography) or similar methods, it appears as fluorescent spots; however, since blood flow is low in those areas, LSFG depicts them as low values.
If the area surrounding the leakage site also has low blood flow, the leakage site will only show a distribution of low values on LSFG, making it impossible to confirm the presence of leakage.
Dr. Watanabe of Gunma University has published well-organized papers on this topic, which we introduce as references for your review.
Watanabe G, Fujii H, Kishi S: Imaging of Choroidal Hemodynamics in Eyes with Polypoidal Choroidal Vasculopathy Using Laser Speckle Phenomenon Jpn J Ophthalmol 52: 204-210, 2008