Optimal focus evaluated using Monte Carlo simulation in non-invasive neuroimaging in the second near-infrared window

Adjusting the focal plane through the intact scalp of mice is crucial in novel angiography of cerebral vasculature using quantum dots emitting second near-infrared light at a wavelength of 1100 nm. Reagents were administered through the caudal vein. When we focused 0.4 mm below the scalp surface, based on the anatomical properties of mice reported previously, the intensity of clear fluorescence images observed transiently under a microscope became very weak within several seconds. The remaining time was extremely short to repeat adjustment of the focal plane. To investigate focus, photons exciting quantum dots at depths of 0.4, 0.8, 1.4, and 2.0 mm and emission photons were tracked in a four-layered Monte Carlo model including the scalp, skull, cerebrospinal fluid, and cortex. Based on the most near-ballistic photons emitted from quantum dots at 0.4 mm depth and specification of the microscope used, including numerical aperture and depth of field, the optimal focus plane was set. • Novel angiography for cerebrovascular structures was proposed using quantum dots with second near-infrared fluorescence. • Anatomical properties reported previously allowed focusing 0.4 mm below the surface of intact scalp before observation under fluorescence. • Clear images of cerebrovascular structures were attributed to many near-ballistic photons emitted from quantum dots at 0.4 mm depth.