PA Imaging with mvpCu. 4T1 cells were trypsinized, neutralized with growth media, and counted on the Countess II FL Cell Counter (Invitrogen, Thermo Fisher Scientific). Cells were spun down, media was aspirated, and a solution was made in 1:1 serum-free RPMI media to Gel-trex™ LDEV-Free, hESC-Qualified, Reduced Growth Factor Basement Membrane Matrix. Mice were injected in the mammary fat pad with 500,000 cells (50 µL) of the cell solution. Tumors grew for around 1.5-2 weeks and the mice were imaged once tumors reached 100 mm 3 .