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Constraining the distribution of gas and dust in the inner 20 au ofprotoplanetary disks is difficult. At the same time, this region is thought tobe responsible for most planet formation, especially around the water ice lineat 3-10 au. Under the assumption that the gas is in a Keplerian disk, we usethe exquisite sensitivity of the Molecules with ALMA at Planet-forming Scales(MAPS) ALMA large program to construct radial surface brightness profiles witha ~3 au effective resolution for the CO isotopologue J=2-1 lines using the linevelocity profile. IM Lup reveals a central depression in 13CO and C18O that isascribed to a pileup of ~500 $M_\oplus$ of dust in the inner 20 au, leading toa gas-to-dust ratio of around <10. This pileup is consistent with efficientdrift of grains ($\gtrsim$ 100 $M_\oplus$ Myr$^{-1}$) and a local gas-to-dustratio that suggests that the streaming instability could be active. The COisotopologue emission in the GM Aur disk is consistent with a small (~15 au),strongly depleted gas cavity within the ~40 au dust cavity. The radial surfacebrightness profiles for both the AS 209 and HD 163296 disks show a localminimum and maximum in the C18O emission at the location of a known dust ring(~14 au) and gap (~10 au), respectively. This indicates that the dust ring hasa low gas-to-dust ratio ($>$ 10) and that the dust gap is gas-rich enough tohave optically thick C18O.

Molecules with ALMA at Planet-forming Scales (MAPS). XV. Tracing protoplanetary disk structure within 20 au