The gas and dust are spatially segregated in protoplanetary disks due to the vertical settling and radial drift of large grains .
A fuller accounting of the mass content and distribution in disks therefore requires spectral line observations .
We extend the modeling approach presented in Williams & Best ( 26 ) to show that gas surface density profiles can be measured from high fidelity ^ { 13 } CO integrated intensity images .
We demonstrate the methodology by fitting ALMA observations of the HD 163296 disk to determine a gas mass , M _ { gas } = 0.048 M _ { \odot } , and accretion disk characteristic size R _ { c } = 213 au and gradient \gamma = 0.39 .
The same parameters match the C ^ { 18 } O 2–1 image and indicates an abundance ratio [ ^ { 13 } CO ] / [ C ^ { 18 } O ] of 700 independent of radius .
To test how well this methodology can be applied to future line surveys of smaller , lower mass T Tauri disks , we create a large ^ { 13 } CO 2–1 image library and fit simulated data .
For disks with gas masses 3 - 10 M _ { Jup } at 150 pc , ALMA observations with a resolution of 0 \farcs 2 - 0 \farcs 3 and integration times of \sim 20 minutes allow reliable estimates of R _ { c } to within about 10 au and \gamma to within about 0.2 .
Economic gas imaging surveys are therefore feasible and offer the opportunity to open up a new dimension for studying disk structure and its evolution toward planet formation .