Is a computational scientist/engineer a viable professional goal?
Computational science is more of a mode of work than an actual career path—using modeling and mathematical techniques to solve problems in engineering, medicine, finance, and other disciplines. Almost all computational scientists have an area of emphasis within that field (as you mentioned, computational imaging engineer or computational biologist). You can move within areas relatively closely related, but making larger leaps may be more difficult: a computational fluid mechanics expert cannot quickly be substituted for someone working in bioinformatics, and vice versa.