20-Year Prospect
As long as people have bodies, they're going to need sonographers. Even when robots eventually take over the medical field, we're still going to need technicians who know how to operate the machines (and keep them from destroying us all).
The job market for ultrasound technicians is expected to grow thirty-nine percent from 2012-2022, which is super fast—faster than most job markets (source).
Previously, sonography was mostly happening in hospitals; now that smaller clinics have woken up and realized how beneficial it is to have their own sonography department, the job market has exploded. Twenty years from now, you'll still be gainfully employed and making good money.