If you want to know whether the certification is necessary to work as sonographer or say as an ultrasound technician then you are at right place and the answer for your question is, the certification and licensing is not legally requisite but it can improve the chances of getting a job as an ultrasound technician. There are many certification exams that are providing by the various professional organizations for example the ARDMS (American Registry for Diagnostic Medical Sonography). So you can also get certification from this kind of organizations and improve the chances of getting a job as an ultrasound technician. More over you can get more acknowledgements about the many other organizations by visiting at ultrasound technician ross taylor site that are providing certification exams.
Anywhere certificate is necessary otherwise how they will know that you are actually good at working that machinery beforehand by not letting you work or at the interview.