Ulthera is usually done to tighten the neck and jowls. I would doubt that you had the silicone injected into these areas. So as long as you do the ulthera in silicone free areas you will be ok. For people who have silicone nodules, we have been using the Ulthera to shrink down these down. So if you do the ulthera over the areas where you have had silicone, there will probably be changes to the contour.
In most cases, areas of the body treated with silicone injections and areas treated by Ultherapy do not overlap. If you're planning on having Ultherapy performed on regions of the body where it's unlikely that there are silicone injections (such as the face), then I wouldn't be concerned. However, just to be safe, I would contact the doctor who injected you with silicone to learn more information about where you were injected and with how much silicone. That information can prove useful for the specialist who will be performing your Ultherapy treatment.
I woud shy away from areas of the face where there are silicone injectables. Typically the treatment areas that are recommended by Ultherapy are silicone free areas (neck, lateral face) so it would be safe to do the treatments in such areas. I would get detailed knowledge from the previous doctor that injected silicone before an Ultherapy is performed. Read the Ultherapy book on the link below.