Finding nearest value in a SortedDictionary

I have a SortedDictionary

 SortedDictionary<int, CPUOptimizationObject> myDict;

Now I want to find the first value above X. I can do something like this

foreach (var iKey in MyDict.Keys)
{
   if (iKey >= thresholdKey)
   {
       foundKey = iKey;
       break;
   }
}

but this isn't good performance wise.
Any better suggestion?
(is there a method for that in the collections something like Binary search for SortedDictionary ?)