What Goldstein said.
There are also tiling substrates even thinner than the Ditra that could be used. Nobleseal CIS
is the first that springs to mind. I believe their proprietary adhesives permit use directly over OSB, but do check with them to verify that.
There are also some very good vinyl floor coverings today that look a great deal like tile while being substantially thinner.
You could also consider removing the current subfloor, dropping a new 3/4" plywood subfloor flush with the joist tops on cleats attached to the sides of your joists, then covering it all with half-inch plywood subflooring. That would give you a good subfloor while lowering the existing enough for your substrate and tile.
My opinion; worth price charged.