Using greyscale info from a single photo to produce depth rarely produces pleasing reliefs without some additional work. Or, in some cases, a lot of additional work.
(However, If you are making lithophanes you can produce a relief that will work with very little effort.)
There are applications like AGIsoft’s Photoscan which can produce good quality 3d models from MANY pictures.
If you understand how the greyscale representation works to create the depth, you could actually use a program like Photoshop to modify the portrait image so that it will work.
Its also possible to use some 3d modeling apps (ie ZBrush or other) to modify the relief so it looks the way you want. There is a program called Leveller that is very low cost and specifically does relief modeling. Its marketed as a terrain modification or creation app, but it has all the necessary tools for this work.
Modifying images or meshes are your only options for doing this type of work. Nurbs surfaces are not suited for this kind of modification, so if you needed a nurbs model for any reason, the very last step would be to convert any of the sculpting work to nurbs.