C++ Template Arguments not working

Question

I have something similar to this,

class Image {
  template <typename T> using iterator = ImageIteratorBase<Image, T>;
};

template <typename ImageType>
class Filter {
  template <typename PixelType> apply();
};

In my Filter::apply() function, I need to do this:

template <typename ImageType>
template <typename PixelType>
void Filter<ImageType>::apply() {
    using IteratorType = typename ImageType::iterator<PixelType>;
}

But this does not work and yields the error

 error: expected ‘;’ before ‘<’ token

What is wrong in here? Thanks in advance