Could antimatter in Saturn's magnetosphere be used as a power source for future spacecraft?

Estimates are that perhaps 1/4 milligram of antimatter is injected into Saturn's magnetosphere per year. Antimatter is the most efficient energy source known to physics, hundreds of times more efficient per unit mass than nuclear fusion. So, is it conceivable in the future, that this antimatter could somehow be harvested from Saturn's magnetosphere, and utilized as a power/energy source by future interplanetary spacecraft? I've no doubt there would be significant technical obstacles to this, of course. What would these obstacles be, exactly, and, how could they possibly be overcome?