It is possible to use antimatter directly but it sounds like that only uses the pions with magnetic nozzle. The main use of antimatter is to heat reaction mass

It is possible to use antimatter directly but it sounds like that only uses the pions with magnetic nozzle. The main use of antimatter is to heat reaction mass. The advantage over nuclear rocket is that the "temperature" of antimatter is really high so the specific impulse can also be high. One advantage is that can change the amount of reaction mass to get more thrust or better efficiency.