Maybe ignoring it doesn't work well for her. Every child is different and it sounds as though she needs something else. Maybe a calm, close approach rather than moving away from her coming down to her level and trying to show her how you understand her frustration and that she's upset. It cold be even more aggrivating to her to feel misunderstood and frustrated and then have the person ignore you.... it sounds frustrating for a little person that is just learning to effectively communicate :-) Maybe look at the varius situations that trigger these tantrums and try to plan it in advance that she is distracted, the environment is changed, there is somehow *less* frustrating moments for her.... something is clearly upsetting. For example if you know she gets upset at diaper changes try to plan ahead to make it fun or exciting and change the mood to fun, rather than stressful...maybe surprise her with some new cheap dollar store toy to hold while you change her, or hand her a special food item to eat while you do it, turn on a few minutes of her favorite show and smile A LOT....make her think diaper change time is awesome! Same goes for any other tantrum time.... think ahead about times you know she might feel frustrated and try to *fix* or *change* those times into exciting happy times for her....
On the other hand, maybe she has sensory issues, which is what a friend of mine's son has.... he would get very stressed at certain times when he was touched, approached or was overwhelmed in a sensory way. Just a thought :-) I think a behavioral person would check for that if you thought that it might be something to consider.
Hang in there and you will both get through. 18 months is a frustrating time for a little one that knows what they want but can't communicate it very well yet. They must feel so misunderstood! Best wishes!