I Had A Severe Back Spasm Last Friday, Unable To

I had a severe back spasm last Friday, unable to move for a whole day. had to have a GP home visit next day. Prior to that I used warm comprressions and tens machine but nothing helped the whole day. Had diazepam and cocodamols for next 2 days. Pain eased and kept feeling nauseous till yesterday, I have been os[eoporotic for last 15 years and have UC. Just stopped fosamax last 18 months as i was taking for long period,
My question is : Is DEXA scan better or MRI to get good diagnosis for my back?