I Am 63 Years Old, Male. I Experienced Few Urinary

I am 63 years old, male. I experienced few urinary tract infections the past 10 years and was treated by antibiotics. A week ago I felt slight pain while urinating and few drops of blood. Blood did not occur again but the irritation while urinating is still there. Had blood and urine tests which didn't show any abnormality. Had an ultrasound done which showed fullness of the left kidney. I occasionally feel slight pain on my left side. How can I proceed with investigations. Would CT be helpful or I would rather do an MRI. Thanks for your prompt answer