Was a urine Culture & Sensitivity done? If not, I'd insist on it. I have no idea why Vets don't do these very often (they're almost always done in humans when a UTI is suspected). A urinalysis can only show so much; yes, it can show the presence of white blood cells (and blood and sediment, pH, spec gravity, etc) which can denote bacteria present.........but what kind? There are MANY different types and only certain antibiotics work on certain types of bacteria. One size doesn't fit all. Too many Vets just use the standard broad spectrum antibiotic but there are many times it doesn't work, because it was the wrong antibiotic for that particular type of infection......and sadly the poor cat remains suffering with their UTI for longer.
As for a urine culture, I'd start with that wayyyyyyy before xrays or ultrasound.
They'll need to collect a sterile urine sample (they do this in-house, insert a fine needle into a full bladder and withdraw some urine, cat doesn't have to be sedated), send it to the lab, it's place onto an agar plate and after 24-72 hours, they see what type of bacteria is grown. Once the type is identified THEN (and this is key!), they use a whole host of different antibiotics on it to see which are effective.....from those that don't, those that do a little, those that do a lot. THEN, based on that - that's how the Vet should be prescribing the CORRECT antibiotic.
I went through this many times in the past and had to become insistent that they do this on my old cat and sure enough, she often grew uncommon types of bacteria and it was no wonder meds like Baytril didn't work.