This is a case of emphysematous pyelonephritis.  This constitutes only a small percentage of pyelonephritis, largely occurring in diabetic patients and others with an immunosuppressive state.  E. coli is a the commonest responsible organism.

It is most commonly depicted on CT - the gas component being very difficult to observe on ultrasound, even though this modality is very frequently first performed.

Urolithiasis is very often observed in this group of patients, as in this case.