It is a diagnostic technique used to detect diseases of kidney.
It involves direct injection of contrast medium into the collecting system from above or below.
It gives the best views of collecting system and upper tract of kidney.It may sometimes identify the cause of urinary tract obstruction.
Image source - personal
Antegrade pyelography requires insertion of fine needle into the pelvicalyceal system under ultrasound or radiographic control.
Retrograde pyelography performed by inserting catheters into ureteric orifices at cystoscopy.
Bibliography-- Davidson's principles and practice of medicine (22nd edition)