Uses of plain abdominal radiography – urinary tract plain radiography :
– For detection of stones and determination of their size and (to an extent) their location within the kidneys, ureters, and bladder
– Renal calculi a calcification overlying the kidneys is intrarenal if it maintains its relationship to the kidney on inspiratory and expiratory films (i.e. if it moves with the kidney). If in doubt as to whether an opacity overlying the outline of the kidney is intrarenal or not, get an ultrasound (look for the characteristic acoustic shadow ? within the kidney), IVU, or CTU.
– Ureteric calculi sensitivity for detection of renal calculi is in the order of 50 70% (i.e. the false -ve rate is between 30 50%; it misses ureteric stones when these are present in 30 50% of cases). CTU or IVU, which relate the position of the opacity to the anatomical location of the ureters, are required to make a definitive diagnosis of a ureteric stone. However, once the presence of a ureteric stone has been confirmed by another imaging study (CTU or IVU), and as long as it is radio-opaque enough and large enough to be seen, plain radiography is a good way of following the patient to establish whether the stone is progressing distally, down the ureter. Not useful for following ureteric stones which are radiolucent (e.g. uric acid), small (generally a stone must be 3 4mm to be visible on plain X-ray), or when the stones pass through the ureter as it lies over the sacrum. Ability of KUB X-ray to see stones is also dependent on amount of overlying bowel gas.
– Plain tomography (a plain X-ray taken of a fixed coronal plane through the kidneys) can be useful, but is rarely done nowadays with the availability of ultrasound and CT.
– Opacities that may be confused with stones (renal, ureteric) on plain radiography: calcified lymph nodes; pelvic phleboliths (round, lucent centre, usually below the ischial spines).
– Look for the psoas shadow obscured where there is retroperitoneal fluid (pus or blood)