Case y - Injury to renal pelvis and introduction to urinary tract injury
The three images from the initial examination show a large right renal hematoma which expands the perinephric space. On the images provided there is no extraluminal contrast to indicate an arterial hemorrhage. However, there is lack of perfusion in the inferior pole of the right kidney (bottom inage) which suggests either transection, thrombosis, or dissection of a renal artery branch (or accessory renal artery). This patient has no left kidney, which is why you cannot find it.
The delayed scan shows persistent hematoma. Just quickly reviewing the anatomy, notice how the perirenal space is expanded by the large hematoma. There is a delayed nephrogram in the upper kidney. By history, this second scan occured several hours after contrast was given. There should be no contrast left in the kidney. The lower pole does not retain contrast but we suspect vascular injury so it is likely no contrast ever reached this pole in the first place.
There is abnormal high density material seen in the perirenal space, around the renal pelvis, and tracking along the ureter. This represents contrast in the urine. The extra-ureteral / extra-renal location of contrast indicates a renal collecting system tear. The kidenys concentrate IV contrast into the urine. Contrast in the urine is therefore brighter than contrast in the vessels. Contrast collections brighter than the vessels should always make you think of a urinary tract injury. The injury can be to a renal calyx, to the renal pelvis, to the ureter, to the bladder, or to the urethra. The contrast will usually collect adjacent to the injury. In this case, most of the contrast is near the renal pelvis and there is probably a tear near the UPJ..
This concept is explained in the schematic. An arterial hemorrhage is as bright as the arteries. On the arterial phase scan you will often not see a urinary leak. The contrast has not had time to filter through the kidneys. There is no contrast in the urine. On delayed images the arterial hemorrhage will diffuse into a hematoma. The density decreases because the extravasated contrast is diluted by the blood in the hematoma. Contrast in the urine, however, becomes more concentrated with time. The urine leak will therefore be brighter than the arteries. Etravasated contrast should be isodense to contrast in the bladder and renal pelvis. Because urinary tract injuries are not usually seen on arterial phase images, delayed images should always be obtained when there is evidence of renal trauma. 3-5 minute delayed images are usually adequate.
In case, why is the renal parenchyma bright? There is a delayed nephrogram. Delayed nephrograms are seen with acute tubular necrosis (ATN), acute renal vein thrombosis, contrast reactions, and acute renal obstruction. In the setting of trauma a delayed nephrogram is likely due to ATN (secondary to vascular injury or shock), obstruction (from clot in the collecting system), or contrast reaction. In this case, the nephrogram is likely secondary to ATN from the renal hematoma (since there is a urine leak there is no obstruction; contrast reaction is always a possibility). Since the contrast that persists in the renal cortex is just as concentrated as the contrast in the collecting system, we expect the two to be equally bright. If there were no delayed nephrogram then the extravasated urine would be brighter than the renal cortex.
Injury to the renal pelvis, ureter, and bladder all look the same. There is extravasation of very dense urine. The extravasated urine will be brighter than the arteries. Extravasated contrast should be isodense to contrast in the bladder and renal pelvis.
Take home points