This is a catastrophic injury to the left kidney due to stretch injury of the main renal artery pedicle, which is evidenced by irregularity and narrowing of the proximal artery. This results in multiple wedge-shaped areas of nonenhancement due to infarct. There is perinephric and retroperitoneal blood, which likely reflects venous injury and hemorrhage due to stretch injury as well. This does not have associated active extravasation or accumulate on delayed images.

Deceleration causes stretching and tearing of the intima because it is less elastic than the media and adventitia. The intimal flap results in thrombosis, which can result in complete or segmental/subsegmental infarction, as in this case.

A great teaching point in this case is the use of the chest images to reconstruct arterial phase images of the left renal artery. At my institution (and many others), the trauma protocol CAP CT involves an arterial phase chest followed by portal venous phase abdomen and pelvis. Thus, the overlapping abdominal images obtained during the chest acquisition will essentially be arterial-phase, giving you a partial arterial phase abdomen. There are three major utilities to these images:

1. Look for active arterial extravasation if there are upper abdominal injuries, particularly liver and spleen. This can provide helpful information in a manner similar to delayed-phase images.

2. Reformat the available images of the upper abdomen to function like an angiography, like in this case. I asked the technologist to make axial, coronal, and sagittal 1 mm reformats of the arterial phase abdominal images, which show the renal arterial anatomy very well. At this point, no further angiographic images are needed, avoiding an additional study and load of contrast for this patient. I have uploaded these images for review.

3. Look for arterial enhancing hepatic lesions indicating HCC, particularly if the patient is cirrhotic. These may be isoattenuating on the portal venous phase abdomen and only seen on the arterial phase.  

This patient was managed conservatively with antiplatelet therapy and follow-up renal Doppler imaging.

Here is a companion case with identical imaging findings.