These images were obtained from a patient with no known co-morbidities, who presented with acute right flank pain. There was no history of fever, hematuria, or other symptoms suggestive of urinary tract infection.

Grayscale ultrasound shows hypoechoic areas in the upper half of the right kidney, giving an overall heterogeneous appearance to the renal parenchyma. Color Doppler and power Doppler images show absent/minimal blood flow to the upper kidney, suggestive of renal infarction in the given clinical context.

It is important to note that grayscale findings are neither sensitive nor specific for renal infarction. Moreover, Doppler (color or power) is not that sensitive for detecting low blood flows or delineating small areas of infarction. Pyelonephritis can have a similar appearance and both these conditions can present with fever/hematuria. Therefore, contrast-enhanced CT scan should be obtained to better identify the extent of renal involvement when infarction is suspected. Here are the CT images from the above patient demonstrating a geographic non-enhancement of a large area of the lateral right kidney suggestive of infarction. It also showed tiny sub-centimeter areas of hypoattenuation in the lower pole of left kidney concerning for infarcts, which could not be identified on POCUS.

In patients with renal failure in whom iodinated contrast might be contraindicated, Contrast-enhanced ultrasound (CEUS) is a good alternative, if available. In fact, the diagnostic performance of CEUS for ischemia is similar to that of CT. Infarcts appear on CEUS as wedge-shaped areas showing no enhancement while they can be readily differentiated from cortical necrosis due to the technique’s increased spatial resolution. These contrast agents have an excellent safety profile, are not contraindicated in renal dysfunction and remain strictly within the blood vessels resulting in increased sensitivity in differentiating viable from necrotic tissue.

No apparent embolic source could be identified in the above patient and was discharged on oral anti-coagulation therapy after adequate pain control. Interestingly, renal infarction can be idiopathic in a significant proportion of patients. We previously reported one such case.

Here is another example of right renal infarction in a patient with abdominal aortic thrombosis at the level of renal arteries. Note the echogenic clot in the aorta. CT scan showed multiple small right renal infarcts, which could not be picked up by the ultrasound. CT also reported possible non-occlusive thrombus in the right renal artery. Renal cortical echogenicity is increased likely because of underlying diabetic nephropathy.