Renal infarcts in dogs: a few images
Renal infarcts in cats are common and, in my experience, usually associated with a history of ureteral obstruction. Renal infarcts in dogs are altogether less common.
This is the left kidney of a Cocker Spaniel with a history of severe IMHA one year previous:

Longitudinal plane view of the left kidney in a 10 y.o. Cocker Spaniel with a history of recurrent immune-mediated hemolytic anemia. Presumed chronic infarcts arrowed.

..and in transverse plane. These smaller ones are quite subtle.

Larger ones are more obvious
Now, this is a more acute presentation: a dog with a really severe, ultimately fatal peritonitis with SIRS. The illness only began 10 days prior to sonography so these are relatively fresh infarcts and (to me) still surprisingly hyperechoic.
Power Doppler certainly looks to confirm a lack of perfusion: