Dr. Zwingenberger at Veterinary Radiology blog offers some preliminary findings about the “ultrasonographic signs of Aminopterin in dogs and cats.”

Update: From this Washington Post story:

At Suburban Animal Hospital in Arlington County, veterinarian Gary Schrader said that after learning rat poison could have sickened pets, he reexamined the ultrasounds of four cats brought in with renal failure in recent days. The records on all four showed a strange brightness around the kidney — usually an indication of poisoning, Schrader said.