This is a 5mnth old shih tzu puppy referred for abdominal u/s. He had history of screaming in pain and biting at rear  end for one month. He had been treated with several agents including buprinex,gabapentin etc. The pup is small. we immediately noticed he had difficulty standing for any length of time. he was painful at tail base. the abdominal ultrasound was normal. the pup does have retained testicles but they were not seen. the perianal images were normal. we obtained a radiograph and we see this chnage at l5 ,s1,c1. Any ideas?? This is a puzzle to me?

This is a 5mnth old shih tzu puppy referred for abdominal u/s. He had history of screaming in pain and biting at rear  end for one month. He had been treated with several agents including buprinex,gabapentin etc. The pup is small. we immediately noticed he had difficulty standing for any length of time. he was painful at tail base. the abdominal ultrasound was normal. the pup does have retained testicles but they were not seen. the perianal images were normal. we obtained a radiograph and we see this chnage at l5 ,s1,c1. Any ideas?? This is a puzzle to me? Would CT be better modality?  Thanks