How safe are ultrasounds?

I think ultrasounds are safe. I have never thought about that during my pregnancy. Now you got me scared a bit. But, I still think ultrasound imaging is safe (when used responsibly, I guess). However, scans today are more powerful that before. Some experts say that pregnant women who have a series of scans are more likely to have left-handed kids. My kid is right-handed and I had many scans.