The informal way: Radiólogo.
The academic title (In Colombia and Ecuador): Tecnólogo en radiología.
EDIT: After further research, it turns out that the "Radiólogo" is an actual MD with a specialization in radiology, while the "Tecnólogo" only had to study for 3 years to achieve this degree. In Ecuador, you can get a degree of "Tecnólogo" in 3 years and a degree of "Licenciado" after 4-5 years. To become an MD you have to study even longer.