respiratory trigger

[El-khatib Hassan]

Hello,
I'm just a student and i want to know how does an mri examination is performed.My question is if all MRI devices have a system for respiratory trigger or there are some MRI devices where the clinicians must say to the patient to hold there breath.
Can you give me some more information about the respiratory trigger?
what kind and which are the respiratory trigger systems for mri that exist in this moment? Are they expensive?
Can you give me some explanation in what it consists exactly? Some photos with and whithout the respiratory trigger?
Thank you very much !