By far the most commonly used for MRI
1.5T is still the most clinically common
3T is gaining
7T is the highest field strength produced (and technically supported) by major manufacturers but they are still considered research systems.
Contrast Sources in MRI
Proton density - Image intensity proportional to density of hydrogen nuclei in sample
Longitudinal Relaxation Times - emphasizes differences in tissues' T₂ characteristics
Transverse Relaxation Times - emphasizes differences in tissues' T₁ characteristics