MRI field strengths: 1.5 T, 3 T and 7 T

Signal in MRI rises roughly with magnetic field strength, so a 3 T scanner can trade that extra signal for finer resolution or faster scans. That is why 3 T is preferred for multiparametric prostate MRI, brain tumour imaging with perfusion and spectroscopy, and breast MRI, and why it dominates research. But higher fields bring longer T1 relaxation, more susceptibility artefact around air, bone and metal, dielectric shading in the abdomen, higher radiofrequency heating limits that slow some sequences, louder gradients and a longer list of implants that cannot be scanned. 1.5 T therefore remains the majority installed system for body and general oncology imaging and is often better for patients with implants or for MR-guided procedures. 7 T systems were cleared for clinical brain and knee imaging in 2017 (Siemens MAGNETOM Terra) and are used to map cortex and small vessels in glioma research rather than for routine staging.

The field is also moving down: 0.55 T systems with deep-learning reconstruction and portable scanners below 0.1 T cost less, tolerate implants and lung air better, and could put MRI where none exists. Helium-free sealed magnets reduce siting cost. For a patient the practical points are that field strength matters less than coil quality, protocol and reading expertise, and that the scanner they can be scanned on safely is the right one.