Steady-state MR imaging sequences: physics, classification, and clinical applications.

Area of Science:

• Medical Imaging
• Physics

Background:

• Steady-state sequences utilize fast gradient-echo acquisitions.
• They maintain constant longitudinal magnetization (LM) and transverse magnetization (TM) by using a repetition time shorter than the T2 relaxation time.

Purpose of the Study:

• To explain the physics, classification, and clinical applications of steady-state sequences.
• To highlight their importance in modern MR imaging.

Main Methods:

• Classification based on sampled signals: postexcitation refocused (S+), preexcitation refocused (S-), and fully refocused (S+ and S-).
• Exploration of signal formation including preexcitation (S-) and postexcitation (S+) signals.
• Discussion of refocused echo paths contributing to signal in tissues with long T2 relaxation times.

Main Results:

• Steady-state sequences achieve a nonzero steady state for both LM and TM.
• Two signal types are formed: preexcitation signal (S-) and postexcitation signal (S+).
• All tissues with long T2 relaxation times exhibit additional signals from refocused echo paths.

Conclusions:

• Steady-state sequences have significantly advanced cardiac imaging, becoming standard for functional assessment and myocardial viability.
• They offer superior signal-to-noise and contrast-to-noise ratios with faster acquisition speeds.
• Applications extend to abdominal, fetal, and interventional MR imaging, with growing importance for radiologists.