Slice offset frequency and shim adjustment for interactive steady-state free-precession (SSFP) imaging

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Medical Physics
• Image Processing

Background:

• Off-resonance artifacts degrade image quality in Magnetic Resonance Imaging (MRI).
• Optimizing radiofrequency (RF) pulse parameters like center frequency (CF) and magnetic field shimming is crucial for minimizing these artifacts.
• Interactive imaging sequences require dynamic adjustments to these parameters.

Purpose of the Study:

• To develop a novel method for real-time optimization of center frequency (CF) and shim parameters.
• To enable this optimization within an interactive steady-state free-precession (SSFP) MRI sequence.
• To achieve this by reformatting a pre-acquired field map to match the orientation of the interactive acquisition.

Main Methods:

• Acquired field maps in a phantom and a volunteer.
• Modified an SSFP sequence to relay slice position and rotation data to an external program.
• The external program reformatted the field map, calculated CF and shim offsets, and returned them to the SSFP sequence.

Main Results:

• CF measurements in the phantom correlated well with scanner-determined offsets (Bland-Altman bias: -14 Hz, limits of agreement: -28 to 0 Hz).
• Qualitative improvements in image quality were observed in the volunteer using the optimized CF and shim parameters.
• The method demonstrated effective real-time CF and shim adjustment for interactive slice positioning.

Conclusions:

• The proposed method successfully enables real-time optimization of CF and shim for interactive MRI acquisitions.
• This approach effectively reduces off-resonance artifacts in SSFP sequences.
• The technique is applicable to any interactively positioned slice, enhancing overall image quality.