Linear Bloch-Siegert phase-encoded low-field MRI: RF coils, pulse sequence, and image reconstruction

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radio Frequency (RF) Engineering
• Biophysics

Background:

• Conventional MRI gradient systems are expensive and bulky.
• There is a need for new spatial encoding methods in MRI.
• The Bloch-Siegert (BS) shift offers potential for novel MRI techniques.

Purpose of the Study:

• To develop an RF gradient encoding system and pulse sequence for phase encoding using the BS shift.
• To address limitations of conventional gradient systems in low-field MRI.
• To provide new degrees of freedom for spatial encoding and system design.

Main Methods:

• Designed and constructed optimized BS spatial encoding coils with bucking windings.
• Developed compatible homogeneous imaging coils for excitation and signal reception.
• Performed BS phase-encoded imaging and RF pulse simulations on a 47.5-mT scanner.
• Designed pulse sequences for linear k-space stepping and implemented them experimentally.
• Calculated Specific Absorption Ratio (SAR) for safety assessment.

Main Results:

• Achieved higher coil linearity (R² = 0.9909 and 0.9921) compared to previous work.
• Validated phantom and wrist imaging coils experimentally, producing peak fields of 1.5 G and 0.8 G.
• Attained nominal imaging resolutions of 5.22 mm and 7.21 mm in the RF phase-encoded dimension.

Conclusions:

• Successfully developed coil systems, pulse sequences, and image reconstructions for linear RF phase encoding using the BS shift.
• Validated the system on a 47.5-mT open low-field scanner.
• Established a key component for gradient-free imaging at low magnetic field strengths.