Concept for gradient-free MRI on twin natural slices

Area of Science:

• Medical Imaging
• Physics
• Engineering

Background:

• Designing MRI for space necessitates minimal mass, complexity, and power, with strict external stray field limits (3.2 Gauss at 7 cm).
• Radio frequency (RF) encoding with Halbach magnets and spatially non-uniform magnetic fields offers a promising solution for reduced hardware complexity in MRI.
• Transmit Array Spatial Encoding (TRASE) utilizes natural slices within these non-uniform fields, but requires separation of signals from twin slices for reconstruction.

Purpose of the Study:

• To develop a mathematical framework and provide simulation examples for simultaneously imaging on twin slices in an inhomogeneous magnetic field.
• To describe a novel MRI design that meets stringent space-based requirements for mass, power, and stray magnetic fields.
• To enable the use of Halbach magnets with built-in axial gradients for MRI applications.

Main Methods:

• Utilized RF encoding with a Transmit Array Spatial Encoding (TRASE) approach.
• Employed spatially non-uniform magnetic fields with foliations of isomagnetic surfaces (natural slices).
• Developed a system using multiple receiver coils with spatially varying RF profiles for twin slice separation and image reconstruction.

Main Results:

• A design for simultaneously imaging on twin slices using multiple receiver coils with spatially varying RF profiles was mathematically described and simulated.
• The proposed design requires at least three TRASE transmit coils (twisted solenoids) and two receive coils (solenoids) for RF encoding and twin slice separation.
• The MRI design integrates RF encoding (TRASE), spatial encoding magnetic fields (SEM) with natural slices, and receive coil spatial profiles for image reconstruction.

Conclusions:

• The presented MRI design successfully combines TRASE, SEM, and specialized receive coils for gradient-free imaging.
• This novel design enables the use of Halbach magnets with integrated axial gradients for MRI.
• The outcome is a new gradient-free TRASE MRI system capable of imaging pairs of electronically selectable axial slices, suitable for space applications.