A simple head-sized phantom for realistic static and radiofrequency characterization at high fields.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Physics

Background:

• High-field MRI systems (e.g., 7T) offer enhanced signal-to-noise ratio but present challenges in static (B0) and radiofrequency (RF) field uniformity.
• Accurate characterization of these fields is crucial for image quality, safety, and developing new MRI techniques.
• Existing phantoms may not fully replicate the complex dielectric properties and anatomical variations of the human head.

Purpose of the Study:

• To develop and validate a simple, head-sized phantom for realistic characterization of static and RF fields in high-field MRI systems.
• To assess the phantom's ability to mimic in vivo field distributions and enable accurate MR thermometry.

Main Methods:

• A head-sized phantom was constructed with an ellipsoidal compartment and a spherical cavity simulating the nasal cavity.
• The phantom was filled with a polyvinylpyrrolidone (PVP) solution to emulate brain tissue dielectric properties.
• Static and RF field distributions were measured on a 7T MRI system, compared with in vivo data and simulations, and MR thermometry was performed.

Main Results:

• The phantom accurately reproduced in vivo static and RF field patterns, with the spherical cavity significantly influencing these distributions.
• MR thermometry and transmit efficiency (B1+) measurements closely matched simulation and fiber optic probe data (peak agreement within 0.3 °C and 0.02 μT/√W; RMSE < 0.18 °C).

Conclusions:

• The developed head-sized phantom effectively mimics the B0 and B1+ nonuniformities found in the human head.
• It provides a suitable reference for RF validation and methodological advancements in high-field MRI, particularly for accurate MR thermometry.