Abstract
To demonstrate a simple head-sized phantom for realistic static and RF field characterization in high field systems. The head-sized phantom was composed of an ellipsoidal compartment and a spherical cavity to mimic the nasal cavity. The phantom was filled with an aqueous solution of polyvinylpyrrolidone (PVP), to mimic the average dielectric properties of brain tissue. The static and RF field distributions were characterized on a 7T MRI system and compared to in vivo measurements and simulations. MR thermometry was performed, and the results were compared to thermal simulations for RF validation purposes. Accurate reproduction of both static and RF fields patterns observed in vivo was confirmed experimentally and was shown to be strongly affected by the inclusion of the spherical cavity. MR thermometry and transmit efficiency ( B1+) measurements were obtained in close agreement with simulations (peak values agreeing within 0.3 °C and 0.02 μT/√W) as well as fiber optic thermal probes (RMSE < 0.18 °C). A simple head-sized phantom has been presented that produces B0 and B1+ nonuniformities similar to those encountered in the human head and allows for accurate MR thermometry measurements, making this a suitable reference phantom for RF validation and methodological development in high field MRI.
Highlights
Tissue-mimicking magnetic resonance (MR) phantoms are instrumental for various MR applications including sequence development as well as system characterization and quality assurance
The second part of this study evaluates the utility of the phantom for MR thermometry using the custom birdcage coil
The importance of including a spherical air cavity within the otherwise homogeneous phantom is illustrated in Figure 2, which compares simulated B1+ fields in the heterogeneous head model, a head model with homogeneous dielectric properties, and the ellipsoidal phantom without and with the spherical air cavity in place
Summary
Tissue-mimicking magnetic resonance (MR) phantoms are instrumental for various MR applications including sequence development as well as system characterization and quality assurance. Tissue properties that are often mimicked include MR relaxation times such as T1 and T2 [1,2], magnetic susceptibility [3], diffusive properties [4], flow [5] and dielectric properties [6]. Inhomogeneities in the transmit RF (B1+) field are generally governed by the bulk dielectric properties of the sample, leading to a shortening of the RF wavelength, as well as being sensitive to the geometry and positioning of the sample [6,11]. RF safety analyses generally involve heterogeneous body models with many different tissue types to capture such mechanisms and determine safe power limits at which MR systems can be operated
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