RESUMO
OBJECTIVE: A volume coil with squared slots-end ring was developed to attain improved sensitivity for imaging of rat's brain at 7 T. MATERIAL AND METHODS: The principles of the high cavity resonator for the low-pass case and the law of Biot-Savart were used to derive a theoretical expression of [Formula: see text]. The slotted-end ring resonator showed a theoretical 2.22-fold sensitivity improvement over the standard birdcage coil with similar dimensions. Numerical studies were carried out for the electromagnetic fields and specific absorption rates for our coil and a birdcage coil loaded with a saline-filled spherical phantom and a digital brain of a rat. RESULTS: An improvement of the signal-to-noise ratio (SNR) can be observed for the slotted volume coil over the birdcage regardless of the load used in the electromagnetic simulations. The specific absorption rate simulations show a decrement for the digital brain and quite similar values with the saline solution phantom. Phantom and rat's brain images were acquired at 7 T to prove the viability of the coil design. The experimental noise figure of our coil design was four times less than the standard birdcage with similar dimensions, which showed a 44.5% increase in experimental SNR. DISCUSSION: There is remarkable agreement among the theoretical, numerical and experimental sensitivity values, which all demonstrate that the coil performance for MR imaging of small rodents can be improved using slotted end-rings.
Assuntos
Encéfalo , Imageamento por Ressonância Magnética , Ratos , Animais , Imageamento por Ressonância Magnética/métodos , Razão Sinal-Ruído , Imagens de Fantasmas , Encéfalo/diagnóstico por imagem , Cabeça , Desenho de EquipamentoRESUMO
OBJECTIVE: A scaled-down slotted surface radio frequency (RF) coil was built, and the specific absorbance rate (SAR) in 100 mg of tissue (SAR100 mg) produced in a rat brain phantom was computed at 7 T. MATERIALS AND METHODS: A slotted coil 2-cm in diameter with six circular slots was developed. Its theoretical and experimental performance was computed and compared using the signal-to-noise ratio (SNR) expression and phantom images obtained with a spin echo sequence. Electromagnetic simulations were performed using the finite integral method with saline sphere and rat brain phantoms. SAR100 mg was computed for the circular coil, by varying its radius, and was also computed for the slotted coil. RESULTS: The slotted coil quality factor gave a twofold increment over the circular coil, and noise was reduced by 17%. The experimental SNR of the slotted coil produced a 30% improvement for points near the coil plane. The theoretical and experimental results showed substantial agreement. Axial map histograms and profiles showed greater SAR100 mg values for the circular coil than for the slotted coil. CONCLUSIONS: The slotted surface coil offers improved performance and low SAR100 mg for rat brain imaging at 7 T. This approach may be used with new RF coils to investigate SAR in humans.