Abstract
Abstract Dipole radiofrequency (RF) elements have been successfully used to compose multi-channel RF coils for ultrahigh fields (UHF) magnetic resonance imaging (MRI). As magnetic components of RF fields (B1) can be very inhomogeneous at UHF (B0≥7T), dielectric pads with high dielectric constants were proposed to improve the B1 efficiency and homogeneity [1]. Dielectric pads can be used as a passive B1 shimmimg technique thanks to inducing a strong secondary magnetic field in their vicinity. The use of such dielectric pads affect not only the B1 field but also the electric field. This in turn affects the specific absorption rate (SAR) and consequently the temperature distribution inside the patient’s body. To study these effects, a 29 cm-long transmission dipole RF coil element terminated by two meander was used for 7T MRI [2]. Using a cylindrical agarose-gel phantom, numerical and experimental results were analyzed with respect to homogeneity and amplitude of the magnetic and electric fields generated by the RF element in various configurations with and without dielectric pads. Calculated and measured B1 results were cross-checked and found to be in good agreement. When using dielectric pads B1 homogeneity and magnitude increase in regions where it was previously weak or insufficient. Calculations suggest that SAR distribution will change when using the pads.
Highlights
In magnetic resonance imaging (MRI) both transmission and receiving of signal are done through RF elements (RF coils)
At ultrahigh fields (UHF) the magnetic component B1 of RF fields can be very inhomogeneous inside the body, since the wavelength becomes comparable to the dimensions of the human head
It shows that using dielectric pads, higher B1 values can be achieved in comparison to Config. 1, presenting similar distribution for a region x > 30cm
Summary
In MRI both transmission and receiving of signal are done through RF elements (RF coils). When a specific RF coil is designed, it is impossible to predict all the usages that the MRI operator can handle: when different sized and shaped objects are analyzed using this coil, there is a detuning and impedance mismatch caused by the different load in the coil, which already generates intrinsic B1 inhomogeneity. This is a very common problem for patients geometry that differs from the standard human model
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