# Use of the Perturbing Sphere Method for the Estimation of Radiofrequency Coils’ Efficiency in Magnetic Resonance Applications: Experience from an Electromagnetic Laboratory

## TL;DR

This paper reviews the use of the perturbing sphere method to accurately and efficiently estimate the performance of radiofrequency coils in magnetic resonance imaging.

## Contribution

The paper presents a validated and efficient method for RF coil efficiency estimation using the perturbing sphere technique.

## Key findings

- The perturbing sphere method allows accurate and rapid estimation of RF coil efficiency and magnetic field mapping.
- The method's accuracy was verified through simulations and workbench tests on various RF coils.
- The method has been successfully applied for periodic quality control checks of RF coils.

## Abstract

Radiofrequency (RF) transmitter and receiver coils are employed in in magnetic resonance (MR) applications to, respectively, excite the nuclei in the object to be imaged and to pick up the signals emitted by the nuclei with a high signal-to-noise ratio (SNR). The ability to obtain high-quality images and spectra in MR strongly depends on the RF coil’s efficiency. Local coil efficiency can be estimated with magnetic field mapping methods evaluated at a fixed point in space. Different methods have been described in the literature, divided into electromagnetic bench tests and MR techniques. In this paper, we review our experience in designing and testing RF coils for MR in our electromagnetic laboratory with the use of the perturbing sphere method, which permits coil efficiency and magnetic field mapping to be estimated with great accuracy and in a short space of time, which is useful for periodic coil quality control checks. The method’s accuracy has been verified with simulations and workbench tests performed on RF coils with different surfaces and of different volumes. Furthermore, all the precautions taken to improve the measurement sensitivity are also included in this review, in addition to the various applications of the method that have been described over the last twenty years of research in our electromagnetic laboratory.