Experimental and numerical investigations of a small animal coil for ultra-high field magnetic resonance imaging (7T)

Maíra M Garcia,Khallil T Chaim,Maria C G Otaduy,Waldemar Zylka,Tiago R Oliveira,Daniel Erni,Daniel Papoti

doi:10.1515/cdbme-2019-0132

Abstract

Abstract The purpose of this work was to develop and investigate a radiofrequency (RF) coil to perform image studies on small animals using the 7T magnetic resonance imaging (MRI) system, installed in the imaging platform in the autopsy room (Portuguese acronym PISA), at the University of Sao Paulo, Brazil, which is the unique 7T MRI scanner installed in South America. Due to a high demand to create new specific coils for this 7T system, it is necessary to carefully assess the distribution of electromagnetic (EM) fields generated by the coils and evaluate the patient/object safety during MRI procedures. To achieve this goal 3D numerical methods were used to design and analyse a 8-rungs transmit/receive linearly driven birdcage coil for small animals. Calculated magnetic field (B1) distributions generated by the coil were crosschecked with measured results, indicating good confidence in the simulated results. Electric field results were post-processed and predictions of local specific absorption rate (SAR) values were achieved for a spherical phantom filled with muscle-like tissue, indicating that the sample would not suffer any unsafe deposition of energy. Post mortem abdomen images obtained from a rat presented good image quality and no artifacts related to field non-homogeneity were observed.

Highlights

Magnetic resonance imaging is a non-invasive and nonionizing imaging technique, which uses energy in the radiofrequency range
The calculated magnetic field generated by this coil presented a very homogeneous and symmetrical internal distribution, especially for a region of interest (ROI) comparable with a small animal body
The aim of the presented work was to develop a workflow that could permit to help in the traditional process of coil design and analysis, and especially, to ensure patient safety when using a designed coil in 7T magnetic resonance imaging (MRI) procedures at PISA

Summary

Introduction

Magnetic resonance imaging is a non-invasive and nonionizing imaging technique, which uses energy in the radiofrequency range. During MRI procedures a patient’s body can absorb RF energy, due to the electromagnetic response of the patient tissue. The excessive energy absorbed may result in tissue heating, which can be significant for systems that extensively use high B1 fields [2]. For ultra-high fields (B0 ≥7T), electrical and magnetic components of RF fields, as well as the SAR profile in the object imaged, are highly complex and spatially non-uniform. To overcome these challenges, RF coils can be designed for different body regions and operations, ensuring a specific EM field distribution and controlling the deposition of EM energy in tissues. The prediction of RF field distribution, as well as the estimation of SAR and temperature increase in the patient, are normally based on 3D numerical simulations using advanced software tools

Objectives

Results

Conclusion