Specific Absorption Rate Measurement of Birdcage Coil for 3.0-T Magnetic Resonance Imaging System Employing Thermographic Method

Magnetic resonance imaging (MRI) is a noninvasive imaging techniques used to visualize the inside of the human body based on nuclear magnetic resonance. Currently, RF coils that transmit high frequency and high power of electromagnetic (EM) wave pulses are being investigated to obtain high-quality images and to shorten the imaging time. Due to these approaches, the specific absorption rate (SAR) rise inside the human body has to be concerned during the imaging. Thus far, the SAR has been evaluated by calculating the EM fields inside the human body, and the safety of the human body has been reported. Meanwhile, according to the IEC standards, there are two methods to measure the total absorption energy of the human body model, although these methods cannot be used to evaluate detailed SAR distribution inside the body. It is important to comprehend detailed SAR distribution by measurement in order to consider reducing the SAR. In this paper, by use of the thermographic method, detailed SAR distribution inside the human head model was intended to measure employing birdcage coil for a 3.0-T MRI system. In the thermographic method, short time radiation and a sufficient temperature rise are required to obtain accurate SAR distribution. However, the effect of temperature diffusion inside the phantom caused by long time radiation depending on circumstances in the experiment must also be considered. In order to determine the temperature diffusion, temperature transition inside the phantom during the experiment was also measured by using a fluoroptic thermosensor. Then, multiplying the correction coefficients, which were calculated from the temperature transition by the measured SAR values, it was found that accurate SAR values could be obtained despite the long radiation time, and in addition, the measurement and calculation of SAR distribution were in good agreement. From these results, it has been verified that the proposed measurement method has a possibility of measuring detailed SAR distribution inside the phantom when any conventional RF coil is used.

I. Introduction

In recent years, magnetic resonance imaging (MRI) is widely used to obtain clear images inside the human body, especially those of high water-content tissues. The fundamental principle of the MRI is to receive nuclear magnetic resonance (NMR) signals induced by radiating electromagnetic (EM) wave pulses to human body, which is placed inside a strong static magnetic field. The MRI system is composed of various elements including an RF coil, which plays an essential role in imaging [1]. Several types of RF coils, such as saddle coils, birdcage coils, surface coils, TEM coils, etc. have been developed and used depending on the body part to be imaged [1]–[3]. These RF coils operate as an antenna that “radiates an EM wave pulse to the body in order to generate a NMR signal” and “receives the signal emitted from the body.” In this paper, the birdcage coil, which is the most popular of volume coils, was chosen and used for the research.