PIERS ONLINE, VOL. 3, NO. 6, 2007 886 On the Radio-frequency Power Requirements of Human MRI L. Tang 2 and T. S. Ibrahim 1,2 1 Departments of Radiology and Bioengineering, University of Pittsburgh Pittsburgh, Pennsylvania, USA 2 School of Electrical and Computer Engineering and Bioengineering Center University of Oklahoma, Norman, Oklahoma, USA Abstract— In high and ultrahigh field magnetic resonance imaging (MRI) research, computa- tional electromagnetic techniques are now taking an important role in the design and evaluation of MRI radiofrequency (RF) coils. This paper focuses on the RF power requirements and specific absorption rate (SAR) associated with the MRI operation at different field strength. This paper also presents new techniques for achieving high-quality transmit field homogeneity simultaneously with lower total RF power deposition. The studies are done utilizing the finite difference time domain (FDTD) method and the validation of the methods is performed using ultra high field MRI volume coils. DOI: 10.2529/PIERS061007225757 1. INTRODUCTION Since magnetic resonance imaging (MRI) technique has been in clinical and research use over the last 30 years, operation at higher magnetic field strength has been a constant goal for the advancement of this diagnostic tool. Although it faces some difficulties such as technical complexity and an increased financial burden, operation at high field MRI is greatly desirable as a result of the associated higher signal-to-noise ratio, contrast-to-noise ratio, and shorter scanning time. Operation at high field MRI and therefore increased frequencies is also associated with complicated interactions of the electromagnetic waves with the tissue since the operating wavelength becomes comparable to or less than the dimensions of the load (human head/body) and RF coil. This can potentially cause severe operational problems such as the presence of inhomogeneous excitation and reception, increased power absorption, and higher local specific absorption rate (SAR). In human MRI, the total RF power deposition and SAR have been characterized by many researchers [1–4]. For example, at low magnetic field MRI where the wavelength is relatively large compared to the load and RF coil dimensions, quasistatic field approximations were used in the design and assessing the performance of RF coils [5]. Conversely at high or ultrahigh (≥ 7 Tesla) magnetic fields for designing and evaluating RF coils, the significant interactions of the electromagnetic waves with the load invalidate the use of quasistatic approximations and require the application of full wave techniques [6–8]. In this work, a full wave computational electromagnetic method, namely the finite difference time domain (FDTD) technique is implemented in a rigorous fashion by treating the coil and the load as a single system [3] to predict the RF power requirements and SARs of human MRI at high and ultrahigh fields. This computational model is then utilized to design new techniques that can achieve high-quality transmit field homogeneity simultaneously with total RF power deposition lower than that achieved with the standard quadrature excitation [5] for 7 and 9.4 Tesla human MRI. 2. METHODS 2.1. Simulation Model The model we used is a 16-element TEM resonator [9], which is based on multi-conductor trans- mission line theory [10], loaded with an anatomically detailed human head mesh [11] as shown in Figure 1. By using the FDTD [12] technique, both the RF coil and the load were modeled as a single system [13] and bounded using perfect matched layers (PML) [14]. In such a modeling approach, the electromagnetic effects on the load due to the coil and on the coil due to the load are included. From the multi-conductor transmission line theory [10], 9 modes at 9 different frequencies exist in a 16-element TEM coil, where the second mode (mode 1) produces a linearly polarized field (when coil is empty) that can be utilized for imaging. Similar to experiment, the coil was tuned while loaded with the human head mesh by adjusting the gap between the tuning stubs (coil elements).