# Three-Line Microstrip Array for Whole-Body MRI System at 7 T
## 1 Introduction  
Magnetic resonance imaging (MRI) has been a useful tool for diagnosing health problems and studying the functions of the human body [1; 2; 3]. MRI technology has continued to improve to achieve better image resolution and reduce the scan time. As a result, studies on the use of MR scanners, such as 7 T, are increasing. As these strong main magnetic (\(\,\mathrm{I}\,\mathrm{B}_{0}\,\mathrm{I}\,\))-field systems receive approval by the safety authorities for clinical use, some aspects of the radiofrequency (RF) transmission and hardware still need to be investigated, such as the use of whole-body systems [4; 5; 6; 7].  
The use of whole-body MRI (wb-MRI) has been recommended for early cancer detection [8], bone marrow malignancies [9], angiography, and cardiac function [10; 11]. To obtain these types of images, an RF transmitter that can produce a uniform magnetic (\(\,\mathrm{I}\,\mathrm{B}_{1}{}^{+}\,\mathrm{I}\,\))-field across the whole body is necessary. In addition to a uniform \(\,\mathrm{I}\,\mathrm{B}_{1}{}^{+}\,\mathrm{I}\,\)-field, the RF transmitter should also exhibit lower energy absorption in the body (specific absorption rate (SAR)), which is the main reason that 7 T MRI systems are not yet fully approved for clinical use.  
A birdcage coil is one of the reference systems for imaging at a lower field strength of 4 T and below, as it can produce a uniform \(\,\mathrm{I}\,\mathrm{B}_{1}{}^{+}\,\mathrm{I}\,\)-field. However, at 7 T, the field produced by the birdcage coil decreases, given that the operational frequency is higher, and thereby reduces the wavelength. The reduction in the wavelength is further affected by interaction with human tissues [12; 13].  
One method that has been proposed for whole-body imaging at higher frequencies is the use of patch antennas and waveguide with the bore of a magnet [14; 15]. However, this technology depends on the geometry and the modes that can be created. The use of microstrips for 7 T MRI has also been proposed, which has shown promising results [16; 17].  
In this study, we investigated the use of a modified conventional microstrip line set in an array configuration. We proposed the use of a triple-line microstrip-transmission line (t-MTL) in which the main line is connected to a voltage source and the adjacent lines arecoupled. As a result, the transmitted field can be increased in a wider area. We compared the performance of the proposed array with those of the reference birdcage coil and the traveling wave antenna. The use of a triple-line t-MTL is a novelty application for MRI, as it has only been applied for microwave line couplers. The use of the MTL array has the advantage that it allows adjustment to the magnetic field propagation, so that it can be more uniform when it interacts with the human tissues.  
The paper will first introduce the theory of t-MTL, followed by a comparison between different configurations to demonstrate the reason for the design. The second part of the method section describes the array of the triple-line microstrips compared with two other configurations: a travel wave patch antenna and a whole-body birdcage coil. We present the \(\mid\)B\({}_{1}\)\({}^{+}\)\(\mid\)-fields from each configuration in the results section.