New Design Concept of Monopole Antenna Array for UHF 7T MRI Suk-Min Hong, Joshua Haekyun Park, Myung-Kyun Woo, Young-Bo Kim, and Zang-Hee Cho* Purpose: We have developed and evaluated a monopole antenna array that can increase sensitivity at the center of the brain for 7T MRI applications. Methods: We have developed a monopole antenna array that has half the length of a conventional dipole antenna with eight channels for brain imaging with a 7T MRI. The eight-channel monopole antenna array and conventional eight-channel trans- ceiver surface coil array were evaluated and compared in terms of transmit properties, specific absorption ratio (SAR), and sensitivity. The sensitivity maps were generated by divid- ing the SNR map by the flip angle distribution. Results: A single surface coil provides asymmetric sensitivity resulting in reduced sensitivity at the center of the brain. In contrast, a single monopole antenna provides higher sensitivity at the center of the brain. Moreover, the monopole antenna array provides uniform sensitivity over the entire brain, and the sensitivity gain was 1.5 times higher at the center of the brain compared with the surface coil array. Conclusion: The monopole antenna array is a promising can- didate for MRI applications, especially for brain imaging in a 7T MRI because it provides increased sensitivity at the center of the brain. Magn Reson Med 71:1944–1952, 2014. V C 2013 Wiley Periodicals, Inc. Key words: monopole antenna; monopole antenna array; 7T MRI; dipole antenna UHFs such as in 7T MRI have begun to find numerous applications. A UHF MRI provides a high SNR, a high susceptibility contrast, a high phase contrast, and an increased T 1 value, which provides unsurpassed contrast in MR angiography (1–6). UHF MR imaging of the center of the brain, however, is still a challenge, because a receive-only surface coil array provides a high SNR gain only near the surface coils (7,8). The transceiver surface coil array also provides a high SNR gain but only near the surface coils (9). Moreover, the short wavelength and increased conductivity of the sample attenuate the B 1 field of the UHF MRI (10). In addition, the circularly polarized components (B 1 þ ,B 1  ) have an asymmetric pattern near the surface coil (11). Although this asymmetric pattern could be used to increase the g- factor, it could degrade the sensitivity at the center of the brain. To increase the SNR gain of a target located deeply inside the body (e.g., the brain), increasing the size of the surface coils or the number of detector elements has pro- ven to be of no particular benefit. For instance, a surface coil of 10  10 cm 2 provides lower SNR compared to a sur- face coil 1/4 of its size over a 3-cm depth when the sample loss dominates (12). Additionally, the results of another study exhibited similar sensitivity at a depth of 10 cm, although the configurations of the array were changed from 1  1 to 4  4 or 8  8 to cover the same area (13). In a more recent study, a single-side adapted dipole antenna was introduced for 7T body imaging to increase B 1 þ penetration (14). Similarly, an electric dipole antenna array has been applied to 7T brain imaging to generate the optimum current mode (15). The length of the dipole antenna, however, was not suitable for MRI brain imaging applications at 300 MHz because the length of the dipole antenna (50 cm) was too long to place a human head at the center of the coil. The geome- try of the dipole antenna, therefore, required an optimi- zation of the length, such as a folded design (14) or the use of an appropriate dielectric substrate (15), when it is to be used with a 7T MRI. However, the folded design had a radiation loss in the folded area (14). One study used a dielectric substrate to minimize the reflected waves from the body, but it required direct contact with the skin or near direct contact (15). Therefore, it appears that the use of a dielectric substrate would not be a suit- able choice for head imaging. In this study, we have proposed a new design concept for a monopole antenna array to alleviate the problems in the dipole design where a long antenna length practically deters the use of the dipole antenna in human brain imag- ing. To prove the usefulness of the monopole antenna con- cept, we have constructed an eight-channel monopole antenna array for 7T, and brain imaging was performed experimentally. Additionally, the SNR, transmit proper- ties, sensitivity, and SAR of the monopole antenna array were evaluated and compared to those of the conventional transceiver eight-channel surface coil array. The first trial of the monopole antenna was used as a component of a traveling wave with a patch antenna with a 9.4T MRI (16); however, application to the near field in the form of a con- ventional array has not been reported yet. METHODS Dipole and Monopole Antenna Theory A dipole antenna is composed of two wires (or rods), each with a length of a quarter wavelength as shown in Neuroscience Research Institute, Gachon University, Incheon, Korea. Grant sponsor: National Research Foundation of Korea, Ministry of Educa- tion, Science and Technology; Grant numbers: 2006-2005137 and 2012K1A3A1A24025536. *Correspondence to: Zang-Hee Cho, Ph.D., Neuroscience Research Insti- tute, Gachon University, 1198 Kuwol-dong, Namdong-gu, Incheon, 405- 760, Korea. E-mail: zcho@gachon.ac.kr Received 24 March 2013; revised 15 May 2013; accepted 23 May 2013 DOI 10.1002/mrm.24844 Published online 1 July 2013 in Wiley Online Library (wileyonlinelibrary.com). Magnetic Resonance in Medicine 71:1944–1952 (2014) V C 2013 Wiley Periodicals, Inc. 1944