New design concept of monopole antenna array for UHF 7T MRI
2013; Wiley; Volume: 71; Issue: 5 Linguagem: Inglês
10.1002/mrm.24844
ISSN1522-2594
AutoresSuk‐Min Hong, Joshua Haekyun Park, Myung‐Kyun Woo, Young-Bo Kim, Zang‐Hee Cho,
Tópico(s)Advanced NMR Techniques and Applications
ResumoMagnetic Resonance in MedicineVolume 71, Issue 5 p. 1944-1952 Full PaperFree Access New design concept of monopole antenna array for UHF 7T MRI Suk-Min Hong, Suk-Min Hong Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorJoshua Haekyun Park, Joshua Haekyun Park Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorMyung-Kyun Woo, Myung-Kyun Woo Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorYoung-Bo Kim, Young-Bo Kim Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorZang-Hee Cho, Corresponding Author Zang-Hee Cho Neuroscience Research Institute, Gachon University, Incheon, KoreaCorrespondence to: Zang-Hee Cho, Ph.D., Neuroscience Research Institute, Gachon University, 1198 Kuwol-dong, Namdong-gu, Incheon, 405-760, Korea. E-mail: zcho@gachon.ac.krSearch for more papers by this author Suk-Min Hong, Suk-Min Hong Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorJoshua Haekyun Park, Joshua Haekyun Park Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorMyung-Kyun Woo, Myung-Kyun Woo Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorYoung-Bo Kim, Young-Bo Kim Neuroscience Research Institute, Gachon University, Incheon, KoreaSearch for more papers by this authorZang-Hee Cho, Corresponding Author Zang-Hee Cho Neuroscience Research Institute, Gachon University, Incheon, KoreaCorrespondence to: Zang-Hee Cho, Ph.D., Neuroscience Research Institute, Gachon University, 1198 Kuwol-dong, Namdong-gu, Incheon, 405-760, Korea. E-mail: zcho@gachon.ac.krSearch for more papers by this author First published: 01 July 2013 https://doi.org/10.1002/mrm.24844Citations: 25AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract 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 transceiver surface coil array were evaluated and compared in terms of transmit properties, specific absorption ratio (SAR), and sensitivity. The sensitivity maps were generated by dividing 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 candidate 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. © 2013 Wiley Periodicals, Inc. 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 T1 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 B1 field of the UHF MRI (10). In addition, the circularly polarized components (B1+, B1−) 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 proven to be of no particular benefit. For instance, a surface coil of 10 × 10 cm2 provides lower SNR compared to a surface 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 B1+ 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 geometry of the dipole antenna, therefore, required an optimization 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 suitable 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 imaging. To prove the usefulness of the monopole antenna concept, we have constructed an eight-channel monopole antenna array for 7T, and brain imaging was performed experimentally. Additionally, the SNR, transmit properties, 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 conventional 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 Figure 1a. In contrast, a monopole antenna is composed of one wire (or rod) with the length of a quarter wavelength perpendicular to the ground plate or plane as shown in Figure 1b. A monopole antenna is formed by replacing half of the dipole antenna with a ground plate; therefore, a monopole antenna has a similar radiation pattern as a dipole antenna but half the length of the dipole antenna. The dipole and monopole antenna have a length l according to the formulas: (1) (2)where λ0 is the wavelength, c is the speed of light ( , 3.0 × 108 m/s in free space), f is the frequency, and k is an adjustment factor. The adjustment factor compensates for the propagation speed and is close to 1 if the diameter of the wire is thin compared to the wavelength in free space (17). Therefore, the length of the monopole antenna is 25 cm at a frequency of 300 MHz in free space. Figure 1Open in figure viewerPowerPoint a: Schematics of dipole antenna. (b) Schematics of monopole antenna. c: Top view of the configuration of the monopole antenna array. d: Eight channel monopole antenna array. e: Eight channel surface coil array. An additional advantage of the monopole antenna is a higher antenna gain (3 dBI) compared to the dipole antenna, which has a gain of 2.15 dBI. The impedance of the half- and quarter- wavelength antennas are 72 + i52 Ω and 36 + i26 Ω; therefore, the corresponding reflection coefficient (S11) values are −7.4 dB and −10.9 dB (18), respectively. 7T MRI System The 7T MRI system (Magnetom, Siemens, Germany) is composed of a 90-cm bore superconducting magnet (Magnex Magnet Technology, Oxford, UK) connected to a Siemens Syngo console. The system has a whole-body gradient coil with an inner diameter of 60 cm, which is coupled to the Siemens GPA (2000 V, 625 A). This provides a maximum gradient strength of 40 mT/m within 200 μs, leading to a slew rate of 200 T/m/s. In the RF portion, a 6-kW RF amplifier (CPC, Brentwood, NY, USA) is coupled to an eight-port power splitter. For independent transmit/receive (TR), we used an interface box that is composed of TR switches and preamps. Experimental Setup for the Eight-Channel Monopole Antenna Array Vs. the Surface Coil Array The length of the monopole antenna was adjusted to 20 cm instead of 25 cm because of the permittivity of the acrylic former and head loading. The 3-dB bandwidth of S11 was over 40 MHz in the head-loaded condition. Each monopole antenna of the eight-channel array was connected to a coaxial cable via a ground plate (Fig. 1b). The ground plate and monopole antenna array were made with copper tape. The width of the monopole antennas was 1 cm. The ground plate had a dimension of 40 × 40 cm2, and the upper corners of the ground plate were cut to fit in the MRI bore, as shown in Figure 1d. To minimize the eddy currents induced on the ground plate, the ground plate was cut by a number of slots with a size of 5 × 10 cm2, and chip capacitors (1 nF) were connected across each slot. The total number of capacitors used to reduce the eddy currents was 130. The top edge of the head was located 10 mm away from the ground plate and isolated with a 10-mm acrylic plate. The ground plate did not generate any artifacts such as an eddy current in the EPI sequence test. Monopole antennas were vertically placed on the ground plate, forming a circle along an acrylic tube with a diameter of 26 cm. Each monopole antenna was placed with a 45° radial space (Fig. 1c). The surface coils used for the experiment had dimensions of 9 × 20 cm2 (Fig. 1e) with a 1-cm gap between each rectangular surface coil. Surface array coils were decoupled using capacitive decoupling. Two capacitors were used to reduce crosstalk between two surface coils and had a value of 2.2 pF. The diameter of surface coil array was 26 cm. The unloaded-to-loaded Q ratio of the monopole antenna and surface coil was measured by shielded inductive probes. When we measured Q of the monopole antenna and surface coil, other elements were placed in an open circuit. FDTD Simulations We have conducted finite-difference time-domain (FDTD) simulations using xFDTD (Remcom, State College, PA, USA). All the geometries were modeled with a high-fidelity head model, which included the shoulders, with a resolution of 2 × 2 × 2 mm3. To evaluate and compare the receiver sensitivity of single elements, we designed the head-mesh model with a single-dipole antenna, monopole antenna, and surface coil. The SNR over the head was calculated by (19) (3)where V is a scaling factor, τ is the duration of the rectangular pulse (3 ms), f is the frequency, Pabs is the absorbed power, and γ is the gyromagnetic ratio. The receiver sensitivity was calculated by dividing the B1− field by the square root of the absorbed power. Additionally, the transmit efficiency was extracted by dividing the B1+ field by the square root of the delivered power. Active voltage ports were placed between the monopole and the ground plate for the single monopole antenna simulation. For the single dipole antenna simulation, an active voltage port was placed between two poles. The single surface coil was ideally driven by four voltage sources with identical amplitude and phase. The eight-channel monopole antenna array was driven by eight voltage sources with identical amplitude but with 45° phase differences. The eight-channel surface coil array was driven by 32 voltage sources with identical amplitude but with 45° phase differences between the coils. Active voltage ports had a 45° phase difference to generate a uniform birdcage-like mode for the eight-channel monopole antenna array. SAR values were normalized and calculated with a 90° pulse having a 3-ms duration (19) at three different ROIs. MR Sequence Parameters To compare the flip angle maps of the surface array and monopole array, an actual flip angle imaging (AFI) sequence (voxel size = 4 × 4 × 4 mm3, BW = 330 Hz/pixel, acquisition time = 6 min 38 s, matrix size = 64 × 62 × 64) was used (20). The ratio of TR1/TR2 was 5, and the flip angle was 30°. We determined the flip angle non-uniformity as the ratio of the standard deviation to the mean value for the entire head. To compare the SNR and imaging sensitivity, proton-density-weighted images (TR = 1000 ms, TE = 3 ms, FA = 30°, voxel size = 1 × 1 × 4 mm3, BW = 500 Hz/pixel, acquisition time = 4 min 16 s) were acquired at axial, sagittal, and coronal orientations. The reference voltage of the monopole antenna array and surface coil array for imaging were 236 V and 310 V, respectively. These reference voltages were required for a 180° pulse at the center of the brain and adjusted to generate a similar flip angle at the center of the brain. SNR maps were generated by dividing the image value by the standard deviation of the noise outside the head image. The SNR maps of the proton-density-weighted image are a function of tissue properties, flip angle, and receiver sensitivity. Because the flip angle distribution could vary depending on the calibration and B1+ shimming, a receiver sensitivity map, instead of an SNR map, should be considered for comparison. However, it is difficult to distinguish the receiver sensitivity map from an SNR map divided by the flip angle distribution. Therefore, we measured the sensitivity, which is a combination of the receiver sensitivity and tissue properties. The sensitivity map can be generated by dividing the SNR map by the flip angle distribution. To demonstrate the capability of the monopole antenna array for 7T human brain imaging, high-resolution anatomical images were acquired (TR = 750 ms, TE = 17.1 ms, voxel size = 0.25 × 0.25 × 2 mm3, BW = 40 Hz/pixel, acquisition time = 10 min 49 s) and compared with a surface coil array. The local ethics committee approved the experiment, and the volunteers signed an informed consent form. RESULTS Figure 2 shows the noise correlation matrix of the monopole antenna array and surface coil array. The noise correlation of the eight-channel monopole antenna array ranged from 4% to 55%, with an average of 21% for the off-diagonal elements. For the eight-channel surface coil array, these numbers ranged from 2% to 41%, with an average of 17%. For the monopole antenna array, crosstalk (S21) between the nearest neighbors ranged from −9.3 dB to −10.6 dB, whereas crosstalk (S21) between the next nearest neighbors had an average of −15 dB. The crosstalk (S21) of the surface coil array between the nearest neighbors was from −12.4 dB to −14.3 dB, and that of the next nearest neighbors had an average of −18 dB when capacitive decoupling was used. Figure 2Open in figure viewerPowerPoint Noise correlation matrix of monopole antenna array and surface coil array. Figure 3 shows the simulation models and their receiver sensitivity and transmit efficiency of a single element of a dipole antenna (a, d, g, j, m), monopole antenna (b, e, h, k, n), and surface coil (c, f, i, l, o). The lengths of the dipole antenna and monopole antenna were 46 cm and 20 cm, respectively, and were adjusted through bench work with head loading. The surface coil provided a strong receiver sensitivity only near the coil and also provided an asymmetric receiver sensitivity pattern, resulting in low receiver sensitivity at the center of the brain. As observed, the receiver sensitivity of the monopole antenna resembled that of the dipole antenna with an additional improvement at the center as well as the superior aspect of the brain. Simulated receiver sensitivity was measured at the ROIs shown in the square boxes in Figure 3g–i (5 × 5 cm2 area). Additionally, the transmit efficiency was measured at the ROIs shown in the square boxes in Figure 3m–o. The receiver sensitivity values of the single dipole antenna, monopole antenna, and surface coil were 314, 385, and 223, respectively. Additionally, the transmit efficiency values were 203, 327, and 219 for the dipole antenna, monopole antenna, and surface coil, respectively. The simulated power efficiencies η (Pdissipated in tissue/Pinput) of the single dipole antenna, monopole antenna, and surface coil were 0.39, 0.71, and 0.93, respectively. The unloaded Q (QUL) and loaded Q (QL) of a single monopole antenna were 7.8 and 3.3, respectively, and the ratio of QUL/QL was 2.3. QUL and QL of a single surface coil were 150 and 60, respectively, and QUL/QL was 2.5. Figure 3Open in figure viewerPowerPoint Design of simulation model and the simulated results of receiver sensitivity and transmit efficiency of single dipole antenna, single monopole antenna and surface coil, respectively. Images (a–c) are simulation models of single dipole antenna, monopole antenna, and surface coil, respectively. Images (d, g, j, and m) are simulated receiver sensitivity and transmit efficiency of single dipole antenna in sagittal and axial orientation; (e, h, k, and n) are simulated receiver sensitivity and transmit efficiency of single monopole antenna in sagittal and axial orientation; (f, i, l, and o) are simulated receiver sensitivity and transmit efficiency of single surface coil in sagittal and axial orientation. Figure 4 shows the transmit properties of the monopole antenna array and surface coil array obtained by experiment and simulation, respectively. The monopole antenna array (Fig. 4a–c) exhibited a strong flip angle distribution at the center and superior aspect of the brain, whereas the surface coil array (Fig. 4d–f) has strong flip angle distributions only at the center of the brain. Figure 4g–l show the corresponding simulated B1+ field distributions obtained with the FDTD simulation. The results of the simulation appeared quite similar to the measured flip angle maps in Figure 4a–f. The simulated B1+ data in Figure 4 was calibrated with ROI 1. The monopole antenna array provided an average flip angle of 20.9° with a standard deviation of 9.5° for the entire head; for the surface coil array, the values are 19.5° and 6.84°, respectively. The nonuniformity of the monopole array and surface array were 45% and 35%, respectively. In Table 1, the simulated SAR results of two arrays are shown. The average and maximum SAR values were calculated with three different ROIs because the location of the 90° calibration could influence the SAR values. The size of the ROIs was 2 × 2 mm2. The surface coil array displayed a minimum SAR value when it was calibrated at ROI 1, whereas a minimum SAR was observed at ROI 2 for the monopole antenna array. Figure 4Open in figure viewerPowerPoint Evaluation of the transmit properties of the eight channel monopole antenna array and the eight channel surface coil array. Images (a–f) are measured flip angle map in sagittal, coronal and axial orientation and (g–l) are corresponding simulated B1+ distribution. The arrows at (k) indicate the locations of normalization point for SAR calculation. Table 1. Global and Maximum 10 g SAR Obtained from FDTD Simulation SARglobal (W/kg) SARMax. 10 g (W/kg) ROI 1 Surface array 1.05 3.12 Monopole array 1.09 7.48 ROI 2 Surface array 1.59 4.71 Monopole array 0.86 5.90 ROI 3 Surface array 2.43 7.23 Monopole array 1.85 12.71 Global SAR and maximum 10 g SAR values were evaluated over the entire head. Input power was scaled to achieve a 90° pulse with 3 ms duration at the three different ROIs. ROI 1 is isocenter of monopole antenna and surface array. ROI 2 is located at +3 cm in the z-direction and ROI 3 is located at +3 cm in the x-direction. Figure 5 shows the experimentally acquired individual sensitivity maps of the eight-channel monopole antenna array and surface coil array. The individual sensitivity maps were generated by dividing the SNR maps of uncombined images by the flip angle distribution (Fig. 4c,f). Table 2 shows the sensitivity values for the individual elements in square boxes (Fig. 5). The single elements of the monopole antenna array provided an average sensitivity value of 359, whereas the single elements of the surface coil array provided an average sensitivity value of 226 at the center of the brain. Figure 5Open in figure viewerPowerPoint Experimentally obtained individual sensitivity of single monopole antenna and surface coils. The individual sensitivity maps were generated by dividing the SNR map of uncombined images by flip angle distribution. The square boxes indicate the location of ROIs where sensitivity was measured. Table 2. Measured Sensitivity of Individual Monopole Antenna and Surface Coil at the Center of the Brain Channel number Monopole antenna Surface coil 1 381 218 2 315 322 3 361 196 4 375 177 5 333 179 6 321 272 7 391 198 8 394 252 The sensitivity was measured at the center of the brain with 5 × 5 cm2 ROI. Figure 6 shows the combined SNR maps and sensitivity maps in the sagittal, coronal, and axial orientations. The monopole antenna array provided a strong SNR at the center as well as the superior aspect of the brain, whereas the surface coil array provided a maximum SNR near the surface coils. The surface coil array provided a strong sensitivity only near the surface coils within a depth of 5 cm, whereas the monopole antenna array provided homogeneous sensitivity over the entire brain. Figure 7 shows the sensitivity profiles of Figure 6g–l. In the sagittal view profile in Figure 7, the monopole antenna array provided higher sensitivity over the entire range. In the axial profile in Figure 7, the surface coil array provided strong sensitivity only near the surface coils, whereas the monopole antenna array provided uniform sensitivity profiles with higher sensitivity at the center of the brain. The surface coil array provided strong sensitivity near the surface coil, but the monopole antenna array provided uniform sensitivity over the entire brain and a sensitivity gain at center and superior aspect of the brain. The sensitivity of the surface array ranged from 530 to 2400, with an average of 1184 in the axial profile (Fig. 7). For the monopole antenna array, these numbers ranged from 903 to 1724, with an average of 1120. The sensitivity of the surface array ranged from 460 to 1124, with an average of 591 in the sagittal profile (Fig. 7). For the monopole array, these numbers ranged from 218 to 1463, with an average of 908. Figure 6Open in figure viewerPowerPoint Images (a–c) are the combined SNR map of monopole antenna array in sagittal, coronal, and axial orientation. Images (g–i) are corresponding sensitivity, and (d–f) are the SNR maps of surface coil array in sagittal, coronal, and axial orientation. Images (j–l) are corresponding sensitivity. Figure 7Open in figure viewerPowerPoint Sensitivity profiles of two arrays through the lines on Figure 6g, 6i, 6j and 6l. In sagittal view profiles, monopole antenna array provides higher sensitivity over the whole range. In axial view profiles, surface coil array provide strong sensitivity only at the periphery, while the monopole antenna array provides uniform profile with higher sensitivity nearly 1.5 fold at center of the brain. Figure 8 shows a set of T2*-weighted gradient echo images of the monopole and surface array in the sagittal and axial orientations. The T2*-weighted gradient echo images display anatomical features such as the detailed structure of the brain stem and small venous structures. In spite of the low sensitivity of the monopole antenna array compared with the surface coil array at the cerebellum, the gray/white matter contrast is fairly uniform throughout the hemisphere compared with the surface coil array. Figure 8Open in figure viewerPowerPoint Images (a) and (b) are T2*-weighted gradient echo images of monopole antenna array in sagittal and axial orientation; (c) and (d) are T2*-weighted gradient echo images of surface coil array. These images are without intensity correction. DISCUSSION The measured and simulated results for single elements show that the monopole antenna provides symmetric and higher receiver sensitivity at the center of the brain. On the other hand, the surface coil provides strong sensitivity near the surface coil with an asymmetric receiver sensitivity pattern, resulting in degradation of the receiver sensitivity at the center of the brain. In contrast, both the dipole antenna and monopole antenna provide a symmetric receiver sensitivity pattern and a bigger penetration depth. The receiver sensitivity pattern of the monopole antenna and dipole antenna seems to be a property of radiative antennas. Radiative antennas such as dipole antennas generate a poynting vector that is directed toward the object (14). A monopole antenna has a similar receiver sensitivity pattern as a dipole antenna in single-element simulations. A recent study showed that the dipole antenna provided a lower SNR compared with a surface coil array (15). However, our simulation results (Fig. 3) show that the dipole antenna provided higher sensitivity at the center of the brain. This lower SNR for the dipole antenna array could be due to its folded design. The monopole antenna had a 1.23-fold gain of receiver sensitivity compared with the dipole antenna. Therefore, the monopole antenna is better suited for the design of a 7T MRI, rather than the folded design (15). QUL and QL of the monopole antenna were 7.8 and 3.3, respectively. These values were significantly lower than those of the surface coil. These low Q values of the monopole antenna were probably due to the impedance of the monopole antenna. The intrinsic impedance of the monopole antenna was 36 + i26 Ω, whereas the surface coil has only a conductor resistance and sample-induced resistance, which are considerably smaller than the impedance of the monopole antenna (the Q factor of an RLC circuit is Q = w0L/R). Although a monopole antenna has an extremely low Q value, it provided bigger penetration depth for transmission and sensitivity. This bigger penetration was evaluated with a simulation (Fig. 3). The surface coil array required a 310-V reference voltage for a 180° pulse. The surface coil array was less efficient than the monopole antenna array, but this result was not correlated with the Q value. However, the transmit efficiency shows that the monopole antenna provides bigger penetration at the center of the brain. The power efficiency was highest for the surface coil, yet it had the lowest sensitivity at the center of the brain. The power efficiency does not account for the field distribution, including the penetration depth and symmetry, because this value is the ratio of the absorbed power divided by the input power over the entire head. The surface coil provided a higher transmit efficiency and sensitivity near the surface coil. However, the asymmetric field pattern of the surface coil degraded the penetration depth and has the lowest sensitivity at the center of the brain. A minor drawback of the monopole antenna array was that it showed strong sensitivity only within a 10-cm coverage in the z-direction and weak sensitivity in the inferior aspect of the brain (brain stem area). Additionally, the sensitivity value was degraded in the area at the cerebellum. The length of the monopole antenna could be extended by a series connection of the capacitors that might allow for an increase in the sensitivity for the inferior aspect of the brain. Modification of the ground plate is another way to overcome the short coverage. For example, the dome shape at the ground plate would allow the head to be placed deeper in the z-direction, thus increasing the sensitivity. The monopole antenna array provided a strong flip angle distribution at the center as well as the superior aspect of the brain, whereas the surface coil array provided a strong flip angle only at the center of the brain. These results were similar to the simulated B1+ field distribution. Both the monopole antenna array and the surface coil array exhibited a center brightening pattern. Further improvement of the B1+ field uniformity in the axial orientation might be achieved by the application of RF shimming (21, 22), such as magnitude and phase optimization. The maximum 10-g SAR of the monopole antenna array was higher compared to that of the surface coil array for all three cases of the calibration location (ROI 1, 2, and 3). However, the global SAR values of the monopole antenna array were similar or lower than that of the surface coil array. This difference between the maximum 10-g SAR and global SAR was because of the averaging method used, because the monopole antenna array generates a strong electric field mostly at the upper part of the head compared to the surface coil array. The global SAR values were acquired from the chin to the top of the head. The monopole antenna had a low S11 of −10.9 dB. Moreover, the connection of the series capacitance cancelled out the imaginary impedance, so the reflection coefficient could be reduced to as low as −15 dB. However, this matching was not available with head loading. After head loading, S11 returned to −11 dB. Additionally, the capacitive coupling matching network degraded the matching. Therefore, further study would be needed for the matching network of the monopole antenna for MRI applications. The noise correlation matrix exhibited a strong coupling effect for the monopole antenna array. The maximum S21 of the monopole antenna array and surface coil array were −9.3 dB and −12.4 dB, respectively. Further increases of the number of monopole antennas would require careful decoupling design because the maximum S21 between nearest neighbors was −9.3 dB. Although there are some decoupling methods for the surface coil array (23-25), these methods are not yet applicable for a monopole antenna array, and further research is needed. At high frequency, the tuning of the lumped element coils became difficult because the value of the discrete capacitance became comparable to the stray capacitance. Additionally, the increased radiation loss limits the use of lumped element coils. To solve these problems, transmission line structures were utilized (26, 27). Such distributed elements can be easily operated up to 500 MHz and can also reduce radiation loss. A radiative antenna (dipole or monopole) could be easily operated at high frequencies with bigger penetration depth because these antennas are also distributed elements. However, the radiative antenna intrinsically has a large radiation loss; therefore, the performance of the monopole antenna needs to be compared with the transmission line coil for a high field application in further study. CONCLUSION The monopole antenna appears to provide symmetric and higher sensitivity at the center of the brain compared to the surface coil. In addition, the monopole antenna exhibits a gain in sensitivity compared to the dipole antenna. Therefore, the monopole antenna is found to be ideally suited for brain imaging with 7T MRI. In summary, a new design concept for a monopole antenna is performed and studied by experiment. The experimental monopole array is built with an eight-channel antenna array specifically designed for 7T MRI. Because the monopole antenna array provides uniform sensitivity and higher sensitivity at the center as well as the superior aspects of the brain compared to an eight-channel surface coil array, it appears promising for many future applications such as brain imaging of deep structures with UHF MRI, including 7T MRI. However, further studies are needed for the extension of the visual field (FoV) in the z-direction, especially near the brain stem, where many interesting areas are to be studied. ACKNOWLEDGMENTS We are grateful to Dr. Christopher M. Collins for longstanding collaboration and advice of FDTD simulation. We are also grateful to Frank H. Geschewski and Jörg Felder for the advice and collaboration, especially on the monopole antenna and flip angle mapping. Suk-Min Hong and Joshua Haekyun Park contributed equally to this work. REFERENCES 1Vaughan JT, Garwood M, Collins CM, et al. 7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images. Magn Reson Med 2001; 46: 24– 30. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar 2Abduljalil AM, Schmalbrock P, Novak V, Chakeres DW. Enhanced gray and white matter contrast of phase susceptibility-weighted images in ultra-high-field magnetic resonance imaging. J Magn Reson Imaging 2003; 18: 284– 290. Wiley Online LibraryPubMedWeb of Science®Google Scholar 3Cho ZH, Han JY, Hwang SI, Kim DS, Kim KN, Kim NB, Kim SJ, Chi JG, Park CW, Kim YB. Quantitative analysis of the hippocampus using images obtained from 7.0 T MRI. Neuroimage 2010; 49: 2134– 2140. 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