AvdievichGPSSH20183NIAvdievichIAGiapitzakisAPfrommerGShajanKSchefflerAHenning2018-07-00Epub aheadOne of the major challenges in constructing multi‐channel and multi‐row transmit (Tx) or transceiver (TxRx) arrays is the decoupling of the array's loop elements. Overlapping of the surface loops allows the decoupling of adjacent elements and also helps to improve the radiofrequency field profile by increasing the penetration depth and eliminating voids between the loops. This also simplifies the design by reducing the number of decoupling circuits. At the same time, overlapping may compromise decoupling by generating high resistive (electric) coupling near the overlap, which cannot be compensated for by common decoupling techniques. Previously, based on analytical modeling, we demonstrated that electric coupling has strong frequency and loading dependence, and, at 9.4 T, both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping the loops. As a result, excellent decoupling was obtained between adjacent loops of an eight‐loop single‐row (1 × 8) human head tight‐fit TxRx array. In this work, we designed and constructed a 9.4‐T (400‐MHz) 16‐loop double‐row (2 × 8) overlapped TxRx head array based on the results of the analytical and numerical electromagnetic modeling. We demonstrated that, simply by the optimal overlap of array loops, a very good decoupling can be obtained without additional decoupling strategies. The constructed TxRx array provides whole‐brain coverage and approximately 1.5 times greater Tx efficiency relative to a transmit‐only/receive‐only (ToRo) array, which consists of a larger Tx‐only array and a nested tight‐fit 31‐loop receive (Rx)‐only array. At the same time, the ToRo array provides greater peripheral signal‐to‐noise ratio (SNR) and better Rx parallel performance in the head–feet direction. Overall, our work provides a recipe for a simple, robust and very Tx‐efficient design suitable for parallel transmission and whole‐brain imaging at ultra‐high fields.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Decoupling of a double‐row 16‐element tight‐fit transceiver phased array for human whole‐brain imaging at 9.4 T150171882115017PfrommerH20183APfrommerAHenning2018-03-00Epub aheadPurpose
The ultimate intrinsic signal-to-noise ratio (UISNR) represents an upper bound for the achievable SNR of any receive coil. To reach this threshold a complete basis set of equivalent surface currents is required. This study systematically investigated to what extent either loop- or dipole-like current patterns are able to reach the UISNR threshold in a realistic human head model between 1.5 T and 11.7 T. Based on this analysis, we derived guidelines for coil designers to choose the best array element at a given field strength. Moreover, we present ideal current patterns yielding the UISNR in a realistic body model.
Methods
We distributed generic current patterns on a cylindrical and helmet-shaped surface around a realistic human head model. We excited electromagnetic fields in the human head by using eigenfunctions of the spherical and cylindrical Helmholtz operator. The electromagnetic field problem was solved by a fast volume integral equation solver.
Results
At 7 T and above, adding curl-free current patterns to divergence-free current patterns substantially increased the SNR in the human head (locally >20%). This was true for the helmet-shaped and the cylindrical surface. On the cylindrical surface, dipole-like current patterns had high SNR performance in central regions at ultra-high field strength. The UISNR increased superlinearly with B0 in most parts of the cerebrum but only sublinearly in the periphery of the human head.
Conclusion
The combination of loop and dipole elements could enhance the SNR performance in the human head at ultra-high field strength.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0The ultimate intrinsic signal-to-noise ratio of loop- and dipole-like current patterns in a realistic human head model15017AvdievichGPBH20173NIAvdievichIAGiapitzakisAPfrommerTBorbathAHenning2018-02-00231113Ultra-high-field (UHF, ≥7 T) human magnetic resonance imaging (MRI) provides undisputed advantages over low-field MRI (≤3 T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight-fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx-only arrays, which are larger in order to fit multi-channel receive (Rx)-only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high-power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16-channel tight-fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx-only vertical loops positioned along the central axis (parallel to the magnetic field B0) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16-element array provided 30% improvement of central signal-to-noise ratio (SNR) in vivo relative to a surface loop eight-element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published12Combination of surface and "vertical" loop elements improves receive performance of a human head transceiver array at 9.4 T15017AvdievichGPH20173NIAvdievichI-AGiapitzakisAPfrommerAHenning2018-02-002791200–1211Purpose
To improve the decoupling of a transceiver human head phased array at ultra-high fields (UHF, ≥ 7T) and to optimize its transmit (Tx) and receive (Rx) performance, a single-row eight-element (1 × 8) tight-fit transceiver overlapped loop array was developed and constructed. Overlapping the loops increases the RF field penetration depth but can compromise decoupling by generating substantial mutual resistance.
Methods
Based on analytical modeling, we optimized the loop geometry and relative positioning to simultaneously minimize the resistive and inductive coupling and constructed a 9.4T eight-loop transceiver head phased array decoupled entirely by overlapping loops.
Results
We demonstrated that both the magnetic and electric coupling between adjacent loops is compensated at the same time by overlapping and nearly perfect decoupling (below -30 dB) can be obtained without additional decoupling strategies. Tx-efficiency and SNR of the overlapped array outperformed that of a common UHF gapped array of similar dimensions. Parallel Rx-performance was also not compromised due to overlapping the loops.
Conclusion
As a proof of concept we developed and constructed a 9.4T (400 MHz) overlapped transceiver head array based on results of the analytical modeling. We demonstrated that at UHF overlapping loops not only provides excellent decoupling but also improves both Tx- and Rx-performance.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published-1200Decoupling of a tight-fit transceiver phased array for human brain imaging at 9.4T: Loop overlapping rediscovered15017AvdievichGPH20183NAvdievichI-AGiapitzakisAPfrommerAHenning2018-01-00nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/accepted0Decoupling of a Double-Row 16-element Tight-Fit Transceiver Phased Array for Human Whole Brain Imaging at 9.4T15017AvdievichPGH20173NAvdievichAPfrommerIAGiapitzakisAHenning2017-10-001030113Ultrahigh-field (UHF) (≥7 T) transmit (Tx) human head surface loop phased arrays improve both the Tx efficiency (B1+/√P) and homogeneity in comparison with single-channel quadrature Tx volume coils. For multi-channel arrays, decoupling becomes one of the major problems during the design process. Further insight into the coupling between array elements and its dependence on various factors can facilitate array development. The evaluation of the entire impedance matrix Z for an array loaded with a realistic voxel model or phantom is a time-consuming procedure when performed using electromagnetic (EM) solvers. This motivates the development of an analytical model, which could provide a quick assessment of the Z-matrix. In this work, an analytical model based on dyadic Green's functions was developed and validated using an EM solver and bench measurements. The model evaluates the complex coupling, including both the electric (mutual resistance) and magnetic (mutual inductance) coupling. Validation demonstrated that the model does well to describe the coupling at lower fields (≤3 T). At UHFs, the model also performs well for a practical case of low magnetic coupling. Based on the modeling, the geometry of a 400-MHz, two-loop transceiver array was optimized, such that, by simply overlapping the loops, both the mutual inductance and the mutual resistance were compensated at the same time. As a result, excellent decoupling (below −40 dB) was obtained without any additional decoupling circuits. An overlapped array prototype was compared (signal-to-noise ratio, Tx efficiency) favorably to a gapped array, a geometry which has been utilized previously in designs of UHF Tx arrays.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published12Analytical modeling provides new insight into complex mutual coupling between surface loops at ultrahigh fields15017PfrommerH20173APfrommerAHenning2017-05-00530116The ultimate intrinsic signal-to-noise ratio (SNR) is a coil independent performance measure to compare different receive coil designs. To evaluate this benchmark in a sample, a complete electromagnetic basis set is required. The basis set can be obtained by curl-free and divergence-free surface current distributions, which excite linearly independent solutions to Maxwell's equations. In this work, we quantitatively investigate the contribution of curl-free current patterns to the ultimate intrinsic SNR in a spherical head-sized model at 9.4 T. Therefore, we compare the ultimate intrinsic SNR obtained with having only curl-free or divergence-free current patterns, with the ultimate intrinsic SNR obtained from a combination of curl-free and divergence-free current patterns. The influence of parallel imaging is studied for various acceleration factors. Moreover results for different field strengths (1.5 T up to 11.7 T) are presented at specific voxel positions and acceleration factors. The full-wave electromagnetic problem is analytically solved using dyadic Green's functions. We show, that at ultra-high field strength (B0⩾7T) a combination of curl-free and divergence-free current patterns is required to achieve the best possible SNR at any position in a spherical head-sized model. On 1.5- and 3T platforms, divergence-free current patterns are sufficient to cover more than 90% of the ultimate intrinsic SNR.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published15On the Contribution of Curl-Free Current Patterns to the Ultimate Intrinsic Signal-to-Noise Ratio at Ultra-High Field Strength15017AvdievichHSGSH20163NIAvdievichJHoffmannGShajanAPfrommerIAGiapitzakisKSchefflerAHenning2017-02-00230112Ultra-high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B1+/√P) in comparison with Tx-only arrays, which are usually larger to fit receive (Rx)-only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx-only/Rx-only RF system, a single-row eight-element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx-only array, which could also be used in combination with an 18-channel Rx-only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system.
Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B1+/√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets.
We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published11Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T150171501718821PfrommerH2017_37APfrommerAHenningVerona, Italy2017-09-00684687In this study, the increase of the ultimate intrinsic signal-to-noise ratio (UISNR) with regard to main magnetic field strength B0 is investigated. A simplified spherical phantom of human head size is used. In the center of the sphere, the UISNR grows more than quadratically. Within the volume, in which the distance to the center is smaller than 85% of the sphere's radius, the UISNR increases superlinearly. At the surface, the UISNR grows only sublinearly. The SNR of curl-free current patterns grows more than cubically in the center, whereas the SNR of divergence-free current patterns increases quadratically. However, this does not imply, that curl-free modes result in higher SNR than divergence-free modes.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published3On the Superlinear Increase of the Ultimate Intrinsic Signal-to-Noise Ratio with Regard to Main Magnetic Field Strength in a Spherical Sample15017PfrommerH2015_37APfrommerAHenningPhoenix, AZ, USA2015-05-0014Parallel magnetic resonance imaging is limited by the SNR of the MR-signal detected by an antenna array. To fully exploit the SNR of circular surface loops surrounding a spherical head phantom, we developed an optimization routine to minimize the array's noise enhancement. We optimized the positioning of each array element and investigated the optimal loop radius. As a result we show optimal setups for 8, 16 and 32 array elements at 400 MHz with different rates of k-space undersampling.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published3Optimal Arrangement of Finite Element Loop Arrays for Parallel Magnetic Resonance Imaging in the Human Head at 400 MHz15017PfrommerH2018_27APfrommerAHenningParis, France2018-06-19For the first time, we present a systematic framework to assess the intrinsic SNR performance of loop-only and dipole-only receive arrays in a realistic human head model. Thereby, we distribute generic current patterns on a helmet-like and a cylindrical coil holder. These current patterns form a basis set for any kind of receive element one could place on the holder. We demonstrate how to design an ideal receive array for human head applications by using complementary current patterns.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0About the Ideal Receive Array for Human Head MRI15017PfrommerAH2016_27APfrommerNIAvdievichAHenningSingapore2016-05-11In this study we investigated the effect of an RF shield on the mutual coupling between adjacent and non-adjacent array elements in a simple model mimicking our previously developed cylindrical eight channel transceiver head array. Both numerical EM simulations and experimental measurements suggest that at 124 MHz and 400 MHz an RF shield can substantially decrease S12 for non-adjacent-array elements.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Effect of the RF Shield on the Mutual Coupling Between Adjacent and Non-Adjacent Array Elements15017AvdievichPGH20167NIAvdievichAPfrommerIAGiapitzakisAHenningSingapore2016-05-10Decoupling of multi-channel ultra-high field (>7T) transmit and transceiver arrays is a major issue. Analytical modeling of the coupling can facilitate the array optimization. We developed an analytical model describing the impedance matrix for two rectangular loops placed on a cylindrical surface and mimicking the human head array geometry. The developed model was comprehensively validated and allows for the optimization of the geometry and positioning of the loops. The latter enabled simultaneous cancellation of resistive and inductive coupling without additional decoupling circuits. The resulting overlapped array element arrangement improves both transmit and receive performance in comparison to conventional gapped arrays.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Analytical Modeling of the Coupling within a Human Head Surface Loop Transmit Phased Array at Ultra-High Fields15017PfrommerH2015_47APfrommerAHenningToronto, Canada2015-06-01Parallel imaging with a finite number of array elements is limited by the g-factor enhancement for high k-space undersampling. To fully exploit the unfolding potential of circular surface coils surrounding a spherical head phantom, we developed an optimization routine to minimize the maximum value of the g-factor inside the “head” region. As a result we showed optimal arrangements for 8, 16 and 32 channels at 9.4 T with different acceleration rates. Moreover we precisely specified the range of possible gmax values for each setup including optimal and worst case positioning of the loops.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Optimal Arrangement of Finite Element Loop Arrays for Parallel Imaging in a Spherical Geometry at 9.4 T15017PfrommerAH20147APfrommerNAvdievichAHenningMilano, Italy2014-05-12RF coils for functional magnetic resonance spectroscopy at ultra-high field strength must be designed with high SNR, high transmit efficiency and optimized to guarantee SAR safety. With numerical EM simulations we compared two possible 4 channel RF coil setups for the application in the human visual cortex. It turned out that overlapping loop elements can provide 12.5 % more B1+ /√SAR(10g) than without overlap for this particular case. Based on the simulation we have constructed a tight fit 4-channel transceiver head phased array. We could reach a B1+ of 63 µT in a 12.4x12.4 mm² sized voxel in the visual cortex region in a human head-and-shoulder phantom.nonotspecifiedhttp://www.kyb.tuebingen.mpg.de/published0Four Channel Transceiver Array for Functional Magnetic Resonance Spectroscopy in the Human Visual Cortex at 9.4 T15017PfrommerH2017_210APfrommerAHenningPfrommerH201610APfrommerAHenningPfrommerAH201610APfrommerNIAvdievichAHenningAvdievichGPH201610NIAvdievichIAGiapitzakisAPfrommerAHenningPfrommerH201510APfrommerAHenningAvdievichPHCSH201410NIAvdievichAPfrommerJHoffmannGLChadzynskiKSchefflerAHenning