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--- Timezone: CEST
Creation date: 2017-08-19
Creation time: 09-17-20
--- Number of references
15
article
AvdievichPGH2017
Analytical modeling provides new insight into complex mutual coupling between surface loops at ultrahigh fields
NMR in Biomedicine
2017
6
Epub ahead
Ultrahigh-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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://onlinelibrary.wiley.com/doi/10.1002/nbm.3759/epdf
10.1002/nbm.3759
navdieviNAvdievich
apfrommerAPfrommer
igiapitzakisIAGiapitzakis
ahenningAHenninmg
article
AvdievichGPH2017
Decoupling of a tight-fit transceiver phased array for human brain imaging at 9.4T: Loop overlapping rediscovered
Magnetic Resonance in Medicine
2017
6
Epub ahead
Purpose
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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://onlinelibrary.wiley.com/doi/10.1002/mrm.26754/epdf
10.1002/mrm.26754
navdieviNIAvdievich
igiapitzakisI-AGiapitzakis
apfrommerAPfrommer
ahenningAHenning
article
PfrommerH2017
On the Contribution of Curl-Free Current Patterns to the Ultimate Intrinsic Signal-to-Noise Ratio at Ultra-High Field Strength
NMR in Biomedicine
2017
5
30
5
1-16
The 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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://onlinelibrary.wiley.com/doi/10.1002/nbm.3691/epdf
10.1002/nbm.3691
e3691
apfrommerAPfrommer
ahenningAHenning
article
AvdievichHSGSH2016
Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T
NMR in Biomedicine
2017
2
30
2
1-12
Ultra-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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
Department Scheffler
http://onlinelibrary.wiley.com/doi/10.1002/nbm.3680/epdf
10.1002/nbm.3680
e3680
navdieviNIAvdievich
tatumJHoffmann
shajangGShajan
apfrommerAPfrommer
igiapitzakisIAGiapitzakis
schefflerKScheffler
ahenningAHenning
inproceedings
PfrommerH2015_3
Optimal Arrangement of Finite Element Loop Arrays for Parallel Magnetic Resonance Imaging in the Human Head at 400 MHz
2015
5
1-4
Parallel 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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7166753
IEEE
Piscataway, NJ, USA
Phoenix, AZ, USA
IEEE MTT-S International Microwave Symposium (IMS 2015)
978-1-4799-8276-9
10.1109/MWSYM.2015.7166753
apfrommerAPfrommer
ahenningAHenning
poster
PfrommerAH2016_2
Effect of the RF Shield on the Mutual Coupling Between Adjacent and Non-Adjacent Array Elements
2016
5
11
2159
In 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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://www.ismrm.org/16/program_files/TP08.htm
Singapore
24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016)
apfrommerAPfrommer
navdieviNIAvdievich
ahenningAHenning
poster
AvdievichPGH2016
Analytical Modeling of the Coupling within a Human Head Surface Loop Transmit Phased Array at Ultra-High Fields
2016
5
10
3525
Decoupling 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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://www.ismrm.org/16/program_files/EP10.htm
Singapore
24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016)
navdieviNIAvdievich
apfrommerAPfrommer
igiapitzakisIAGiapitzakis
ahenningAHenning
poster
PfrommerH2015_4
Optimal Arrangement of Finite Element Loop Arrays for Parallel Imaging in a Spherical Geometry at 9.4 T
2015
6
1
3165
Parallel 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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://www.ismrm.org/15/program_files/MonEPS03.htm
Toronto, Canada
23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015)
apfrommerAPfrommer
ahenningAHenning
poster
PfrommerAH2014
Four Channel Transceiver Array for Functional Magnetic Resonance Spectroscopy in the Human Visual Cortex at 9.4 T
2014
5
12
2014
1305
RF 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.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
http://www.ismrm.org/14/program_files/TP04.htm
Milano, Italy
Joint Annual Meeting ISMRM-ESMRMB 2014
apfrommerAPfrommer
navdieviNAvdievich
ahenningAHenning
conference
PfrommerH2017_2
The ultimate intrinsic SNR in a spherical phantom with regard to an open-pole surface current distribution at 9.4T
2017
4
26
460
RF coils for human head imaging need to provide access for the human neck and cannot be entirely closed. In this work, we investigate the ultimate intrinsic signal-to-noise ratio (UISNR) in a spherical phantom due to an open-pole surface current distribution, where the generic surface current patterns run on a spherical cap. The influence of the cap’s opening angle ϑ0 on UISNR, parallel imaging performance and on the contribution of curl-free and divergence-free current patterns to UISNR is studied.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
Abstract Talk
http://www.ismrm.org/17/program_files/CE11.htm
Honolulu, HI, USA
25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)
apfrommerAPfrommer
ahenningAHenning
conference
PfrommerH2016
Comparison of the ultimate intrinsic SNR in a spherical
phantom vs a realisitc human head model at 9.4 T
Magnetic Resonance Materials in Physics, Biology and Medicine
2016
10
1
29
Supplement 1
S308-S309
Purpose/Introduction: To have a performance measure for RF
receive coils at ultra-high field strength the ultimate intrinsic signal to-noise ratio (UISNR) was first calculated within a simple spherical model (1–3). Recently with the advance of ultrafast volume integral solvers (4) the UISNR could also be evaluated within a realistic
human body model (5, 6). In this study we compared the UISNR
obtained in a spherical phantom versus a realistic human head model.
Subjects and Methods: We used the voxel model Duke with 5 mm
isotropic resolution (7). The homogeneous spherical phantom had a radius of 10.3 cm with relative permittivity of 40 and conductivity of 0.6 S/m. For both setups (Fig 1) a generic surface current with curlfree and divergence-free patterns was running on a spherical surface of radius 14 cm. Thereby a basis set of vector spherical harmonics
was used with expansion order of 60. The electromagnetic fields in Duke created by the basis current set were obtained with MARIE (8), following the procedure described in (6). For the spherical phantom the electromagnetic field problem was solved with dyadic Green’s functions (9) in a similar way as in (3).
Results: In Fig. 2 we show the spatial distribution of the unaccelerated UISNR for Duke and the spherical phantom. In both models the maximum UISNR is located in the periphery. Regarding the spherical model the UISNR in the center is about 30 dB lower than in the periphery. The SNR damping in Duke is more than 40 dB in the midbrain and cerebellum and about 30 dB averaged over grey and white matter. Additionally we examined parallel imaging performance
and plotted exemplary g-factor maps for 5 9 5 acceleration in LR- and AP direction (Fig. 2 bottom). The maximum g-factor occurring in the sphere is 1.68 as opposed to 1.25 in Duke, corresponding to an overestimation of 34 %. In Fig. 3 we visualized the contribution of loop-like current patterns to the UISNR, which is complemented by dipole-like current patterns to reach the maximum possible SNR. If loops are the only receiving elements, for the spherical phantom it is possible to gain the total SNR in the center
whereas only 67 % are reached in Duke’s midbrain.
Discussion/Conclusion: The comparison of the UISNR in a realistic human head model with a simple spherical phantom revealed significant differences. As RF arrays are built for the human body the UISNR should consequently be evaluated in a body model.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
Abstract Talk
http://link.springer.com/content/pdf/10.1007%2Fs10334-016-0570-3.pdf
Wien, Austria
33rd Annual Scientific Meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRMB 2016)
10.1007/s10334-016-0570-3
apfrommerAPfrommer
ahenningAHenning
conference
PfrommerAH2016
About the Ultimate SNR for Cylindrical and Spherical RF Arrays in a Realistic Human Head Model
2016
5
9
0175
In this work we investigated differences in the ultimate SNR in a realistic human head model for two configurations with the RF array elements distributed on either a cylindrical or a spherical holder. The basis set of solutions in our approach was created by vector cylindrical and spherical harmonics, which are known to form a complete set of eigenfunctions to Maxwell’s equations in free-space. Assuming both surfaces have the same radius, the spherical geometry yielded higher SNR in grey and white matter compared to the cylindrical one. Moreover it allowed higher acceleration factors with the same g-factors.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
Abstract Talk
http://www.ismrm.org/16/program_files/O40.htm
Singapore
24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016)
apfrommerAPfrommer
navdieviNIAvdievich
ahenningAHenning
conference
AvdievichGPH2016
Optimization of the Transceiver Phased Array for Human Brain Imaging at 9.4T: Loop Overlapping Rediscovered
2016
5
9
0169
Ultra-high field (UHF) (>7T) transmit (Tx) and transceiver surface loop phased arrays improve Tx-efficiency and homogeneity for human brain imaging. Overlapping the loops enhances Tx-efficiency and SNR by increasing the penetration depth. However, overlapping can compromise decoupling and SNR by generating a substantial mutual resistance. Therefore, UHF Tx-arrays are commonly constructed using gapped loops. Based on analytical optimization we constructed a 9.4T 8-loop head transceiver array. Both the magnetic and electric coupling were compensated at the same time by overlapping and excellent decoupling was obtained. Tx- and Rx-performance of the array was compared favorably to that of a gaped array.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
Abstract Talk
http://www.ismrm.org/16/program_files/O40.htm
Singapore
24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2016)
navdieviNIAvdievich
igiapitzakisIAGiapitzakis
apfrommerAPfrommer
ahenningAHenning
conference
PfrommerH2015
On the Contribution of Electric-Type Current Patterns to UISNR for a Spherical Geometry at 9.4 T
2015
6
4
23
0856
Parallel imaging is intrinsically limited by Maxwell’s equations. A complete set of vector solutions to the Helmholtz equation consists of both curl-free and divergence-free fields. In this study we investigated the contribution of electric-type current patterns to UISNR for different voxel positions and acceleration factors in a spherical model at 9.4T. For moderate acceleration the electric mode increased UISNR by maximally 55%. For very high acceleration, however, UISNR was mostly caused by the magnetic mode. The reason for this might be the much faster growing power loss of the electric mode with respect to the expansion order.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
Abstract Talk
http://www.ismrm.org/15/program_files/ThuSci07.htm
Toronto, Canada
23rd Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2015)
apfrommerAPfrommer
ahenningAHenning
conference
AvdievichPHCSH2014
Tranceive Phased Array with High Transmit Performance for Human Brain Application at 9.4 T
2014
5
14
0622
Surface loop phased arrays have been shown to improve transmit performance and B1 homogeneity for head imaging up to 9.4T. However, transmit arrays enlarged to fit receive arrays often cannot satisfy the requirements in B1 and bandwidth for ultra-high field spectroscopic imaging. We have developed and constructed a tight fit 400MHz 8-channel transceiver array. The array improved transmit efficiency and homogeneity in the axial slab through the phantom’s center when used in CP mode. B1+ averaged over the central axial slice measured 55.6 nT/V, which corresponds to 12.4 uT per 1 kW of RF power delivered directly to the array.
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
http://www.kyb.tuebingen.mpg.de
Research Group Henning
Department Scheffler
Abstract Talk
http://www.ismrm.org/14/program_files/Session49.htm
Milano, Italy
Joint Annual Meeting ISMRM-ESMRMB 2014
navdieviNIAvdievich
apfrommerAPfrommer
tatumJHoffmann
gzchadzyGLChadzynski
schefflerKScheffler
ahenningAHenning