Decoupling of a tight‐fit transceiver phased array for human brain imaging at 9.4T: Loop overlapping rediscovered

Avdievich, NI; Giapitzakis, IA; Pfrommer, A; Henning, A

doi:10.1002/mrm.26754

Cited by 29 publications

(63 citation statements)

References 31 publications

(86 reference statements)

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“…Figure A shows the S 12 matrix measured for the surface loops whilst all vertical loops were actively detuned. As demonstrated previously, at 400 MHz, overlapping provides perfect decoupling for a pair of loops of this size with a closely located sample (tight fit). As seen from Figure A, the presence of actively detuned vertical loops did not compromise the decoupling of the surface loops during transmission, and decoupling of about –30 dB or better was obtained for all adjacent surface loops.…”

Section: Resultssupporting

confidence: 72%

“…Next, we performed numerical EM simulations for the 16‐loop array and compared its performance with that of the array consisting of eight overlapped surface loops only . Figure B, C shows examples of B 1 + and SAR simulations for the 16‐loop array.…”

Section: Resultsmentioning

confidence: 99%

“…Vertical loops are positioned along the central axis (parallel to magnetic field B 0 ) of each TxRx loop, perpendicular to its surface, so as to produce an RF magnetic field perpendicular to B 0 . We evaluated the effect of these additional Rx‐only loops on the Tx efficiency of the array and the maximum local specific absorption rate (SAR), as well as the improvement in SNR and parallel Rx performance relative to an array consisting of eight surface loops only …”

Section: Introductionmentioning

confidence: 99%

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Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

et al. 2017

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Ultra-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 B ) 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.

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Section: Resultssupporting

confidence: 72%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

et al. 2017

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“…To decrease radiation losses and coupling between non‐adjacent loops, the array is shielded with the cylindrical shield placed at a 40‐mm distance from the surface loops located in the second row. The distance to the shield was chosen based on previously published data for UHF human head size arrays . Because the original 2 × 8 TxRx‐array lacks RF elements at the superior locations of a head, both Tx and Rx performances in this area are rather poor .…”

Section: Methodsmentioning

confidence: 99%

“…Previously, we demonstrated that at 400 MHz, a 2 × 8 tight‐fit human head TxRx surface loop array can be well decoupled by optimally overlapping adjacent loops without any additional decoupling methods, which substantially simplifies the entire design. This is based on our previous analytical and experimental findings that at 400 MHz the mutual resistance between adjacent overlapped loops of a tight‐fit 8‐loop single‐row (1 × 8) array is minimal, and both the mutual inductance and mutual resistance can be minimized at the same time by optimally overlapping the loops . Therefore, the 2 × 8 loop tight‐fit TxRx array decoupled by overlapping provides for a robust and safe initial design of our new ultimate head array.…”

Section: Introductionmentioning

confidence: 99%

Double‐row 18‐loop transceive–32‐loop receive tight‐fit array provides for whole‐brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T

Avdievich

Giapitzakis

Bause

et al. 2018

Magnetic Resonance in Med

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Purpose To improve the transmit (Tx) and receive (Rx) performance of a human head array and provide whole‐brain coverage at 9.4T, a novel 32‐element array design was developed, constructed, and tested. Methods The array consists of 18 transceiver (TxRx) surface loops and 14 Rx‐only vertical loops all placed in a single layer. The new design combines benefits of both TxRx and transmit‐only–receive‐only (ToRo) designs. The general idea of the design is that the total number of array elements (both TxRx and Rx) should not exceed the number of required Rx elements. First, the necessary number of TxRx loops is placed around the object tightly to optimize the Tx performance. The rest of the elements are loops, which are used only for reception. We also compared the performance of the new array with that of a state‐of‐the‐art ToRo array consisting of 16 Tx‐only loops and 31 Rx‐only loops. Results The new array provides whole‐brain coverage, ~1.5 times greater Tx efficiency and 1.3 times higher SNR near the brain center as compared to the ToRo array, while the latter delivers higher (up to 1.5 times) peripheral SNR. Conclusion In general, the new approach of constructing a single‐layer array consisting of both TxRx‐ and Rx‐only elements simplifies the array construction by minimizing the total number of elements and makes the entire design more robust and, therefore, safe. Overall, our work provides a recipe for a Tx‐ and Rx‐efficient head array coil suitable for parallel transmission and reception as well as whole‐brain imaging at UHF.

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Metabolite‐cycled STEAM and semi‐LASER localization for MR spectroscopy of the human brain at 9.4T

Giapitzakis

Shao

Avdievich

et al. 2017

Magnetic Resonance in Med

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Decoupling of a tight‐fit transceiver phased array for human brain imaging at 9.4T: Loop overlapping rediscovered

Cited by 29 publications

References 31 publications

Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

Combination of surface and ‘vertical’ loop elements improves receive performance of a human head transceiver array at 9.4 T

Double‐row 18‐loop transceive–32‐loop receive tight‐fit array provides for whole‐brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T

Metabolite‐cycled STEAM and semi‐LASER localization for MR spectroscopy of the human brain at 9.4T

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