The ultimate intrinsic signal‐to‐noise ratio of loop‐ and dipole‐like current patterns in a realistic human head model

Pfrommer, A; Henning, A

doi:10.1002/mrm.27169

Cited by 28 publications

(60 citation statements)

References 45 publications

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“…Interestingly, increase of SNR at the periphery in the transversal slice was substantially less (10–30%), which indicates that at UHF the RF field produced by vertical loops decreases substantially slower with increasing depth than that of surface loops (i.e., vertical loops have a greater penetration depth). This fact is consistent with previously published data obtained using ultimate intrinsic SNR (uISNR) calculations . It is also important that the new array delivers ~1.3 times SNR enhancement near the brain center as compared to the ToRo array.…”

Section: Discussionmentioning

confidence: 95%

“…A realistic array holder has to include openings for the face and neck, which is difficult to incorporate into a simplified analytical model. The presence of these openings, which do not have any elements, affects the array performance at the center …”

Section: Discussionmentioning

confidence: 99%

“…Adding a curl‐free pattern, which is partially associated with dipole antennas, helps to further improve SNR. The situation is even more pronounced for a cylindrical holder, where it was shown that z ‐directed (i.e., dipole‐like) current provides higher contribution to the central SNR than divergent‐free current at fields higher than 9.4 T. However, even within these numerical studies, it is not yet feasible to model an optimal current distribution on a surface representing a realistic array holder and to make theoretical predictions relevant to a comparison between specific array designs.…”

Section: Discussionmentioning

confidence: 99%

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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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Section: Discussionmentioning

confidence: 95%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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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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“…However, for small TR under 10 ms (that results in a stronger microvascular signal contribution, see Figure ), the signal change always increases with field strength for all sequence types. Note that all calculations assume a constant M 0 and do not include the increase of SNR with B 0 with the power of 1.5–1.7 …”

Section: Discussionmentioning

confidence: 99%

The BOLD sensitivity of rapid steady‐state sequences

Scheffler

Heule

Báez-Yáñez

et al. 2018

Magnetic Resonance in Med

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Purpose Relaxation and dephasing of water protons embedded in a vascular network is driven by local magnetic field inhomogeneities around deoxygenated blood vessels. These effects strongly depend on the relation between mean diffusion length and diameter of blood vessels, as well as on the chosen imaging sequence. In this work, the BOLD sensitivity of steady‐state sequences as a function of vessel size, field strength, and sequence parameters are analyzed. Methods Steady‐state magnetization within a network of artificial cylinders is simulated with Monte Carlo methods for different coherence pathways. In addition, measurements on microspheres were performed to confirm theoretical results. Results Simulations and phantom results demonstrate a vessel size‐dependent signal attenuation effect of all coherence pathways. Both the FID and ECHO pathways show a signal profile similar to spin echo sequences where in the static dephasing regime the effect of larger vessels is suppressed. Conclusion The BOLD effect measured in steady‐state sequences is most sensitive to microvessels and might therefore be closer to the underlying neuronal event compared to gradient echo sequences.

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“…magnetic field strengths of 7 T and higher). In fact, the SNR has been shown to increase at a super‐linear rate with respect to the field strength . For magnetic resonance spectroscopic imaging (MRSI), the use of higher field strength can lead to images with higher spatial resolution and spectra with higher spectral resolution …”

Section: Introductionmentioning

confidence: 99%

Over‐discretized SENSE reconstruction and B₀ correction for accelerated non‐lipid‐suppressed ¹H FID MRSI of the human brain at 9.4 T

et al. 2018

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The aim of this work was to use post‐processing methods to improve the data quality of metabolite maps acquired on the human brain at 9.4 T with accelerated acquisition schemes. This was accomplished by combining an improved sensitivity encoding (SENSE) reconstruction with a B0 correction of spatially over‐discretized magnetic resonance spectroscopic imaging (MRSI) data. Since MRSI scans suffer from long scan duration, investigating different acceleration techniques has recently been the focus of several studies. Due to strong B0 inhomogeneity and strict specific absorption rate (SAR) limitations at ultra‐high fields, the use of a low‐SAR sequence combined with an acceleration technique that is compatible with dynamic B0 shim updating is preferable. Hence, in this study, a non‐lipid‐suppressed ultra‐short TE and TR 1H free induction decay MRSI sequence is combined with an in‐plane SENSE acceleration technique to obtain high‐resolution metabolite maps in a clinically feasible scan time. One of the major issues in applying parallel imaging techniques to non‐lipid‐suppressed MRSI is the presence of strong lipid aliasing artifacts, which if not thoroughly resolved will hinder the accurate quantification of the metabolites of interest. To achieve a more robust reconstruction, an over‐discretized SENSE reconstruction (with direct control over the shape of the spatial response function) was combined with an over‐discretized B0 correction. This method is compared with conventional SENSE reconstruction for seven acceleration schemes on four healthy volunteers. The over‐discretized method consistently outperformed conventional SENSE, resulting in an average of 23 ± 1.2% higher signal‐to‐noise ratio and 8 ± 2.9% less error in the fitting of the N‐acetylaspartate signal over a whole brain slice. The highest achievable acceleration factor with the proposed technique was determined to be 4. Finally, using the over‐discretized method, high‐resolution (97 μL nominal voxel size) metabolite maps can be acquired in 3.75 min at 9.4 T. This enables the acquisition of high‐resolution metabolite maps with more spatial coverage at ultra‐high fields.

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The ultimate intrinsic signal‐to‐noise ratio of loop‐ and dipole‐like current patterns in a realistic human head model

Abstract: The combination of loop and dipole elements could enhance the SNR performance in the human head at ultra-high field strength.

Cited by 28 publications

References 45 publications

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

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

The BOLD sensitivity of rapid steady‐state sequences

Over‐discretized SENSE reconstruction and B₀ correction for accelerated non‐lipid‐suppressed ¹H FID MRSI of the human brain at 9.4 T

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The ultimate intrinsic signal‐to‐noise ratio of loop‐ and dipole‐like current patterns in a realistic human head model

Abstract: The combination of loop and dipole elements could enhance the SNR performance in the human head at ultra-high field strength.

Cited by 28 publications

References 45 publications

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

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

The BOLD sensitivity of rapid steady‐state sequences

Over‐discretized SENSE reconstruction and B0 correction for accelerated non‐lipid‐suppressed 1H FID MRSI of the human brain at 9.4 T

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Over‐discretized SENSE reconstruction and B₀ correction for accelerated non‐lipid‐suppressed ¹H FID MRSI of the human brain at 9.4 T