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  • 1
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    Online Resource
    Bent folded‐end dipole head array for ultrahigh‐field MRI turns “dielectric resonance” from an enemy to a friend
    Wiley ; 2020
    In:  Magnetic Resonance in Medicine Vol. 84, No. 6 ( 2020-12), p. 3453-3467
    Details
    In: Magnetic Resonance in Medicine, Wiley, Vol. 84, No. 6 ( 2020-12), p. 3453-3467
    Abstract: To provide transmit whole‐brain coverage at 9.4 T using an array with only eight elements and improve the specific absorption rate (SAR) performance, a novel dipole array was developed, constructed, and tested. Methods The array consists of eight optimized bent folded‐end dipole antennas circumscribing a head. Due to the asymmetrical shape of the dipoles (bending and folding) and the presence of an RF shield near the folded portion, the array simultaneously excites two modes: a circular polarized mode of the array itself, and the TE mode (“dielectric resonance”) of the human head. Mode mixing can be controlled by changing the length of the folded portion. Due to this mixing, the new dipole array improves longitudinal coverage as compared with unfolded dipoles. By optimizing the length of the folded portion, we can also minimize the peak local SAR (pSAR) value and decouple adjacent dipole elements. Results The new array improves the SEE ( 〈 〉 /√pSAR) value by about 50%, as compared with the unfolded bent dipole array. It also provides better whole‐brain coverage compared with common single‐row eight‐element dipole arrays, or even to a more complex double‐row 16‐element surface loop array. Conclusion In general, we demonstrate that rather than compensating for the constructive interference effect using additional hardware, we can use the “dielectric resonance” to improve coverage, transmit field homogeneity, and SAR efficiency. Overall, this design approach not only improves the transmit performance in terms of the coverage and SAR, but substantially simplifies the common surface loop array design, making it more robust, and therefore safer.
    Type of Medium: Online Resource
    ISSN: 0740-3194 , 1522-2594
    URL: Issue
    URL: Article
    DOI: 10.1002/mrm.v84.6
    DOI: 10.1002/mrm.28336
    Language: English
    Publisher: Wiley
    Publication Date: 2020
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    detail.hit.zdb_id: 1493786-4
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  • 2
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    A novel method to measure T 1 ‐relaxation times of macromolecules and quantification of the macromolecular resonances
    Wiley ; 2021
    In:  Magnetic Resonance in Medicine Vol. 85, No. 2 ( 2021-02), p. 601-614
    Details
    In: Magnetic Resonance in Medicine, Wiley, Vol. 85, No. 2 ( 2021-02), p. 601-614
    Abstract: Click here for author‐reader discussions
    Type of Medium: Online Resource
    ISSN: 0740-3194 , 1522-2594
    URL: Issue
    URL: Article
    DOI: 10.1002/mrm.v85.2
    DOI: 10.1002/mrm.28484
    Language: English
    Publisher: Wiley
    Publication Date: 2021
    detail.hit.zdb_id: 605774-3
    detail.hit.zdb_id: 1493786-4
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  • 3
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    Double‐row dipole/loop combined array for human whole brain imaging at 7 T
    Wiley ; 2022
    In:  NMR in Biomedicine Vol. 35, No. 10 ( 2022-10)
    Details
    In: NMR in Biomedicine, Wiley, Vol. 35, No. 10 ( 2022-10)
    Abstract: Important issues in designing radiofrequency (RF) coils for human head imaging at ultra‐high field (UHF; ≥7 T) are the inhomogeneity and longitudinal coverage (along the magnet axis) of the transmit (Tx) RF field. Both the homogeneity and coverage produced by Tx volume coils can be improved by means of three‐dimensional (3D) RF shimming, which requires the use of multirow Tx‐arrays. In addition, according to recent findings of the ultimate intrinsic signal‐to‐noise ratio (UISNR) theory, the loop‐only receive (Rx) arrays do not provide optimal SNR near the brain center at UHF. The latter can be obtained by combining complementary conductive structures carrying different current patterns (e.g., loops and dipole antennas). In this work, we developed, constructed, and evaluated a novel 32‐element hybrid array design for human head imaging at 7 T. The array consists of 16 transceiver loops placed in two rows circumscribing the head and 16 folded‐end Rx‐only dipoles positioned in the centers of loops. By placing all elements in a single layer, we increased RF power deposition into the tissue and, thus, preserved the Tx‐efficiency. Using this hybrid design also simplifies the coil structure by minimizing the total number of array elements. The array demonstrated whole brain coverage, 3D RF shimming capability, and high SNR. It provided ~15% higher SNR near the brain center and, depending on the RF shim mode, from 20% to 40% higher Tx‐efficiency than a common commercial head array coil.
    Type of Medium: Online Resource
    ISSN: 0952-3480 , 1099-1492
    URL: Issue
    URL: Article
    DOI: 10.1002/nbm.v35.10
    DOI: 10.1002/nbm.4773
    Language: English
    Publisher: Wiley
    Publication Date: 2022
    detail.hit.zdb_id: 2002003-X
    detail.hit.zdb_id: 1000976-0
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  • 4
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    7T head volume coils: Improvements for rostral brain imaging
    Wiley ; 2009
    In:  Journal of Magnetic Resonance Imaging Vol. 29, No. 2 ( 2009-02), p. 461-465
    Details
    In: Journal of Magnetic Resonance Imaging, Wiley, Vol. 29, No. 2 ( 2009-02), p. 461-465
    Abstract: To improve the performance of 7T head coils over the rostral head regions. Due to radiofrequency (RF) field/tissue interactions, the RF magnetic field profile produced by 7T volume head coils is very inhomogeneous, with enhanced sensitivity near the center of the human brain and substantially reduced in the periphery. Materials and Methods Two head‐sized quadrature volume coils of similar diameters but substantially different lengths (17 and 10 cm) were constructed and tested using a 7T Varian Inova system. Results Experimental data demonstrated that by using a shorter volume head‐sized coil or simply by partially moving a head out of the coil, coil efficiency near the top of a head can be improved by 20%. The homogeneity also improved, largely resulting from an increase in peripheral B 1 values. This resulted in 10%–20% variation in axial slices located near the top of a head. Conclusion We have demonstrated a less deeply positioned head or substantially shorter volume coil can significantly improve coil performance and homogeneity for the rostral head at ultrahigh magnetic fields (7T and above). For studies that target superior brain regions, this coil arrangement can be highly effective. J. Magn. Reson. Imaging 2009;29:461–465. © 2009 Wiley‐Liss, Inc.
    Type of Medium: Online Resource
    ISSN: 1053-1807 , 1522-2586
    URL: Issue
    URL: Article
    DOI: 10.1002/jmri.v29:2
    DOI: 10.1002/jmri.21660
    Language: English
    Publisher: Wiley
    Publication Date: 2009
    detail.hit.zdb_id: 1146614-5
    detail.hit.zdb_id: 1497154-9
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  • 5
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    Safety testing and operational procedures for self‐developed radiofrequency coils
    Wiley ; 2016
    In:  NMR in Biomedicine Vol. 29, No. 9 ( 2016-09), p. 1131-1144
    Details
    In: NMR in Biomedicine, Wiley, Vol. 29, No. 9 ( 2016-09), p. 1131-1144
    Abstract: The development of novel radiofrequency (RF) coils for human ultrahigh‐field (≥7 T), non‐proton and body applications is an active field of research in many MR groups. Any RF coil must meet the strict requirements for safe application on humans with respect to mechanical and electrical safety, as well as the specific absorption rate (SAR) limits. For this purpose, regulations such as the International Electrotechnical Commission (IEC) standard for medical electrical equipment, vendor‐suggested test specifications for third party coils and custom‐developed test procedures exist. However, for higher frequencies and shorter wavelengths in ultrahigh‐field MR, the RF fields may become extremely inhomogeneous in biological tissue and the risk of localized areas with elevated power deposition increases, which is usually not considered by existing safety testing and operational procedures. In addition, important aspects, such as risk analysis and comprehensive electrical performance and safety tests, are often neglected. In this article, we describe the guidelines used in our institution for electrical and mechanical safety tests, SAR simulation and verification, risk analysis and operational procedures, including coil documentation, user training and regular quality assurance testing, which help to recognize and eliminate safety issues during coil design and operation. Although the procedure is generally applicable to all field strengths, specific requirements with regard to SAR‐related safety and electrical performance at ultrahigh‐field are considered. The protocol describes an internal procedure and does not reflect consensus among a large number of research groups, but rather aims to stimulate further discussion related to minimum coil safety standards. Furthermore, it may help other research groups to establish their own procedures. Copyright © 2015 John Wiley & Sons, Ltd.
    Type of Medium: Online Resource
    ISSN: 0952-3480 , 1099-1492
    URL: Issue
    URL: Article
    DOI: 10.1002/nbm.v29.9
    DOI: 10.1002/nbm.3290
    Language: English
    Publisher: Wiley
    Publication Date: 2016
    detail.hit.zdb_id: 2002003-X
    detail.hit.zdb_id: 1000976-0
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  • 6
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    Online Resource
    Decoupling of folded‐end dipole antenna elements of a 9.4 T human head array using an RF shield
    Wiley ; 2020
    In:  NMR in Biomedicine Vol. 33, No. 9 ( 2020-09)
    Details
    In: NMR in Biomedicine, Wiley, Vol. 33, No. 9 ( 2020-09)
    Abstract: Dipole antennas have recently been introduced to the field of MRI and successfully used, mostly as elements of ultra‐high field (UHF, ≥ 7 T) human body arrays. Usage of dipole antennas for UHF human head transmit (Tx) arrays is still under development. Due to the substantially smaller size of the sample, dipoles must be made significantly shorter than in the body array. Additionally, head Tx arrays are commonly placed on the surface of rigid helmets made sufficiently large to accommodate tight‐fit receive arrays. As a result, dipoles are not well loaded and are often poorly decoupled, which compromises Tx efficiency. Commonly, adjacent array elements are decoupled by circuits electrically connected to them. Placement of such circuits between distantly located dipoles is difficult. Alternatively, decoupling is provided by placing passive antennas between adjacent dipole elements. This method only works when these additional components are sufficiently small (compared with the size of active dipoles). Otherwise, RF fields produced by passive elements interfere destructively with the RF field of the array itself, and previously reported designs have used passive dipoles of about the size of array dipoles. In this work, we developed a novel method of decoupling for adjacent dipole antennas, and used this technique while constructing a 9.4 T human head eight‐element transceiver array. Decoupling is provided without any additional circuits by simply folding the dipoles and using an RF shield located close to the folded portion of the dipoles. The array reported in this work demonstrates good decoupling and whole‐brain coverage.
    Type of Medium: Online Resource
    ISSN: 0952-3480 , 1099-1492
    URL: Issue
    URL: Article
    DOI: 10.1002/nbm.v33.9
    DOI: 10.1002/nbm.4351
    Language: English
    Publisher: Wiley
    Publication Date: 2020
    detail.hit.zdb_id: 2002003-X
    detail.hit.zdb_id: 1000976-0
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  • 7
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    Online Resource
    Dynamic parallel imaging at 9.4 T using reconfigurable receive coaxial dipoles
    Wiley ; 2024
    In:  NMR in Biomedicine Vol. 37, No. 6 ( 2024-06)
    Details
    In: NMR in Biomedicine, Wiley, Vol. 37, No. 6 ( 2024-06)
    Abstract: Parallel imaging is one of the key MRI technologies that allow reduction of image acquisition time. However, the parallel imaging reconstruction commonly leads to a signal‐to‐noise ratio (SNR) drop evaluated using a so‐called geometrical factor ( g ‐factor). The g ‐factor is minimized by increasing the number of array elements and their spatial diversity. At the same time, increasing the element count requires a decrease in their size. This may lead to insufficient coil loading, an increase in the relative noise contribution from the RF coil itself, and hence SNR reduction. Previously, instead of increasing the channel number, we introduced the concept of electronically switchable time‐varying sensitivities, which was shown to improve parallel imaging performance. In this approach, each reconfigurable receive element supports two spatially distinct sensitivity profiles. In this work, we developed and evaluated a novel eight‐element human head receive‐only reconfigurable coaxial dipole array for human head imaging at 9.4 T. In contrast to the previously reported reconfigurable dipole array, the new design does not include direct current (DC) control wires connected directly to the dipoles. The coaxial cable itself is used to deliver DC voltage to the PIN diodes located at the ends of the antennas. Thus, the novel reconfigurable coaxial dipole design opens a way to scale the dynamic parallel imaging up to a realistic number of channels, that is, 32 and above. The novel array was optimized and tested experimentally, including in vivo studies. It was found that dynamic sensitivity switching provided an 8% lower mean and 33% lower maximum g ‐factor (for R y  ×  R z  = 2 × 2 acceleration) compared with conventional static sensitivities.
    Type of Medium: Online Resource
    ISSN: 0952-3480 , 1099-1492
    URL: Issue
    URL: Article
    DOI: 10.1002/nbm.v37.6
    DOI: 10.1002/nbm.5118
    Language: English
    Publisher: Wiley
    Publication Date: 2024
    detail.hit.zdb_id: 2002003-X
    detail.hit.zdb_id: 1000976-0
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  • 8
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    Online Resource
    Signal‐to‐noise ratio versus field strength for small surface coils
    Wiley ; 2024
    In:  NMR in Biomedicine
    Details
    In: NMR in Biomedicine, Wiley
    Abstract: The increasing signal‐to‐noise ratio (SNR) is the main reason to use ultrahigh field MRI. Here, we investigate the dependence of the SNR on the magnetic field strength, especially for small animal applications, where small surface coils are used and coil noise cannot be ignored. Measurements were performed at five field strengths from 3 to 14.1 T, using 2.2‐cm surface coils with an identical coil design for transmit and receive on two water samples with and without salt. SNR was measured in a series of spoiled gradient echo images with varying flip angle and corrected for saturation based on a series of flip angle and T 1 measurements. Furthermore, the noise figure of the receive chain was determined and eliminated to remove instrument dependence. Finally, the coil sensitivity was determined based on the principle of reciprocity to obtain a measure for ultimate SNR. Before coil sensitivity correction, the SNR increase in nonconductive samples is highly supralinear with B 0 1.6–2.7 , depending on distance to the coil, while in the conductive sample, the growth is smaller, being around linear close to the surface coil and increasing up to a B 0 2.0 dependence when moving away from the coil. After sensitivity correction, the SNR increase is independent of loading with B 0 2.1 . This study confirms the supralinear increase of SNR with increasing field strengths. Compared with most human measurements with larger coil sizes, smaller surface coils, as mainly used in animal studies, have a higher contribution of coil noise and thus a different behavior of SNR at high fields.
    Type of Medium: Online Resource
    ISSN: 0952-3480 , 1099-1492
    URL: Article
    DOI: 10.1002/nbm.5168
    Language: English
    Publisher: Wiley
    Publication Date: 2024
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  • 9
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    Online Resource
    Identification of the Cu 2+ Binding Sites in the N-Terminal Domain of the Prion Protein by EPR and CD Spectroscopy
    American Chemical Society (ACS) ; 2000
    In:  Biochemistry Vol. 39, No. 45 ( 2000-11-01), p. 13760-13771
    Details
    In: Biochemistry, American Chemical Society (ACS), Vol. 39, No. 45 ( 2000-11-01), p. 13760-13771
    Type of Medium: Online Resource
    ISSN: 0006-2960 , 1520-4995
    URL: Article
    DOI: 10.1021/bi001472t
    RVK:
    WA 15000
    Language: English
    Publisher: American Chemical Society (ACS)
    Publication Date: 2000
    detail.hit.zdb_id: 1108-3
    detail.hit.zdb_id: 1472258-6
    SSG: 12
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  • 10
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    Molecular Features of the Copper Binding Sites in the Octarepeat Domain of the Prion Protein
    American Chemical Society (ACS) ; 2002
    In:  Biochemistry Vol. 41, No. 12 ( 2002-03-01), p. 3991-4001
    Details
    In: Biochemistry, American Chemical Society (ACS), Vol. 41, No. 12 ( 2002-03-01), p. 3991-4001
    Type of Medium: Online Resource
    ISSN: 0006-2960 , 1520-4995
    URL: Article
    DOI: 10.1021/bi011922x
    RVK:
    WA 15000
    Language: English
    Publisher: American Chemical Society (ACS)
    Publication Date: 2002
    detail.hit.zdb_id: 1108-3
    detail.hit.zdb_id: 1472258-6
    SSG: 12
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