skip to main content


Title: Multi‐channel helical‐antenna inner‐volume RF coils for ultra‐high field MR scanners
Abstract

RFcoil design for human ultra‐high field (7 T and higher) magnetic resonance (MR) imaging is an area of intense development, to overcome difficult challenges such asRFexcitation spatial heterogeneity and lowRFtransfer efficiency into the spin system. This article proposes a novel category of multi‐channelRFvolume coil structures at both 7 T and 10.5 T based on a subject‐loaded multifilar helical‐antennaRFcoil that aims at addressing these problems. In some prior applications of helix antennas asMR RFcoils at 7 T, the imaged sample was positioned outside the helix. Here, we introduce a radically different approach, with the inner volume of a helix antenna being utilized to image a sample. The new coil uniquely combines traveling‐wave behavior through the overall antenna wire structure and near‐fieldRFinteraction between the conducting elements and the imaged tissues. It thus benefits from the congruence of far‐ and near‐field regimes. Design and analysis of the novel inner‐volume coils are performed by numerical simulations using multiple computational electromagnetics techniques. The fabricated coil prototypes are tested, validated, and evaluated experimentally in 7‐T and 10.5‐TMRhuman wide bore (90‐cm) MRscanners. Phantom data at 7 T show good consistency between numerical simulations and experimental results. SimulatedB1+transmit efficiencies, in T/√W, are comparable to those of some of the conventional and state‐of‐the‐artRFcoil designs at 7 T. Experimental results at 10.5 T show the scalability of the helix coil design.

 
more » « less
NSF-PAR ID:
10462738
Author(s) / Creator(s):
 ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering
Volume:
48B
Issue:
4
ISSN:
1552-5031
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    A novel method for excitation of RFB1field in high‐field (3‐T) magnetic resonance imaging (MRI) systems using a subject‐loaded quadrifilar helical antenna as an RF coil is proposed, evaluated, and demonstrated. Design, analysis, characterization, and evaluation of the novel coil when situated in a 3‐T MRI bore and loaded with different phantoms are performed and cross‐validated by extensive numerical simulations using multiple computational electromagnetics techniques. The results for the quadrifilar helical‐antenna RF body coil show (a) strong field penetration in the entire phantoms; (b) excellent right‐hand circular polarization (RCP); (c) high spatial uniformity of RCP RF magnetic field, B1+, throughout the phantoms; (d) large field of view (FOV); (e) good transmit efficiency; and (f) low local specific absorption rate (SAR). The examples show that the new RF coil provides substantially better B1+‐field uniformity and much larger FOV than any of the previously reported numerical and experimental results for the existing RF coil designs at 3 T in literature that enable comparison. In addition, helical RF body coils of different lengths can, for instance, easily provide an excellent RCP and highly uniform B1+‐field within the MRI maximum FOV length of 50 cm, and even 100 cm. The proposed MRI RF coil yields a remarkable improvement in the field uniformity in the longitudinal direction, for various phantoms, with comparable efficiency and SAR levels.

     
    more » « less
  2. Purpose

    To investigate how high‐permittivity materials (HPMs) can improve SNR when placed between MR detectors and the imaged body.

    Methods

    We used a simulation framework based on dyadic Green’s functions to calculate the electromagnetic field inside a uniform dielectric sphere at 7 Tesla, with and without a surrounding layer of HPM. SNR‐optimizing (ideal) current patterns were expressed as the sum of signal‐optimizing (signal‐only) current patterns and dark mode current patterns that minimize sample noise while contributing nothing to signal. We investigated how HPM affects the shape and amplitude of these current patterns, sample noise, and array SNR.

    Results

    Ideal and signal‐only current patterns were identical for a central voxel. HPMs introduced a phase shift into these patterns, compensating for signal propagation delay in the HPMs. For an intermediate location within the sphere, dark mode current patterns were present and illustrated the mechanisms by which HPMs can reduce sample noise. High‐amplitude signal‐only current patterns were observed for HPM configurations that shield the electromagnetic field from the sample. For coil arrays, these configurations corresponded to poor SNR in deep regions but resulted in large SNR gains near the surface due to enhanced fields in the vicinity of the HPM. For very high relative permittivity values, HPM thicknesses corresponding to even multiples of λ/4 resulted in coil SNR gains throughout the sample.

    Conclusion

    HPMs affect both signal sensitivity and sample noise. Lower amplitude signal‐only optimal currents corresponded to higher array SNR performance and could guide the design of coils integrated with HPM.

     
    more » « less
  3. Summary

    Light signal provides the spatial and temporal information for plants to adapt to the prevailing environmental conditions. Alterations in light quality and quantity can trigger robust changes in global gene expression. InArabidopsis thaliana, two groups of key factors regulating those changes in gene expression areCONSTITUTIVE PHOTOMORPHOGENESIS/DEETIOLATED/FUSCA(COP/DET/FUS) and a subset of basic helix‐loop‐helix transcription factors calledPHYTOCHROMEINTERACTING FACTORS(PIFs). Recently, rapid progress has been made in characterizing the E3 ubiquitin ligases for the light‐induced degradation ofPIF1,PIF3 andPIF4; however, the E3 ligase(s) forPIF5 remains unknown. Here, we show that theCUL4COP1–SPAcomplex is necessary for the red light‐induced degradation ofPIF5. Furthermore,COP1 andSPAproteins stabilizePIF5 in the dark, but promote the ubiquitination and degradation ofPIF5 in response to red light through the 26S proteasome pathway. Genetic analysis illustrates that overexpression ofPIF5can partially suppress bothcop1‐4andspaQseedling de‐etiolation phenotypes under dark and red‐light conditions. In addition, thePIF5 protein level cycles under both diurnal and constant light conditions, which is also defective in thecop1‐4andspaQbackgrounds. Bothcop1‐4andspaQshow defects in diurnal growth pattern. Overexpression ofPIF5partially restores growth defects incop1‐4andspaQunder diurnal conditions, suggesting that theCOP1–SPAcomplex plays an essential role in photoperiodic hypocotyl growth, partly through regulating thePIF5 level. Taken together, our data illustrate how theCUL4COP1–SPAE3 ligase dynamically controls thePIF5 level to regulate plant development.

     
    more » « less
  4. Abstract

    Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008TC3was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities toCI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of severalCC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3OCand/orCC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived fromEC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless inVNIR, and have an ~2.7 μm absorption band due toOHin phyllosilicates. Spectral modeling, using mixtures of laboratoryVNIRreflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008TC3consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% ofOC,EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm−3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm−3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008TC3in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when aCC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread inCREages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.

     
    more » « less
  5. Dimethylsulfoniopropionate (DMSP) is produced by many species of marine phytoplankton and has been reported to provide a variety of beneficial functions including osmoregulation. Dinoflagellates are recognized as majorDMSPproducers; however, accumulation has been shown to be highly variable in this group. We explored the effect of hyposaline transfer inGambierdiscus belizeanusbetween ecologically relevant salinities (36 and 31) onDMSPaccumulation, Chla, cell growth, and cell volume, over 12 d. Our results showed thatG. belizeanusmaintained an intracellularDMSPcontent of 16.3 pmol cell−1and concentration of 139 mMin both salinities. Although this intracellular concentration was near the median reported for other dinoflagellates, the cellular content achieved byG. belizeanuswas the highest reported of any dinoflagellate thus far, owing mainly to its large size.DMSPlevels were not significantly affected by salinity treatment but did change over time during the experiment. Salinity, however, did have a significant effect on the ratio ofDMSP:Chla, suggesting that salinity transfer ofG. belizeanusinduced a physiological response other thanDMSPadjustment. A survey ofDMSPcontent in a variety ofGambierdiscusspecies and strains revealed relatively highDMSPconcentrations (1.0–16.4 pmol cell−1) as well as high intrageneric and intraspecific variation. We conclude that, althoughDMSPmay not be involved in long‐term (3–12 d) osmoregulation in this species,G. belizeanusand otherGambierdiscusspecies may be important contributors toDMSPproduction in tropical benthic microalgal communities due to their large size and high cellular content.

     
    more » « less