Sensitivity and Power Deposition in a High-Field Imaging Experiment

Hoult, D. I.

doi:10.1002/1522-2586(200007)12:1<46::aid-jmri6>3.0.co;2-d

Cited by 448 publications

(403 citation statements)

References 22 publications

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“…18 It should be expected that B 1 + twisting and the shoulder’s asymmetric location in the body result in distinct B 1 + patterns in the left and right shoulders, requiring distinct shim weights. Full-wave simulations using a current mode expansion with the Dyadic Green functions 19,20 were used to demonstrate the effect of B 1 + twisting in a shoulder-like configuration, where the transmit array does not fully surround the imaged object.…”

Section: Methodsmentioning

confidence: 99%

Design and Application of Combined 8-Channel Transmit and 10-Channel Receive Arrays and Radiofrequency Shimming for 7-T Shoulder Magnetic Resonance Imaging

Brown

Deniz

Zhang

et al. 2014

Investigative Radiology

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Objective The objective of the study was to investigate the feasibility of 7-T shoulder magnetic resonance imaging by developing transmit and receive radiofrequency (RF) coil arrays and exploring RF shim methods. Materials and Methods A mechanically flexible 8-channel transmit array and an anatomically conformable 10-channel receive array were designed and implemented. The transmit performance of various RF shim methods was assessed through local flip angle measurements in the right and left shoulders of 6 subjects. The receive performance was assessed through signal-to-noise ratio measurements using the developed 7-T coil and a baseline commercial 3-T coil. Results The 7-T transmit array driven with phase-coherent RF shim weights provided adequate B1+ efficiency and uniformity for turbo spin echo shoulder imaging. B1+ twisting that is characteristic of high-field loop coils necessitates distinct RF shim weights in the right and left shoulders. The 7-T receive array provided a 2-fold signal-to-noise ratio improvement over the 3-T array in the deep articular shoulder cartilage. Conclusions Shoulder imaging at 7-T is feasible with a custom transmit/receive array either in a single-channel transmit mode with a fixed RF shim or in a parallel transmit mode with a subject-specific RF shim.

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Section: Methodsmentioning

confidence: 99%

Design and Application of Combined 8-Channel Transmit and 10-Channel Receive Arrays and Radiofrequency Shimming for 7-T Shoulder Magnetic Resonance Imaging

Brown

Deniz

Zhang

et al. 2014

Investigative Radiology

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show abstract

“…In this region, manual adjustments, in particular B 0 shimming was performed utilizing a shim volume adapted to the region of interest. As MRI at higher field strength is known to be susceptible to transmit B 1 field inhomogeneities due to the reduced Larmor wavelength [21], static shimming of the eight individual transmit phases of the RF coil was performed to mitigate potential limitations and to achieve a more homogeneous excitation of the upper abdomen [22]. A phase increment of 90 degrees and equal amplitudes between neighboring elements (circularly polarized mode CP 2+ ) was found to be most compatible for this study in terms of image quality as it constrained the signal voids to a periaortal focus as shown in Figure 1.…”

Section: Methodsmentioning

confidence: 99%

Non-Enhanced T1-Weighted Liver Vessel Imaging at 7 Tesla

et al. 2014

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ObjectivesAim of the study was to assess the feasibility and to compare three non-enhanced T1-weighted (w) sequences for liver vessel imaging at 7 Tesla (T).Material and Methods12 healthy volunteers were examined on a 7 T whole-body MR-system. The following non-enhanced sequences were acquired: T1w 2D FLASH, T1w 3D FLASH and Time of flight (TOF)-MRA. Qualitative image analysis was performed by two radiologists including over all image quality as well as vessel delineation of the liver arteries, liver veins and portal vein and the presence of artifacts using a five-point scale (5 = excellent vessel delineation to 1 = non-diagnostic). Contrast ratios (CR), SNR und CNR of the above named vessels in correlation to adjacent liver tissue were calculated for quantitative assessment. For statistical analysis, a Wilcoxon Rank Test was applied.ResultsAll three sequences provided a homogenous hyperintense delineation of the assessed liver vessels. Qualitative image analysis demonstrated the superiority of TOF-MRA, providing best overall image quality (TOF 4.17, 2D FLASH 3.42, 3D FLASH 3.46; p<0.01) as well as highest image quality values for all analyzed liver vessel segments. TOF-MRA was least impaired by B1 inhomogeneity (4.13) and susceptibility artifacts (4.63) out of all three sequences (p<0.01). Quantitative image analysis confirmed the superiority of TOF MRA showing significant higher CR values for all liver vessels (e.g. right hepatic artery TOF 0.47, 2D FLASH 0.09, 3D FLASH 0.11 with p = 0.02 and 0.01, respectively). Providing the lowest standard deviation in noise, TOF showed highest values for SNR and CNR.ConclusionsNon-enhanced T1w imaging in general and TOF MRA in particular, appear to be promising techniques for high quality non-enhanced liver vessel assessment at 7 T.

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“…The B 1 + is denoted as the transmit magnetic field can be extracted from the simulations by [20]:

B_{1}^{+} = \frac{B_{x} + i B_{y}}{2} .

These arrays were excited in birdcage-like mode, applying 1 W power to each port, with sequential 45 degree phase increments. For the unoptimized array with D d =0 cm, the B 1 + field is low near the decoupling elements, as indicated by the white circles in Figure 3A.…”

Section: Resultsmentioning

confidence: 99%

Optimizing the ICE decoupling element distance to improve monopole antenna arrays for 7 Tesla MRI

Yan

Zhang

Xue

et al. 2016

Magnetic Resonance Imaging

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The induced current elimination (ICE) method has been previously applied to decouple monopole coil arrays in ultrahigh field MRI. However, the method creates low B1+ spots near the decoupling elements. In this study, we aim to improve the performance of ICE-decoupled monopole array in human head imaging at 7 Tesla. Eight-channel ICE-decoupled monopole arrays were optimized by varying the position of the decoupling elements. A series of numerical studies were performed using the co-simulation method. In simulation, decoupling performance, quality (Q−) values and transmit field (B1+) were comparatively investigated. In addition, we constructed an optimized ICE-decoupled monopole array and compared its performance with the unoptimized array. The simulation results showed that a good trade-off between decoupling and B1+ loss can be obtained when decoupling elements were moved 2.5-cm away from coil elements. This was validated by in-vivo MR imaging using the constructed array. Compared with the unoptimized ICE decoupled monopole array, the optimized array had a more homogeneous transmit field and no dark spots or signal cancellations in the MR images.

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Sensitivity and Power Deposition in a High-Field Imaging Experiment

Cited by 448 publications

References 22 publications

Design and Application of Combined 8-Channel Transmit and 10-Channel Receive Arrays and Radiofrequency Shimming for 7-T Shoulder Magnetic Resonance Imaging

Design and Application of Combined 8-Channel Transmit and 10-Channel Receive Arrays and Radiofrequency Shimming for 7-T Shoulder Magnetic Resonance Imaging

Non-Enhanced T1-Weighted Liver Vessel Imaging at 7 Tesla

Optimizing the ICE decoupling element distance to improve monopole antenna arrays for 7 Tesla MRI

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