Eight‐channel transmit/receive body MRI coil at 3T

Vernickel, P.; Röschmann, P.; Findeklee, Christian; Lüdeke, K.‐M.; Leussler, Christoph; Overweg, Johan; Katscher, Ulrich; Gräßlin, I.; Schünemann, K.

doi:10.1002/mrm.21294

Cited by 107 publications

(111 citation statements)

References 15 publications

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“…The elements are terminated with their characteristic impedance, which prevents the retransmission of received energy from other elements. In practice this can be accomplished by matching the coil to the impedance of the RF amplifiers or by the use of circulators (20).…”

Section: Methodsmentioning

confidence: 99%

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

Bergen

Berg

Klomp

et al. 2009

Magnetic Resonance Imaging

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Purpose: To determine the best radiofrequency (RF) shimming method for 7 T body imaging that provides sufficient B 1 ϩ excitation inside the target region while energy deposition (SAR) and power demands are as low as possible and that does not incorporate anatomy specific electric field information inside the patient models, as this information is not available in practice. Materials and Methods:Finite difference time domain (FDTD) simulations were used to evaluate five RF shimming strategies for the pelvis inside a body coil. The results were compared to the theoretical best solution that could be achieved if the electric field inside the patient was known.Results: Most of the RF shimming strategies were successful. However, between the different strategies a factor of two difference in average SAR reduction, a factor of three difference in local maximum SAR reduction, and a factor of 20 difference in power efficiency was observed. Phase matching was found to be the most promising RF shimming method for the body coil used and patient models. Conclusion:RF shimming can reduce the SAR and improve power efficiency in an accurate patient model without knowing the electric field. However, choosing the right method is critical to prevent unexpected behavior in local SAR deposition. THE INCREASE in main magnetic field strength for human magnetic resonance imaging (MRI) toward 7 T has shown large benefits for head imaging, where the higher signal-to-noise ratio can be used to increase scan speed and/or to obtain a higher resolution (1-4) and where the higher spectral resolution (5,6) and the increased sensitivity for various contrast mechanisms such as blood oxygenation level-dependent (BOLD) (4,7,8) have proven useful.Recent studies have shown that 7 T MRI is not limited to the head, but that imaging of the body is also possible (9 -11). However, this ultrahigh magnetic field imaging is hampered by difficulties related to the control of the radiofrequency (RF) excitation field. In the head these difficulties are not too severe and imaging is possible with traditional imaging concepts. In the body this becomes increasingly challenging due to its larger anatomical dimensions. As the Larmor frequency increases with the main magnetic field strength, the wavelength of the RF excitation field becomes smaller, which leads to more pronounced interference effects both in the RF excitation (B 1 ϩ ) field and in the accompanying electric field. The former can result in inhomogeneous B 1 ϩ fields with excitation voids in the region of interest (ROI) (9,12,13) and the latter can cause local maxima in the electric field, which result in large local energy depositions (specific absorption rate, SAR). This poses a safety concern because of possible local overheating of the patient. A second effect of the increase in Larmor frequency is that the RF penetration depth becomes smaller. A larger amount of power is therefore required to reach the center of the body with an increasing SAR as a result. In addition, the chemical shift also in...

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Section: Methodsmentioning

confidence: 99%

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

Bergen

Berg

Klomp

et al. 2009

Magnetic Resonance Imaging

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“…Each bottle had a square cross-section of 5 Â 5 cm 2 , yielding an overall phantom size of 25 Â 25 cm 2 . This phantom was used to determine the spatial transmit sensitivity distributions S n (r) of the N ¼ 8 elements of a multichannel body coil (20), integrated in a Philips 3T Achieva (Philips Medical System, Best, The Netherlands) (13). The transmit sensitivities were measured by actual flip angle imaging (17), including RF spoiling (21) (TR1/ TR2/echo time ¼ 40/200/2 ms, a ¼ 60 , spatial resolution 20 Â 20 Â 10 mm 3 ).…”

Section: Determination Of Encoding Functionsmentioning

confidence: 99%

RF encoding using a multielement parallel transmit system

Katscher

Lisinski

Börnert

2010

Magnetic Resonance in Med

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Radiofrequency (RF) encoding using spatially variant RF transmission fields represents an alternative to the conventional signal-encoding techniques applied in MRI, which are based on main field gradients. Thus, RF encoding might allow omitting the use of all main field gradients, alleviating acoustic noise and other main field gradient-related problems. This study investigates the potential of RF transmit encoding using spatially nonlinear RF fields generated by an eight-channel parallel transmit system equipped with the corresponding transmit array. Appropriate spatial encoding functions are determined iteratively by randomized superposition of the array element sensitivities and judging their performance. Besides RF amplitude-based signal encoding, the possibility of RF phase-dominated encoding is investigated to allow future fast imaging applications. The theoretical background is given and experimental phantom results are compared with predictions from corresponding simulations to prove the basic feasibility of the approach for low-resolution MRI. Deviations of roughly 10-15% between main field and RF encoded images were found for an in-plane resolution of 5 3 5 mm 2 . The approach of RF encode MRI does not seem to be able to replace standard main field gradient encoding completely, but a suitable combination of both concepts could find promising applications.

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“…[11]) by superimposing their results after signal acquisition. 81,82 With the development of multielement-transmit coils, [102][103][104][105] parallel transmission has been used to perform basic RF shimming [75][76][77][78] by varying amplitudes and phases for the individual channels.…”

Section: Experimental Proofmentioning

confidence: 99%

“…107 Left: Transmit-receive RF coil with eight cylindrically arranged transverse electromagnetic elements, 105 and quality phantom. Center: Corresponding cabinets showing eight RF amplifiers, two circulator boxes (central cabinet) and the fully integrated spectrometer (right cabinet).…”

Section: Figmentioning

confidence: 99%

Parallel Magnetic Resonance Imaging

Katscher

Börnert

2007

Neurotherapeutics

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Summary: Parallel MRI started with the introduction of coil arrays in improving radiofrequency (RF) acquisition (what is called parallel imaging) and continued with an analogous development for RF transmission (parallel transmission). Based on differences in the spatial sensitivity distributions of the involved array elements, both techniques try to shorten the respective k-space trajectory. Parallel imaging refers to the acquisition of k-space data, whereas parallel transmission is dealing with the deposition of RF energy packages in the excitation k-space.However, parallel transmission is not simply the reciprocal of parallel imaging. The main goal of parallel imaging is the shortening of the acquisition time. The main goal of parallel transmission is the shortening of the pulse duration of spatially selective RF pulses. The present article describes the basic concept, the state of the art, and the similarities and differences of both technologies.

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Eight‐channel transmit/receive body MRI coil at 3T

Cited by 107 publications

References 15 publications

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

RF encoding using a multielement parallel transmit system

Parallel Magnetic Resonance Imaging

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