Franz Hebrank

Parallel RF transmission with eight channels at 3 tesla

Spatially selective RF waveforms were designed and demonstrated for parallel excitation with a dedicated eight‐coil transmit array on a modified 3T human MRI scanner. Measured excitation profiles of individual coils in the array were used in a low‐flip‐angle pulse design to achieve desired spatial target profiles with two‐ (2D) and three‐dimensional (3D) k‐space excitation with simultaneous transmission of RF on eight channels. The 2D pulse excited a high‐resolution spatial pattern in‐plane, while the 3D trajectory produced high‐quality slice selection with a uniform in‐plane excitation despite the highly nonuniform individual spatial profiles of the coil array. The multichannel parallel RF excitation was used to accelerate the 2D excitation by factors of 2–8, and experimental results were in excellent agreement with simulations based on the measured coil maps. Parallel RF transmission may become critical for robust and routine human studies at very high field strengths where B1 inhomogeneity is commonly severe. Magn Reson Med, 2006.

Slice-Selective RF pulses for In-vivo B1+ Inhomogeneity Mitigation at 7 Tesla using Parallel RF Excitation with a 16-Element Coil

Slice‐selective RF waveforms that mitigate severe B  1+ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16‐element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least‐squares (MLS) criteria with an optimized k‐space excitation trajectory to significantly improve profile uniformity compared to conventional least‐squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in‐plane flip angle (FA) with slice selection in the z‐direction were demonstrated and compared with conventional sinc‐pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B  1+ on the excitation FA. The optimized parallel RF pulses for human B  1+ mitigation were only 67% longer than a conventional sinc‐based excitation, but significantly outperformed RF shimming. For example the standard deviations (SDs) of the in‐plane FA (averaged over six human studies) were 16.7% for conventional sinc excitation, 13.3% for RF shimming, and 7.6% for parallel excitation. This work demonstrates that excitations with parallel RF systems can provide slice selection with spatially uniform FAs at high field strengths with only a small pulse‐duration penalty. Magn Reson Med 60:1422–1432, 2008.

Degenerate mode band-pass birdcage coil for accelerated parallel excitation.

An eight‐rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this “loop coil” basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B1 field pattern, and the higher‐order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one‐, four‐, six‐, and eightfold‐accelerated spatially tailored RF excitations with 2D and 3D k‐space excitation trajectories. Transmit accelerations of up to six‐fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B1 transmit inhomogeneities using parallel excitation. Magn Reson Med 57:1148–1158, 2007.

Broadband slab selection with B1+ mitigation at 7T via parallel spectral-spatial excitation.

Chemical shift imaging benefits from signal‐to‐noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B  1+ mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B  1+ mitigation methods, which are based on a type of echovolumnar trajectory referred to as “spokes” or “fast‐kz”. Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B  1+ inhomogeneities and achieve relatively short pulse durations with slice‐selective excitations, they exhibit a narrow‐band off‐resonance response and may not be suitable for applications that require B  1+ mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral‐spatial excitation that achieves a target‐error minimization simultaneously over a bandwidth of frequencies and a specified spatial‐domain. The technique is demonstrated for slab‐selective excitation with in‐plane B  1+ mitigation over a 600‐Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight‐channel transmit array system. The results show significant increases in the pulses spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B  1+ mitigation compared to the standard spoke‐based parallel RF design. Magn Reson Med 61:493–500, 2009.

High-flip-angle slice-selective parallel RF transmission with 8 channels at 7 T.

At high magnetic field, B(1)(+) non-uniformity causes undesired inhomogeneity in SNR and image contrast. Parallel RF transmission using tailored 3D k-space trajectory design has been shown to correct for this problem and produce highly uniform in-plane magnetization with good slice selection profile within a relatively short excitation duration. However, at large flip angles the excitation k-space based design method fails. Consequently, several large-flip-angle parallel transmission designs have recently been suggested. In this work, we propose and demonstrate a large-flip-angle parallel excitation design for 90 degrees and 180 degrees spin-echo slice-selective excitations that mitigate severe B(1)(+) inhomogeneity. The method was validated on an 8-channel transmit array at 7T using a water phantom with B(1)(+) inhomogeneity similar to that seen in human brain in vivo. Slice-selective excitations with parallel RF systems offer means to implement conventional high-flip excitation sequences without a severe pulse-duration penalty, even at very high B(0) field strengths where large B(1)(+) inhomogeneity is present.

Comparison of the threshold for peripheral nerve stimulation during gradient switching in whole body MR systems

To compare thresholds for peripheral nerve stimulation from gradient switching in whole body magnetic resonance (MR) equipment of different design.

Stimulation threshold comparison of time-varying magnetic pulses with different waveforms

To clarify whether sinusoidal pulses possess lower thresholds than rectangular ones at perception threshold, a statement often made that contradicts the theory of stimulation.

Physikalische Parameter bei der Anwendung der MRT Begrenzung durch physiologische Reaktionen und Richtlinien

ZusammenfassungZielsetzung: Die in der MRT für den Schutz von Patienten und Anwender gültigen Normen und Vorschriften werden zusammengestellt. Die daraus resultierenden Auswirkungen auf die physikalischen Parameter bei der Bildgebung durch MRT werden beurteilt. Material und Methodik: Die Aspekte statisches Magnetfeld, Hochfrequenzerwärmung und Lärm werden dargestellt und die dafür derzeit gültigen Begrenzungen mit dem Stand von Wissenschaft und Technik verglichen. In einer klinischen Prüfung wurden Stimulationschwellen durch gepulste Gradientenfelder bestimmt. Ergebnisse: Viele für den Normalbetrieb empfohlene Parameter werden heute bereits in der Routinebildgebung überschritten. Anhand der klinischen Prüfung wird nachgewiesen, daß die Patientensicherheit in bezug auf periphere Stimulationen auch bei Betriebsparametern, die über das Doppelte der empfohlenen Werte hinausgehen, gewährleistet ist. Schlußfolgerungen: Die anzuwendenden nationalen und internationalen Regeln weisen z.T. gravierende Unterschiede bei den Begriffsdefinitionen und den Zahlenwerten auf. Eine Harmonisierung würde ihre Umsetzung wesentlich vereinfachen. Die in den MR-spezifischen Standards geregelten Werte sollten dem aktuellen Kenntnisstand angepaßt (erhöht) werden.SummaryPurpose: The standards and regulations concerning the protection of patients and operator staff within the context of MRI are compiled. Resulting consequences regarding physical parameters are evaluated. Material and methods: The static magnetic field, heating effects caused by RF-fields and acoustical noise are outlined. The actual boundaries of these parameters are compared against the relevant published standards. Peripheral stimulation limits due to pulsed gradient fields have been determined in a new clinical study. Results: Many parameters recommended for the normal operating mode are already exceeded during routine MRI. Referring to our clinical study, we found that limits recommended in the MRI relevant standards are unnecessarily conservative and can actually be doubled. Conclusions: The applicable national and international standards and regulations show (at least partly) that serious differences in the definition of terms and values exist. The application of these standards would be much easier if they were made uniform. The values defined in the MR-specific standards should be adapted to actual knowledge concerning patients’ safety.