Martin Hergt

MR Spectroscopy of the Human Brain With Enhanced Signal Intensity at Ultrashort Echo Times on a Clinical Platform at 3T and 7T

Recently, the spin‐echo full‐intensity acquired localized (SPECIAL) spectroscopy technique was proposed to unite the advantages of short TEs on the order of milliseconds (ms) with full sensitivity and applied to in vivo rat brain. In the present study, SPECIAL was adapted and optimized for use on a clinical platform at 3T and 7T by combining interleaved water suppression (WS) and outer volume saturation (OVS), optimized sequence timing, and improved shimming using FASTMAP. High‐quality single voxel spectra of human brain were acquired at TEs below or equal to 6 ms on a clinical 3T and 7T system for six volunteers. Narrow linewidths (6.6 ± 0.6 Hz at 3T and 12.1 ± 1.0 Hz at 7T for water) and the high signal‐to‐noise ratio (SNR) of the artifact‐free spectra enabled the quantification of a neurochemical profile consisting of 18 metabolites with Cramér‐Rao lower bounds (CRLBs) below 20% at both field strengths. The enhanced sensitivity and increased spectral resolution at 7T compared to 3T allowed a two‐fold reduction in scan time, an increased precision of quantification for 12 metabolites, and the additional quantification of lactate with CRLB below 20%. Improved sensitivity at 7T was also demonstrated by a 1.7‐fold increase in average SNR (= peak height/root mean square [RMS]‐of‐noise) per unit‐time. Magn Reson Med, 2009.

Investigation of high-resolution functional magnetic resonance imaging by means of surface and array radiofrequency coils at 7 T ☆

In this investigation, high-resolution, 1x1x1-mm(3) functional magnetic resonance imaging (fMRI) at 7 T is performed using a multichannel array head coil and a surface coil approach. Scan geometry was optimized for each coil separately to exploit the strengths of both coils. Acquisitions with the surface coil focused on partial brain coverage, while whole-brain coverage fMRI experiments were performed with the array head coil. BOLD sensitivity in the occipital lobe was found to be higher with the surface coil than with the head array, suggesting that restriction of signal detection to the area of interest may be beneficial for localized activation studies. Performing independent component analysis (ICA) decomposition of the fMRI data, we consistently detected BOLD signal changes and resting state networks. In the surface coil data, a small negative BOLD response could be detected in these resting state network areas. Also in the data acquired with the surface coil, two distinct components of the positive BOLD signal were consistently observed. These two components were tentatively assigned to tissue and venous signal changes.

Characterization of commercial IGBT modules for pulsed power applications

One of the key components of pulsed power technology is the switch, which is increasingly realized using semiconductor switches. Use of solid state switches, if properly designed, provides longer lifetime, reliability, and reduces maintenance as compared to the conventional spark gaps which are used in Marx generators or other devices for pulsed power applications today. An analysis of commercial semiconductor switches favors the IGBT for pulsed power applications, in particular for high average power, high pulse repetition rate applications, due to its widespread use in drive applications and its availability. High power IGBT modules rated at 4.5 kV / 800 A of two different technologies have been investigated in this work: the planar technology and the trench3 technology. Both types of semiconductor switches were tested in a special low inductance setup to characterize the IGBT for pulsed power applications. For this characterization, the development of a dedicated gate drive unit enables the IGBT to generate fast rise times for the collector current and fast fall times for the collector-emitter voltage. The results show that the planar technology is preferable for pulsed power applications. The IGBT with the planar technology was characterized at a DC link voltage of 4kV and a peak current of 2kA. The switching time of the IGBT stays in the region of 200ns (tfall time(20-80%)) of the collector-emitter voltage, while the rise time of the collector current is 160ns (trise time(10-90%)) with peak power losses of 1.41MW. The associated junction temperature of the chip will be increased by approximately 1K only. This allows to use the IGBT at higher pulse repetition rates (PRF) up to 2kHz, at a pulse duration of 1μs, without additional cooling. The switching speed of the IGBT can be influenced by the matching network and depends on the application which will be realized with the IGBT. The IGBT with the trench3 technology shows gate voltage oscillations at peak currents above 1 kA, which infers that the gate source capacitance will be slowly destroyed by overvoltage. These oscillations can be explained with the higher gate source capacitance of the trench3 technology as compared to the planar technology, in combination with the unavoidable gate inductance. The planar technology, on the other hand, is realized with a low inductance gate runner topology and can thus be used at shorter pulse rise times. The results present a commercial semiconductor which is suitable for a pulsed power application. The IGBT with the planar technology can be used with the right choice of the driver matching network for a pulsed power application. Furthermore there is no need to design a switch using small, discrete semiconductor devices. That saves cost and keeps the circuit development simple. Only the gate drive unit is developed in-house particularly for pulsed power applications. The technical functions and the economic efficiency are accordingly balanced as well.

Compact 3.5 kW semiconductor RF modules based on SiC-VJFETs for accelerator applications

We present first prototypes of compact low cost high power semiconductor RF amplifier modules based on silicon carbide (SiC) normally-on vertical JFETs that can operate at VHF frequencies. The RF amplifiers have a modified parallel push pull topology (floating bridge or circlotron) with two parallelized transistors per side. They were tested in pulsed mode at drain voltage levels up to 500V and at an operation frequency of 150 MHz. The RF amplifier modules delivered an effective output power of 3500 W into a 12,5 Ω resistive load at a duty cycle of 1:1000. The achieved power gain was 9,5 dB at the 1dB compression point. The corresponding efficiency was 45%. Simulations and first measurements indicate that a new optimized transistor generation will provide a factor of four higher output power in a similar RF module circuit.

A three-stage inductive voltage adder for industrial applications

We report on an Inductive Voltage Adder (IVA) [1] development designated for industrial applications like electroporation, environmental applications, etc. The IVA described [2]-[4] is a three-stage demonstrator which shows the feasibility of using conventional high power IGBT semiconductor switch modules instead of spark gaps or arrays of low-power IGBTs. The pulse generator described produces pulses with peak voltages and currents of up to 12kV and 6kA, respectively, at a pulse duration of typically 1μs FWHM (full width at half maximum). The IVA is tested in single pulse mode and at low repetition rates due to load and power supply constrictions. Each IVA stage is designed as a radial transmission line fed by several parallel pulse modules (“bricks”) which contain the electrical components, like capacitors, inductors, and switches. The combined power of an individual stage is added to the preceding stages in a section called “transformer”. The transformer matches the electric and magnetic fields and is realized as a combination of radial and coaxial transmission lines. Each stage of the IVA is matched to the next stage and is connected in series with a coaxial transmission line. The mechanical dimensions of the three-stage IVA demonstrator are: outer diameter 820 mm, outer diameter of the coaxial transmission line 210 mm, height of the IVA 352 mm. The I VA geometry, in particular the most critical parts radial transmission line and transformer section, is simulated by a transient electromagnetic field solver to analyze the reflections and transmission coefficients of the device.

Concept of an inductive voltage adder for industrial applications

The concept of an inductive voltage adder (IVA) which is suitable for industrial pulsed power applications is described. The pulse generator is designed to produce a double exponential waveform at voltage amplitudes of tens of kV to >100 kV, and current amplitudes of typically 10 kA into a matched load, at a pulse duration of typically 1 μs FWHM (full width at half maximum). In the IVA concept, the power is added through vector addition of electromagnetic fields rather than connecting a large number of semiconductor switches in series as in standard (Marx generator) technologies. This work comprises conceptual and simulation work. Fundamental components of the generator like the magnetic coupling (“transformer” section), magnetic isolation and the combining of power have been designed on the basis of 3D electromagnetic (EM) simulation work using CST Microwave Suite. Complementary experimental work based on these simulations comprises the verification of a single pulse module [1], single IVA stages incorporating several parallel pulse modules [2], and a multi-module, multi-stage IVA [3]. The IVA concept and results of these simulations are presented.

High power SiC solid state RF-modules

Solid state amplifiers begin to replace traditional vacuum tube technology (e. g. Klystrons) in several accelerator applications [1]. They offer the perspective of lower cost, better reliability and reduced maintenance [2]. Due to their modular construction, power levels can be scaled easily to meet the target application requirements. The new direct drive concept [3] offers the benefit of simple triggering and the possibility to individually control phase and power in each cavity segment, which increases operation mode flexibility of the final particle accelerator. We present development results of compact high power solid state RF-modules based on novel SiC transistors. The SiC transistor layout and the packaging technology was optimized for high frequency operation and we already reported previously that SiC transistors can provide RF output power levels well above 1 kW per device at 150 MHz [4]. We now present our second generation of high power solid state RF modules based on normally-on SiC vertical JFETs with significantly increased power ratings in the range of 5 – 25 kW per module depending on supply voltage, input power and pulse duration. An 84 kW RF-source was built by power combining of 32 RF-modules running at relatively low voltage of 160 V.

Semi-planar power combiner structure for IGBT-based pulsed power modulators

A semi-planar, rotationally symmetric power combiner has been realized using a hard-wired parallel circuit of four power transfer stages which feed into a common radial transmission line. For testing purposes, the radial transmission line is terminated with a matched ohmic resistor. The power combiner is designed to produce a double exponential pulse by optimizing the transmission line geometry with the help of electrodynamic modeling using CST Microwave Suite. Each of the four stages contains three parallel capacitors of 200nF each, the geometrical circuit inductance, a switch, and are discharged into the common resistive load. The switch is realized with an industrial type IGBT module, at a DC link voltage of up to 4.5 kV. Lifetime estimations show a permissible peak current of up to 2kA for a single IGBT module, at pulse durations of around 1 μs. Hence, a peak current of over 6 kA can be achieved by paralleling four of these power transfer stages in the semi-planar power combiner structure. First experimental results show that the semi-planar power combiner is a suitable functional unit for pulsed power applications. The circuit was characterized at a DC link voltage of up to 4 kV, a peak current of 1.68kA, and a pulse duration of 1μs per IGBT module. The low switching losses of the IGBT when using a hard gate drive allow using the IGBT at high pulse repetition rates (PRF) up to kHz, at pulse durations around 1μs. The circuit presented is suitable to be used as a modular component of an inductive voltage adder to increase the available voltage and peak power levels, respectively.