Oliver Heid

Reduction of eddy-current-induced distortion in diffusion MRI using a twice-refocused spin echo

Image distortion due to field gradient eddy currents can create image artifacts in diffusion‐weighted MR images. These images, acquired by measuring the attenuation of NMR signal due to directionally dependent diffusion, have recently been shown to be useful in the diagnosis and assessment of acute stroke and in mapping of tissue structure. This work presents an improvement on the spin‐echo (SE) diffusion sequence that displays less distortion and consequently improves image quality. Adding a second refocusing pulse provides better image quality with less distortion at no cost in scanning efficiency or effectiveness, and allows more flexible diffusion gradient timing. By adjusting the timing of the diffusion gradients, eddy currents with a single exponential decay constant can be nulled, and eddy currents with similar decay constants can be greatly reduced. This new sequence is demonstrated in phantom measurements and in diffusion anisotropy images of normal human brain. Magn Reson Med 49:177–182, 2003.

Prospective acquisition correction for head motion with image-based tracking for real-time fMRI.

In functional magnetic resonance imaging (fMRI) head motion can corrupt the signal changes induced by brain activation. This paper describes a novel technique called Prospective Acquisition CorrEction (PACE) for reducing motion‐induced effects on magnetization history. Full three‐dimensional rigid body estimation of head movement is obtained by image‐based motion detection to a high level of accuracy. Adjustment of slice position and orientation, as well as regridding of residual volume to volume motion, is performed in real‐time during data acquisition. Phantom experiments demonstrate a high level of consistency (translation < 40μm; rotation < 0.05°) for detected motion parameters. In vivo experiments were carried out and they showed a significant decrease of variance between successively acquired datasets compared to retrospective correction algorithms. Magn Reson Med 44:457–465, 2000.

Magnetization preparation during the steady state: Fat-saturated 3D TrueFISP

A novel fat saturation scheme is proposed which combines spectral fat saturation with a steady‐state 3D true Fast Imaging with Steady Precesson (TrueFISP) sequence. Fat saturation consisted of a conventional frequency‐selective excitation pulse surrounded by spoiler gradients. This saturation block was periodically repeated within a continuously running TrueFISP sequence. Except for the fat signals, the steady‐state signal formation and the resulting image contrast of TrueFISP was not modified by the periodically inserted fat saturation block. This was achieved by a α/2 flip‐back pulse before the fat saturation block, which stores the established steady‐state transverse magnetization as pure longitudinal magnetization. After fat saturation this longitudinal magnetization was excited by a α/2 preparation pulse to continue the TrueFISP acquisition. The resulting images show contrast identical to conventional TrueFISP images, but without the usually very bright fat signals. The short repetition time allows acquisition of a 3D data set of the abdomen within a single breath‐hold. Magn Reson Med 45:1075–1080, 2001.

Method and apparatus for shimming a magnet system of a nuclear magnetic resonance tomography system

In a method and apparatus for shimming a magnet system of a nuclear magnetic resonance tomography system, an apparatus-specific matrix is determined, which indicates the effect on the magnetic field for each shim channel. A phase difference matrix is subsequently formed by means of calculation of the phase differences between three-dimensional spatially resolved nuclear magnetic resonance raw data sets obtained with different echo times. This phase of difference matrix gives the field deviation for each pixel. In this phase difference matrix, the phase differences between consecutive pixels are calculated in several spatial directions, thereby determining a phase error data set. Finally, on the basis of the predefined matrix and of this phase error data set, currents for the individual shim channels are determined.

Induction of electric fields due to gradient switching: A numerical approach

Since the first observations of peripheral nerve stimulation in MRI, it has been clear that the underlying mechanism is the activation of the nervous system by induced electric fields. However, compared to experimental investigations little work has been done on calculating these electric fields with adequate accuracy. In this article a numerical analysis of the electric fields induced by a complete whole body gradient system is presented. The calculations were carried out on three human body models of different complexities. The numerical results correlate better to the experimental observations with a body model that resembles the human body. Applying a model with inhomogeneous conductivity, numerical stability was not reached. The results were compared to the limits given in the upcoming IEC 60601‐2‐33 standard. The comparison shows that the derived peak electric fields depend substantially on the body model used, which dictates that limits have to refer to a body model that is exactly defined. Magn Reson Med 48:731–734, 2002.

On-coil multiple channel transmit system based on class-D amplification and pre-amplification with current amplitude feedback

A complete high‐efficiency transmit amplifier unit designed to be implemented in on‐coil transmit arrays is presented. High power capability, low power dissipation, scalability, and cost minimization were some of the requirements imposed to the design. The system is composed of a current mode class‐D amplifier output stage and a voltage mode class‐D preamplification stage. The amplitude information of the radio frequency pulse was added through a customized step‐down DC‐DC converter with current amplitude feedback that connects to the current mode class‐D stage. Benchtop measurements and imaging experiments were carried out to analyze system performance. Direct control of B1 was possible and its load sensitivity was reduced to less than 10% variation from unloaded to full loaded condition. When using the amplifiers in an array configuration, isolation above 20 dB was achieved between neighboring coils by the amplifier decoupling method. High output current operation of the transmitter was proved on the benchtop through output power measurements and in a 1.5T scanner through flip angle quantification. Finally, single and multiple channel excitations with the new hardware were demonstrated by receiving signal with the body coil of the scanner. Magn Reson Med, 2013.

MRI Pulse Sequence Design With First-Order Gradient Moment Nulling in Arbitrary Directions by Solving a Polynomial Program

We suggest a polynomial program for the calculation of optimized gradient waveforms for magnetic resonance tomography pulse sequences. Such non-linear mathematical programs can describe gradient system capabilities, meet k-space trajectory specifications, and capture sequence timing conditions. Moreover they allow the incorporation of gradient moment nulling constraints in one or several arbitrary spatial directions, which can reduce flow motion artifacts in the images. We report first experiences in solving such automatic pulse sequence design programs with the interior point solver Ipopt.

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.

Method for operating a magnetic resonance imaging apparatus to obtain a high T2 contrast

Nuclear spins are excited in an examination subject by a sequence of radio-frequency pulses that are emitted in under the influence of slice selection gradients, with n different slices of the examination subject being excited. Due to read-out gradients, n gradient echoes respectively allocated to a slice are generated such that each gradient echo allocated to a radio-frequency pulse comes to lie between two successive radio-frequency pulses. Further slices can thus be excited within the echo time of a gradient echo of a specific slice. A T2 * contrast image can thus be obtained with a significantly shorter examination time than was heretofore achievable.

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.