Report on the development of open-source reference hardware (RF coil, exciter, modulators, RF power amplifiers) and open-source control software, including traceable measurement procedures that allow testing of implant under parallel transmission (pTx) MR conditions

Confidentiality Status: PU - Public, fully open (remember to deposit public deliverables in a trusted repository) Deliverable Cover Sheet Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or EURAMET. Neither the European Union nor the granting authority can be held responsible for them. The project has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. 1 of 14 21NRM05 STASIS D3: Report on the development of open-source reference hardware (RF coil, exciter, modulators, RF power amplifiers) and open-source control software, including traceable measurement procedures that allow testing of implant under parallel transmission (pTx) MR conditions Organisation name of the lead participant for the deliverable: German Cancer Research Centre (DKFZ) Due date of the deliverable: 30th of September 2025 Actual submission date of the deliverable: 30th of September 2025 2 of 14 Glossary B1+ Circularly polarized flux density, transmit field DAC Digital-to-analogue converter FPGA Field programmable gate array GUI Graphical user interface IQ-Modulator Inphase/quadrature modulator LDMOS Laterally diffused metal-oxide-semiconductor MR Magnetic resonance MRI Magnetic resonance imaging RF Radio frequency RMS Root mean square 𝜀𝑟 Relative permittivity 𝜎 Conductivity 21NRM05 STASIS 3 of 14 TABLE OF CONTENTS 1 Summary ................................................................................................................................. 4 2 Introduction .............................................................................................................................. 4 2.1 Purpose ............................................................................................................................. 4 2.2 System Overview ............................................................................................................... 5 3 RF array ................................................................................................................................... 5 3.1 Introduction ........................................................................................................................ 5 3.2 Mechanical Design ............................................................................................................. 6 3.3 Electrical Design ................................................................................................................ 7 3.4 RF field simulation results .................................................................................................. 8 4 RF Generation System ............................................................................................................. 9 4.1 Overview ............................................................................................................................ 9 4.2 Timing .............................................................................................................................. 10 4.3 Modulation ....................................................................................................................... 10 5 RF power amplifiers ............................................................................................................... 11 6 Graphical User Interface ........................................................................................................ 12 6.1 Overview .......................................................................................................................... 12 6.2 Main window .................................................................................................................... 12 6.3 Pulse Tool ........................................................................................................................ 12 6.4 Calibration ........................................................................................................................ 13 7 Summary and Conclusion ...................................................................................................... 14 8 References ............................................................................................................................ 14 21NRM05 STASIS 4 of 14 1 Summary This report provides an overview of the hardware and software components of the STASIS open-source exposure system. The various parts of the system are briefly described. As the system is open source, detailed information is available in the corresponding repositories. Photographs of the RF coil array and the transmit chain are shown in Figure 1. Figure 1: The 8-channel transmit array (left) and the control system and amplifiers in a 19” rack (right). 2 Introduction 2.1 Purpose The STASIS RF exposure system is an open-source hardware and software platform designed for implant testing under exposure to radiofrequency (RF) fields, such as those encountered in the environment of an MRI system. For this purpose, the target RF frequency is 128 MHz, corresponding to the resonant frequency of protons in a 3 Tesla magnetic field. The system is capable of transmitting on eight independent RF channels, enabling parallel transmission. To ensure the design is easily accessible to potential users, all design files and software are publicly available and can be accessed via the following repositories: https://github.com/sOrzada/STASIS_Exposure_Hardware https://github.com/sOrzada/STASIS_Control All hardware components of the exposure system are licensed under the CERN-OHL-W while the software is licensed under a GNU GPL v3. In addition, the project has been submitted to the conformity assessment body of the Open Source Imaging Initiative e.V. (https://www.opensourceimaging.org) to assess its open-source status according to DIN SPEC 3105. To further improve accessibility, standard components with long production lifespans were selected wherever possible. For example, standard logic components were used instead of specialised microcontrollers or FPGAs. 21NRM05 STASIS 5 of 14 2.2 System Overview Figure 2: Schematic overview of the STASIS exposure system. The STASIS exposure system consists of 4 main parts as shown in Figure 2. The user interacts with the system via a graphical user interface (GUI), from which the control hardware can be programmed. The control system manages timing and generates eight independent RF signals, which are transmitted to eight separate RF amplifiers. These amplifiers then deliver high-peak and average power signals to the antenna array, creating the RF fields to which the object under test is exposed. The antenna array is an eight-channel microstrip line array with an inner diameter of 59.5 cm, matching the size of a standard clinical MRI system bore. A more detailed description is provided in Section 3. The RF generation system comprises all electronics required to drive the RF chain in a manner that simulates an MRI environment. It handles transmission timing and pseudo-reception (pauses), generates the RF centre frequency, and modulates the baseband signals onto the centre frequency for all eight channels. Further details can be found in Section 4. The RF amplifiers are solid-state power amplifiers capable of peak output powers exceeding 1 kW. They are also capable of continuous wave (CW) transmission at lower root mean square (RMS) power levels. These amplifiers operate as stand-alone units and can potentially be used independently of the STASIS system. A detailed description is given in Section 5. To provide a user-friendly operating experience, custom software with an intuitive graphical user interface was developed. To ensure maintainability, the software is written in Python and follows an object-oriented design. The GUI allows full system configuration and all required calibrations, such as gradient linearization. A full description of the software is provided in Section 6. 3 RF array 3.1 Introduction The RF array is designed to resemble an eight-channel transmit array of a size and type suitable for use within a 3 Tesla MRI system. Its design is closely based on the configuration proposed by Vernickel et al. (1) in 2007. The array consists of eight microstrip antenna elements, each electrically connected to its adjacent neighbours. A photograph of the RF array mounted on its trolley is shown in Figure 3. 21NRM05 STASIS 6 of 14 Figure 3: Photo of the 8-channel RF array. 3.2 Mechanical Design The coil housing is constructed from poly(methyl methacrylate) (PMMA). A schematic is shown in Figure 4. The overall length of the housing is 820 mm, comprising 800 mm cylindrical walls and 10 mm end rings on each side. The inner diameter of the housing is 594 mm, closely matching the inner diameter of a Siemens 60 cm bore (actual diameter: 595 mm). The inner cylinder of the housing has a wall thickness of 8 mm, while the outer cylinder’s wall thickness is 6 mm. The outer diameter of the housing is 700 mm, resulting in a 39 mm gap between the two cylinders, within which the antenna elements are positioned. The inner surface of the outer cylinder is lined with adhesive copper foil, serving as the ground plane for the microstrip antennas. As the array is not intended for use in an MR environment, there is no requirement for eddy current mitigation; therefore, the copper cladding is continuous. The ground plane consists of four large sections of copper foil, joined by copper bands with electrically conductive adhesive applied across the seams. 21NRM05 STASIS 7 of 14 Figure 4: Sketch of the array including the housing. The inner side of the outer cylinder is clad with copper foil. Note that the small holders for the trimmer rods are placed directly in line with the center of the micro-strip lines. For more information refer to the CAD model in the Data. Each antenna element has a width of 100 mm and a length of 450 mm. The distance between the centre of the microstrip and the ground plane is 37.3 mm, maintained by 3D-printed mechanical holders made from PLA. The copper layer of the microstrips is located on the side of the board facing the ground plane. Consequently, the capacitors are also mounted on this side, which allows the microstrips to be positioned closer to the inner cylinder of the housing. 3.3 Electrical Design The electrical design of the array closely follows the configuration proposed by Vernickel et al. (1). The circuit schematic of a single antenna element, including its electrical connections to adjacent elements, is shown in Figure 5. The component values indicated in the schematic represent total capacitances, inclusive of parasitic capacitances from the PCBs. To achieve a homogeneous current distribution along the microstrip, each element is divided into five sections, interconnected by pairs of 33 pF capacitors. A 22 pF capacitor is placed at the end of the strip. To minimize coupling between adjacent elements, a decoupling network is implemented. This consists of a 560 pF capacitor connected to two coaxial cables, each 130 mm in length, for each directly adjacent element. These cables are terminated with 7.6 pF capacitors, which interface with the corresponding coaxial lines from neighbouring elements. Simulations revealed that significant peak voltages can occur across these coupling capacitors. In the actual implementation, it is therefore necessary to replace each with multiple capacitors in series to ensure voltage handling capability and reliability. 21NRM05 STASIS 8 of 14 Figure 5: Schematic showing one microstrip element with the connection to the neighboring antennas. The capacitor values given in this schematic are total capacitances. Some of the lumped elements on the actual coil have different values due to parasitic capacitances of the PCBs. 3.4 RF field simulation results All simulations of the RF array were carried out in CST Microwave Studio (Dassault Système, France). For the simulations the array was loaded with a homogeneous phantom with electrical properties that correspond to the mean of the human body at 128 MHz (𝜀𝑟=52; 𝜎=0.47 S/m). A transverse view of the B1+ field distribution for the birdcage mode and a stimulated power of 1W is shown in Figure 6. It shows the pattern typical for 3T body imaging with a high intensity in the center and two lower intensity bands. Figure 6: Simulated B1+ field distribution of the CP+ mode (birdcage mode) for stimulated power of 1W. The typical pattern with a high intensity center and two lower intensity bands as known from 3T body imaging is visible. 21NRM05 STASIS 9 of 14 Figure 7 shows the B1+ for the birdcage mode and stimulated power of 8 kW on a central line on the z-axis within the phantom. The values vary between 18 µT and 22µT. This field strength is sufficient to emulate the transmit fields of an MRI system. Figure 7: B1+ for the birdcage mode and a stimulated power of 8 kW on a central line on the z-axis through the phantom. The values vary between 18 µT and 22 µT. 4 RF Generation System 4.1 Overview The RF Generation System (Figure 8) produces both digital and analogue signals required for the operation of the RF exposure system. It emulates the signal and timing generation of an MRI system, allowing the user to configure a sequence of RF pulses and pauses that replicate an actual imaging sequence. The system is built around IEC 60603-2 (formerly DIN 41612) connectors and housed in a Eurocard rack system (IEC 60297), facilitating modularity and ease of component exchange. For example, by adding additional racks with extra modulator cards, the number of RF channels can be easily expanded. Figure 8: A picture of the front of the Control System. A simplified schematic of the RF Generation System is shown in Figure 9. The Control System comprises a signal generator that provides a stable 128 MHz RF signal, a timing control module that switches between transmit and pseudo-receive modes, and modulator modules capable of adjusting the amplitude and phase of