A Semi-Blind Calibration and Compensation Method for Dynamic Range Recovery of Low-Power Pre-Amplifiers in MRI Receive Chains.

To enable wireless MRI receive arrays, per-channel power consumption must be reduced by a significant factor. To address this, a low-power SiGe alternative to industry standard MRI pre-amplifier blocks has been proposed and its impact on imaging performance evaluated in a benchtop environment. The SiGe amplifier reduces power consumption 28x, but exhibits increased non-linearity and reduced dynamic range relative to industry standard amplifiers. This distorts the images, causing reduced contrast and a blurring of fine features. In conjunction with the amplifier, a semi-blind calibration and compensation framework has been proposed to remove artifacts caused by this non-linearity. Requiring the knowledge of the calibration signal bandwidth, the associated peak transmit powers, and the distorted baseband signals, a second non-linearity is constructed that when cascaded with the receive chain produces a linear response. This method was evaluated for both knee and phantom image datasets of peak input power -20dBm with a -40dBm peak input power image as reference. In the benchtop environment, industry standard amplifiers produced input normalized RMSEs of 0.0199 and 0.0310 for phantom and knee datasets, respectively. The low-power SiGe amplifier resulted in RMSEs of 0.0869 and 0.1130 which were reduced to 0.0158 and 0.0168 following compensation, for phantom and knee images respectively. The ability to effectively compensate for this reduced dynamic range encourages further investigation of low-power SiGe amplifiers for power limited MRI receive arrays.

An 8 channel parallel transmit system with current sensor feedback for MRI-guided interventional applications.

Background.Parallel transmit (pTx) has introduced many benefits to magnetic resonance imaging (MRI) with regard to decreased specific absorption rates and improved transmit field homogeneity, of particular importance in applications at higher magnetic field strengths. PTx has also been proposed as a solution to mitigating dangerous RF induced heating of elongated conductive devices such as those used in cardiac interventions. In this work we present a system that can augment a conventional scanner with pTx, in particular for use in interventional MRI for guidewire safety, by adjusting the amplitude and phase of each channel right before the start of the imaging pulses.Methods.The pTx system was designed to work in-line with a 1.5 T MRI while the RF synthesis and imaging control was maintained on the host MR scanner. The add-on pTx system relies on the RF transmit signal, unblanking pulse, and a protocol driven trigger from the scanner. The RF transmit was split into multiple fully modulated transmit signals to drive an array of custom transceiver coils. The performance of the 8-channel implementation was tested with regards to active and real-time control of RF induced currents on a standard guidewire, heating mitigation tests, and anatomical imaging in sheep.Results. The pTx system was intended to update RF shims in real-time and it was demonstrated that the safe RF shim could be determined while the guidewire is moved. The anatomical imaging demonstrated that cardiac anatomy and neighbouring superficial structures could be fully characterized with the pTx system inline.Conclusion.We have presented the design and performance of a real-time feedback control pTx system capable of adding such capabilities to a conventional MRI with the focus of guidewire imaging in cardiac interventional MRI applications.

Biopsy marker localization with thermo-acoustic ultrasound for lumpectomy guidance.

PURPOSE: Almost one in four lumpectomies fails to fully remove cancerous tissue from the breast, requiring reoperation. This high failure rate suggests that existing lumpectomy guidance methods are inadequate for allowing surgeons to consistently identify the proper volume of tissue for excision. Current guidance techniques either provide little information about the tumor position or require surgeons to frequently switch between making incisions and manually probing for a marker placed at the lesion site. This article explores the feasibility of thermo-acoustic ultrasound (TAUS) to enable hands-free localization of metallic biopsy markers throughout surgery, which would allow for continuous visualization of the lesion site in the breast without the interruption of surgery. In a TAUS-based localization system, microwave excitations would be transmitted into the breast, and the amplification in microwave absorption around the metallic markers would generate acoustic signals from the marker sites through the thermo-acoustic effect. Detection and ranging of these signals by multiple acoustic receivers on the breast could then enable marker localization through acoustic multilateration. METHODS: Physics simulations were used to characterize the TAUS signals generated from different markers by microwave excitations. First, electromagnetic simulations determined the spatial pattern of the amplification in microwave absorption around the markers. Then, acoustic simulations characterized the acoustic fields generated from these markers at various acoustic frequencies. TAUS-based one-dimensional (1D) ranging of two metallic markers-including a biopsy marker that is FDA-approved for clinical use-immersed in saline was also performed using a bench-top setup. To perform TAUS acquisitions, a microwave applicator was driven by 2.66 GHz microwave signals that were amplitude-modulated by chirps at the desired acoustic excitation frequencies, and the resulting TAUS signal from the markers was detected by an ultrasonic transducer. RESULTS: The simulation results show that the geometry of the marker strongly impacts the quantity and spatial pattern of both the microwave absorption around the marker and the resulting TAUS signal generated from the marker. The simulated TAUS signal maps and acoustic frequency responses also make clear that the marker geometry plays an important role in determining the overall system response. Using the bench-top setup, TAUS detection and 1D localization of the markers were successfully demonstrated for multiple different combinations of microwave applicator and metallic marker. These initial results indicate that TAUS-based localization of biopsy markers is feasible. CONCLUSIONS: Through microwave excitations and acoustic detection, TAUS can be used to localize metallic biopsy markers. With further development, TAUS opens new avenues to enable a more intuitive lumpectomy guidance system that could help to achieve better lumpectomy outcomes.

Thermo-acoustic ultrasound for noninvasive temperature monitoring at lead tips during MRI.

PURPOSE: We explore the use of thermo-acoustic ultrasound (TAUS) to monitor temperature at the tips of conductive device leads during MRI. THEORY: In TAUS, rapid radiofrequency (RF) power deposition excites an acoustic signal via thermoelastic expansion. Coupling of the MRI RF transmit to device leads causes SAR amplification at lead tips, allowing MRI RF transmitters to excite significant lead tip TAUS signals. Because the amplitude of the TAUS signal depends on temperature, it becomes feasible to monitor the lead tip temperature during MRI by tracking the TAUS amplitude. METHODS: The TAUS temperature dependence was characterized in a phantom and in tissue. To perform TAUS acquisitions in an MRI scanner, amplitude modulated RF chirps were transmitted by the body coil, and the lead tip TAUS signal was detected by an ultrasonic transducer. The TAUS signal level was correlated with the RF current induced on the lead and the associated B1 artifacts in MRI. TAUS signals acquired during RF-induced heating were used to estimate the lead tip temperature. RESULTS: The TAUS signal exhibited strong dependence on temperature, increasing over 30% with 10∘ C of heating both in the phantom and in tissue. A lead tip TAUS signal was observed for a 100 mA rms current induced on a lead. During RF-induced heating, the TAUS signal appeared to accurately approximate the peak lead tip temperature. CONCLUSIONS: TAUS allows for noninvasive monitoring of lead tip temperature in an MRI environment. With further development, TAUS opens new avenues to improve RF device safety during MRI scans.

Wireless Resonant Circuits Printed Using Aerosol Jet Deposition for MRI Catheter Tracking.

Interventional magnetic resonance imaging (MRI) could allow for diagnosis and immediate treatment of ischemic stroke; however, such endovascular catheter-based procedures under MRI guidance are inherently difficult. One major challenge is tracking the tip of the catheter, as standard fabrication methods for building inductively coupled coil markers are rigid and bulky. Here, we report a new approach that uses aerosol jet deposition to three-dimensional (3-D) print an inductively coupled RF coil marker on a polymer catheter. Our approach enables lightweight conforming markers on polymer catheters and these low-profile markers allow the catheter to be more safely navigated in small caliber vessels. Prototype markers with an inductor with the geometry of a double helix are incorporated on catheters for in vitro studies, and we show that these markers exhibit good signal amplification. We report temperature measurements and, finally, demonstrate feasibility in a preliminary in vivo experiment. We provide material properties and electromagnetic simulation performance analysis. This paper presents fully aerosol jet-deposited and functional wireless resonant markers on polymer catheters for use in 3T clinical scanners.

Safe guidewire visualization using the modes of a PTx transmit array MR system.

PURPOSE: MRI-guided cardiovascular intervention using standard metal guidewires can produce focal tissue heating caused by induced radiofrequency guidewire currents. It has been shown that safe operation is made possible by using parallel transmit radiofrequency coils driven in the null current mode, which does not induce radiofrequency currents and hence allows safe tissue visualization. We propose that the maximum current modes, usually considered unsafe, be used at very low power levels to visualize conductive wires, and we investigate pulse sequences best suited for this application. METHODS: Spoiled gradient echo, balanced steady-state free precession, and turbo spin echo sequences were evaluated for their ability to visualize a conductive guidewire embedded in a gel phantom when run in maximum current modes at very low power level. Temperature at the guidewire tip was monitored for safety assessment. RESULTS: Excellent guidewire visualization could be achieved using maximum current modes excitation, with the turbo spin echo sequence giving the best image quality. Although turbo spin echo is usually considered to be a high-power sequence, our method reduced all pulses to 1% amplitude (0.01% power), and heating was not detected. In addition, visualization of background tissue can be achieved using null current mode, also with no recorded heating at the guidewire tip even when running at 100% (reported) specific absorption rate. CONCLUSION: Parallel transmit is a promising approach for both guidewire and tissue visualization using maximum and null current modes, respectively, for interventional cardiac MRI. Such systems can switch excitation mode instantaneously, allowing for flexible integration into interactive sequences.

Design and implementation of a low-cost, tabletop MRI scanner for education and research prototyping.

While access to a laboratory MRI system is ideal for teaching MR physics as well as many aspects of signal processing, providing multiple MRI scanners can be prohibitively expensive for educational settings. To address this need, we developed a small, low-cost, open-interface tabletop MRI scanner for academic use. We constructed and tested 20 of these scanners for parallel use by teams of 2-3 students in a teaching laboratory. With simplification and down-scaling to a 1 cm FOV, fully-functional scanners were achieved within a budget of $10,000 USD each. The design was successful for teaching MR principles and basic signal processing skills and serves as an accessible testbed for more advanced MR research projects. Customizable GUIs, pulse sequences, and reconstruction code accessible to the students facilitated tailoring the scanner to the needs of laboratory exercise. The scanners have been used by >800 students in 6 different courses and all designs, schematics, sequences, GUIs, and reconstruction code is open-source.

An MRI Compatible RF MEMs Controlled Wireless Power Transfer System.

In magnetic resonance imaging (MRI), wearable wireless receive coil arrays are a key technology goal. An MRI compatible wireless power transfer (WPT) system will be needed to realize this technology. An MRI WPT system must withstand the extreme electromagnetic environment of the scanner and cannot degrade MRI image quality. Here, a WPT system is developed for operation in MRI scanners using new microelectromechanical RF switch (RF MEMs) technology. The WPT system includes a class-E power amplifier, RF MEMs automated impedance matching, a primary coil array employing RF MEMs power steering, and a flexible secondary coil with class E rectification. To adapt WPT technology to MRI, techniques are developed for operation at high magnetic field, and to mitigate the RF interactions between the scanner and WPT system. A major challenge was the identification and suppression of noise and harmonic interference, by gating, filtering, and rectifier topologies. The system can achieve 63% efficiency while exceeding 13 W delivery over a coil distance of 3.5 cm. For continuous WPT beyond 5W, added filters and full-wave class E rectification lowers harmonic generation at some cost to efficiency, while image SNR reaches about 32% of the ideal. RF-gated WPT, which interrupts power transfer in the MRI signal acquisition interval, achieves SNR performance to within 1 dB of the ideal. With further refinement, the inclusion of WPT technology in MRI scanners appears completely feasible.

Evaluation of a Flexible 12-Channel Screen-printed Pediatric MRI Coil.

Background Screen-printed MRI coil technology may reduce the need for bulky and heavy housing of coil electronics and may provide a better fit to patient anatomy to improve coil performance. Purpose To assess the performance and caregiver and clinician acceptance of a pediatric-sized screen-printed flexible MRI coil array as compared with conventional coil technology. Materials and Methods A pediatric-sized 12-channel coil array was designed by using a screen-printing process. Element coupling and phantom signal-to-noise ratio (SNR) were assessed. Subjects were scanned by using the pediatric printed array between September and November 2017; results were compared with three age- and sex-matched historical control subjects by using a commercial 32-channel cardiac array at 3 T. Caregiver acceptance was assessed by asking nurses, technologists, anesthesiologists, and subjects or parents to rate their coil preference. Diagnostic quality of the images was evaluated by using a Likert scale (5 = high image quality, 1 = nondiagnostic). Image SNR was evaluated and compared. Results Twenty study participants were evaluated with the screen-printed coil (age range, 2 days to 12 years; 11 male and nine female subjects). Loaded pediatric phantom testing yielded similar noise covariance matrices and only slightly degraded SNR for the printed coil as compared with the commercial coil. The caregiver acceptance survey yielded a mean score of 4.1 ± 0.6 (scale: 1, preferred the commercial coil; 5, preferred the printed coil). Diagnostic quality score was 4.5 ± 0.6. Mean image SNR was 54 ± 49 (paraspinal muscle), 78 ± 51 (abdominal wall muscle), and 59 ± 35 (psoas) for the printed coil, as compared with 64 ± 55, 65 ± 48, and 57 ± 43, respectively, for the commercial coil; these SNR differences were not statistically significant (P = .26). Conclusion A flexible screen-printed pediatric MRI receive coil yields adequate signal-to-noise ratio in phantoms and pediatric study participants, with similar image quality but higher preference by subjects and their caregivers when compared with a conventional MRI coil. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Lamb in this issue.

Thermo-Acoustic Ultrasound for Detection of RF-Induced Device Lead Heating in MRI.

Patients who have implanted medical devices with long conductive leads are often restricted from receiving MRI scans due to the danger of RF-induced heating near the lead tips. Phantom studies have shown that this heating varies significantly on a case-by-case basis, indicating that many patients with implanted devices can receive clinically useful MRI scans without harm. However, the difficulty of predicting RF-induced lead tip heating prior to scanning prevents numerous implant recipients from being scanned. Here, we demonstrate that thermo-acoustic ultrasound (TAUS) has the potential to be utilized for a pre-scan procedure assessing the risk of RF-induced lead tip heating in MRI. A system was developed to detect TAUS signals by four different TAUS acquisition methods. We then integrated this system with an MRI scanner and detected a peak in RF power absorption near the tip of a model lead when transmitting from the scanner's body coil. We also developed and experimentally validated simulations to characterize the thermo-acoustic signal generated near lead tips. These results indicate that TAUS is a promising method for assessing RF implant safety, and with further development, a TAUS pre-scan could allow many more patients to have access to MRI scans of significant clinical value.

Optimal Broadband Noise Matching to Inductive Sensors: Application to Magnetic Particle Imaging.

Inductive sensor-based measurement techniques are useful for a wide range of biomedical applications. However, optimizing the noise performance of these sensors is challenging at broadband frequencies, owing to the frequency-dependent reactance of the sensor. In this work, we describe the fundamental limits of noise performance and bandwidth for these sensors in combination with a low-noise amplifier. We also present three equivalent methods of noise matching to inductive sensors using transformer-like network topologies. Finally, we apply these techniques to improve the noise performance in magnetic particle imaging, a new molecular imaging modality with excellent detection sensitivity. Using a custom noise-matched amplifier, we experimentally demonstrate an 11-fold improvement in noise performance in a small animal magnetic particle imaging scanner.

An RF-gated wireless power transfer system for wireless MRI receive arrays.

In MRI systems, cable-free receive arrays would simplify setup while reducing the bulk and weight of coil arrays and improve patient comfort and throughput. Since battery power would limit scan time, wireless power transfer (WPT) is a viable option to continuously supply several watts of power to on-coil electronics. To minimize added noise and decouple the wireless power system from MRI coils, restrictions are placed on the coil geometry of the wireless power system, which are shown to limit its efficiency. Continuous power harvesting can also cause a large increase in the background noise of the image due to diode rectifier up-conversion of noise around the frequency of the transmitted power. However, by RF gating the transmitted power off during the MRI receive time while continuing to supply power from a storage capacitor, WPT is demonstrated to have minimal impact on image quality at received power levels up to 11 W. The integration of WPT with a 1.5T scanner is demonstrated.

A Millimeter-Wave Digital Link for Wireless MRI.

A millimeter (mm) wave radio is presented in this work to support wireless MRI data transmission. High path loss and availability of wide bandwidth make mm-waves an ideal candidate for short range, high data rata communication required for wireless MRI. The proposed system uses a custom designed integrated chip (IC) mm-wave radio with 60 GHz as radio frequency carrier. In this work, we assess performance in a 1.5 T MRI field, with the addition of optical links between the console room and magnet. The system uses ON-OFF keying (OOK) modulation for data transmission and supports data rates from 200 Mb/s to 2.5 Gb/s for distances up-to 65 cm. The presence of highly directional, linearly polarized, on-chip dipole antennas on the mm-wave radio along with the time division multiplexing (TDM) circuitry allows multiple wireless links to be created simultaneously with minimal inter-channel interference. This leads to a highly scalable solution for wireless MRI.

Depletion-Mode GaN HEMT Q-Spoil Switches for MRI Coils.

Q-spoiling is the process of decoupling an MRI receive coil to protect the equipment and patient. Conventionally, Q-spoiling is performed using a PIN diode switch that draws significant current. In this work, a Q-spoiling technique using a depletion-mode Gallium Nitride HEMT device was developed for coil detuning at both 1.5 T and 3 T MRI. The circuits with conventional PIN diode Q-spoiling and the GaN HEMT device were implemented on surface coils. SNR was measured and compared for all surfaces coils. At both 1.5 T and 3 T, comparable SNR was achieved for all coils with the proposed technique and conventional Q-spoiling. The GaN HEMT device has significantly reduced the required power for Q-spoiling. The GaN HEMT device also provides useful safety features by detuning the coil when unpowered.

14-Vertex Heteroboranes with 14 Skeletal Electron Pairs: An Experimental and Computational Study.

Three isomers of [(Cp*Ru)2 C2 B10 H12 ], the first examples of 14-vertex heteroboranes containing 14-skeletal electron pairs, have been synthesized by the direct electrophilic insertion of a {Cp*Ru(+) } fragment into the anion [4-Cp*-4,1,6-RuC2 B10 H12 ](-) . All three compounds have the same unique polyhedral structure having an approximate Cs symmetry and featuring a four-atom trapezoidal face. X-ray diffraction studies could confidently identify only one of the two cage C atoms in each structure. The other C atom position has been established by a combination of i) best fitting of computed and experimental (11) B and (1) H NMR chemical shifts, and ii) consideration of the lowest computed energy for series of isomers studied by DFT calculations. In all three isomers, one cage C atom occupies a degree-4 vertex on the short parallel edge of the trapezium.

Screen-printed flexible MRI receive coils.

Magnetic resonance imaging is an inherently signal-to-noise-starved technique that limits the spatial resolution, diagnostic image quality and results in typically long acquisition times that are prone to motion artefacts. This limitation is exacerbated when receive coils have poor fit due to lack of flexibility or need for padding for patient comfort. Here, we report a new approach that uses printing for fabricating receive coils. Our approach enables highly flexible, extremely lightweight conforming devices. We show that these devices exhibit similar to higher signal-to-noise ratio than conventional ones, in clinical scenarios when coils could be displaced more than 18 mm away from the body. In addition, we provide detailed material properties and components performance analysis. Prototype arrays are incorporated within infant blankets for in vivo studies. This work presents the first fully functional, printed coils for 1.5- and 3-T clinical scanners.

A semiflexible 64-channel receive-only phased array for pediatric body MRI at 3T.

PURPOSE: To design, construct, and validate a semiflexible 64-channel receive-only phased array for pediatric body MRI at 3T. METHODS: A 64-channel receive-only phased array was developed and constructed. The designed flexible coil can easily conform to different patient sizes with nonoverlapping coil elements in the transverse plane. It can cover a field of view of up to 44 × 28 cm(2) and removes the need for coil repositioning for body MRI patients with multiple clinical concerns. The 64-channel coil was compared with a 32-channel standard coil for signal-to-noise ratio and parallel imaging performances on different phantoms. With IRB approval and informed consent/assent, the designed coil was validated on 21 consecutive pediatric patients. RESULTS: The pediatric coil provided higher signal-to-noise ratio than the standard coil on different phantoms, with the averaged signal-to-noise ratio gain at least 23% over a depth of 7 cm along the cross-section of phantoms. It also achieved better parallel imaging performance under moderate acceleration factors. Good image quality (average score 4.6 out of 5) was achieved using the developed pediatric coil in the clinical studies. CONCLUSION: A 64-channel semiflexible receive-only phased array has been developed and validated to facilitate high quality pediatric body MRI at 3T. Magn Reson Med 76:1015-1021, 2016. © 2015 Wiley Periodicals, Inc.

The exopolyhedral ligand orientation (ELO) in 3-(nitrato-κO)-3,3-bis(triphenylphosphane-κP)-3-rhoda-1,2-dicarba-closo-dodecaborane(11) dichloromethane 2.2-solvate.

In the title compound, [Rh(C2H11B9)(NO3)(C18H15P)2]·2.2CH2Cl2, studied as a 2.2-solvate of what was assumed to be dichloromethane, the nitrate ligand lies cis with respect to both cage C atoms. Accordingly, the compound displays a pronounced preferred exopolyhedral ligand orientation (ELO) which is traced to both the greater trans influence of the cage B over the cage C atoms and the greater trans influence of the triphenylphosphane ligands over the nitrate ligand. The overall molecular architecture therefore agrees with that of a number of similar 3-L-3,3-L'2-3,1,2-closo-MC2B9H11 species in the literature.

Controlling radiofrequency-induced currents in guidewires using parallel transmit.

PURPOSE: Elongated conductors, such as pacemaker leads, neurostimulator leads, and conductive guidewires used for interventional procedures can couple to the MRI radiofrequency (RF) transmit field, potentially causing dangerous tissue heating. The purpose of this study was to demonstrate the feasibility of using parallel transmit to control induced RF currents in elongated conductors, thereby reducing the RF heating hazard. METHODS: Phantom experiments were performed on a four-channel parallel transmit system at 1.5T. Parallel transmit "null mode" excitations that induce minimal wire current were designed using coupling measurements derived from axial B1 (+) maps. The resulting current reduction performance was evaluated with B1 (+) maps, current sensor measurements, and fluoroptic temperature probe measurements. RESULTS: Null mode excitations reduced the maximum coupling mode current by factors ranging from 2 to 80. For the straight wire experiment, a current null imposed at a single wire location was sufficient to reduce tip heating below detectable levels. For longer insertion lengths and a curved geometry, imposing current nulls at two wire locations resulted in more distributed current reduction along the wire length. CONCLUSION: Parallel transmit can be used to create excitations that induce minimal RF current in elongated conductors, thereby decreasing the RF heating risk, while still allowing visualization of the surrounding volume.