Tx Efficiency Research Articles

Self-image-guided ultrasonic transceiver design for reliably powering mm-sized implants

This paper presents the design of a self-image-guided ultrasonic (SIG-US) transceiver for reliable wireless power transmission (WPT) to miniaturized biomedical implants. In the receiver (RX) mode, this paper proposes a method for high precision time interval measurement (TIM) between two signals independent of their amplitudes. In the transmitter (TX) mode, a 15 V Class-DE driver by stacking 3.3 V transistors of 180 nm standard CMOS technology is designed to drive a 32-element linear array. The designed TX efficiency is 90.93%. The post-layout and the Matlab k-Wave toolbox simulation results show that the designed system is capable of transmitting power to the implant with less than 0.9% reduction compared to the ideal SIG-US system over the ±45° steering range. Considering the mismatch effect between transistors, the power reduction is less than 2% in the worst case. The RX with a power consumption of 628.83 μW from a nominal supply voltage of 1.8 V, is capable of detecting time delays between two signals over the input range of 1 mV to 10 mV, with high accuracy.

Double-row 16-element folded-end dipole transceiver array for 3D RF shimming of the whole human brain at 9.4 T.

Homogeneity and longitudinal coverage of transmit (Tx) human head RF coils at ultrahigh field (UHF, ≥7 T) can be improved by 3D RF shimming, which requires using multi-row Tx arrays. Examples of 3D RF shimming using double-row UHF loop transceiver (TxRx) and Tx arrays have been described previously. Dipole antennas provide unique simplicity and robustness while offering comparable Tx efficiency and signal-to-noise ratio to conventional loop designs. Single-row Tx and TxRx human head UHF dipole arrays have been previously described by multiple groups. Recently, we developed a novel type of dipole antenna, a folded-end dipole, and presented single-row eight-element array prototypes for human head imaging at 7 and 9.4 T. These studies have shown that the novel antenna design can improve the longitudinal coverage and minimize peak local specific absorption rate (SAR) as compared with common unfolded dipoles. In this work, we developed, constructed, and evaluated a 16-element double-row TxRx folded-end dipole array for human head imaging at 9.4 T. To minimize cross-talk between neighboring dipoles located in different rows, we used transformer decoupling, which decreased coupling to a level below -20 dB. The developed array design was demonstrated to be capable of 3D static RF shimming and can be potentially used for dynamic shimming using parallel transmission. For optimal phase shifts between the rows, the array provides 11% higher SAR efficiency and 18% higher homogeneity than a folded-end dipole single-row array of the same length. The design also offers a substantially simpler and more robust alternative to the common double-row loop array with about 10% higher SAR efficiency and better longitudinal coverage.

A 2 nJ/bit, 2.3% FSK Error Fully Integrated Sub-2.4 GHz Transmitter With Duty-Cycle Controlled PA for Medical Band

This paper proposed a fully integrated MBAN (2360–2400 MHz) continuous phase modulated transmitter (TX) with tunable less than 0dBm output power for medical band. A duty-cycle tuning strategy was proposed for the power amplifier (PA) featuring adaptive optimized efficiency for different output powers. A fully on-chip transformer-based match network was proposed to suppress the 2nd harmonic using a series <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> resonator and to suppress the 3rd harmonic by introducing a transformer inter-winding capacitor feedback path. A fractional-N all-digital phase locked loop (ADPLL) with a transformer-based digitally controlled oscillator (DCO) is employed to reduce power consumption as well as improve modulation quality. The transmitter was fabricated in 40-nm CMOS technology, occupying an active area of 0.48mm2. Experimental results show a 26% drain efficiency with −10dBm PA output and 4dB tunable range. A 2mW total power consumption was measured with a TX efficiency of 5% and an energy efficiency of 2nJ/bit. The measured 2nd and 3rd harmonic distortion of the output were −44.3dBm and −57.2dBm, respectively, with on-chip matching network. The measured FSK error of CPM was 2.3% with an M of 2 and 1.57% with an M of 4.

Scalable and modular 8-channel transmit and 8-channel flexible receive coil array for 19 F MRI of large animals.

To introduce an RF coil system consisting of an 8-channel transmit (Tx) and 8-channel receive (Rx) coil arrays for 19 F MRI of large animals. The Tx efficiency and homogeneity of the 8-element loop coil array (loop size: 6× 15 cm2 ) were simulated for two different pig models rendered from MR images. An 8-channel Rx coil array consisting of a flexible 6-channel posterior and a 2-channel planar anterior array was designed to fit on the abdomen of an average-sized pig in supine position. Measurements were performed in a grid phantom and ex vivo on a pig model with perfluoroctylbromide (PFOB)-filled tubes inserted in the thorax. Measured and simulated Tx efficiency and homogeneity for the 8-channel and 5-channel arrays were in good agreement: 1.87 ± 0.22μT/√kW versus 1.96 ± 0.29μT/√kW, and 2.29 ± 0.39μT/√kW versus 2.41 ± 0.37μT/√kW. An isolation of 38 ± 8dB is achieved between the 19 F Tx and Rx elements, and over 30dB between the 1 H and 19 F elements. The PFOB-filled vials could be clearly identified within the cadaver abdomen with an SNR of 275 ± 51 for a 3D gradient-echo sequence with 2-mm isotropic resolution and 12 averages, acquired in 9:52 min:s. Performance of the Tx array was robust against phase and amplitude mismatches at the input ports. A modular and scalable Tx array offers improved Tx efficiency in 19 F MRI of large animals with various sizes. Although conventional birdcage coils have superior Tx efficiency within the target region of interest, scalability of the Tx array to animal size is a major benefit. The described 19 F coil provides homogeneous excitation and high sensitivity detection in large pig models.

A 900MHz GFSK and 16-FSK TX Achieving Up to 63.9% TX Efficiency and 76.2% PA Efficiency via a DC-DC-Powered Class-D VCO and a Class-E PA

This brief presents a highly efficient transmitter capable of radiating GFSK and 16-FSK modulated signals in the 900MHz ISM band. Multiple low power techniques are exploited such as a class-D VCO and a class-E single stage PA to enhance the TX efficiency. Using an on-chip DC-DC converter, the VCO can operate with down to a 250mV supply and generate 750mV signals to the input of the PA. A phased locked loop (PLL) is implemented for rapid calibration of the LO frequency and is locked to a 2MHz crystal, which also provides the clock for the DC-DC converter. The fabricated integrated circuit occupies 1mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area in 65nm LP CMOS, achieves a peak 76.2% PA efficiency and 63.9% TX efficiency, while being capable of delivering up to 3dBm to a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50\Omega $ </tex-math></inline-formula> antenna.

A 32-element loop/dipole hybrid array for human head imaging at 7T.

To improve whole-brain SNR at 7Tesla, a novel 32-element hybrid human head array coil was developed, constructed, and tested. Our general design strategy is based on 2 major ideas: Firstly, following suggestions of previous works based on the ultimate intrinsic SNR theory, we combined loops and dipoles for improvement of SNR near the head center. Secondly, we minimized the total number of array elements by using a hybrid combination of transceive (TxRx) and receive (Rx) elements. The new hybrid array consisted of 8 folded-end TxRx-dipole antennas and 3 rows of 24 Rx-loops all placed in a single layer on the surface of a tight-fit helmet. The developed array significantly improved SNR in vivo both near the center (∼20%) and at the periphery (∼20% to 80%) in comparison to a common commercial array coil with 8 transmit (Tx) and 32 Rx-elements. Whereas 24 loops alone delivered central SNR very similar to that of the commercial coil, the addition of complementary dipole structures provided further improvement. The new array also provided ∼15% higher Tx efficiency and better longitudinal coverage than that of the commercial array. The developed array coil demonstrated advantages in combining complementary TxRx and Rx resonant structures, that is, TxRx-dipoles and Rx-loops all placed in a single layer at the same distance to the head. This strategy improved both SNR and Tx-performance, as well as simplified the total head coil design, making it more robust.

A 24–28-GHz Four-Element Phased-Array Transceiver Front End With 21.1%/16.6% Transmitter Peak/OP1dB PAE and Subdegree Phase Resolution Supporting 2.4 Gb/s in 256-QAM for 5-G Communications

This article presents a power efficient 24-28-GHz four-element phased-array transceiver (TRX) front end (FE) for 5-G base stations and user equipment communications. A power efficient hybrid quadrature vector interpolation-based phase-shifting architecture is proposed to increase the TX and RX power efficiency and phase resolution. A transformer-based embedded T/R switch is proposed which minimizes TX and RX losses to ensure high TX output power and power efficiency while maintaining a low noise figure (NF) and extremely compact chip area. The 24-28-GHz four-element TRX FE is implemented in a 65-nm CMOS process (1P9M) for large-scale phased-array applications. The TX and RX paths achieve 0.8° phase resolution with <; 0.47°/0.17-dB RMS phase/gain error as well as 24-dB gain control range with 0.75-dB gain step and <; 0.1°/0.4-dB RMS gain/phase error over 24-30 GHz. The full TX path (including splitter and T/R switch) also achieves a measured 16.6-/18.9-dBm OP1dB/ P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> with 21.1%/16.6% PAE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> /OP1dB for each FE element while maintaining a minimum 4.4-dB NF in the full RX path (including combiner, T/R switch, and OFF-mode PA) at 28 GHz. Under the 64-/256-QAM modulation measurements, the full TX path achieves a state-of-the-art average output power of 13.5-/10.2-dBm per element (8.4%/3.5% full TX path PAE) with -26.4/-30.8 dB EVM and 27.8-/30.1-dB ACLR at 28 GHz, making the proposed TRX FE can be effectively deployed in both base stations and user equipment for 5-G communications. The chip size of the four-element TRX FE excluding I/O pads is only 3.4×1.1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

A Multiband FDD SAW-Less Transmitter for 5G-NR Featuring a BW-Extended N-Path Filter-Modulator, a Switched-BB Input, and a Wideband TIA-Based PA Driver

This article reports a multiband frequency-division duplex (FDD) SAW-less transmitter (TX) for 5G new radio (5G-NR). It features a bandwidth-extended N-path filter-modulator (BW-Ext FIL-MOD) to enable high-Q bandpass filtering at a flexible RF and the synthesis of a complex-pole pair via merging positive- and negative-feedback networks (PFN/NFN) enhances its bandpass characteristic, surmounting the tradeoff between the passband flatness and out-of-band (OB) rejection, a switched-baseband (BB) input network to avoid the mutual loading effect between the BW-Ext FIL-MOD and itself, and a transimpedance amplifier (TIA)-based power-amplifier driver (PAD) to absorb both the bias and signal currents of the BW-Ext FIL-MOD for better linearity and power efficiency. Fabricated in 28-nm CMOS, the proposed TX manifests a 20-MHz passband BW and a consistently low OB noise (≤-157.5 dBc/Hz) for different 5G-NR bands between 1.4 and 2.7 GHz. The TX achieves sufficient output power (3 dBm), high TX efficiency (2.8%-3.6%), and high linearity (ACLR1 <; 44 dBc and EVM <; 2%). The active area is 0.31 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

A 0.85mm2 BLE Transceiver Using an On-Chip Harmonic-Suppressed RFIO Circuitry With T/R Switch

This article presents a small-area Bluetooth Low-Energy (BLE) transceiver (TRX) for short-range Internet-of-Things (IoT) applications in 65-nm CMOS. An integrated Radio-Frequency Input-Output (RFIO) circuitry embedded with transmitter/receiver (TX/RX) switch function and on-chip impedance matching is proposed. A hybrid-loop TRX structure based on a wide-bandwidth fractional-N digital phase-locked loop (DPLL) is implemented to achieve the maximum power reduction. A -94dBm receiver sensitivity is achieved with 2.3mW receiver power consumption, while a RF receiving bypass route integration enhances the input power tolerance. The BLE transceiver delivers -6dBm output power while consuming 2.6mW and delivers 0dBm output power while achieving 18.5% maximum TX efficiency. Thanks to the RFIO with harmonic suppression, -56dBc of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> -order harmonic distortion (HD2) and -48dBc of 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> -order harmonic distortion (HD3) suppression are achieved with 0.85mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on-chip area. This transceiver satisfied the BLE radio specification without the need for external filters and with low-power consumption, which enables minimum size and long life-time modules.

Decoupling of folded-end dipole antenna elements of a 9.4 T human head array using an RF shield.

Dipole antennas have recently been introduced to the field of MRI and successfully used, mostly as elements of ultra-high field (UHF, ≥ 7 T) human body arrays. Usage of dipole antennas for UHF human head transmit (Tx) arrays is still under development. Due to the substantially smaller size of the sample, dipoles must be made significantly shorter than in the body array. Additionally, head Tx arrays are commonly placed on the surface of rigid helmets made sufficiently large to accommodate tight-fit receive arrays. As a result, dipoles are not well loaded and are often poorly decoupled, which compromises Tx efficiency. Commonly, adjacent array elements are decoupled by circuits electrically connected to them. Placement of such circuits between distantly located dipoles is difficult. Alternatively, decoupling is provided by placing passive antennas between adjacent dipole elements. This method only works when these additional components are sufficiently small (compared with the size of active dipoles). Otherwise, RF fields produced by passive elements interfere destructively with the RF field of the array itself, and previously reported designs have used passive dipoles of about the size of array dipoles. In this work, we developed a novel method of decoupling for adjacent dipole antennas, and used this technique while constructing a 9.4 T human head eight-element transceiver array. Decoupling is provided without any additional circuits by simply folding the dipoles and using an RF shield located close to the folded portion of the dipoles. The array reported in this work demonstrates good decoupling and whole-brain coverage.

An 8-element Tx/Rx array utilizing MEMS detuning combined with 6 Rx loops for 19 F and 1 H lung imaging at 1.5T.

To firstly improve the attainable image SNR of 19 F and 1 H C3 F8 lung imaging at 1.5 tesla using an 8-element transmit/receive (Tx/Rx) flexible vest array combined with a 6-element Rx-only array, and to secondly evaluate microelectromechanical systems for switching the array elements between the 2 resonant frequencies. The Tx efficiency and homogeneity of the 8-element array were measured and simulated for 1 H imaging in a cylindrical phantom and then evaluated for in vivo 19 F/1 H imaging. The added improvement provided by the 6-element Rx-only array was quantified through simulation and measurement and compared to the ultimate SNR. It was verified through the measurement of isolation that microelectromechanical systems switches provided broadband isolation of Tx/Rx circuitry such that the 19 F tuned Tx/Rx array could be effectively used for both 19 F and 1 H nuclei. For 1 H imaging, the measured Tx efficiency/homogeneity (mean ± percent SD; ) was comparable to that simulated ( ). The 6 additional Rx-only loops increased the mean Rx sensitivity when compared to the 8-element array by a factor of 1.41× and 1.45× in simulation and measurement, respectively. In regions central to the thorax, the simulated SNR of the 14-element array achieves ≥70% of the ultimate SNR when including noise from the matching circuits and preamplifiers. A measured microelectromechanical systems switching speed of 12 µs and added minimum 22 dB of isolation between Tx and Rx were sufficient for Tx/Rx switching in this application. The described single-tuned array driven at 19 F and 1 H, utilizing microelectromechanical systems technology, provides excellent results for 19 F and 1 H dual-nuclear lung ventilation imaging.

Foamy Virus Based Vectors for CAR-T Cell Development

Introduction A novel approach that can cover the therapeutic gap in NHL treatment are the autologous T cells, expressing Chimeric Antigen Receptors (CAR-T cells) against tumor markers. Such clinical-grade products based on Lenti (LV) or Retro- vectors have hit the market. An alternative vector system for CAR gene transfer in T-cells are Foamy Viruses (FV). To evaluate the potential of FV vectors in CAR-T cell development, we synthesized an antiCD19 scFv cDNA and cloned it in both an FV and an LV backbone; both vectors were tested in paired experiments Material and Methods The anti-CD19 CAR was under the control of the EF1a promoter; EGFP expression was under the control of an IRES2 element. The anti-CD19 CAR sequence was deduced from published data. FV vectors were made with a 4-plasmid vector system in 293T cells. 2nd generation LV vectors were purchased from Addgene. Cord blood (CB), healthy donor peripheral blood (PB) and CLL patients' PB was used as a source for CD3+ cells using immunomagnetic enrichment. Informed consent has been obtained in all cases of human sample use. T cells were activated by antiCD3/CD28 beads and transduced with antiCD19 LV or FV vectors. Transduction efficiency was assayed by flow cytometry (FCM) using a PE-conjugated anti-mouse Fab antibody. FV and LV CAR-T cells were expanded with Rapid Expansion Protocol (REP) and their cytotoxicity assays was evaluated against the CD19+ cell lines Raji and Daudi. The CLL patient derived CAR-Ts were evaluated against autologous B cells. Cytotoxicity was evaluated with an FCM protocol using CFSE-stained target cells vs unstained effector CARTs in different ratios. At the end of the incubation cells were stained with 7AAD to discriminate against live/dead cells. CAR-T cell activation was also assayed by INF-γ ELISA, following cocultures with target cells at a ratio of 1:1 for 24h. Results Vector titers: LV vector titers were between 3-5x10^5 TU/ml for both LV vectors (with or without EGFP cassette). FV vector titers were between 2-4x10^5 TU/ml regardless of the presence of the EGFP cassette. Tx efficiency: FV can mediate efficient gene transfer on T cells in the presence of heparin at an effective dose of 20-40 U/ml using a spinoculation technique. Transduction efficiency ranged from 40-65% at MOI=3-5, and was comparable to the transduction efficiency of LV vectors at a much higher MOI (10 to 30). Cytotoxicity data on lines: Following REP, the cell population consisted mostly (close to 96% purity) of CAR-T cells regardless of the vector used or of the T cell source. Effector cells were cocultured with the CD19+ cell lines, Daudi and Raji at varying ratios. With cord blood derived FV-CAR-T cells, at 4h post coculture we observed a 39.4% cell lysis at a ratio of 10:1 effector to target (n=1). Similar results were obtained for LV vectors. Peripheral blood derived CAR-T cells at THE same ratio (10:1), demonstrated 83.9% and 93.1% cell lysis for FV-CART and LV-CART cells respectively (n=2). Cytotoxicity data on CLL cells: T-cells from peripheral blood of CLL patients were used to generate LV- and FV-CAR-T cells. At the ratio of 10:1, we observed 73.1% and 69,8% cytotoxicity for FV-CAR-Ts and 70.1% and 70.7% with LV-CAR-Ts, in 2 independent paired experiments. IFN as activation marker: In two paired activation experiments, CB-derived FV-CAR-T cells secrete 560 and 437pg/ml of IFN-γ; similarly, LV-CAR-Ts secrete 534 and 554pg/ml IFN-γ. Untransduced control cells, produced 68pg/ml and 12pg/ml for FV-CAR-T and LV-CAR-T experimental arm respectively. Conclusion In the current work, we developed and tested FV vectors for anti- CD19 CAR-T cell production. We proved that FV viral vectors are capable of mediating efficient gene transfer to human T cells. We developed a method to efficiently transfer FV vectors into T-cells, using a clinically relevant protocol with heparin. The FV-derived CAR T cells demonstrate the same cytotoxic properties in vitro as their LV-derived counterpart and the same activation levels in the presence of CD19 expressing target cells as measured by IFN-γ secretion. FV CARTs derived from PB of CLL patients were capable of mediating comparable cytotoxicity levels as their LV-derived counterparts. Overall, we provide a proof of concept that FVs could be a safe and efficient alternative to LV derived vectors for CAR-T cells. Disclosures No relevant conflicts of interest to declare.

Analysis and Design of an Ultra-Low-Power Bluetooth Low-Energy Transmitter With Ring Oscillator-Based ADPLL and 4$\times$ Frequency Edge Combiner

In this paper, we present an all-digital ring oscillator (RO)-based Bluetooth low-energy (BLE) transmitter (TX) for ultra-low-power radios in short range Internet-of-Things (IoT) applications. The power consumption of state-of-the-art BLE TXs has been limited by the relatively power-hungry local oscillator (LO) due to the use of LC oscillators for superior phase noise (PN) performance. This paper addresses this issue by analyzing the PN limit of a BLE TX and proposes an RO-based solution for power and cost savings. The proposed transmitter features: 1) a wideband all-digital phase-locked loop (ADPLL) featuring an $f_{\mathrm {RF}} / {4}$ RO, with an embedded 5-bit TDC; 2) a 4 $\times $ frequency edge combiner to generate the 2.4-GHz signal; and 3) a switch-capacitor digital PA optimized for high efficiency at low transmit power levels. These not only help reduce the power consumption and improve PN performance but also enhance the TX efficiency for short range applications. The TX is prototyped in 40-nm CMOS, occupies an active area of 0.0166 mm2, and consumes 486 $\mu \text{W}$ in its low-power mode, while configured as a non-connectable advertiser. The TX has been validated by wirelessly communicating beacon messages to a mobile phone.

Evaluation of short folded dipole antennas as receive elements of ultra-high-field human head array.

To improve the receive (Rx) performance of a human head transceiver (TxRx) array at 9.4T without compromising its transmit (Tx) performance, a novel 16-element array was developed, constructed, and tested. We designed and constructed a phased array, which consists of 8 TxRx surface loops placed in a single row and circumscribing a head, and 8 Rx-only short folded dipole antennas. Dipoles were positioned along the central axis of each transceiver loop perpendicular to its surface. We evaluated the effect of Rx dipoles on the Tx efficiency of the array and maximum local specific absorption rate (SAR) as compared to the array of 8 surface loops only. We also compared the new array to a 16-channel array of the same size consisting of 8 TxRx surface loops and 8 Rx-only vertical loops in terms of Tx efficiency, SAR, and signal-to-noise ratio (SNR). The new array improves both peripheral (up to 2 times) and central (1.17 times) SNR as compared to the 16-element array of the same geometry consisting of 8 TxRx surface loops and 8 Rx-only vertical loops. We demonstrated that an addition of actively detuned Rx-only dipole elements produces only a small decrease (~7%) of the transmit field and a small increase (<7%) of the maximum local SAR. As a proof of concept, we developed and constructed a prototype of a 9.4T (400 MHz) head array consisting of 8 TxRx surface loops and 8 Rx-only short optimized folded dipoles. We demonstrated that at ultra-high field, dipoles outperformed Rx-only vertical loops in vivo.

Double-row 18-loop transceive-32-loop receive tight-fit array provides for whole-brain coverage, high transmit performance, and SNR improvement near the brain center at 9.4T.

To improve the transmit (Tx) and receive (Rx) performance of a human head array and provide whole-brain coverage at 9.4T, a novel 32-element array design was developed, constructed, and tested. The array consists of 18 transceiver (TxRx) surface loops and 14 Rx-only vertical loops all placed in a single layer. The new design combines benefits of both TxRx and transmit-only-receive-only (ToRo) designs. The general idea of the design is that the total number of array elements (both TxRx and Rx) should not exceed the number of required Rx elements. First, the necessary number of TxRx loops is placed around the object tightly to optimize the Tx performance. The rest of the elements are loops, which are used only for reception. We also compared the performance of the new array with that of a state-of-the-art ToRo array consisting of 16 Tx-only loops and 31 Rx-only loops. The new array provides whole-brain coverage, ~1.5 times greater Tx efficiency and 1.3 times higher SNR near the brain center as compared to the ToRo array, while the latter delivers higher (up to 1.5 times) peripheral SNR. In general, the new approach of constructing a single-layer array consisting of both TxRx- and Rx-only elements simplifies the array construction by minimizing the total number of elements and makes the entire design more robust and, therefore, safe. Overall, our work provides a recipe for a Tx- and Rx-efficient head array coil suitable for parallel transmission and reception as well as whole-brain imaging at UHF.

Decoupling of a double‐row 16‐element tight‐fit transceiver phased array for human whole‐brain imaging at 9.4 T

One of the major challenges in constructing multi-channel and multi-row transmit (Tx) or transceiver (TxRx) arrays is the decoupling of the array's loop elements. Overlapping of the surface loops allows the decoupling of adjacent elements and also helps to improve the radiofrequency field profile by increasing the penetration depth and eliminating voids between the loops. This also simplifies the design by reducing the number of decoupling circuits. At the same time, overlapping may compromise decoupling by generating high resistive (electric) coupling near the overlap, which cannot be compensated for by common decoupling techniques. Previously, based on analytical modeling, we demonstrated that electric coupling has strong frequency and loading dependence, and, at 9.4T, both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping the loops. As a result, excellent decoupling was obtained between adjacent loops of an eight-loop single-row (1×8) human head tight-fit TxRx array. In this work, we designed and constructed a 9.4-T (400-MHz) 16-loop double-row (2×8) overlapped TxRx head array based on the results of the analytical and numerical electromagnetic modeling. We demonstrated that, simply by the optimal overlap of array loops, a very good decoupling can be obtained without additional decoupling strategies. The constructed TxRx array provides whole-brain coverage and approximately 1.5 times greater Tx efficiency relative to a transmit-only/receive-only (ToRo) array, which consists of a larger Tx-only array and a nested tight-fit 31-loop receive (Rx)-only array. At the same time, the ToRo array provides greater peripheral signal-to-noise ratio (SNR) and better Rx parallel performance in the head-feet direction. Overall, our work provides a recipe for a simple, robust and very Tx-efficient design suitable for parallel transmission and whole-brain imaging at ultra-high fields.

Combination of surface and 'vertical' loop elements improves receive performance of a human head transceiver array at 9.4T.

Ultra-high-field (UHF, ≥7T) human magnetic resonance imaging (MRI) provides undisputed advantages over low-field MRI (≤3T), but its development remains challenging because of numerous technical issues, including the low efficiency of transmit (Tx) radiofrequency (RF) coils caused by the increase in tissue power deposition with frequency. Tight-fit human head transceiver (TxRx) arrays improve Tx efficiency in comparison with Tx-only arrays, which are larger in order to fit multi-channel receive (Rx)-only arrays inside. A drawback of the TxRx design is that the number of elements in an array is limited by the number of available high-power RF Tx channels (commonly 8 or 16), which is not sufficient for optimal Rx performance. In this work, as a proof of concept, we developed a method for increasing the number of Rx elements in a human head TxRx surface loop array without the need to move the loops away from a sample, which compromises the array Tx performance. We designed and constructed a prototype 16-channel tight-fit array, which consists of eight TxRx surface loops placed on a cylindrical holder circumscribing a head, and eight Rx-only vertical loops positioned along the central axis (parallel to the magnetic field B0 ) of each TxRx loop, perpendicular to its surface. We demonstrated both experimentally and numerically that the addition of the vertical loops has no measurable effect on the Tx efficiency of the array. An increase in the maximum local specific absorption rate (SAR), evaluated using two human head voxel models (Duke and Ella), measured 3.4% or less. At the same time, the 16-element array provided 30% improvement of central signal-to-noise ratio (SNR) in vivo relative to a surface loop eight-element array. The novel array design also demonstrated an improvement in the parallel Rx performance in the transversal plane. Thus, using this method, both the Rx and Tx performance of the human head array can be optimized simultaneously.

Analytical modeling provides new insight into complex mutual coupling between surface loops at ultrahigh fields.

Ultrahigh-field (UHF) (≥7T) transmit (Tx) human head surface loop phased arrays improve both the Tx efficiency (B1+ /√P) and homogeneity in comparison with single-channel quadrature Tx volume coils. For multi-channel arrays, decoupling becomes one of the major problems during the design process. Further insight into the coupling between array elements and its dependence on various factors can facilitate array development. The evaluation of the entire impedance matrix Z for an array loaded with a realistic voxel model or phantom is a time-consuming procedure when performed using electromagnetic (EM) solvers. This motivates the development of an analytical model, which could provide a quick assessment of the Z-matrix. In this work, an analytical model based on dyadic Green's functions was developed and validated using an EM solver and bench measurements. The model evaluates the complex coupling, including both the electric (mutual resistance) and magnetic (mutual inductance) coupling. Validation demonstrated that the model does well to describe the coupling at lower fields (≤3T). At UHFs, the model also performs well for a practical case of low magnetic coupling. Based on the modeling, the geometry of a 400-MHz, two-loop transceiver array was optimized, such that, by simply overlapping the loops, both the mutual inductance and the mutual resistance were compensated at the same time. As a result, excellent decoupling (below -40dB) was obtained without any additional decoupling circuits. An overlapped array prototype was compared (signal-to-noise ratio, Tx efficiency) favorably to a gapped array, a geometry which has been utilized previously in designs of UHF Tx arrays.

Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4T.

Ultra-high field (UHF, ≥7T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B1+ /√P) in comparison with Tx-only arrays, which are usually larger to fit receive (Rx)-only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx-only/Rx-only RF system, a single-row eight-element prototype of a 400MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx-only array, which could also be used in combination with an 18-channel Rx-only array. Data were acquired on the Siemens Magnetom whole body 9.4T human MRI system. Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B1+ /√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets. We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated.

Class-C PA-VCO Cell for FSK and GFSK Transmitters

In this paper, a Class-C PA-VCO cell tailored to FSK/GFSK transmitters is presented. In the proposed solution, a Class-C VCO and a common-gate stage PA are stacked in a current-reuse architecture operating with 1.2 V power supply. The PA and the VCO efficiencies are maximized by adjusting their voltage headroom without the use of any DC-DC converters. The PA-VCO is inserted in a transmitter based on an open-loop architecture. The presented prototype, fabricated in $0.13~\mu \text{m}$ CMOS technology, occupies an active area of 0.2 mm2. A maximum TX efficiency of 17.5% is achieved while the TX is delivering an output power of −1 dBm at 2.45 GHz. A phase noise of −129 dBc/Hz at 2.5 MHz frequency offset results in a carrier-frequency drift below 7 Hz/s and an FSK error below 0.7%, which allows the transmitter to operate in open-loop while delivering long data-packets. The transmitter is also compliant to BLE specifications when FSK and GFSK modulations with index of 0.5 are applied.