Shajan Gunamony

Ultra-high-field MRI promises major gains in BOLD sensitivity but introduces severe challenges such as RF field inhomogeneity, B0 field inhomogeneity, motion and vibration-induced field variations. We report the first whole-brain resting-state and task-based fMRI experiments at 11.7T. A 3D-EPI sequence was optimized using tailored parallel-transmit RF pulses, gradient reshaping to suppress vibration-induced B0 fluctuations, and prospective motion correction using servo navigation combined with retrospective phase equalization (PEERS). Four participants were scanned at 11.7T with whole-brain 1.2 mm isotropic resolution with servo navigation and PEERS that considerably reduced residual motion and increased temporal SNR. The Default Mode Network in resting state could be detected and a physiological-noise-dominated regime was identified. Four additional but different participants were scanned at 7T for tSNR comparison and validation of the protocol, data quality and processing pipeline. Task-based fMRI likewise was conducted at 11.7T and yielded robust, spatially specific activations across motor, visual, mathematical, and language networks, again with improved sensitivity and cleaner BOLD responses when using servo navigation and PEERS. These results demonstrate the feasibility and reliability of whole-brain human fMRI at 11.7T and constitute a first quality control milestone to further increase resolution at that field strength.

Purpose: To implement and evaluate chemical exchange saturation transfer (CEST) at ultra‐high field on a whole‐body 11.7 T scanner using parallel transmission (pTx). Methods: Tailored pTx CEST saturation pulses were optimized to achieve a spatially uniform target B ₁ ⁺ while satisfying SAR and power constraints either defined by conservatively defined radio‐frequency (RF) power limits or by a virtual observation point (VOP)–based framework. The resulting pulse performance at 11.7 T was assessed through numerical simulations and compared against corresponding designs at 7 T. The proposed saturation pulses were integrated into a two‐dimensional multi‐slice EPI sequence. Experimental testing of the sequence was performed in vitro using metabolite‐doped phantoms and in vivo in four healthy volunteers. Results: The pTx pulse designs optimized under VOP‐based SAR constraints at 11.7 T achieved comparable performance to 7 T designs despite higher intrinsic B ₁ ⁺ inhomogeneity. In vitro experiments validated the sequence implementation and confirmed detection of the targeted metabolites. In vivo experiments demonstrated acceptable saturation homogeneity (nRMSE ≤ 12.1%) in a slab of thickness 1 cm, with only minimal additional improvement achieved through B ₁ ⁺ post‐processing correction. Across the cohort, robust gray matter–white matter contrast was consistently observed for multiple CEST contrasts. Conclusion: VOP‐based SAR‐constrained pTx CEST saturation pulse design enabled robust and reproducible contrast at 11.7 T by effectively mitigating B ₁ ⁺ inhomogeneities while remaining within safety limits, facilitating translation to future in vivo clinical research applications.

Purpose: To design and develop an 8‐channel transmit 32‐channel receive 11.7T head coil for the strongest human MRI scanner to date, demonstrate safety, and evaluate signal‐to‐noise ratio (SNR) gains compared to 7T, the most used ultra‐high‐field (UHF) platform. Methods: The transmit array design incorporated a folded‐end radiofrequency (RF) shield to mitigate wave propagation in the scanner bore. The receive array consisted of 32‐overlapped loops constructed on a helmet size‐matched with the industry standard 7T head coil. The transmit array was characterized using B ₁ ⁺ mapping and MR thermometry, whereas the receive array performance was evaluated by measuring SNR at 7T and 11.7T using a lightbulb‐shaped phantom filled with a tissue equivalent solution. Results: Loss due to wave propagation was 4.5% of the input power for the chosen RF shield. The mean magnitude of the complex correlations between the simulated and measured single channel B ₁ ⁺ maps was 94%, which highlights minimal interaction between the transmit and receive arrays. An SNR gain of three‐fold was achieved in the phantom centre. In the periphery, representing the cortex, the SNR gain was 2.65‐fold, and the average gain over the whole volume was a factor of 2.69. The g‐factors of the 11.7T array were lower than the reference 7T coil due to the distinct coil sensitivities at higher frequency. Conclusion: We report substantial SNR gains over the whole volume at 11.7T using a fully overlapped 32‐channel loop array. The central SNR gain of B ₀ ^2.14 agrees with the theoretical prediction, while we also gain supra‐linearly in the periphery.

Introduction: Ultra-high field (UHF) 7 tesla (7T) MRI offers unique diagnostic opportunities for clinical neuroimaging. However, broader clinical implementation remains limited because the reduced radiofrequency (RF) wavelength at 7T causes RF transmit field ( B 1 + ) inhomogeneity, resulting in spatial variation in image signal and tissue contrast. These effects are particularly pronounced in the skull base, temporal lobes, and posterior fossa, regions frequently implicated in neurological disease. RF parallel transmission (pTx) techniques can improve B 1 + homogeneity through optimization of multiple transmit fields, while parallel imaging with multiple receive elements enables accelerated acquisitions. To fully exploit these advantages at UHF, RF coils combining high-performance transmit and high-density receive arrays are required. Methods: An open-face eight-channel transmit 64-channel receive (8Tx64Rx) 7T head coil was developed with electrical and mechanical ergonomic features designed to improve usability and imaging homogeneity. The coil was evaluated in an in vivo validation study assessing ease of use, participant comfort, diagnostic value, and image quality using questionnaires and structured scoring sheets. Imaging performance of the 8Tx64Rx coil was compared with that of a commercially available regulatory approved 7T head coil featuring single transmit (sTx) and a 32-channel receive array. Diagnostic image quality acquired using circularly polarized (CP) mode and pTx mode was additionally assessed and compared. Results: The 8Tx64Rx coil demonstrated image quality that was equivalent or superior to the commercial 7T head coil across evaluated measures. In particular, improved signal homogeneity was achieved within the posterior fossa and temporal lobes. The coil also supported practical clinical use through favorable ergonomic performance and user comfort during scanning. Diagnostic image quality was enhanced when using pTx approaches compared with conventional transmit configurations. Discussion: The developed 8Tx64Rx 7T head coil demonstrates the potential to advance clinical neuroimaging at UHF by improving B 1 + homogeneity in anatomically challenging brain regions while maintaining usability and diagnostic performance. These findings support the integration of combined high-density receive arrays and pTx technology to overcome key limitations of 7T MRI and facilitate broader clinical adoption.

Purpose: To mitigate artifacts related to motion and field changes in high‐resolution ‐weighted human brain imaging using servo navigation at ultra‐high fields up to 11.7 T. Methods: MR‐based servo navigators were integrated into a segmented 3D‐EPI sequence to allow for prospective correction of involuntary head motion and first‐order shim changes. Seven subjects were scanned with whole‐brain protocols at 0.3 mm isotropic resolution with and without correction at 7 and 11.7 T. Validation was performed on detailed brain vasculature in scans with involuntary motion. Results: Blurring of small veins was reduced by servo navigation for all subjects and across field strengths. In case of involuntary large motion, the method preserved image quality, while uncorrected motion led to severe artifacts. In case of microscopic motion, reduced blurring and shading in the frontal lobe demonstrate the additional benefit of prospective field drift correction. Conclusion: Servo‐navigated segmented 3D‐EPI improves 0.3 mm isotropic whole‐brain ‐weighted imaging under realistic motion and field changes within 5.5 to 11 min scan time at 11.7 and 7 T.

Purpose: To develop a 7T neurovascular head and neck (NVHN) coil with an extended longitudinal coverage of the brain and cervical spine, with eight transceiver (TxRx) channels and 56 receive (Rx) channels for dynamic parallel‐transmit (pTx) applications. Methods: A dual‐row transceiver array with six elements in the upper row and two elements in the lower row was designed using combined electromagnetic and circuit optimization and constructed. A 56Rx array covering the brain and cervical spine was designed and combined with the transceiver array. The performance of the 8TxRx56Rx NVHN coil such as , signal‐to‐noise ratio, and g‐factor were validated in phantom and in vivo studies and compared with an in‐house 8Tx64Rx head coil. High‐resolution in vivo images were acquired with the NVHN and head coil. Results: The average in phantom while exciting the upper six channels and all eight channels are 43.45 nT/V and 45.80 nT/V, respectively, demonstrating that the available field is seamlessly distributed in the brain and/or cervical spine, depending on the chosen excitation. The 8TxRx56Rx NVHN coil increases the SNR in the cervical spine and central brain by a factor of 2.18 and 1.16, respectively, compared with the 8Tx64Rx head coil. Furthermore, it demonstrates similar 1/g‐factor performance for acceleration factors up to 5 × 5 compared with the head coil and provides diagnostic‐quality images of the brain and spinal cord in a single acquisition. Conclusion: The extended longitudinal coverage of the NVHN coil promises to improve the clinical application of the current generation of pTx 7T MRI systems with 8Tx channels.

Purpose: The newly operational 11.7T Iseult scanner provides an improved global SNR in the human brain. This gain in SNR can be pushed even further locally by designing region‐focused dense receive arrays. The temporal lobes are particularly interesting to neuroscientists as they are associated with language and concept recognition. Our main goal was to maximize the SNR in the temporal lobes and provide high‐acceleration capabilities for fMRI studies.Methods: We designed and developed a 32‐channel receive array made of non‐overlapped hexagonal loops. The loops were arranged in a honeycomb pattern and targeted the temporal lobes. They were placed on a flexible neoprene cap closely fitting the head. A new stripline design with a high impedance was proposed and applied for the first time at 11.7T. Specific homebuilt miniaturized low‐impedance preamplifiers were directly mounted on the loops, providing preamplifier decoupling in a compact and modular design. Using an anatomical phantom, we experimentally compared the SNR and parallel imaging performance of the region‐focused cap to a 32‐channel whole‐brain receive array at 11.7T.Results: The experimental results showed a 1.7‐time higher SNR on average in the temporal lobes compared to the whole brain receive array. The g‐factor is also improved when undersampling in the antero‐posterior and head‐foot directions.Conclusion: A significant SNR boost in the temporal lobes was demonstrated at 11.7T compared to the whole‐brain receive array. The parallel imaging capabilities were also improved in the temporal lobes in some acceleration directions.

Purpose: Compared with lower field strengths, DWI at 7 T faces the combined challenges of increased distortion and blurring due to B₀ inhomogeneity, and increased signal dropouts due to B₁⁺ inhomogeneity. This study addresses the B₁⁺ limitations using slice‐specific static parallel transmission (pTx) in a multi‐shot, readout‐segmented EPI diffusion imaging sequence.Methods: DWI was performed in 7 healthy subjects using MRI at 7 T and readout‐segmented EPI. Data were acquired with non‐pTx circular‐polarized (CP) pulses (CP‐DWI) and static pTx pulses (pTx‐DWI) using slice‐specific B₁⁺ shim coefficients. Each volunteer underwent two scan sessions on the same day, with two runs of each sequence in the first session and one run in the second. The sequences were evaluated by assessing image quality, flip‐angle homogeneity, and intrasession and intersession repeatability in ADC estimates.Results: pTx‐DWI significantly reduced signal voids compared with CP‐DWI, particularly in inferior brain regions. The use of pTx also improved RF uniformity and symmetry across the brain. These effects translated into improved intrasession and intersession repeatability for pTx‐DWI. Additionally, re‐optimizing the pTx pulse between repeat scans did not have a negative effect on ADC repeatability.Conclusion: The study demonstrates that pTx provides a reproducible image‐quality increase in multishot DWI at 7 T. The benefits of pTx also extend to quantitative ADC estimation with regard to the improvement in intrasession and intersession repeatability. Overall, the combination of multishot imaging and pTx can support the development of reliable, high‐resolution DWI for clinical studies at 7 T.

Purpose: Driven by the Lorentz force, acoustic noise may arguably be the next physiological challenge associated with ultra‐high field MRI scanners and powerful gradient coils. This work consisted of isolating and mitigating the main sound pathway in the NexGen 7 T scanner equipped with the investigational Impulse head gradient coil.Methods: Sound pressure level (SPL) measurements were performed with and without the RF coil to assess its acoustic impact. Vibration measurements were carried out on the gradient coil, the RF coil, and on the patient table to distinguish the different vibration mechanisms and pathways. Vibrations of the RF coil were modified by either making contact with the patient bore liner with padding material or by changing directly the RF shield with phosphor bronze mesh material.Results: SPL and vibration measurements demonstrated that eddy‐currents induced in the RF shield were the primary cause of acoustic noise. Replacing the conventional solid copper shield with phosphor bronze mesh material altered the vibrations of the RF shield and decreased SPL by 6 to 8 dB at the highest frequencies in EPI, depending on the gradient axis, while boosting the transmit B₁⁺ field by 15%. Padding led to slightly less sound reduction on the X and Z gradient axes, but with minimal impact for the Y axis.Conclusion: This study demonstrates the potential importance of eddy‐current induced vibrations in the RF coil in terms of acoustic noise and opens new horizons for mitigation measures.

Purpose: Optimization of transmit array performance is crucial in ultra‐high‐field MRI scanners such as 11.7T because of the increased RF losses and RF nonuniformity. This work presents a new workflow to investigate and minimize RF coil losses, and to choose the optimum coil configuration for imaging.Methods: An 8‐channel transceiver loop‐array was simulated to analyze its loss mechanism at 499.415 MHz. A folded‐end RF shield was developed to limit radiation loss and improve the efficiency. The coil element length, and the shield diameter and length were further optimized using electromagnetic (EM) simulations. The generated EM fields were used to perform RF pulse design (RFPD) simulations under realistic constraints. The chosen coil design was constructed to demonstrate performance equivalence in bench and scanner measurements.Results: The use of conventional RF shields at 11.7T resulted in significantly high radiation losses of 18.4%. Folding the ends of the RF shield combined with optimizing the shield diameter and length increased the absorbed power in biological tissue and reduced the radiation loss to 2.4%. The peak of the optimal array was 42% more than the reference array. Phantom measurements validated the numerical simulations with a close match of within 4% of the predicted .Conclusion: A workflow that combines EM and RFPD simulations to numerically optimize transmit arrays was developed. Results have been validated using phantom measurements. Our findings demonstrate the need for optimizing the RF shield in conjunction with array element design to achieve efficient excitation at 11.7T.