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A complementary safety assessment of the specific absorption rate (SAR) of the electromagnetic energy was performed in a prototype 8Tx/16Rx RF array for cardiac magnetic resonance imaging (MRI) at 7 T. The study aimed to address two critical aspects of 7-T SAR safety not always explicitly examined by coil vendors: (i) the influence of an RF-array position on a peak SAR value, and (ii) the risk of exceeding the permitted maximal SAR in the tissue surrounding conductive passive implants. The full-wave 3D electromagnetic simulations for the thorax with shifted array position and the whole-body volume in the presence of a dental retainer, an intrauterine contraceptive device (IUD), and a hip joint implant, were performed for two human voxel models. The effect of the array displacement on the SAR was simulated for seven array locations on the thorax shifted from the central position in different directions on 50 mm. The peak SAR values for both models were analyzed for the three phase-only transmit vectors optimized for B1 + homogeneity and transmit efficiency. Peak SAR values due to the shifts of the array position increase up to ≈50%. The worst-case peak SAR value for a dental retainer was found to be in the range of 10% of the maximal SAR in the tissue within the array's borders. For the IUD and artificial hip joint implants the effect was found to be negligible (peak SAR < 1% of the SAR within array borders). In addition to simulations for cardiac MRI, we performed a preliminary B1 + shimming and SAR-safety analysis for the same RF-array at various positions lower on the body trunk to assess a potential application in imaging abdominopelvic organs (prostate, kidney, and liver). The most promising target for an ad hoc alternative application of the array was found to be the prostate.
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Imageamento por Ressonância Magnética , Tórax , Masculino , Humanos , Imagens de Fantasmas , Imageamento por Ressonância Magnética/métodos , Coração/diagnóstico por imagem , PróstataRESUMO
INTRODUCTION: MRI of excised hearts at ultra-high field strengths ([Formula: see text]≥7 T) can provide high-resolution, high-fidelity ground truth data for biomedical studies, imaging science, and artificial intelligence. In this study, we demonstrate the capabilities of a custom-built, multiple-element transceiver array customized for high-resolution imaging of excised hearts. METHOD: A dedicated 16-element transceiver loop array was implemented for operation in parallel transmit (pTx) mode (8Tx/16Rx) of a clinical whole-body 7 T MRI system. The initial adjustment of the array was performed using full-wave 3D-electromagnetic simulation with subsequent final fine-tuning on the bench. RESULTS: We report the results of testing the implemented array in tissue-mimicking liquid phantoms and excised porcine hearts. The array demonstrated high efficiency of parallel transmits characteristics enabling efficient pTX-based B1+-shimming. CONCLUSION: The receive sensitivity and parallel imaging capability of the dedicated coil were superior to that of a commercial 1Tx/32Rx head coil in both SNR and T2*-mapping. The array was successfully tested to acquire ultra-high-resolution (0.1 × 0.1 × 0.8 mm voxel) images of post-infarction scar tissue. High-resolution (isotropic 1.6 mm3 voxel) diffusion tensor imaging-based tractography provided high-resolution information about normal myocardial fiber orientation.
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Inteligência Artificial , Imagem de Tensor de Difusão , Suínos , Animais , Razão Sinal-Ruído , Desenho de Equipamento , Imageamento por Ressonância Magnética/métodos , Imagens de FantasmasRESUMO
To improve parallel transmit (pTx) and receive performance for cardiac MRI (cMRI) in pigs at 7 T, a dedicated transmit/receive (Tx/Rx), 16-element antisymmetric dipole antenna array, which combines L-shaped and straight dipoles, was designed, implemented, and evaluated in both cadavers and animals in vivo. Electromagnetic-field simulations were performed with the new 16-element dipole antenna array loaded with a pig thorax-shaped phantom and compared with an eight-element array of straight dipoles. The new dipole array was interfaced to a 7 T scanner in pTx mode (8Tx/16Rx). Imaging performance of the novel array was validated through MRI measurements in a pig phantom, an 85 kg pig cadaver, and two pigs in vivo (74 and 81 kg). Due to the improved decoupling between interleaved L-shaped and straight dipole elements, the 16-element dipole array fits within the same outer dimensions as an eight-element array of straight dipoles. This provides improvement of both transmit and receive characteristics and additional degrees of freedom for B1+ shimming. The antisymmetric dipole array demonstrated efficient suppression of destructive interferences in the B1+ field, with up to 25% improvement in the B1+ homogeneity achieved using static pTx-RFPA B1+ shimming in comparison with the hardware-adjusted state, which was optimized for single transmit. High-resolution (0.5 × 0.5 × 4 mm3 ) anatomical images of the heart after cardiac arrest proved good transmit and receive characteristics of the novel array design. Parallel imaging with an acceleration factor up to R = 6 was possible while maintaining a mean g factor of 1.55 within the pig heart. CINE images acquired in vivo in two pigs demonstrated SNR and parallel imaging capabilities similar to those of a reference 8Tx/16Rx dedicated loop array for cMRI in pigs.
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Coração , Imageamento por Ressonância Magnética , Animais , Cadáver , Desenho de Equipamento , Coração/diagnóstico por imagem , Imageamento por Ressonância Magnética/métodos , Imagens de Fantasmas , Razão Sinal-Ruído , SuínosRESUMO
Cardiac myocyte sodium (Na+) homoeostasis is pivotal in cardiac diseases and heart failure. Intracellular Na+ ([Na+]i) is an important regulator of excitation-contraction coupling and mitochondrial energetics. In addition, extracellular Na+ ([Na+]e) and its water-free storage trigger collagen cross-linking, myocardial stiffening and impaired cardiac function. Therefore, understanding the allocation of tissue Na+ to intra- and extracellular compartments is crucial in comprehending the pathophysiological processes in cardiac diseases. We extrapolated [Na+]e using a three-compartment model, with tissue Na+ concentration (TSC) measured by in vivo 23Na-MRI, extracellular volume (ECV) data calculated from T1 maps, and [Na+]i measured by in vitro fluorescence microscopy using Na+ binding benzofuran isophthalate (SBFI). To investigate dynamic changes in Na+ compartments, we induced pressure overload (TAC) or myocardial infarction (MI) via LAD ligation in mice. Compared to SHAM mice, TSC was similar after TAC but increased after MI. Both TAC and MI showed significantly higher [Na+]i compared to SHAM (around 130% compared to SHAM). Calculated [Na+]e increased after MI, but not after TAC. Increased TSC after TAC was primarily driven by increased [Na+]i, but the increase after MI by elevations in both [Na+]i and [Na+]e.
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Experimentação Animal , Insuficiência Cardíaca , Infarto do Miocárdio , Camundongos , Animais , Sódio/metabolismo , Insuficiência Cardíaca/metabolismo , Miócitos Cardíacos/metabolismo , Infarto do Miocárdio/metabolismo , Imageamento por Ressonância Magnética/métodosRESUMO
BACKGROUND: 7 T cardiac magnetic resonance imaging (MRI) studies may enable higher precision in clinical metrics like cardiac function, ventricular mass, and more. Higher precision may allow early detection of functional impairment and early evaluation of treatment responses in clinical practice and pre-clinical studies. METHODS: Seven female German Landrace pigs were scanned prior to and at three time points (3-4 days, 7-10 days, and ~60 days) post myocardial infarction using a whole body 7 T system and three radiofrequency (RF) coils developed and built in-house to accompany animal growth. RESULTS: The combination of dedicated RF hardware and 7 T MRI enables a longitudinal study in a pig model of acute and chronic infarction, providing consistent blood tissue contrast and high signal-to-noise ratio (SNR) in measurements of cardiac function, as well as low coefficients of variation (CoV) for ejection fraction (CoVintra-observer: 2%, CoVinter-observer: 3.8%) and infarct size (CoVintra-observer: 8.4%, CoVinter-observer: 3.8%), despite drastic animal growth. CONCLUSIONS: Best results are achieved via manual segmentation. We define state-of-the-art procedures for large animal studies at 7 T.
In magnetic resonance imaging (MRI), scanners use magnets to generate detailed images of structures in the body, such as the heart. Stronger magnets can produce stronger magnetic fields, which can be leveraged for better image quality and developing new methods for disease diagnosis. In clinical practice, such systems using strong magnets are not yet used for imaging of the heart and some safety aspects remain challenging. We apply such an imaging approach in pigs, in which heart structure and function are similar to humans. We focus on the most important clinical imaging aspects following a heart attack, namely heart function and scar detection. We demonstrate that the high magnetic strength system enabled consistent image quality and accuracy. These findings may help to guide future developments in MRI of the heart, for example in patients who have had a heart attack.
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Cardiac magnetic resonance (CMR) imaging allows precise non-invasive quantification of cardiac function. It requires reliable image segmentation for myocardial tissue. Clinically used software usually offers automatic approaches for this step. These are, however, designed for segmentation of human images obtained at clinical field strengths. They reach their limits when applied to preclinical data and ultrahigh field strength (such as CMR of pigs at 7 T). In our study, eleven animals (seven with myocardial infarction) underwent four CMR scans each. Short-axis cine stacks were acquired and used for functional cardiac analysis. End-systolic and end-diastolic images were labelled manually by two observers and inter- and intra-observer variability were assessed. Aiming to make the functional analysis faster and more reproducible, an established deep learning (DL) model for myocardial segmentation in humans was re-trained using our preclinical 7 T data (n = 772 images and labels). We then tested the model on n = 288 images. Excellent agreement in parameters of cardiac function was found between manual and DL segmentation: For ejection fraction (EF) we achieved a Pearson's r of 0.95, an Intraclass correlation coefficient (ICC) of 0.97, and a Coefficient of variability (CoV) of 6.6%. Dice scores were 0.88 for the left ventricle and 0.84 for the myocardium.
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Aprendizado Profundo , Modelos Animais de Doenças , Infarto do Miocárdio , Animais , Infarto do Miocárdio/diagnóstico por imagem , Infarto do Miocárdio/fisiopatologia , Suínos , Reprodutibilidade dos Testes , Processamento de Imagem Assistida por Computador/métodos , Imagem Cinética por Ressonância Magnética/métodos , Humanos , Coração/diagnóstico por imagem , Coração/fisiopatologia , Volume Sistólico , Imageamento por Ressonância Magnética/métodosRESUMO
A key step in translational cardiovascular research is the use of large animal models to better understand normal and abnormal physiology, to test drugs or interventions, or to perform studies which would be considered unethical in human subjects. Ultrahigh field magnetic resonance imaging (UHF-MRI) at 7â T field strength is becoming increasingly available for imaging of the heart and, when compared to clinically established field strengths, promises better image quality and image information content, more precise functional analysis, potentially new image contrasts, and as all in-vivo imaging techniques, a reduction of the number of animals per study because of the possibility to scan every animal repeatedly. We present here a solution to the dual use problem of whole-body UHF-MRI systems, which are typically installed in clinical environments, to both UHF-MRI in large animals and humans. Moreover, we provide evidence that in such a research infrastructure UHF-MRI, and ideally combined with a standard small-bore UHF-MRI system, can contribute to a variety of spatial scales in translational cardiovascular research: from cardiac organoids, Zebra fish and rodent hearts to large animal models such as pigs and humans. We present pilot data from serial CINE, late gadolinium enhancement, and susceptibility weighted UHF-MRI in a myocardial infarction model over eight weeks. In 14 pigs which were delivered from a breeding facility in a national SARS-CoV-2 hotspot, we found no infection in the incoming pigs. Human scanning using CINE and phase contrast flow measurements provided good image quality of the left and right ventricle. Agreement of functional analysis between CINE and phase contrast MRI was excellent. MRI in arrested hearts or excised vascular tissue for MRI-based histologic imaging, structural imaging of myofiber and vascular smooth muscle cell architecture using high-resolution diffusion tensor imaging, and UHF-MRI for monitoring free radicals as a surrogate for MRI of reactive oxygen species in studies of oxidative stress are demonstrated. We conclude that UHF-MRI has the potential to become an important precision imaging modality in translational cardiovascular research.
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The development of novel multiple-element transmit-receive arrays is an essential factor for improving B1+ field homogeneity in cardiac MRI at ultra-high magnetic field strength (B0 > = 7.0T). One of the key steps in the design and fine-tuning of such arrays during the development process is finding the default driving phases for individual coil elements providing the best possible homogeneity of the combined B1+-field that is achievable without (or before) subject-specific B1+-adjustment in the scanner. This task is often solved by time-consuming (brute-force) or by limited efficiency optimization methods. In this work, we propose a robust technique to find phase vectors providing optimization of the B1-homogeneity in the default setup of multiple-element transceiver arrays. The key point of the described method is the pre-selection of starting vectors for the iterative solver-based search to maximize the probability of finding a global extremum for a cost function optimizing the homogeneity of a shaped B1+-field. This strategy allows for (i) drastic reduction of the computation time in comparison to a brute-force method and (ii) finding phase vectors providing a combined B1+-field with homogeneity characteristics superior to the one provided by the random-multi-start optimization approach. The method was efficiently used for optimizing the default phase settings in the in-house-built 8Tx/16Rx arrays designed for cMRI in pigs at 7T.
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Coração/diagnóstico por imagem , Imagem Cinética por Ressonância Magnética/instrumentação , Algoritmos , Animais , Desenho de Equipamento , Fenômenos Magnéticos , Modelos Animais , Razão Sinal-Ruído , SuínosRESUMO
A novel mono-surface antisymmetric 16-element transmit/receive (Tx/Rx) coil array was designed, simulated, constructed, and tested for cardiac magnetic resonance imaging (cMRI) in pigs at 7 T. The cardiac array comprised of a mono-surface 16-loops with two central elements arranged anti-symmetrically and flanked by seven elements on either side. The array was configured for parallel transmit (pTx) mode to have an eight channel transmit and 16-channel receive (8Tx/16Rx) coil array. Electromagnetic (EM) simulations, bench-top measurements, phantom, and MRI experiments with two pig cadavers (68 and 46 kg) were performed. Finally, the coil was used in pilot in-vivo measurements with a 60 kg pig. Flip angle (FA), geometry factor (g-factor), signal-to-noise ratio (SNR) maps, and high-resolution cardiac images were acquired with an in-plane resolution of 0.6 mm × 0.6 mm (in-vivo) and 0.3 mm × 0.3 mm (ex-vivo). The mean g-factor over the heart was 1.26 (R = 6). Static phase [Formula: see text] shimming in a pig body phantom with the optimal phase vectors makes possible to improve the [Formula: see text] homogeneity by factor > 2 and transmit efficiency by factor > 3 compared to zero phases (before RF shimming). Parallel imaging performed in the in-vivo measurements demonstrated well preserved diagnostic quality of the resulting images at acceleration factors up to R = 6. The described hardware design can be adapted for arrays optimized for animals and humans with a larger number of elements (32-64) while maintaining good decoupling for various MRI applications at UHF (e.g., cardiac, head, and spine).
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Engenharia Biomédica/instrumentação , Coração/diagnóstico por imagem , Imageamento por Ressonância Magnética/instrumentação , Animais , Cadáver , Radiação Eletromagnética , Desenho de Equipamento , Imageamento por Ressonância Magnética/métodos , Imagens de Fantasmas , Razão Sinal-Ruído , SuínosRESUMO
The design, simulation, assembly and testing of a novel dedicated antisymmetric transmit/receive (Tx/Rx) coil array to demonstrate the feasibility of cardiac magnetic resonance imaging (cMRI) in pigs at 7â¯T was described. The novel antisymmetric array is composed of eight elements based on mirrored and reversed loop orientations to generate varying B1+ field harmonics for RF shimming. The central four loop elements formed together a pair of antisymmetric L-shaped channels to allow good decoupling between all neighboring elements of the entire array. The antisymmetric array was compared to a standard symmetric rectilinear loop array with an identical housing dimension. Both arrays were driven in the parallel transmit (pTx) mode forming an 8-channel transmit and 16-channel receive (8Tx/16Rx) coil array, where the same posterior array was combined with both anterior arrays. The hardware and imaging performance of the dedicated cardiac arrays were validated and compared by means of electromagnetic (EM) simulations, bench-top measurements, phantom, and ex-vivo MRI experiments with 46â¯kg female pig. Combined signal-to-noise ratio (SNR), geometry factor (g-factor), noise correlation maps, and high resolution ex-vivo cardiac images were acquired with an in-plane resolution of 0.3â¯mmâ¯×â¯0.3â¯mm using both arrays. The novel antisymmetric array enhanced the SNR within the heart by about two times and demonstrated good decoupling and improved control of the B1+ field distributions for RF shimming compared to the standard coil array. Parallel imaging with acceleration factor (R) up to 4 was possible using the novel antisymmetric coil array while maintaining the mean g-factor within the heart region of 1.13.
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Coração/diagnóstico por imagem , Imageamento por Ressonância Magnética/instrumentação , Animais , Desenho de Equipamento , Estudos de Viabilidade , Imagens de Fantasmas , Ondas de Rádio , Razão Sinal-Ruído , SuínosRESUMO
Potassium-39 ((39)K) magnetic resonance imaging (MRI) is a noninvasive technique which could potentially allow for detecting intracellular physiological variations in common human pathologies such as stroke and cancer. However, the low signal-to-noise ratio (SNR) achieved in (39)K-MR images hampered data acquisition with sufficiently high spatial and temporal resolution in animal models so far. Full wave electromagnetic (EM) simulations were performed for a single-loop copper (Cu) radio frequency (RF) surface resonator with a diameter of 30 mm optimized for rat brain imaging at room temperature (RT) and at liquid nitrogen (LN2) with a temperature of 77 K. A novel cryogenic Cu RF surface resonator with home-built LN2 nonmagnetic G10 fiberglass cryostat system for small animal scanner at 9.4 T was designed, built and tested in phantom and in in vivo MR measurements. Aerogel was used for thermal insulation in the developed LN2 cryostat. In this paper, we present the first in vivo (39)K-MR images at 9.4 T for both healthy and stroke-induced rats using the developed cryogenic coil at 77 K. In good agreement with EM-simulations and bench-top measurements, the developed cryogenic coil improved the SNR by factor of 2.7 ± 0.2 in both phantom and in in vivo MR imaging compared with the same coil at RT.