The development of novel multiple-element transmit-receive arrays is an essential factor for improving B.sub.1 .sup.+ field homogeneity in cardiac MRI at ultra-high magnetic field strength (B.sub.0 > ...= 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 B.sub.1 .sup.+ -field that is achievable without (or before) subject-specific B.sub.1 .sup.+ -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 B.sub.1 -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 B.sub.1 .sup.+ -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 B.sub.1 .sup.+ -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.
PurposeFunctional cardiac MRI scans employing balanced steady‐state free precession sequences suffer from dark band artifacts in the myocardium due to B0 inhomogeneity. We recently introduced a novel ...method for the theoretical derivation of B0 distributions in the human heart. This study aims to simulate the B0 distributions in the heart across the cardiac cycle using structural MR images and validate the simulations via in vivo measured cardiac phase‐specific B0 maps on the same subjects at 3T.MethodsCardiac phase‐specific B0 field maps were acquired from eight healthy subjects at 3T. B0 conditions were simulated based on tissue masks created from the cardiac‐phase specific structural images from the in vivo B0 map scan and anatomical images from a thoracic MRI scan, adopting our recently published approach. The simulations and in vivo measurements were compared by calculating the spatial correlation of their B0 distributions and temporal correlation of the derived spherical harmonic coefficients throughout the cardiac cycle.ResultsThe spatial comparison of B0 maps between the simulation and in vivo measurement indicates an overall average correlation coefficient of 0.91 across the cardiac cycle in all subjects. Both groups show consistent high‐level B0 patterns. Temporal variations of B0 conditions exhibit sinusoidal characteristics and are strongly correlated between simulation and in vivo.ConclusionTheoretical simulations employing regional anatomical features were validated by direct in vivo B0 mapping in the same subjects. The spatial B0 condition throughout the cardiac cycle exhibits oscillatory characteristics due to structural distortions of cardiac motion.
Two 16-element radio frequency (RF) transceiver antisymmetric coil arrays with two different implementation designs aiming for improved characteristics for static phase <inline-formula> <tex-math ...notation="LaTeX">B_{1}^{+} </tex-math></inline-formula> shimming and parallel receive were developed for human cardiac MRI at 7 T. Both cardiac arrays consist of a weakly curved lightweight housing fit to an average human thorax shape as an anterior array in combination with a flat posterior array. The first array (Design 1) comprised eight identical antisymmetric loops for both anterior and posterior sections. Based on initial testing and to improve parallel imaging capabilities and <inline-formula> <tex-math notation="LaTeX">B_{1}^{+} </tex-math></inline-formula> field homogeneity, the second array (Design 2) was composed of 12-loops for the anterior section and four antisymmetric loops of larger size for the posterior section. Electromagnetic-field (EMF) simulations were carried out for both antisymmetric arrays loaded with an elliptical-shaped thorax phantom and two human voxel models (Duke and Ella). Static phase <inline-formula> <tex-math notation="LaTeX">B_{1}^{+} </tex-math></inline-formula> shimming has been carried out within Duke and Ella models using two optimization cost functions aiming to maximize the transmit efficiency and weighted combination of both <inline-formula> <tex-math notation="LaTeX">B_{1}^{+} </tex-math></inline-formula> field homogeneity and efficiency. The hardware and imaging performance of the two developed antisymmetric cardiac arrays was validated through EMF simulations, benchtop measurements, and MR measurements in a thorax phantom. Both antisymmetric arrays were compared to two commercial 16-element transceiver arrays (a 1Tx/16Rx in single transmit mode and an 8Tx/16Rx in parallel transmit mode). MRI measurements were performed with Design 2 using a 70-kg fresh pig cadaver (10-15 min postmortem). Parallel imaging with an acceleration factor up to <inline-formula> <tex-math notation="LaTeX">R = 6 </tex-math></inline-formula> was possible using Design 2 while maintaining a mean g-factor of 1.47 within the pig heart. <inline-formula> <tex-math notation="LaTeX">T_{2}^{\ast } </tex-math></inline-formula>-weighted images of the pig heart were acquired using up to <inline-formula> <tex-math notation="LaTeX">R = 5 </tex-math></inline-formula> with a spatial resolution of <inline-formula> <tex-math notation="LaTeX">0.35 \times 0.35 \times 4 </tex-math></inline-formula> mm 3 .
Introduction
MRI of excised hearts at ultra-high field strengths (
B
0
≥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 B
1
+
-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 T
2
*-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 mm
3
voxel) diffusion tensor imaging-based tractography provided high-resolution information about normal myocardial fiber orientation.
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.
Background
To investigate the effects of B
1
-shimming and radiofrequency (RF) parallel transmission (pTX) on the visualization and quantification of the degree of stenosis in a coronary artery ...phantom using 7 Tesla (7 T) magnetic resonance imaging (MRI).
Methods
Stenosis phantoms with different grades of stenosis (0%, 20%, 40%, 60%, 80%, and 100%; 5 mm inner vessel diameter) were produced using 3D printing (clear resin). Phantoms were imaged with four different concentrations of diluted Gd-DOTA representing established arterial concentrations after intravenous injection in humans. Samples were centrally positioned in a thorax phantom of 30 cm diameter filled with a custom-made liquid featuring dielectric properties of muscle tissue. MRI was performed on a 7 T whole-body system. 2D-gradient-echo sequences were acquired with an 8-channel transmit 16-channel receive (8 Tx / 16 Rx) cardiac array prototype coil with and without pTX mode. Measurements were compared to those obtained with identical scan parameters using a commercially available 1 Tx / 16 Rx single transmit coil (sTX). To assess reproducibility, measurements (
n
= 15) were repeated at different horizontal angles with respect to the B
0
-field.
Results
B
1
-shimming and pTX markedly improved flip angle homogeneity across the thorax phantom yielding a distinctly increased signal-to-noise ratio (SNR) averaged over a whole slice relative to non-manipulated RF fields. Images without B
1
-shimming showed shading artifacts due to local B
1
+
-field inhomogeneities, which hampered stenosis quantification in severe cases. In contrast, B
1
-shimming and pTX provided superior image homogeneity. Compared with a conventional sTX coil higher grade stenoses (60% and 80%) were graded significantly (p<0.01) more precise. Mild to moderate grade stenoses did not show significant differences. Overall, SNR was distinctly higher with B
1
-shimming and pTX than with the conventional sTX coil (inside the stenosis phantoms 14%, outside the phantoms 32%). Both full and half concentration (10.2 mM and 5.1 mM) of a conventional Gd-DOTA dose for humans were equally suitable for stenosis evaluation in this phantom study.
Conclusions
B
1
-shimming and pTX at 7 T can distinctly improve image homogeneity and therefore provide considerably more accurate MR image analysis, which is beneficial for imaging of small vessel structures.
ABSTRACT
White dwarfs that accrete from non-degenerate companions show anomalous carbon and nitrogen abundances in the photospheres of their stellar components have been postulated to be descendants ...of supersoft X-ray binaries. Measuring the carbon-to-nitrogen abundance ratio may provide constraints on their past evolution. We fit far-ultraviolet spectroscopy of the cataclysmic variable HS 0218 + 3229 taken with the Cosmic Origins Spectrograph using Markov chain Monte Carlo methods, and found the carbon-to-nitrogen ratio is about one tenth of the Solar value $(\rm{\log \mathrm{C/N}}=-0.56\pm 0.15)$. We also provide estimates of the silicon and aluminium abundances, and upper limits for iron and oxygen. Using the parameters we derived for HS 0218 + 3229 we reconstruct its past. We calculated a grid of mesa models and implemented Gaussian process fits in order to determine its most likely initial binary configuration. We found that an initial mass of the donor of $M_{\rm donor;i}=0.90-0.98,\rm{\mathrm{M}_{\odot }}$ and an initial orbital period of Porb; i = 2.88 d (Porb; i = 3.12–3.16 d) for an assumed initial white dwarf mass of $\rm{M_{\mathrm{WD}}}_\mathrm{;i}=0.83\, \rm{\mathrm{M}_{\odot }}$$(\rm{M_{\mathrm{WD}}}_{\rm ;i}=0.60\, \rm{\mathrm{M}_{\odot }})$ can replicate the measured parameters. The low mass ratio, $M_{\rm donor;i} / \rm{M_{\mathrm{WD}}}_{\rm ;i} =1.08-1.18\, (1.5-1.63)$, suggests that the system did not go through a phase of hydrogen-burning on the white dwarf’s surface. However, we can not exclude a phase of thermal time-scale mass transfer in the past. We predict that HS 0218 + 3229 will evolve below the ≃ 76.2 ± 1 min period minimum for normal cataclysmic variables.
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.
A novel dedicated transceiver 16‐element antisymmetric dipole antenna array (8Tx/16Rx) combining L‐shaped and straight dipoles was designed and tested to improve parallel transmit and receive performance for cardiac MRI in pigs at 7 T.
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 comparable to those of a reference 8Tx/16Rx loop array.
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 B
1
+
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 B
1
+
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.
Subendocardial damage is among the first cardiac manifestations of hypertension and is already present in asymptomatic disease states. Accordingly, markers of subendocardial impairment may facilitate ...early detection of cardiac damages and risk stratification under these conditions. This study aimed to investigate the impact of subendocardial damage on myocardial microstructure and function to elucidate early pathophysiologic processes and to identify corresponding diagnostic measures. Mice (n=38) were injected with isoproterenol to induce isolated subendocardial scarring or saline as corresponding control. Cardiac function and myocardial deformation were determined by high-frequency echocardiography. The cardiac stress response was assessed in a graded exercise test and during dobutamine stress echocardiography. Myocardial microstructure was studied ex vivo by 7 T diffusion tensor magnetic resonance imaging at a spatial resolution of 100×100×100 µm. Results were correlated with histology and biomarker expression. Subendocardial fibrosis was accompanied by diastolic dysfunction, impaired longitudinal deformation (global peak longitudinal strain LS−12.5±0.5% versus −15.6±0.5%; P<0.001) and elevated biomarker expression (ANP atrial natriuretic peptide, Galectin-3, and ST2). Systolic function and cardiac stress response remained preserved. Diffusion tensor magnetic resonance imaging revealed a left-shift in helix angle towards lower values in isoproterenol-treated animals, which was mainly determined by subepicardial myofibers (mean helix angle2.2±0.8° versus 5.9±1.0°; P<0.01). Longitudinal strain and subepicardial helix angle were highly predictive for subendocardial fibrosis (sensitivity, 82%–92% and specificity, 89%–90%). The results indicate that circumscribed subendocardial damage alone can cause several hallmarks observed in cardiovascular high-risk patients. Microstructural remodeling under these conditions involves also remote regions, and corresponding changes in longitudinal strain and helix angle might serve as diagnostic markers.
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