Papers by Pierre-Francois Van de Moortele
2017 International Conference on Electromagnetics in Advanced Applications (ICEAA), 2017
A sixteen-channel array consisting of eight pairs of stacked loop and dipole antennas was simulat... more A sixteen-channel array consisting of eight pairs of stacked loop and dipole antennas was simulated for head MRI applications at 10.5 Tesla. Remcom XFdtd was used to calculate isolation between loop and dipole elements modeled with a dielectric load with varying space between loop and dipole elements. A loop-dipole array assembly was simulated for a candidate element configuration with a head-shaped dielectric phantom.
Introduction: Simultaneous MultiSlice (SMS) MR imaging (1) using MultiBand (MB) RF pulses is beco... more Introduction: Simultaneous MultiSlice (SMS) MR imaging (1) using MultiBand (MB) RF pulses is becoming increasingly popular in the neuroimaging community (2-4). Recently, there has been an interest in utilizing multielement RF arrays in combination with multichannel (pTx) MB pulse design to reduce transmit B1 (B1+) inhomogeneity and SAR (5,6) for SMS/MB imaging. Meanwhile, it has been shown that the use of transmit coil elements that approximately align with the slice direction, such as Z-stacked arrays with azimuthally distributed elements in two rings displaced from each other along the Z-direction versus axial slices, can provide improved RF performance for pTx non-MB pulses at 3T (7) and 7T (8), as compared to conventional single ring arrays. In this study, we evaluate the performance of such transmit coil element/slice geometries for achieving whole brain SMS/MB imaging at 7T by designing pTx MB RF pulses based on electromagnetic (EM) simulations. Method: Three head RF arrays we...
Introduction: Without the need for contrast agents and the risk of ionizing radiation, renal arte... more Introduction: Without the need for contrast agents and the risk of ionizing radiation, renal arterial spin labeling (ASL) perfusion imaging is a well-suited imaging modality for renal disease studies and longitudinal evaluation of renal function after transplantation (1-2). Due to the intrinsically low signal noise ratio nature of ASL imaging, lengthy signal averaging and correspondingly long imaging acquisition times are usually needed in renal perfusion imaging at lower fields, which not only makes imaging sensitive to physiological motion but also imposes critical limitations on its application in patients (2). The increased SNR, prolonged longitudinal relaxation times, and better parallel imaging performance (3) of ultra high field provide the potential to reduce image acquisition time and motion-associated artifacts. The feasibility of single breath-hold renal ASL perfusion imaging at 7T was evaluated, and the results of renal perfusion imaging using single shot fast spin echo ...
Target Audience: RF Engineers, UHF Labs, Researchers interested in pTX, TOF and ASL Purpose The a... more Target Audience: RF Engineers, UHF Labs, Researchers interested in pTX, TOF and ASL Purpose The aim of this work was to develop a 7 Tesla Arterial Spin Labeling (ASL) [1,2] transceiver coil array capable of being used as a stand-alone array [3,4] or in combination with an additional transceiver head array [5]. The transceiver design is potentially beneficial at 7T since it allows for Radio Frequency (RF) transmit efficiency optimization and location-dependent B1 + shimming. In variation to Wiggins pioneering work [3] we utilize the same coil for transmission and reception. This allows for a mechanically smaller housing and potential for improved transmit efficiency. Methods The coil layout and circuitry had to support fully flexible positioning of the ASL array relative to the head array without significant coil interaction. For this a dedicated coil housing for the ASL arrays and the transceiver head coil was built utilizing Fused Deposition Modeling (FDM) (Fig. 1). The 8-channel A...
Magnetic Resonance in Medicine, 2017
To evaluate the feasibility of quantitative single breath-hold renal arterial spin labeling (ASL)... more To evaluate the feasibility of quantitative single breath-hold renal arterial spin labeling (ASL) imaging at 7T. Methods: A single-shot fast spin echo FAIR (flow-sensitive alternating inversion recovery) method was used to perform two studies. First, a multi-delay perfusion study was performed to estimate the spin labeling temporal bolus width achievable with a local transceiver array coil at 7T. Second, with a conservatively defined bolus width, a quantitative perfusion study was performed using the single subtraction approach. To address issues of B þ 1 inhomogeneity/efficiency and excessive short-term specific absorption rates, various strategies were used, such as dynamic radiofrequency shimming and optimization. Results: A conservative temporal bolus width of 600 ms determined from the multi-delay study was applied for singlesubtraction imaging to measure the renal blood flow in the cortex and medulla: 303 6 31.8 and 91.3 6 15.2 (mL/100 g/min), respectively. The estimated spatial and temporal signal-to-noise ratios of renal perfusion measurements were 3.8 6 0.7 and 2.4 6 0.6 for the cortex, and 2.2 6 0.6 and 1.4 6 0.2 for the medulla. Conclusion: With proper management of field strength specific challenges, quantitative renal ASL imaging can be achieved at 7T within a single breath-hold. Magn Reson Med 000:000-000, 2017. V
Magnetic resonance in medicine, Jan 23, 2017
To develop and evaluate a technique for imaging electrical properties ((EPs), conductivity and pe... more To develop and evaluate a technique for imaging electrical properties ((EPs), conductivity and permittivity) of an animal tumor model in vivo using MRI. Electrical properties were reconstructed from the calculated EP gradient, which was derived using two sets of measured transmit B1 magnitude and relative phase maps with the sample and radiofrequency (RF) coil oriented in the positive and negative z-directions, respectively. An eight-channel transceiver microstrip array RF coil fitting the size of the animal was developed for generating and mapping B1 fields to reconstruct EPs. The technique was evaluated at 7 tesla using a physical phantom and in vivo on two Copenhagen rats with subcutaneously implanted AT-1 rat prostate cancer on a hind limb. The reconstructed EPs in the phantom experiment was in good agreement with the target EP map determined by a dielectric probe. Reconstructed conductivity map of the animals revealed the boundary between tumor and healthy tissue consistent wit...
Magnetic Resonance in Medicine, 2016
Purpose: To develop and evaluate a robust motion-insensitive Bloch-Siegert shift based B þ 1 mapp... more Purpose: To develop and evaluate a robust motion-insensitive Bloch-Siegert shift based B þ 1 mapping method in the heart. Methods: Cardiac Bloch-Siegert B þ 1 mapping was performed with interleaved positive and negative off-resonance shifts and diastolic spoiled gradient echo imaging in 12 heartbeats. Numerical simulations were performed to study the impact of respiratory motion. The method was compared with three-dimensional (3D) actual flip angle imaging (AFI) and two-dimensional (2D) saturated double angle method (SDAM) in phantom scans. Cardiac B þ 1 maps of three different views were acquired in six healthy volunteers using Bloch-Siegert and SDAM during breath-hold and free breathing. In vivo maps were evaluated for interview consistency using the correlation coefficients of the B þ 1 profiles along the lines of intersection between the views. Results: For the Bloch-Siegert sequence, numerical simulations indicated high similarity between breath-hold and free breathing scans, and phantom results indicated low deviation from the 3D AFI reference (normalized root mean square error [NRMSE] ¼ 2.0%). Increased deviation was observed with 2D SDAM (NRMSE ¼ 5.0%) due to underestimation caused by imperfect excitation slice profiles. Breath-hold and free breathing Bloch-Siegert in vivo B þ 1 maps were visually comparable with no significant difference in the interview consistency (P > 0.36). SDAM showed strongly impaired B þ 1 map quality during free breathing. Interview consistency was significantly lower than with the Bloch-Siegert method (breath-hold: P ¼ 0.014, free breathing: P < 0.0001). Conclusion: The proposed interleaved Bloch-Siegert sequence enables cardiac B þ 1 mapping with improved interview consistency and high resilience to respiratory motion. Magn Reson Med
2015 IEEE MTT-S International Microwave Symposium, 2015
In this paper, we present the design of a probe for a travelling-wave 16.4T small-bore animal res... more In this paper, we present the design of a probe for a travelling-wave 16.4T small-bore animal research MRI system. The probe is a 698-MHz coaxially-fed microstrip patch designed to give a circularly polarized magnetic field when placed in the bore cavity. Images of a water phantom using the patch probe are obtained and compared with simulations. Additionally, a periodic axial strip cylinder is inserted into the bore, resulting in a 7-fold increase in SNR, and enabling both gradient recalled echo and spin echo imaging of the phantom. The modified mode content in the image is compared to full-wave simulations.
Cerebral cortex (New York, N.Y. : 1991), Jan 3, 2014
The precise delineation of auditory areas in vivo remains problematic. Histological analysis of p... more The precise delineation of auditory areas in vivo remains problematic. Histological analysis of postmortem tissue indicates that the relation of areal borders to macroanatomical landmarks is variable across subjects. Furthermore, functional parcellation schemes based on measures of, for example, frequency preference (tonotopy) remain controversial. Here, we propose a 7 Tesla magnetic resonance imaging method that enables the anatomical delineation of auditory cortical areas in vivo and in individual brains, through the high-resolution visualization (0.6 × 0.6 × 0.6 mm(3)) of intracortical anatomical contrast related to myelin. The approach combines the acquisition and analysis of images with multiple MR contrasts (T1, T2*, and proton density). Compared with previous methods, the proposed solution is feasible at high fields and time efficient, which allows collecting myelin-related and functional images within the same measurement session. Our results show that a data-driven analysis...
Progress in Biomedical Optics and Imaging - Proceedings of SPIE, 2006
Multi-channel acquisition is employed in MRI to decrease total imaging time. In this paper, artif... more Multi-channel acquisition is employed in MRI to decrease total imaging time. In this paper, artifact free images are calculated by utilizing the difference in spatial encoding of the MR signal from neighboring channels. The encoding functions are estimated in the presence of noise and motion. For fMRI studies, the temporal stability of the signal is essential, since neuronal activity in
Ultra High Field Magnetic Resonance Imaging, 2006
... Kâmil Uğurbil, Gregor Adriany, Can Akgün, Peter Andersen, Wei Chen, Michael Garwood, Rolf Gru... more ... Kâmil Uğurbil, Gregor Adriany, Can Akgün, Peter Andersen, Wei Chen, Michael Garwood, Rolf Gruetter, Pierre-Gilles Henry, Malgorzata Marjanska, Steen Moeller, Pierre-François Van de Moortele, Klaas Prüssmann,* Ivan Tkac, J. Thomas Vaughan, Florian Wiesinger,* Essa ...
NMR in Biomedicine, 2011
Increased sensitivity and chemical shift dispersion at ultra-high magnetic fields (UHF) enable pr... more Increased sensitivity and chemical shift dispersion at ultra-high magnetic fields (UHF) enable precise quantification of an extended range of brain metabolites from 1 H MR spectra. However, all previous neurochemical profiling studies using single-voxel MRS at 7 T were limited to data acquired from the occipital lobe with half-volume coils. Challenges of 1 H MRS of the human brain at 7 T include short T 2 and complex B 1 distribution that imposes limitations in maximum achievable B 1 strength. In this study, the feasibility of acquiring and quantifying short-echo (TE = 8 ms), single voxel 1 H MR spectra from multiple brain regions was demonstrated by utilizing a 16-channel transceiver array coil with 16 independent transmit channels allowing local transmit B 1 (B 1 +) shimming. Spectra were acquired from volumes-of-interest of 1-8 mL in brain regions that are of interest for various neurological disorders: frontal white matter, posterior cingulate, putamen, substantia nigra, pons and cerebellar vermis. Local B 1 + shimming substantially increased transmit efficiency, especially in the peripheral and ventral brain regions. By optimizing a STEAM sequence for utilization with a 16-channel coil, artifact-free spectra were acquired with a small chemical shift displacement error (< 5% /ppm/direction) from all regions. The high SNR enabled the quantification of neurochemical profiles consisting of at least 9 metabolites including GABA, glutamate and glutathione in all brain regions. Significant differences in neurochemical profiles were observed between brain regions. For example, GABA levels were highest in the substantia nigra, total creatine highest in the cerebellar vermis and total choline highest in the pons, consistent with known biochemistry of these regions. These findings demonstrate that single voxel 1 H MRS at UHF can reliably detect region-specific neurochemical patterns in the human brain and has the potential to objectively detect alterations in neurochemical profiles associated with neurological diseases.
NeuroImage, 2012
Ultra high fields (UHF) permit unprecedented explorations of functional organizations and insight... more Ultra high fields (UHF) permit unprecedented explorations of functional organizations and insight into basic neuronal processes. Increases in the signal and contrast to noise ratios have allowed increases in the spatial resolution of T(2) weighted gradient echo (GE) echo planar imaging (EPI). Furthermore, while the use of T(2) weighted imaging methods at UHF (e.g. spin echo (SE) EPI, gradient and spin echo (GRASE) EPI) can also permit higher resolution images, they in addition allow for increased spatial specificity of functional responses, permitting the in-vivo study of functional organizations down to the columnar level of the cortex. The study of the visual cortex has, thus far, benefitted the most from higher resolution T(2) weighted studies as achieving the required transmit B(1) magnitude at 7T is more challenging in other brain regions, such as the auditory cortex. As such, auditory fMRI studies at UHF have been limited to T(2) weighted GE sequences. Recent advances in multi-channel RF transmission (e.g. B(1) shimming) have enabled procedures to efficiently address deficiencies in transmit B(1) profiles. However, these techniques, shown to be advantageous in anatomical imaging at UHF, are not generally utilized to facilitate T(2) weighted fMRI studies. Here we investigate the feasibility of applying B(1) shimming to achieve efficient RF transmission in the human auditory cortex. We demonstrate that, with B(1) shimming, functional responses to simple tones and to complex sounds (i.e. voices, speech, animal cries, tools and nature) can be efficiently measured with T(2) weighted SE-EPI in the bilateral human auditory cortex at 7T without exceeding specific absorption rate (SAR) limits.
Nature Communications, 2013
To date, the functional organization of human auditory subcortical structures can only be inferre... more To date, the functional organization of human auditory subcortical structures can only be inferred from animal models. Here we use high-resolution functional magnetic resonance imaging at ultra-high magnetic fields (7T) to map the organization of spectral responses in the human inferior colliculus, a subcortical structure fundamental for sound processing. We reveal a tonotopic map with a spatial gradient of preferred frequencies approximately oriented from dorsolateral (low frequencies) to ventromedial (high frequencies) locations. Furthermore, we observe a spatial organization of spectral selectivity (tuning) of functional magnetic resonance imaging responses in the human inferior colliculus. Along isofrequency contours, functional magnetic resonance imaging tuning is narrowest in central locations and broadest in the surrounding regions. Finally, by comparing subcortical and cortical auditory areas we show that functional magnetic resonance imaging tuning is narrower in human inferior colliculus than on the cortical surface. Our findings pave the way to noninvasive investigations of sound processing in human subcortical nuclei and for studying the interplay between subcortical and cortical neuronal populations.
Magnetic Resonance in Medicine, 2007
The origin of the stimulus/task-induced signal changes in spin echo (SE) functional MRI (fMRI) at... more The origin of the stimulus/task-induced signal changes in spin echo (SE) functional MRI (fMRI) at high magnetic fields is dynamic averaging due to diffusion in the presence of field gradients surrounding deoxyhemoglobin-containing microvasculature. The same mechanism is expected to be operative in stimulated echoes (STE). Compared to SE-fMRI, however, STE-fMRI has the potential for larger diffusion weighting and consequently larger stimulus/task-induced signal changes as a result of an additional delay, the mixing time, T(M). In the present study, functional signal changes were quantified for both primary echo (PRE) and STE as a function of echo and mixing time. The relative blood oxygenation level dependent (BOLD) signal changes in STE were larger than in PRE at the same echo time and increased with both mixing and echo time. The contrast-to-noise ratio (CNR) of the STE, however, is close to the CNR of the PRE, indicating an increase of physiological noise with longer mixing times. In addition, the signal attenuation due to diffusion in the presence of magnetic field gradients near blood vessels was modeled using Monte Carlo simulations. They support the hypothesis that the sensitivity of the STE to fluctuations of susceptibility-induced magnetic field gradients near microvasculature is enhanced as a result of an extended diffusion time.
Magnetic Resonance in Medicine, 2012
The purpose of this study was to detail a strategy for performing non-contrast enhanced renal ang... more The purpose of this study was to detail a strategy for performing non-contrast enhanced renal angiography (rMRA) studies at 7.0 Tesla. It is demonstrated that with proper B 1 + management, these studies can be successfully performed at ultrahigh fields within local specific absorption rate (SAR) constraints. An inversion prepared gradient echo acquisition, standard for non-contrast rMRA studies, required RF pulse specific B 1 + shimming solutions to be dynamically applied to address the field dependent increases in both B 0 and B 1 + inhomogeneity as well as to accommodate limitation in available power. By using more efficient B 1 + shimming solutions for the inversion preparation and more homogeneous solutions for the excitation, high quality images of the the renal arteries were obtained without venous and background signal artifacts while working within hardware and safety constraints. Finite difference time domain simulations confirmed in vivo measurements with respect to B 1 + distributions and homogeneity for the range of shimming strategies employed, and allowed the calculation of peak local SAR values normalized by input power and B 1 +. Increasing B 1 + homogeneity was accompanied by decreasing local SAR per Watt and increasing local SAR per [B 1 + ] 2 , which must be considered, along with body size and respiratory rate, when finalizing acquisition parameters for a given individual.
Magnetic Resonance Imaging, 2006
Proton magnetic resonance spectroscopy (1 H-MRS) has been used in a number of studies to noninvas... more Proton magnetic resonance spectroscopy (1 H-MRS) has been used in a number of studies to noninvasively assess the temporal changes of lactate in the activated human brain. However, the results have not been consistent. The aim of the present study was to test the sensitivity of 1 H-MRS during functional experiments at the highest magnetic field currently available for human studies (7 T). Stability and reproducibility of the measurements were evaluated from LCModel analysis of time series of spectra measured during a visual stimulation paradigm and by examination of the difference between spectra obtained at rest and during activation. The sensitivity threshold to detect concentration changes was 0.2 Amol/g for most of the quantified metabolites. The possible variations of metabolite concentrations during visual stimulation were within the same range (F0.2 Amol/g). In addition, the influence of a small line-narrowing effect due to the blood oxygenation leveldependent (BOLD) T2* changes on the estimated concentrations was simulated. Quantification of metabolites was, in general, not affected beyond 1% by line-width changes within 0.5 Hz.
Brain Research Reviews, 1998
G. Adriany, P. Van de Moortele, F. Wiesinger, P. Andersen, J. Strupp, X. Zhang, C. J. Snyder, W. ... more G. Adriany, P. Van de Moortele, F. Wiesinger, P. Andersen, J. Strupp, X. Zhang, C. J. Snyder, W. Chen, K. P. Pruessmann, P. Boesiger, J. T. Vaughan, K. Ugurbil Center for MR Research, University of Minnesota Medical School, Minneapolis, United States, Institute for Biomedical Engineering, University and ETH Zurich, Switzerland We present four and eight channel transceive stripline arrays for ultra high field parallel imaging applications. Good coil decoupling between stripline array elements was achieved without preamplifier decoupling. With the four channel array and the eight channel array we achieved high reduction factors and excellent average gfactors. Our results confirm the prediction that the maximal achievable reduction factor increases with field strength. Introduction At ultra high fields, many applications that require EPI or SPIRAL type fast imaging, such as fMRI, are expected to significantly benefit from parallel imaging techniques due to reduction in time for acquisition of k-space data. Furthermore, it is also expected that the maximal feasible reduction factor will increase with higher field strength [1,2]. However, there are a number of challenges that arise for designing dedicated parallel imaging coils at ultra high fields. Strong coupling to the sample at the high frequencies mediates interactions between the separate coils, making it difficult to decouple them. There are image inhomogeneities caused partly by interaction with conductive tissue and partly by the differences between reception and excitation profiles [3]; these inhomogeneities aid parallel imaging strategies but pose problems for constructing coil arrays that cover the brain. Furthermore, the geometrical constraints for a experimental setup that includes separate transmit coil and receive array coils, as well as task presentation for fMRI studies, are severe. Transceive arrays seem particular advantageous in addressing these issues [4] and they additionally allow for transmit SENSE [5,6]. Here we present four and eight channel transceive array coil designs for parallel imaging of the brain at 7T and results obtained with these coil arrays. Methods We built four and eight channel transceive surface coil arrays according to stripline transmission line principles [7-9]. The four channel transceive coils were 13 cm x 12 cm in size with 2.5 cm inter coil spacing and oriented in 0,90,180 and 270 degree position around an elliptical former. The eight channel coils were built from 7cm wide and 14 cm long elements and evenly spaced with 1.5 cm inter coil spacing (Fig.1). All coils were built using 1cm wide copper tape for the coil conductors and 2cm wide copper tape for the ground conductor. All grounds were cut in one position to avoid eddy currents. Imaging experiments were performed on a 7 Tesla Magnet (Magnex Scientific, UK) equipped with a Varian console (Palo Alto, CA) and Siemens gradient amplifier (Erlangen, Germany). We utilized a single 4 kW RF amplifier (CPC, Brentwood, NY) and split the RF power 4-ways or 8-ways. The transmit phase increments for each channel were adjusted for optimal image homogeneity. T/R switches with low insertion loss of 0.2dB in each transmit path blocked transmitter noise during reception and enabled the used of low noise preamplifiers. Results and Discussion Utilizing stripline surface coils we were able to achieve good coil decoupling without preamplifier decoupling. No resonance peak split was observed and the coils could be tuned and matched for each subject. By adjusting the transmit phase and amplitude for each array coil independently we were able to achieve excellent parallel imaging performance while obtaining additionally RF shimming capabilities. Since the profile of the coils is less than 1cm there is sufficient space for task presentation hardware. The average Q0/ QL ratio of the coils was measured to be 285/70, indicating good transmit efficiency. Decoupling in the four channel coil between neighboring coils when loaded with a human head was -18dB and -35dB between opposing coils. We achieved similar values in the eight channel coil when using decoupling capacitors as an additional decoupling method. We compared the transceive arrays with our standard TEM head coils and acquired B1 field maps. A 90deg flip angle in the center of the head with 1kW RF power required a 350μs long square pulse with the four channel array, which is comparable to a volume coil [10]. Peripheral areas however are more efficiently excited with the transceive array and we were able to achieve spin inversion in areas of the lower brain where this previously was not feasible with a quadrature volume coil. Using full k-space acquisition, sum-of-squares from the four channels displayed good homogeneity in image intensity. Using SENSE, we were able to achieve acceleration factors of 3 with the four channel array with an average geometry factor of 1.41 when reducing phase encoding along the long axis and 1.65…
American Journal of Physiology-Endocrinology and Metabolism, 2006
The adult brain relies on glucose for its energy needs and stores it in the form of glycogen, pri... more The adult brain relies on glucose for its energy needs and stores it in the form of glycogen, primarily in astrocytes. Animal and culture studies indicate that brain glycogen may support neuronal function when the glucose supply from the blood is inadequate and/or during neuronal activation. However, the concentration of glycogen and rates of its metabolism in the human brain are unknown. We used in vivo localized 13C-NMR spectroscopy to measure glycogen content and turnover in the human brain. Nine healthy volunteers received intravenous infusions of [1-13C]glucose for durations ranging from 6 to 50 h, and brain glycogen labeling and washout were measured in the occipital lobe for up to 84 h. The labeling kinetics suggest that turnover is the main mechanism of label incorporation into brain glycogen. Upon fitting a model of glycogen metabolism to the time courses of newly synthesized glycogen, human brain glycogen content was estimated at ∼3.5 μmol/g, i.e., three- to fourfold highe...
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Papers by Pierre-Francois Van de Moortele