Publications
2023
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(2023) Magnetic Resonance in Medicine. Abstract
Purpose: Rapid 3D steady-state sequences are widely used but are also known to be sensitive to semi-periodic physiological signal fluctuations due to, for example, cardiac pulsation, breathing, and eye/eyelids movement. This semi-periodicity results in repeating artifacts in the image whose intensity depends on the scan parameters. The purpose of this study is to design a reordering of the 2D phase encodes (within the 3D acquisition) that reduces these artifacts. Methods: A randomized order of the phase encodes can suppress repeating artifact but may also introduce its own apparent noise, for example, in cases of slow subject movement or gradual changes in eddy currents. In a new design a semi-randomized space-filling curve is generated by scrambling the local order of the phase encodes to achieve a controlled frequency selective effect, that is, eliminating artifacts above a chosen (fluctuation) frequency threshold while leaving lower frequencies untouched, thus overcoming the limitations of a randomized order. The method was characterized in simulations and substantiated by human brain imaging at 7 T using two steady-state gradient echo variants that suffer from pulsation, either near blood vessels or near the ventricles. Results: The simulations with a point source show that the maximum artifact intensity can be reduced by factors of 10–50 depending on the scan parameters. In human scanning, the new approach drastically reduced physiologically induced artifacts and was superior in this regard to both full randomization and a generalized Hilbert curve, another semi-randomized approach. Conclusion: The phase-encodes reordering presented here effectively removes artifacts arising from local fluctuations.
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2022
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(2022) Magnetic Resonance in Medicine. 88, 6, p. 2339-2357 Abstract
Purpose: Fast proton (1H) MRSI is an important diagnostic tool for clinical investigations, providing metabolic and spatial information. MRSI at 7 T benefits from increased SNR and improved separation of peaks but requires larger spectral widths. RS-COKE (Readout-Segmented Consistent K-t space Epsi) is an echo planar spectroscopic imaging (Epsi) variant capable to support the spectral width required for human brain metabolites spectra at 7 T. However, mismatches between readout segments lead to artifacts, particularly when subcutaneous lipid signals are not suppressed. In this study, these mismatches and their effects are analyzed and reduced. Methods: The following corrections to the data were performed: i) frequency-dependent phase corrections; ii) k-space trajectory corrections, derived from short reference scans; and iii) smoothing of data at segment transitions to mitigate the effect of residual mismatches. The improvement was evaluated by performing single-slice RS-COKE on a head-shaped phantom with a “lipid” layer and healthy subjects, using varying resolutions and durations ranging from 4.1 × 4.7 × 15 mm3 in 5:46 min to 3.1 × 3.3 × 15 mm3 in 13:07 min. Results: Artifacts arising from the readout-segmented acquisition were substantially reduced, thus providing high-quality spectroscopic imaging in phantom and human scans. LCModel fitting of the human data resulted in a relative Cramer-Rao lower bounds within 6% for NAA, Cr, and Cho images in the majority of the voxels. Conclusion: Using the new reference scans and reconstruction steps, RS-COKE was able to deliver fast 1H MRSI at 7 T, overcoming the spectral width limitation of standard EPSI at this field strength.
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(2022) Scientific Reports. 12, 14088. Abstract
Magnetic resonance imaging (MRI) is a powerful and versatile technique that offers a range of physiological, diagnostic, structural, and functional measurements. One of the most widely used basic contrasts in MRI diagnostics is transverse relaxation time (T2)-weighted imaging, but it provides only qualitative information. Realizing quantitative high-resolution T2 mapping is imperative for the development of personalized medicine, as it can enable the characterization of diseases progression. While ultra-high-field (≥ 7 T) MRI offers the means to gain new insights by increasing the spatial resolution, implementing fast quantitative T2 mapping cannot be achieved without overcoming the increased power deposition and radio frequency (RF) field inhomogeneity at ultra-high-fields. A recent study has demonstrated a new phase-based T2 mapping approach based on fast steady-state acquisitions. We extend this new approach to ultra-high field MRI, achieving quantitative high-resolution 3D T2 mapping at 7 T while addressing RF field inhomogeneity and utilizing low flip angle pulses; overcoming two main ultra-high field challenges. The method is based on controlling the coherent transverse magnetization in a steady-state gradient echo acquisition; achieved by utilizing low flip angles, a specific phase increment for the RF pulses, and short repetition times. This approach simultaneously extracts both T2 and RF field maps from the phase of the signal. Prior to in vivo experiments, the method was assessed using a 3D head-shaped phantom that was designed to model the RF field distribution in the brain. Our approach delivers fast 3D whole brain images with submillimeter resolution without requiring special hardware, such as multi-channel transmit coil, thus promoting high usability of the ultra-high field MRI in clinical practice.
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(2022) Magma (New York, N.Y.). 35, 6, Abstract
This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple "dielectric pads" for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed.
2021
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(2021) Scientific Reports. 11, 15371. Abstract
Ultra-high-field functional magnetic resonance imaging (fMRI) offers the way to new insights while increasing the spatial and temporal resolution. However, a crucial concern in 7T human MRI is the increase in power deposition, supervised through the specific absorption rate (SAR). The SAR limitation can restrict the brain coverage or the minimal repetition time of fMRI experiments. fMRI is based on the well-known gradient-echo echo-planar imaging (GRE-EPI) sequence, which offers ultrafast acquisition. Commonly, the GRE-EPI sequence comprises two pulses: fat suppression and excitation. This work provides the means for a significant reduction in the SAR by circumventing the fat-suppression pulse. Without this fat-suppression, however, lipid signal can result in artifacts due to the chemical shift between the lipid and water signals. Our approach exploits a reconstruction similar to the simultaneous-multi-slice (SMS) method to separate the lipid and water images, thus avoiding undesired lipid artifacts in brain images. The lipid-water separation is based on the known spatial shift of the lipid signal, which can be detected by the multi-channel coils sensitivity profiles. Our study shows robust human imaging, offering greater flexibility to reduce the SAR, shorten the repetition time or increase the volume coverage with substantial benefit for brain functional studies.
2020
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(2020) NMR in Biomedicine. 1, e4421. Abstract
Purpose Moving to ultra-high fields (≥7 T), the inhomogeneity of both RF (B1) and static (B0) magnetic fields increases, which further motivates us to design a realistic head-shaped phantom, especially for spectroscopic imaging. Such phantoms provide images similar to the human brain and serve as a reliable tool for developing and examining methods in MRI. This study aims to develop and characterize a realistic head-shaped phantom filled with brain-mimicking metabolites for MRS and magnetic resonance spectroscopic imaging in a 7 T MRI scanner. Methods A 3D head-shaped container with three sections—mimicking brain, muscle and precranial lipid—was constructed. The phantom was designed to provide robustness to heating, mechanical damage and leakage, with easy refilling. The head’s shape and the agarose mixture were optimized to provide B0 and B1 distributions and T1/T2 relaxation values similar to those of human brain. Eight brain-tissue-mimicking metabolites were included for spectroscopy. The phantom was evaluated for localized spectroscopy, fast spectroscopic imaging and fat suppression. Results The B0 and B1 maps showed distribution similar to that of human brain, with increased B0 inhomogeneity near the nasal and ear areas and reduced B1 in the temporal lobe and brain stem regions, as expected in vivo. The metabolites’ concentrations were verified by single-voxel spectroscopy, showing an average deviation of 11D head-shaped phantom for human brain imaging and spectroscopic imaging in 7 T MRI was demonstrated, making it a realistic phantom for methodology development at 7 T.
2019
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(2019) Magnetic Resonance in Medicine. 82, 3, p. 867-876 Abstract
Purpose: To design and implement a multislice MRSI method for fast spectroscopic imaging, using a modified version of echo planar spectroscopic imaging (EPSI) that offers higher spectral width and/or shorter scan time.Methods: Echo planar spectroscopic imaging suffers from inconsistencies between readout lines acquired with gradients of opposite signs, which has typically been addressed by reconstructing the "positive" and "negative" data sets separately and averaging the two. Nevertheless, consistency between the readout lines of each phase encode can be achieved by interposing the EPSI readouts with alternating "blipped" phase-encode gradients. This method exchanges inconsistencies along the temporal dimension with inconsistencies along the phase-encode dimension, which are straightforward to correct, as is conventionally done in various EPI reconstruction schemes. Such consistent k-t-space EPSI doubles the spectral width in comparison to EPSI, or, in an alternative realization, yields the same spectral width as EPSI, but at half the acquisition time. In this work, multiband CAIPIRINHA (controlled aliasing in parallel imaging results in higher acceleration) slice selection was integrated with consistent k-t-space EPSI to further accelerate the measurement 2-fold.Results: The feasibility of a consistent k-t-space EPSI was demonstrated in both phantoms and in vivo brain imaging at 3 T, and four pulse scheme variants were evaluated. It was demonstrated to be useful in optimizing the spectral width and scan acceleration, both of which are limiting factors in vivo. Dual-band implementation was shown to shorten the duration of the scan 4-fold.Conclusion: The consistent k-t-space EPSI can be used to accelerate MRSI or, alternatively, double its spectral width. Adding dual-band CAIPIRINHA further accelerates the acquisition by a factor of 2.
2017
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(2017) ACS Applied Materials and Interfaces. 9, 40, p. 34618-34624 Abstract
Magnetic resonance imaging and spectroscopy (MRI and MRS) are both widely used techniques in medical diagnostics and research. One of the major thrusts in recent years has been the introduction of ultrahigh-field magnets in order to boost the sensitivity. Several MRI studies have examined further potential improvements in sensitivity using metamaterials, focusing on single frequency applications. However, metamaterials have yet to reach a level that is practical for routine MRI use. In this work, we explore a new metamaterial implementation for MRI, a dual-nuclei resonant structure, which can be used for both proton and heteronuclear magnetic resonance. Our approach combines two configurations, one based on a set of electric dipoles for the low frequency band, and the second based on a set of magnetic dipoles for the high frequency band. We focus on the implementation of a dual-nuclei metamaterial for phosphorus and proton imaging and spectroscopy at an ultrahigh-field strength of 7 T. In vivo scans using this flexible and compact structure show that it locally enhances both the phosphorus and proton transmit and receive sensitivities.
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(2017) Magnetic Resonance in Medicine. 77, 6, p. 2431-2437 Abstract
Purpose: To design and characterize dual-segment dielectric resonators (DR) and assess their performance for in vivo imaging and localized proton spectroscopy at 7 Tesla.Methods: Annular DRs operating in the HEM11 mode at ultrahigh field have advantage of a simple design with an intrinsically circularly polarized homogeneous mode. However, practical realization for extremity imaging requires a splittable design for patient comfort and positioning. Splitting the annulus segments breaks the resonator symmetry and the circularly polarized distribution is lost. Here, we show that one can restore the electromagnetic modes by incorporating copper connectors between the segments. By designing the connectors such that two modes become frequency degenerate, one can also generate circularly polarized modes in a half-annular resonator. Electromagnetic simulations were performed and phantom and in vivo experiments conducted using custombuilt DRs.Results: Electromagnetic simulations showed that using copper interconnectors, the resonant modes could be restored. This was confirmed in phantom experiments, as well as in vivo images of the human knee using a dual-segment splittable annular DR and images and localized spectra of the human calf muscle acquired using a dual-segment half-annular resonator.Conclusion: A new approach incorporating copper connectors into the DR structure allows efficient operation of splittable DRs. (C) 2016 International Society for Magnetic Resonance in Medicine
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(2017) Scientific Reports. 7, 1, 1678. Abstract
Developments in metamaterials and related structures such as metasurfaces have opened up new possibilities in designing materials and devices with unique properties. Here we report a new hybrid metasurface structure, comprising a two-dimensional metamaterial surface and a very high permittivity dielectric substrate, that has been designed to enhance the local performance of an ultra-high field MRI scanner. This new flexible and compact resonant structure is the first metasurface which can be integrated with multi-element close-fitting receive coil arrays that are used for all clinical MRI scans. We demonstrate the utility of the metasurface acquiring in-vivo human brain images and proton MR spectra with enhanced local sensitivity on a commercial 7 Tesla system.
2016
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(2016) IEEE Transactions on Biomedical Engineering. 63, 11, p. 2390-2395 7416180. Abstract
Goal: To design and characterize a new set-up for dual nuclei MRI combining an annular dielectric resonator filled with high permittivity material for phosphorous (P-31) and a traveling wave antenna for proton imaging. Methods: Recent studies have shown that an annular cylinder filled with water can serve as dielectric resonator for proton MRI of the extremities at 7 T. Using a very high permittivity material such as BaTiO3, this type of dielectric resonator can potentially be designed for lower gyromagnetic ratio nuclei. Combining this with a remote antenna for proton imaging, an alternative method for dual frequency imaging at ultra-high field has been implemented. Results: 3D electromagnetic simulations were performed to examine the efficiency of the dielectric resonator. The new dielectric resonator was constructed for 31P acquisition at 121 MHz on a human 7 TMRI system. Phantom and in vivo scans demonstrated the feasibility of the setup, although the current sensitivity of the dielectric resonator is only half that of an equivalently sized birdcage. Conclusion: The new approach offers a simple implementation for dual nuclei imaging at ultra-high field, with several possibilities for further increases in sensitivity. Significance: Utilizing high permittivity materials enables very simple designs for high field RF coils: in the current configuration the interactions between the proton and phosphorous resonators are very low.
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(2016) IEEE Transactions on Medical Imaging. 35, 8, p. 1837-1844 7410074. Abstract
Ultra-high field MRI is an area of great interest for clinical research and basic science due to the increased signal-to-noise, spatial resolution and magnetic-susceptibility-based contrast. However, the fact that the electromagnetic wavelength in tissue is comparable to the relevant body dimensions means that the uniformity of the excitation field is much poorer than at lower field strengths. In addition to techniques such as transmit arrays, one simple but effective method to counteract this effect is to use high permittivity "pads". Very high permittivities enable thinner, flexible pads to be used, but the limiting factor is wavelength effects within the pads themselves, which can lead to image artifacts. So far, all studies have used simple continuous rectangular/circular pad geometries. In this work we investigate how the wavelength effects can be partially mitigated utilizing shaped pad with holes. Several arrangements have been simulated, including low order pre-fractal geometries, which maintain the overall coverage of the pad, but can provide better image homogeneity in the region of interest or higher sensitivity depending on the setup. Experimental data in the form of in vivo human images at 7T were acquired to validate the simulation results.
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(2016) Magnetic Resonance in Medicine. 75, 5, p. 2064-2071 Abstract
Purpose: Evaluate the usefulness of diffusion-weighted spatiotemporally encoded (SPEN) methods to obtain apparent diffusion coefficient (ADC) maps of fibroglandular human breast tissue, in the presence of silicone implants.Methods: Seven healthy volunteers with breast augmentation were scanned at 3 Tesla (T) using customized SPEN sequences yielding separate silicone and water H-1 images in one scan, together with their corresponding diffusion-weightings.Results: SPEN's ability to deliver multiple spectrally resolved images in a single scan, coupled to the method's substantial robustness to magnetic field heterogeneities, served to acquire ADC maps that could be freed from contributions that did not belong to fibroglandular tissue.Conclusion: SPEN-based sequences incorporating spectral discrimination and diffusion-weighting enable the acquisition of reliable ADC maps despite the presence of dominant signals from silicone implants, thereby opening new screening possibilities for the identification of malignancies in breast augmented patients.
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(2016) Magnetic Resonance in Medicine. 75, 5, p. 1935-1948 Abstract
Purpose: Single-shot imaging by spatiotemporal encoding (SPEN) can provide higher immunity to artifacts than its echo planar imaging-based counterparts. Further improvements in resolution and signal-to-noise ratio could be made by rescinding the sequence's single-scan nature. To explore this option, an interleaved SPEN version was developed that was capable of delivering optimized images due to its use of a referenceless correction algorithm. Methods: A characteristic element of SPEN encoding is the absence of aliasing when its signals are undersampled along the low-bandwidth dimension. This feature was exploited in this study to segment a SPEN experiment into a number of interleaved shots whose inaccuracies were automatically compared and corrected as part of a navigator-free image reconstruction analysis. This could account for normal phase noises, as well as for object motions during the signal collection. Results: The ensuing interleaved SPEN method was applied to phantoms and human volunteers and delivered high-quality images even in inhomogeneous or mobile environments. Submillimeter functional MRI activation maps confined to gray matter regions as well as submillimeter diffusion coefficient maps of human brains were obtained. Conclusion: We have developed an interleaved SPEN approach for the acquisition of high-definition images that promises a wider range of functional and diffusion MRI applications even in challenging environments.
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(2016) Journal of Magnetic Resonance. 262, p. 8-14 Abstract
Electrical Properties Tomography (EPT) using MRI is a technique that has been developed to provide a new contrast mechanism for in vivo imaging. Currently the most common method relies on the solution of the homogeneous Helmholtz equation, which has limitations in accurate estimation at tissue interfaces. A new method proposed in this work combines a Maxwell's integral equation representation of the problem, and the use of high permittivity materials (HPM) to control the RF field, in order to reconstruct the electrical properties image. The magnetic field is represented by an integral equation considering each point as a contrast source. This equation can be solved in an inverse method. In this study we use a reference simulation or scout scan of a uniform phantom to provide an initial estimate for the inverse solution, which allows the estimation of the complex permittivity within a single iteration. Incorporating two setups with and without the HPM improves the reconstructed result, especially with respect to the very low electric field in the center of the sample. Electromagnetic simulations of the brain were performed at 3 T to generate the B(1)(+)field maps and reconstruct the electric properties images. The standard deviations of the relative permittivity and conductivity were within 14% and 18%, respectively for a volume consisting of white matter, gray matter and cerebellum. (C) 2015 Elsevier Inc. All rights reserved.
2015
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(2015) JOURNAL OF MAGNETIC RESONANCE. 259, p. 199-206 Abstract
Given their high sensitivity and ability to limit the field of view (FOV), surface coils are often used in magnetic resonance spectroscopy (MRS) and imaging (MRI). A major downside of surface coils is their inherent radiofrequency (RF) B1 heterogeneity across the FOV, decreasing with increasing distance from the coil and giving rise to image distortions due to non-uniform spatial responses. A robust way to compensate for B1 inhomogeneities is to employ adiabatic inversion pulses, yet these are not well adapted to all imaging sequences - including to single-shot approaches like echo planar imaging (EPI). Hybrid spatiotemporal encoding (SPEN) sequences relying on frequency-swept pulses provide another ultrafast MRI alternative, that could help solve this problem thanks to their built-in heterogeneous spatial manipulations. This study explores how this intrinsic SPEN-based spatial discrimination, could be used to compensate for the B1 inhomogeneities inherent to surface coils. Experiments carried out in both phantoms and in vivo rat brains demonstrate that, by suitably modulating the amplitude of a SPEN chirp pulse that progressively excites the spins in a direction normal to the coil, it is possible to compensate for the RF transmit inhomogeneities and thus improve sensitivity and image fidelity.
2014
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(2014) Magnetic Resonance in Medicine. 72, 6, p. 1687-1695 Abstract
Purpose: Ultrafast sequences based on "Hybrid" spatiotemporal encoding (SPEN) replace echo-planar imaging's phase encoding "blips," while retaining a k-space readout acquisition. Hardware imperfections during acquisition may lead to ghosts and striped artifacts along the SPEN dimension; akin to echo-planar imaging's Nyquist ghosts, but weaker. A referenceless method to eliminate these artifacts in Hybrid SPEN is demonstrated.Theory and Methods: Owing to its encoding in direct space, rather than reciprocal space, undersampling in SPEN does not generate an echo-planar-imaging-like aliasing, but instead lowers the spatial resolution. Hybrid SPEN data can be split into two undersampled signals: a reference one comprised of the odd-echos, and an even-echo set that has to be "corrected" for consistency with the former. A simple way of implementing such a correction that enables a joint high-resolution reconstruction is proposed.Results: The referenceless algorithm is demonstrated with various examples, including oblique scans, large in vivo datasets from real-time dynamic contrast-enhanced perfusion experiments, and human brain imaging.Conclusions: The referenceless correction enables robust single-scan imaging under changing conditions-such as patient motion and changes in shimming over time-without the need of ancillary navigators. This opens new options for real-time MRI and interactive scanning.
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(2014) JOURNAL OF MAGNETIC RESONANCE. 240, p. 8-15 Abstract
Hyperpolarized metabolic imaging is a growing field that has provided a new tool for analyzing metabolism, particularly in cancer. Given the short life times of the hyperpolarized signal, fast and effective spectroscopic imaging methods compatible with dynamic metabolic characterizations are necessary. Several approaches have been customized for hyperpolarized 13C MRI, including CSI with a center-out k-space encoding, EPSI, and spectrally selective pulses in combination with spiral EPI acquisitions. Recent studies have described the potential of single-shot alternatives based on spatiotemporal encoding (SPEN) principles, to derive chemical-shift images within a sub-second period. By contrast to EPSI, SPEN does not require oscillating acquisition gradients to deliver chemical-shift information: its signal encodes both spatial as well as chemical shift information, at no extra cost in experimental complexity. SPEN MRI sequences with slice-selection and arbitrary excitation pulses can also be devised, endowing SPEN with the potential to deliver single-shot multi-slice chemical shift images, with a temporal resolution required for hyperpolarized dynamic metabolic imaging. The present work demonstrates this with initial in vivo results obtained from SPEN-based imaging of pyruvate and its metabolic products, after injection of hyperpolarized [1-13C]pyruvate. Multi-slice chemical-shift images of healthy rats were obtained at 4.7 T in the region of the kidney, and 4D (2D spatial, 1D spectral, 1D temporal) data sets were obtained at 7 T from a murine lymphoma tumor model.
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(2014) Magnetic Resonance in Medicine. 71, 2, p. 711-722 Abstract
Purpose Single-scan multislice acquisition schemes play key roles in magnetic resonance imaging. Central among these "ultrafast" experiments stands echo-planar imaging, a technique that although of optimal sampling is challenged by T2* artifacts. Recent studies described alternatives based on spatiotemporal encoding (SPEN), which are particularly robust if implemented in a "full-refocusing" mode. This work extends this modality from the single-slice acquisitions in which it has hitherto been implemented, by introducing a variety of multislice schemes scanning 3D volumes. Methods Multislice SPEN employing either inversion or stimulated echo pulses and timed to fulfill the demands of full refocusing, are demonstrated. The performance of the ensuing methods was examined in "Hybrid" modalities encoding data in k- and direct-space, in low-specific absorption rate stimulated-echo approaches, and in direct-space SPEN approaches. Results When applied in phantoms and in in vivo systems, the ensuing single-shot sequences evidenced similar robustness, sensitivity, and resolution qualities as previously discussed 2D single-slice schemes, while enabling a rapid sampling of the third dimension via multislicing. Conclusion The unique benefits deriving from fully refocused, multislice, single-scan SPEN sequences were corroborated by phantom tests, as well as by in vivo scans at 3 and 7 T. Low specific absorption rate multislice SPEN variants compatible with human studies were demonstrated.
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(2014) Magnetic Resonance in Medicine. 71, 2, p. 746-755 Abstract
Purpose To introduce a method that provides simultaneous spatial and spectral selectivity, whose implementation is less demanding than - and quality comparable to - conventional 2D spectral-spatial counterparts. Theory Spatiotemporal encoding concepts lead to a spatially selective, chemical-shift-dependent echo, with simultaneous dephasing of all other off-resonant species. The approach only requires applying a pair of suitable radiofrequency-swept pulses, and allows arbitrary shaping of the spatial profiles. Methods Based on arguments derived for chirp pulses operating in the sequential-sweep approximation, quadratic-phase SLR excitation and refocusing waveforms were designed and used to collect 2D slice- and shift-selective images on a 7 T microimaging system (phantoms). The same strategy was used to obtain multi-slice echo-planar and spin-echo images of breast on human volunteers in a 3 T scanner. Results The method managed to deliver excellent shift-selective multi-slice images in phantoms and human volunteers. Simultaneous water and fat images were also collected in a single, interleaved acquisition mode on both platforms, using straightforward sequence and reconstruction modifications of the basic scheme. Conclusion A new way to achieve chemical shift selectivity with high quality spatial profiling is achieved, without the usual requirements for playing out fast oscillating gradients in conjunction with carefully timed radiofrequency pulses.
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(2014) Magnetic Resonance Imaging. 32, 1, p. 60-70 Abstract
Recent studies described an "ultrafast" scanning method based on spatiotemporal (SPEN) principles. SPEN demonstrates numerous potential advantages over EPI-based alternatives, at no additional expense in experimental complexity. An important aspect that SPEN still needs to achieve for providing a competitive ultrafast MRI acquisition alternative, entails exploiting parallel imaging algorithms without compromising its proven capabilities. The present work introduces a combination of multi-band frequency-swept pulses simultaneously encoding multiple, partial fields-of-view, together with a new algorithm merging a Super-Resolved SPEN image reconstruction and SENSE multiple-receiving methods. This approach enables one to reduce both the excitation and acquisition times of sub-second SPEN acquisitions by the customary acceleration factor R, without compromises in either the method's spatial resolution, SAR deposition, or capability to operate in multi-slice mode. The performance of these new single-shot imaging sequences and their ancillary algorithms were explored and corroborated on phantoms and human volunteers at 3T. The gains of the parallelized approach were particularly evident when dealing with heterogeneous systems subject to major T2/T2* effects, as is the case upon single-scan imaging near tissue/air interfaces.
2013
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(2013) Magnetic Resonance Materials In Physics Biology And Medicine. 26, 5, p. 477-490 Abstract
Objective: Recent years have seen an increased interest in combining MRI thermometry with devices capable of destroying malignancies by heat ablation. Expected from the MR protocols are accurate and fast thermal characterizations, providing real time feedback on restricted tissue volumes and/or rapidly moving organs like liver. This article explores the potential advantages of relying on spatiotemporally encoded (SPEN) sequences for retrieving real-time thermometric images based on the water's proton resonance frequency (PRF) shifts. Materials and methods: Hybrid spatiotemporal/k-space encoding single-scan MRI experiments were implemented on animal and human scanners, and their abilities to deliver single- and multi-slice real-time thermometric measurements based on PRF-derived phase maps in phantoms and in vivo, were compared against echo planar imaging (EPI) and gradient-echo counterparts. Results: Under comparable acquisition conditions, SPEN exhibited advantages vis-à-vis EPI in terms of dealing with inhomogeneous magnetic field distortions, with shifts arising due to changes in the central frequency offsets, with PRF distributions, and for zooming into restricted fields-of-view without special pulse sequence provisions. Conclusion: This work confirms the ability of SPEN sequences, particularly when implemented under fully-refocused conditions, to exploit their built-in robustness to shift- and field-derived inhomogeneities for monitoring thermal changes in real-time under in vitro and in vivo conditions.
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(2013) Magnetic Resonance in Medicine. 70, 2, p. 382-391 Abstract
A novel method for acquiring and processing quality multislice spectroscopically resolved 2D images in a single shot is introduced and illustrated. By contrast to the majority of single-scan spectroscopic imaging sequences developed so far, the method here discussed is not based on the acquisition of echo planar data in the k/t-space, but rather on the use of recently proposed spatiotemporal encoding methods. These techniques provide a robust alternative to classical techniques, as they can scan two spatial plus one spectral dimension by oscillating a single imaging gradient. This work demonstrates that the use of extended spectral/spatial super-resolution algorithms coupled to new experimental spatiotemporal encoding formulations based on swept inversions rather than on chirped excitations can lead to novel spatiotemporal encoding-based tools for resolving complex multisliced 2D images according to the chemical shifts in subsecond experiments. A number of phantom-based models were explored to clarify the relative merits of this technique and estimate its sensitivity performance. In vivo results of fat and water separation on abdominal imaging of mice at 7 T and on human breast imaging at 3 T are presented.
2006
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(2006) European Journal of Radiology. 59, 2, p. 190-197 Abstract
The purpose of this study: To evaluate the safety and efficacy of an enhanced magnetic resonance-guided focused ultrasound (MRgFUS) emission protocol that results in more extensive treatment by increasing the volume of each focal ablation using the same energy.Materials and methods: Six pigs were treated with an MRgFUS system combined with real-time MR, for imaging and temperature mapping, with 102 "enhanced" and 97 "regular" focal ablations performed on both buttock muscles. Real-time imaging, temperature mapping, and acoustic reflected spectrum data enabled immediate evaluation of the results. MR contrast-enhanced images and pathology examinations were used for confirmation.Results: The location of the ablated volume by "enhanced" sonication is predictable, with a maximum possible shift of 6 mm toward, and 3 mm away, from the transducer. The ablated volume after enhanced sonication was, on average, 1.8 times larger than after a regular sonication of the same energy. Pathology results showed the same thermally induced damage patterns in the enhanced sonications and the regular sonications.Conclusion: Accelerated MRgFUS with enhanced sonication is a safe, controllable, and more effective tissue ablative modality than standard sonication. This new technology may significantly reduce the length of tumor ablation procedures. (Isn't the new technology you're talking about MRgFUS? If so, you don't need to repeat it at the end of this sentence.) (c) 2006 Elsevier Ireland Ltd. All rights reserved.