SPECIAL semi‐LASER with lipid artifact compensation for 1H MRS at 7 T

The measurement of full metabolic profiles at ultrahigh fields including low concentrated or fast‐relaxing metabolites is usually achieved by applying short echo time sequences. One sequence beside stimulated echo acquisition mode that was proposed in this regard is spin echo full intensity‐acquired localized spectroscopy. Typical problems that are still persistent for spin echo full intensity‐acquired localized spectroscopy are B₁ inhomogeneities especially for signal acquisition with surface coils and chemical shift displacement artifacts due to limited B₁ amplitudes when using volume coils. In addition, strong lipid contaminations in the final spectrum can occur when only a limited number of outer volume suppression pulses is used. Therefore, an adiabatic short echo time (= 19 ms) spin echo full intensity‐acquired localized spectroscopy semilocalization by adiabatic selective refocusing sequence is presented that is less sensitive to strong B₁ variations and that offers increased excitation and refocusing pulse bandwidths than regular spin echo full intensity acquired localized spectroscopy. Furthermore, the existence of the systematic lipid artifact is identified and linked to unfavorable effects due to the preinversion localization pulse. A method to control this artifact is presented and validated in both phantom and in vivo measurements. The viability of the proposed sequence was further assessed for in vivo measurements by scanning 17 volunteers using a surface coil and moreover acquiring additional volume coil measurements. The results show well‐suppressed lipid artifacts, good signal‐to‐noise ratio, and reproducible fitting results in accordance with other published studies. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A 32‐channel lattice transmission line array for parallel transmit and receive MRI at 7 tesla

Transmit and receive RF coil arrays have proven to be particularly beneficial for ultra‐high‐field MR. Transmit coil arrays enable such techniques as B₁⁺ shimming to substantially improve transmit B₁ homogeneity compared to conventional volume coil designs, and receive coil arrays offer enhanced parallel imaging performance and SNR. Concentric coil arrangements hold promise for developing transceiver arrays incorporating large numbers of coil elements. At magnetic field strengths of 7 tesla and higher where the Larmor frequencies of interest can exceed 300 MHz, the coil array design must also overcome the problem of the coil conductor length approaching the RF wavelength. In this study, a novel concentric arrangement of resonance elements built from capacitively‐shortened half‐wavelength transmission lines is presented. This approach was utilized to construct an array with whole‐brain coverage using 16 transceiver elements and 16 receive‐only elements, resulting in a coil with a total of 16 transmit and 32 receive channels. Magn Reson Med 63:1478–1485, 2010. © 2010 Wiley‐Liss, Inc.     

Eigenmode analysis of transmit coil array for tailored B1 mapping

In vivo radiofrequency (RF) field B₁ mapping represents an essential prerequisite for parallel transmit applications. However, the large dynamic range of the transmit fields of the individual coil elements challenges the accuracy of MR‐based B₁ mapping techniques. In the present work, the B₁ mapping error and its impact on the RF performance are studied based on a coil eigenmode analysis. Furthermore, the linear properties of the transmit chain are exploited to virtually adjust the weighting of the different coil eigenmodes in the B₁ mapping procedure, resulting in considerably reduced mapping errors. In addition, the weighting of the eigenmodes is tailored to potential target applications, e.g., specific absorption rate (SAR) reduced RF shimming or multidimensional RF pulses, resulting in improved RF performance. The basic theoretic principles of the concept are elaborated and validated by corresponding simulations. Furthermore, results on B₁ mapping and RF shimming experiments, performed on phantoms and in vivo using a 3‐T scanner equipped with an eight‐channel transmit/receive body coil, are presented to prove the feasibility of the approach. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Reduction of transmitter B1 inhomogeneity with transmit SENSE slice‐select pulses

Parallel transmitter techniques are a promising approach for reducing transmitter B₁ inhomogeneity due to the potential for adjusting the spatial excitation profile with independent RF pulses. These techniques may be further improved with transmit sensitivity encoding (SENSE) methods because the sensitivity information in pulse design provides an excitation that is inherently compensated for transmitter B₁ inhomogeneity. This paper presents a proof of this concept using transmit SENSE 3D tailored RF pulses designed for small flip angles. An eight‐channel receiver coil was used to mimic parallel transmission for brain imaging at 3T. The transmit SENSE pulses were based on the fast‐k_z design and produced 5‐mm‐thick slices at a flip angle of 30° with only a 4.3‐ms pulse length. It was found that the transmit SENSE pulses produced more homogeneous images than those obtained from the complex sum of images from all receivers excited with a standard RF pulse. Magn Reson Med 57:842–847, 2007. © 2007 Wiley‐Liss, Inc.     

Transmit B1‐field correction at 7T using actively tuned coupled inner elements

When volume coils are used for ¹H imaging of the human head at 7T, wavelength effects in tissue cause a variation in intensity, that is typically brighter at the center of the head and darker in the periphery. Much of this image nonuniformity can be attributed to variation in the effective transmit B₁ field, which falls by ～ 50% to the left and right of center at mid‐elevation in the brain. Because most of this B₁ loss occurs in the periphery of the brain, we have explored use of actively controlled, off‐resonant loop elements to locally enhance the transmit B₁ field in these regions. When tuned to frequencies above the NMR frequency, these elements provide strong local enhancement of the B₁ field of the transmit coil. Because they are tuned off‐resonance, some volume coil detuning results, but resistive loading of the coil mode remains dominated by the sample. By digitally controlling their frequency offsets, the field enhancement of each inner element can be placed under active control. Using an array of eight digitally controlled elements placed around a custom‐built head phantom, we demonstrate the feasibility of improving the B₁ homogeneity of a transmit/receive volume coil without the need for multiple radio frequency transmit channels. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

MRI using radiofrequency magnetic field phase gradients

Conventionally, MR images are formed by applying gradients to the main static magnetic field (B₀). However, the B₀ gradient equipment is expensive, power‐hungry, complex, and noisy and can induce eddy currents in nearby conducting structures, including the patient. Here, we describe a new silent, B₀ gradient‐free MRI principle, Transmit Array Spatial Encoding (TRASE), based on phase gradients of the radio‐frequency (RF) field. RF phase gradients offer a new method of k‐space traversal. Echo trains using at least two different RF phase gradients allow spin phase to accumulate, causing k‐space traversal. Two such RF fields provide one‐dimensional imaging and three are sufficient for two‐dimensional imaging. Since TRASE is a k‐space method, analogues of many conventional pulse sequences are possible. Experimental results demonstrate one‐dimensional and two‐dimensional RF MRI and slice selection using a single‐channel, transmit/receive, 0.2 T, permanent magnet, human MR system. The experimentally demonstrated spatial resolution is much higher than that provided by RF receive coil array sensitivity encoding alone but lower than generally achievable with B₀ gradients. Potential applications are those in which one or more of the features of simplified equipment, lower costs, silent MRI, or the different physics of the image formation process are particularly advantageous. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

“In situ” observation of guanine radicals induced by ultrasoft X‐ray irradiation around the K‐edge regions of nitrogen and oxygen

Purpose: In order to understand the molecular mechanism of nucleobase damage caused by ultrasoft X‐ray irradiation, guanine radicals have been studied using an X‐band EPR (electron paramagnetic resonance) spectrometer installed in a synchrotron soft X‐ray beamline.

Materials and Methods: Guanine pellets were irradiated under vacuum with ultrasoft X‐rays obtained from a soft X‐ray beamline (BL23SU) in SPring‐8. The energy regions around the nitrogen (0.4?keV) and oxygen (0.5?keV) K‐edges were chosen for the irradiation. The ultrasoft X‐ray irradiation and EPR measurements were carried out simultaneously at low temperature, 20?K and 77?K.

Results: The EPR spectrum observed during irradiation was clearly distinguishable from that of the stable radical, which still exists after exposure to ultrasoft X‐rays at 77?K. The spectrum of the short‐lived radicals consisted of two components, which exhibited different EPR microwave power saturation. The EPR signal intensities increased linearly with increasing dose rate (photon flux density). These signals immediately disappeared when the beam was turned off, even when irradiated at lower temperature (20?K). At the energy of the oxygen K‐resonance excitation (539?eV) the signal intensity was clearly increased to more than five times that obtained on the lower energy side (526?eV). On the other hand, the enhancement was insignificant above and below the nitrogen K‐edge (401?eV). The singlet EPR signal of the stable radical was similar to that reported previously in the literature for γ‐irradiated guanine.

Conclusions: The short‐lived radical species observed were mainly induced as a result of the final state of the resonant Auger process on oxygen atoms existing solely in the carbonyl group in guanine. Auger events at the other atoms in guanine (namely, carbon and nitrogen) do not induce this radical process to any great extent, even though the abundance of these atoms (i.e. the sum of their photoabsorption cross sections) is dominant in the guanine molecule.     

Improved homogeneity of the transmit field by simultaneous transmission with phased array and volume coil

Purpose:

To improve the homogeneity of transmit volume coils at high magnetic fields (≥4 T). Due to radiofrequency (RF) field/tissue interactions at high fields, 4 T to 8 T, the transmit profile from head‐sized volume coils shows a distinctive pattern with relatively strong RF magnetic field B₁ in the center of the brain.

Materials and Methods:

In contrast to conventional volume coils at high field strengths, surface coil phased arrays can provide increased RF field strength peripherally. In theory, simultaneous transmission from these two devices could produce a more homogeneous transmission field. To minimize interactions between the phased array and the volume coil, counter rotating current (CRC) surface coils consisting of two parallel rings carrying opposite currents were used for the phased array.

Results:

Numerical simulations and experimental data demonstrate that substantial improvements in transmit field homogeneity can be obtained.

Conclusion:

We have demonstrated the feasibility of using simultaneous transmission with human head‐sized volume coils and CRC phased arrays to improve homogeneity of the transmit RF B₁ field for high‐field MRI systems. J. Magn. Reson. Imaging 2010;32:476–481. © 2010 Wiley‐Liss, Inc.     

In vivo imaging of a stable paramagnetic probe by pulsed-radiofrequency electron paramagnetic resonance spectroscopy

Imaging of free radicals by electron paramagnetic resonance (EPR) spectroscopy using time domain acquisition as in nuclear magnetic resonance (NMR) has not been attempted because of the short spin-spin relaxation times, typically under 1 μs, of most biologically relevant paramagnetic species. Recent advances in radiofrequency (RF) electronics have enabled the generation of pulses of the order of 10–50 ns. Such short pulses provide adequate spectral coverage for EPR studies at 300 MHz resonant frequency. Acquisition of free induction decays (FID) of paramagnetic species possessing inhomogenously broadened narrow lines after pulsed excitation is feasible with an appropriate digitizer/averager. This report describes the use of time-domain RF EPR spectrometry and imaging for in vivo applications. FID responses were collected from a water-soluble, narrow line width spin probe within phantom samples in solution and also when infused intravenously in an anesthetized mouse. Using static magnetic field gradients and back-projection methods of image reconstruction, two-dimensional images of the spin-probe distribution were obtained in phantom samples as well as in a mouse. The resolution in the images was better than 0.7 mm and devoid of motional artifacts in the in vivo study. Results from this study suggest a potential use for pulsed RF EPR imaging (EPRI) for three-dimensional spatial and spectral-spatial imaging applications. In particular, pulsed EPRI may find use in in vivo studies to minimize motional artifacts from cardiac and lung motion that cause significant problems in frequency-domain spectral acquisition, such as in continuous wave (cw) EPR techniques.     

DREAM-a novel approach for robust, ultrafast, multislice B(1) mapping

A novel multislice B₁‐mapping method dubbed dual refocusing echo acquisition mode is proposed, able to cover the whole transmit coil volume in only one second, which is more than an order of magnitude faster than established approaches. The dual refocusing echo acquisition mode technique employs a stimulated echo acquisition mode (STEAM) preparation sequence followed by a tailored single‐shot gradient echo sequence, measuring simultaneously the stimulated echo and the free induction decay as gradient‐recalled echoes, and determining the actual flip angle of the STEAM preparation radiofrequency pulses from the ratio of the two measured signals. Due to an elaborated timing scheme, the method is insensitive against susceptibility/chemical shift effects and can deliver a B₀ phase map and a transceive phase map for free. The approach has only a weak T₁ and T₂ dependence and moreover, causes only a low specific absorption rate (SAR) burden. The accuracy of the method with respect to systematic and statistical errors is investigated both, theoretically and in experiments on phantoms. In addition, the performance of the approach is demonstrated in vivo in B₁‐mapping and radiofrequency shimming experiments on the abdomen, the legs, and the head on an eight‐channel parallel transmit 3 T MRI system. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Chemical shift encoded water–fat separation using parallel imaging and compressed sensing

Chemical shift encoded techniques have received considerable attention recently because they can reliably separate water and fat in the presence of off‐resonance. The insensitivity to off‐resonance requires that data be acquired at multiple echo times, which increases the scan time as compared to a single echo acquisition. The increased scan time often requires that a compromise be made between the spatial resolution, the volume coverage, and the tolerance to artifacts from subject motion. This work describes a combined parallel imaging and compressed sensing approach for accelerated water–fat separation. In addition, the use of multiscale cubic B‐splines for B₀ field map estimation is introduced. The water and fat images and the B₀ field map are estimated via an alternating minimization. Coil sensitivity information is derived from a calculated k‐space convolution kernel and l₁‐regularization is imposed on the coil‐combined water and fat image estimates. Uniform water–fat separation is demonstrated from retrospectively undersampled data in the liver, brachial plexus, ankle, and knee as well as from a prospectively undersampled acquisition of the knee at 8.6x acceleration. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Development of a PEDRI free-radical imager using a 0.38 T clinical MRI system.

Proton electron double resonance imaging (PEDRI) uses the Overhauser effect to image the distribution of free-radicals in biological samples and animals. Standard MRI hardware and software is used, with the addition of hardware to irradiate the free-radical-of-interest's EPR resonance. For in vivo applications it must be implemented at a sufficiently low magnetic field to result in an EPR irradiation frequency that will penetrate the sample but will not cause excessive nonresonant power deposition therein. Many clinical MRI systems use resistive magnets that are capable of operating at 10-20 mT, and which could thus be used as PEDRI imagers with the addition of a small amount of extra hardware. This article describes the conversion of a 0.38 T whole-body MRI system for operation as a 20.1 mT small-animal PEDRI imager. The magnet power supply control electronics required a small modification to operate at the lower field strength, but no permanent hardware changes to the MRI console were necessary, and no software modification was required. Frequency down- and up-conversion was used on the NMR RF system, together with a new NMR/EPR dual-resonance RF coil assembly. The system was tested on phantoms containing free-radical solution, and was also used to image the distribution of a free-radical contrast agent injected intravenously into anesthetized mice.     

Computational and experimental optimization of a double‐tuned 1H/31P four‐ring birdcage head coil for MRS at 3T

Purpose

To optimize the homogeneity and efficiency of the B₁ magnetic field of a four‐ring birdcage head coil that is double‐tuned at the Larmor frequencies of both ³¹P and ¹H and optimized to acquire magnetic resonance spectroscopy (MRS) data at 3T for the study of infants.

Materials and Methods

We developed a finite difference time domain (FDTD) tool in‐house to iteratively compute and seek the range of geometric and electromagnetic parameters of a dual‐tuned, four‐ring birdcage coil that would produce the desired resonance patterns, optimize homogeneity of the B₁‐field, and maximize efficiency of the coil. To demonstrate the validity of our computational results, we constructed three RF coils: one dual‐tuned coil that was based on the calculated optimized parameters and two single‐tuned coils that had dimensions similar to those of the dual‐tuned coil, but tuned at the Larmor frequencies of both ³¹P and ¹H, respectively. We then tested and compared the performances of the dual‐tuned coil and single‐tuned coils at both of these frequencies.

Results

We found that a dual‐tuned, four‐ring birdcage coil with a diameter of 180 mm, an inner birdcage length of 100–300 mm, and an outer birdcage length of 25–100 mm produces the desired resonance patterns. For the use of this coil with human infants, optimization of the homogeneity of the B₁ field, combined with improved coil efficiency, yielded a dual‐tuned birdcage coil with diameter of 180mm, an inner birdcage length of 150 mm, an outer birdcage length of 25 mm, and corresponding inner and outer capacitances of 17.2 pF and 7.6 pF, respectively. The experimental results from a constructed coil having the sameparameters with the modeled coil agreed well with the computational results from the modeled coil. This optimized design overcame the deficiencies of existing dual‐tuned, four‐ring birdcage coils.

Conclusion

The homogeneity and efficiency of the B₁ field for ³¹P/¹H dual‐tuned, four‐ring birdcage coils can be optimized well using our FDTD tool, especially at high static magnetic fields (B₀). J. Magn. Reson. Imaging 2009;29:13–22. © 2008 Wiley‐Liss, Inc.     

Proton-decoupled, Overhauser-enhanced, spatially localized carbon-13 spectroscopy in humans

Spatially localized, natural abundance, carbon (13C) NMR spectroscopy has been combined with proton (1H) decoupling and nuclear Overhauser enhancement to improve 13C sensitivity up to five-fold in the human leg, liver, and heart. Broadhand-decoupled 13C spectra were acquired in 1 s to 17 min with a conventional 1.5-T imaging/spectroscopy system, an auxiliary 1H decoupler, an air-cooled dual-coil coplanar surface probe, and both depth-resolved surface coil spectroscopy (DRESS) and one-dimensional phase-encoding gradient NMR pulse sequences. The surface coil probe comprised circular and figure-eight-shaped coils to eliminate problems with mutual coupling of coils at high decoupling power levels applied during 13C reception. Peak decoupler RF power deposition in tissue was computed numerically from electromagnetic theory assuming a semi-infinite plane of uniform biological conductor. Peak values at the surface were calculated at 4 to 6 W/kg in any gram of tissue for each watt of decoupler power input excluding all coil and cable losses, warning of potential local RF heating problems in these and related experiments. The average power deposition was about 9 mW/kg per watt input, which should present no systemic hazard. At 3 W input, human 13C spectra were decoupled to a depth of about 5 cm while some Overhauser enhancement was sustained up to about 3 cm depth, without ill effect. The observation of glycogen in localized natural abundance 13C spectra of heart and muscle suggests that metabolites in the citric acid cycle should be observable noninvasively using 13C-labeled substrates.     

Transverse relaxation time (T2) mapping in the brain with off‐resonance correction using phase‐cycled steady‐state free precession imaging

Purpose

To investigate a new approach for more completely accounting for off‐resonance affects in the DESPOT2 (driven equilibrium single pulse observation of T₂) mapping technique.

Materials and Methods

The DESPOT2 method derives T₂ information from fully balanced steady‐state free precession (bSSFP) images acquired over multiple flip angles. Off‐resonance affects, which present as bands of altered signal intensity throughout the bSSFP images, results in erroneous T₂ values in the corresponding calculated maps. Radiofrequency (RF) phase‐cycling, in which the phase of the RF pulse is incremented along the pulse train, offers a potential method for eliminating these artifacts. In this work we present a general method, referred to as DESPOT2, with full modeling (DESPOT2‐FM), for deriving T₂, as well as off‐resonance frequency, from dual flip angle bSSFP data acquired with two RF phase increments.

Results

The method is demonstrated in vivo through the acquisition of whole‐brain, 1 mm³ isotropic T₂ maps at 3T and shown to provide near artifact‐free maps, even in areas with steep susceptibility‐induced gradients.

Conclusion

DESPOT2‐FM offers an efficient method for acquiring high spatial resolution, whole‐brain T₂ maps at 3T with high precision and free of artifact. J. Magn. Reson. Imaging 2009;30:411–417. © 2009 Wiley‐Liss, Inc.     

A multispectral three‐dimensional acquisition technique for imaging near metal implants

Metallic implants used in bone and joint arthroplasty induce severe spatial perturbations to the B₀ magnetic field used for high‐field clinical magnetic resonance. These perturbations distort slice‐selection and frequency encoding processes applied in conventional two‐dimensional MRI techniques and hinder the diagnosis of complications from arthroplasty. Here, a method is presented whereby multiple three‐dimensional fast‐spin‐echo images are collected using discrete offsets in RF transmission and reception frequency. It is demonstrated that this multi acquisition variable‐resonance image combination technique can be used to generate a composite image that is devoid of slice‐plane distortion and possesses greatly reduced distortions in the readout direction, even in the immediate vicinity of metallic implants. Magn Reson Med 61:381–390, 2009. © 2009 Wiley‐Liss, Inc.     

Estimation of bloch model MT spin system parameters from Z-spectral data

Previous studies have described magnetization transfer (MT) Z-spectra in terms of a two-pool Bloch model, with six spin-system parameters K_A, F, T_1A, T_1B, and T_2B. By simulation, we show that a process including nonlinear constrained optimization can be used to accurately and uniquely estimate spin-system parameters from MT 2-spectra prepared by continuous wave (CW) RF irradiation. Experiments producing Z-spectra by pulsed RF irradiation give substantially different magnetization values, relative to MT acquisitions obtained by CW RF irradiation, at small offset frequencies, with a consequence that only T_2B can be uniquely determined. However, several equalities and bounds involving four of the other parameters (K_A, F, T_1A, T_1B) are derived, which are applicable to pulsed data. These relationships allow calculation of “free pool” magnetization corresponding to complete saturation of the restricted pool, without requiring that this complete saturation be experimentally achieved. MT experimental data from pulsed RF irradiation on boiled egg albumin, obtained using a clinical whole-body MRI system, are analyzed using an optimization algorithm and the derived expressions.     

Surface coil cardiac tagging and 31P spectroscopic localization with B1-insensitive adiabatic pulses

A technique is presented for MRI tagging in the presence of inhomogeneous B₁ fields. A rectangular tagging grid is produced with B₁-insensitive adiabatic pulses in a magnetization preparation period that precedes image acquisition. Phantom results demonstrate that the method is well-suited to surface coil experiments. The technique is applied to a canine model of myocardial ischemia to track the spatially dependent wall motion of the left ventricle during the cardiac cycle. Transmural ³¹P spectra are acquired from the same double-tuned surface coil, with tagging and spectroscopy performed for the first time, during normal, ischemic, and recovery conditions for the same animal.     

Fast fat suppression RF pulse train with insensitivity to B1 inhomogeneity for body imaging

In higher‐field magnetic resonance imaging scanners, a spectrally selective fat saturation radiofrequency (RF) pulse does not work well because B₁ inhomogeneity increases. An adiabatic 180° pulse is used to improve nonuniform fat suppression, but requires inversion recovery time. Therefore, a new RF pulse that achieves flip angles near 90° and is B₁ insensitive has been developed. The pulse consists of three sinc‐shaped RF pulses with different flip angles and with different time intervals between each RF pulse. Using the Bloch equations, we analyzed the optimal combination of flip angles. Experimental results demonstrated that M_z was maintained at less than 0.05 M₀ for a B₁ inhomogeneity of ±35%. The optimal net flip angles was adjusted to 95° by varying the time interval between RF pulses. The pulse duration was 77 ms, which is less than half of the 170‐ms inversion recovery time required for the adiabatic pulse. We demonstrated excellent fat suppression for body imaging. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.