Pulsed Gradient Research Articles

The Rate of Apparent Diffusion Coefficient Change With Diffusion Time on Breast Diffusion-Weighted Imaging Depends on Breast Tumor Types and Molecular Prognostic Biomarker Expression.

The aim of this study was to investigate the variation of apparent diffusion coefficient (ADC) values with diffusion time according to breast tumor type and prognostic biomarkers expression. A total of 201 patients with known or suspected breast tumors were prospectively enrolled in this study, and 132 breast tumors (86 malignant and 46 benign) were analyzed. Diffusion-weighted imaging scans with 2 diffusion times were acquired on a clinical 3-T magnetic resonance imaging scanner using oscillating and pulsed diffusion-encoding gradients (effective diffusion times, 4.7 and 96.6 milliseconds) and b values of 0 and 700 s/mm2. Diagnostic performances to differentiate malignant and benign breast tumors for ADC values at short and long diffusion times (ADCshort and ADClong), ΔADC (the rate of change in ADC values with diffusion time), ADC0-1000 (ADC value from a standard protocol), and standard reading including dynamic contrast-enhanced magnetic resonance imaging (BI-RADS) were investigated. The correlations of ADCshort, ADClong, and ΔADC values with hormone receptor expression and breast cancer subtypes were also analyzed. The ADC values were lower, and ΔADC was higher in malignant tumors compared with benign tumors. The specificity of ADC values at all diffusion times and ΔADC values for differentiating malignant and benign breast tumors was superior to that of BI-RADS (87.0%-95.7% vs 73.9%), whereas the sensitivity was inferior (87.2%-90.7% vs 100%). Lower ADCshort and ADC0-1000 in ER-positive compared with ER-negative cancers (false discovery rate [FDR]-adjusted P = 0.037 and 0.018, respectively) and lower ADCshort, ADClong, and ADC0-1000 in progesterone receptor-positive compared with progesterone receptor-negative cancers (FDR-adjusted P = 0.037, 0.036, and 0.018, respectively) were found. Ki-67-positive cancers had larger ΔADCs than Ki-67-negative cancers (FDR-adjusted P = 0.018). The ADC values vary with different diffusion time and vary in correlation with molecular biomarkers, especially Ki-67. Those results suggest that the diffusion time, which should be reported, might be a useful parameter to consider for breast cancer management.

Using NMR to Test Molecular Mobility during a Chemical Reaction.

We evaluate critically the use of pulsed gradient spin-echo nuclear magnetic resonance to measure molecular mobility during chemical reactions. With raw NMR spectra available in a public depository, we confirm the boosted mobility during the click chemical reaction (Wang et al. Science 2020 369, 537-541) regardless of the order of magnetic field gradient (linearly increasing, linearly decreasing, random sequence). We also confirm boosted mobility for the Diels-Alder chemical reaction. The conceptual advantage of the former chemical system is that a constant reaction rate implies a constant catalyst concentration, whereas that of the latter is the absence of a paramagnetic catalyst, precluding paramagnetism as an objection to the measurements. The data and discussion in this paper show the reliability of experiments when one avoids convection, allows decay of nuclear spin magnetization between successive pulses and recovery of its intensity between gradients, and satisfies quasi-steady state during the time window to acquire each datum. Especially important is to make comparisons on the time scale of the actual chemical reaction kinetics. We discuss possible sources of mistaken conclusions that are desirable to avoid.

Diffusion Tensor Imaging of Skeletal Muscle Contraction Using Oscillating Gradient Spin Echo.

Diffusion tensor imaging (DTI) measures water diffusion in skeletal muscle tissue and allows for muscle assessment in a broad range of neuromuscular diseases. However, current DTI measurements, typically performed using pulsed gradient spin echo (PGSE) diffusion encoding, are limited to the assessment of non-contracted musculature, therefore providing limited insight into muscle contraction mechanisms and contraction abnormalities. In this study, we propose the use of an oscillating gradient spin echo (OGSE) diffusion encoding strategy for DTI measurements to mitigate the effect of signal voids in contracted muscle and to obtain reliable diffusivity values. Two OGSE sequences with encoding frequencies of 25 and 50 Hz were tested in the lower leg of five healthy volunteers with relaxed musculature and during active dorsiflexion and plantarflexion, and compared with a conventional PGSE approach. A significant reduction of areas of signal voids using OGSE compared with PGSE was observed in the tibialis anterior for the scans obtained in active dorsiflexion and in the soleus during active plantarflexion. The use of PGSE sequences led to unrealistically elevated axial diffusivity values in the tibialis anterior during dorsiflexion and in the soleus during plantarflexion, while the corresponding values obtained using the OGSE sequences were significantly reduced. Similar findings were seen for radial diffusivity, with significantly higher diffusivity measured in plantarflexion in the soleus muscle using the PGSE sequence. Our preliminary results indicate that DTI with OGSE diffusion encoding is feasible in human musculature and allows to quantitatively assess diffusion properties in actively contracting skeletal muscle. OGSE holds great potential to assess microstructural changes occurring in the skeletal muscle during contraction, and for non-invasive assessment of contraction abnormalities in patients with muscle diseases.

Vibration analysis of functionally graded microbeam under initial stress via a generalized thermoelastic model with dual-phase lags

The aim of this article is to investigate the thermal and mechanical vibration properties of functionally graded microbeams. The governing system of equations is formulated on the basis of classical Euler–Bernoulli beam model incorporating the generalized dual-phase lag model of thermoelasticity rather than the conventional steady-state Fourier heat conduction. In our analysis, it is also assumed that the mechanical and thermal properties such as modulus of elasticity, density and coefficient of thermal conductivity change through the thickness by an exponential law distribution, with the exception of Poisson’s ratio. The effects of system parameters on different field quantities have been depicted for initial stress, material gradient index and pulses duration . The numerical results are compared with benchmark findings, for the sake of verification. Presented results demonstrate the considerable effects of temperature and the material gradients on the dynamic behavior of microscopic structures. Finally, a comparison is conducted with the results discussed in the literature to justify the quality of the present technique.

Frequency-swept pulses for ultrafast spatially encoded NMR

Ultrafast NMR based on spatial encoding yields arbitrary multidimensional spectra in a single scan. The dramatic acceleration afforded by spatial parallelisation makes it possible to capture transient species and processes, and has notably been applied to the monitoring of reactions and the analysis of hyperpolarised species. At the heart of ultrafast NMR lies the spatially sequential manipulation of nuclear spins. This is virtually always achieved by combining a swept radio-frequency pulse with a magnetic field gradient pulse. The dynamics of nuclear spins during these pulse sequence elements is key to understand and design ultrafast NMR experiments, and can often be described by surprisingly simple models. This article describes the spatial encoding of relaxation, chemical shift and diffusion in a common framework and discusses directions for future developments.

Hyperpolarized 13C Spectroscopy with Simple Slice-and-Frequency-Selective Excitation.

Hyperpolarized 13C nuclear magnetic resonance spectroscopy can characterize in vivo tissue metabolism, including preclinical models of cancer and inflammatory disease. Broad bandwidth radiofrequency excitation is often paired with free induction decay readout for spectral separation, but quantification of low-signal downstream metabolites using this method can be impeded by spectral peak overlap or when frequency separation of the detected peaks exceeds the excitation bandwidth. In this work, alternating frequency narrow bandwidth (250 Hz) slice-selective excitation was used for 13C spectroscopy at 7 T in a subcutaneous xenograft rat model of human pancreatic cancer (PSN1) to improve quantification while measuring the dynamics of injected hyperpolarized [1-13C]lactate and its metabolite [1-13C]pyruvate. This method does not require sophisticated pulse sequences or specialized radiofrequency and gradient pulses, but rather uses nominally spatially offset slices to produce alternating frequency excitation with simpler slice-selective radiofrequency pulses. Additionally, point-resolved spectroscopy was used to calibrate the 13C frequency from the thermal proton signal in the target region. This excitation scheme isolates the small [1-13C]pyruvate peak from the similar-magnitude tail of the much larger injected [1-13C]lactate peak, facilitates quantification of the [1-13C]pyruvate signal, simplifies data processing, and could be employed for other substrates and preclinical models.

The Influence of Deep Tissue Massage on Stresses Related Indexes of Middle-aged Women

The purpose of this study is to understand the effect of deep tissue massage on stress of middle-aged women. Deep tissue massage is performed centering on the upper and posterior cervical muscles of the body, and stress measurement using a device was measured three times in a unit of 30 minutes and 1 hour before the experiment. The results of the study are summarized as follows. The pulse gradient and the sympathetic activity were changed significantly by 30-minute massage but those were not changed significantly after 60 minutes massage. The average pulse was decreased significantly by deep tissue massage. The stress index increased from 49.270 before deep tissue massage to 54.900 after 30 minutes (p=0.005), and 52.270 after 60 minutes.

Influence of the number and type of functional groups on self-diffusion of some aromatic compounds in acetone: Nuclear magnetic resonance and molecular dynamics simulations

Self-diffusion of parabens (methylparaben and propylparaben), phenolic acids (syringic and gallic acids) and their derivative (protocatechualdehyde), hydroxycinnamic acids (caffeic, ferulic, sinapic, and p-coumaric acids), and tetramethylsilane in acetone has been investigated by the pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) method and molecular dynamics simulation in ambient conditions. The self-diffusion coefficients obtained by the experimental and theoretical methods are in good agreement with each other. A detailed analysis of the average number of solute – acetone hydrogen bonds has made it possible to trace the influence of the molecular heteroassociation on the self-diffusion of the aromatic compounds. The degree of influence of solute – acetone molecular association on the solute self-diffusion defined as the ratio of the solute self-diffusion coefficient to the self-diffusion coefficient of its monomer has been calculated using the Stokes-Einstein equation. It has been found that for a number of aromatic compounds with the same type and number of functional groups, the parameter expressing the degree of influence of molecular association has similar values. This fact has allowed us to calculate the self-diffusion coefficients of the compounds in the given series at a known self-diffusion coefficient value of one of the series members.

Radiofrequency encoded Only Parahydrogen SpectroscopY

A new pulse sequence aimed to filter out NMR signals coming from thermally polarized protons in PHIP experiments based on the OPSY pulse sequence (Only Parahydrogen SpectroscopY) is presented. In analogy to OPSY, which removes thermal polarization by using a pair of magnetic field gradient pulses with an intensity ratio 1:2 and equal duration, the same effect can be achieved using inhomogeneous radiofrequency fields. The spatial dependence of the radiofrequency field is used to control the Hamiltonian, which results in an effective suppression of thermal contributions in the NMR signal, while PHIP originated signals remain unmodified. A theoretical model for the radiofrequency encoded only parahydrogen (REOPSY) sequence is presented along with an experimental implementation on a birdcage coil in a 7 T magnetic field. The control level achieved by this strategy allows the inclusion of a long train of refocusing pulses. Therefore, the new sequence can be combined with the parahydrogen discriminated PHIP (PhD-PHIP) pulse sequence as a detection block to improve sensitivity and resolution in a single–scan experiment. Experiments with REOPSY and REOPSY+PhD-PHIP are presented in thermally and hyperpolarized samples.

Diffusion Spectrum of Polymer Melt Measured by Varying Magnetic Field Gradient Pulse Width in PGSE NMR.

The translational motion of polymers is a complex process and has a big impact on polymer structure and chemical reactivity. The process can be described by the segment velocity autocorrelation function or its diffusion spectrum, which exhibit several characteristic features depending on the observational time scale—from the Brownian delta function on a large time scale, to complex details in a very short range. Several stepwise, more-complex models of translational dynamics thus exist—from the Rouse regime over reptation motion to a combination of reptation and tube-Rouse motion. Accordingly, different methods of measurement are applicable, from neutron scattering for very short times to optical methods for very long times. In the intermediate regime, nuclear magnetic resonance (NMR) is applicable—for microseconds, relaxometry, and for milliseconds, diffusometry. We used a variation of the established diffusometric method of pulsed gradient spin-echo NMR to measure the diffusion spectrum of a linear polyethylene melt by varying the gradient pulse width. We were able to determine the characteristic relaxation time of the first mode of the tube-Rouse motion. This result is a deviation from a Rouse model of polymer chain displacement at the crossover from a square-root to linear time dependence, indicating a new long-term diffusion regime in which the dynamics of the tube are also described by the Rouse model.

A simple estimate of axon size with diffusion MRI

Noninvasive estimation of mean axon diameter presents a new opportunity to explore white matter plasticity, development, and pathology. Several diffusion-weighted MRI (DW-MRI) methods have been proposed to measure the average axon diameter in white matter, but they typically require many diffusion encoding measurements and complicated mathematical models to fit the signal to multiple tissue compartments, including intra- and extra-axonal spaces. Here, Monte Carlo simulations uncovered a straightforward DW-MRI metric of axon diameter: the change in radial apparent diffusion coefficient estimated at different effective diffusion times, ΔD⊥. Simulations indicated that this metric increases monotonically within a relevant range of effective mean axon diameter while being insensitive to changes in extra-axonal volume fraction, axon diameter distribution, g-ratio, and influence of myelin water. Also, a monotonic relationship was found to exist for signals coming from both intra- and extra-axonal compartments. The slope in ΔD⊥ with effective axon diameter increased with the difference in diffusion time of both oscillating and pulsed gradient diffusion sequences.

An integrated magnetic resonance plant imager for mobile use in greenhouse and field

In this contribution we demonstrate a mobile, integrated MR plant imager that can be handled by one single person and used in the field. Key to the construction of it was a small and lightweight gradient amplifier, specifically tailored to our combination of magnet, gradient coils and the requirements of the desired pulse sequences.To allow imaging of branches and stems, an open C-shaped permanent magnet was used. In the design of the magnet, pole gap width, low weight and robustness were prioritized over homogeneity and field strength. To overcome the adverse effects of short T2*, multi-spin echo imaging was employed, using short echo times and high spectral widths. To achieve microscopic resolution under these constraints requires fast switching field gradients, driven by strong and fast gradient amplifiers. While small-scale spectrometers and RF amplifiers are readily available, appropriate small-scale gradient amplifiers or designs thereof currently are not. We thus constructed a small, 3-channel gradient amplifier on the basis of a conventional current-controlled AB amplifier design, using cheap and well-known parts. The finished device weighs 5 kg and is capable of delivering 40 A gradient pulses of >6 ms in duration.With all components built onto an aluminum hand trolley, the imaging setup weighs 45 kg and is small enough to fit into a car. We demonstrate the mobility and utility of the device imaging quantitative water content and T2, first of an apple tree in an orchard; second, of a beech tree during spring leaf flushing in a greenhouse. The latter experiment ran for a continuous period of 62 days, acquiring more than 6000 images.

How Does the Solvent Composition Influence the Transport Properties of Electrolyte Solutions?

The electrolyte solutions used in lithium ion batteries have been optimized based on the empirical knowledge and concept. Yet the ion transport phenomena in the electrolyte solution, which composed of mixed solvent with salt concentration of about 1 mol kg−1, are complicated and significantly different from those in dilute solutions. In the present study, the characteristics of the electrolyte solutions are reanalyzed by using physical and spectroscopic data for the electrolyte solutions, which were obtained from our experiment, and by referring to recent theoretical studies 1,2) and the experimentally determined diffusion coefficient in previous studies 3,4).We experimentally measured the viscosity η and ionic conductivity σ of the electrolyte solutions of 1 mol kg−1 of LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSA) dissolved in the binary mixture solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in the temperature range of 288 ≤ T/K ≤ 328 with varying the EC content from 0 to 60 vol.%, which translates into the molar fraction of EC in 0 ≤ x EC ≤ 0.6 and the solute to solvent ratio constant (total ca. 11 solvent molecules are present per solute in the system). The diffusion coefficients D of each species, Li+, PF6, FSA−, EC and DMC, were determined by pulse gradient spin echo NMR. The state of molecules around Li+ was examined by the Raman spectra of the solvents and anions.With the increase in x EC, DMC solvating Li+ is replaced by EC, resulting in the increase in the total number of solvents around Li+. Reanalysis of Raman spectra based on Borodin's calculations 2) reassessed the solvation numbers and confirmed that the preferential coordination of EC to Li+, but the extent of the preference was not as strong as previously conceived. The addition of EC, more viscous fluid than DMC, into the system leads to the increase in η, which should be responsible for the monotonous decrease in D of all the species with x EC. On the other hand, σ initially increases until marking the maximum at x EC = ~0.3. The opposite trend between D and σ, i.e., dD/dx EC < 0 while dσ/dx EC > 0, at low x EC suggests that the Nernst-Einstein relation does not hold as it ignores the inter-ionic correlations. The analysis of the Stokes radii r St based on η and D obtained in the present study implies that the increase in x EC results in the decrease in r St for all the species, among which the anions are most influenced. Although only an equivocal picture on the cation-anion correlation, such as the ion pair formation, was obtained from the Raman spectroscopy, the present study is consistent with the view that the highly Li+-solvating EC, with its higher dielectric shielding effect than DMC, liberates the anions from the attraction from Li+, enhancing the cation-anion anti-correlation, which positively contributes to the ionic conductivity until the viscosity prevails at high x EC. The Einstein-Stokes formula and van der Waals radius are certainly "classical" concepts; nonetheless, if properly formulated, they can be consistent with the solvation number obtained from Raman spectra not only qualitatively but also semi-quantitatively. References Borodin, G. V. Zhuang, P. N. Ross and K. Xu, Phys. Chem. C, 117, 7433 – 7444 (2013).Borodin, M. Olguin, P. Ganesh, P. R. C. Kent, J. L. Allen and W. A. Henderson, Chem. Chem. Phys., 18, 164 – 175 (2016).Hayamizu, Y. Aihara, S. Arai, C. Garcia-Martinez, J. Phys. Chem. B, 103, 519 – 524 (1999).Hayamizu, J. Chem. Eng. Data, 57, 2012 – 2017 (2012). Figure 1

Combining Sparse and Dense Features to Improve Multi-Modal Registration for Brain DTI Images.

A new solution to overcome the constraints of multimodality medical intra-subject image registration is proposed, using the mutual information (MI) of image histogram-oriented gradients as a new matching criterion. We present a rigid, multi-modal image registration algorithm based on linear transformation and oriented gradients for the alignment of T2-weighted (T2w) images (as a fixed reference) and diffusion tensor imaging (DTI) (b-values of 500 and 1250 s/mm2) as floating images of three patients to compensate for the motion during the acquisition process. Diffusion MRI is very sensitive to motion, especially when the intensity and duration of the gradient pulses (characterized by the b-value) increases. The proposed method relies on the whole brain surface and addresses the variability of anatomical features into an image stack. The sparse features refer to corners detected using the Harris corner detector operator, while dense features use all image pixels through the image histogram of oriented gradients (HOG) as a measure of the degree of statistical dependence between a pair of registered images. HOG as a dense feature is focused on the structure and extracts the oriented gradient image in the x and y directions. MI is used as an objective function for the optimization process. The entropy functions and joint entropy function are determined using the HOGs data. To determine the best image transformation, the fiducial registration error (FRE) measure is used. We compare the results against the MI-based intensities results computed using a statistical intensity relationship between corresponding pixels in source and target images. Our approach, which is devoted to the whole brain, shows improved registration accuracy, robustness, and computational cost compared with the registration algorithms, which use anatomical features or regions of interest areas with specific neuroanatomy. Despite the supplementary HOG computation task, the computation time is comparable for MI-based intensities and MI-based HOG methods.

A novel approach for b-value optimization in Intravoxel Incoherent Motion Imaging using Metaheuristic algorithm

Intravoxel Incoherent Motion (IVIM) based Magnetic Resonance Imaging (MRI) technique allows the quantitative evaluation of perfusion and diffusion without the use of contrast agents. The correctness of the diagnosis depends upon the accuracy and precision of IVIM parameter estimation. To achieve this, several diffusion weighted images must be acquired. The criteria of selection of diffusion weights varies among researchers. The diffusion weights are incorporated by a factor called ‘b value’, which reflects the intensity and duration of the diffusion gradient pulses used for imaging. IVIM imaging takes more time for image acquisition with multiple b values and the selection of the absolute b values as well as the number of b values, is a real challenge. As the b value count increases the scan time increases, which leads to increased patient discomfort and motion artifacts, resulting in poor image quality. Moreover, in most cases, these b values are found using trial and error methods during image acquisition. These issues can be addressed to a large extent by finding, optimum number and range of b values. In this paper, we propose a population based Metaheuristic algorithm for arriving at the optimal b value count and the range of absolute b values for liver, which is an organ with high perfusion. Three separate models are developed to appropriately choose all possible b values ranging between 0 s/mm2 and 850 s/mm2, to observe the effects of diffusion and perfusion as well as to increase the global search space. The effect of low (0 s/mm2 to 50 s/mm2 ), medium (55 s/mm2 to 220 s/mm2) and high (230 s/mm2 to 800 s/mm2) b values has been studied to find the optimal number of b values that can be used for IVIM imaging. In order to define a b value count that minimizes the error in IVIM parameters, simulation experiments are performed for different b value counts specifically 16, 14, 12, 10, 8, 6 and 4. For each of these experiments, repeated observations are made to analyze the parameter uncertainty. The results obtained show that the b value count can be minimized for a better quantitative estimation of IVIM parameters with the least possible errors and the difference in error between each of these observations is found to be less than 0.001. Minimization of b value count results in the reduction of overall image acquisition time and hence patient discomfort. It is also observed that for a very good SNR, b value count can be reduced to 4 although for a reasonable SNR, 8 or 10 b values are to be used for accurate quantitative estimation of IVIM parameters.

Improved gradient waveforms for oscillating gradient spin-echo (OGSE) diffusion tensor imaging.

The dependence of the diffusion tensor on frequency is of great interest in studies of tissue microstructure because it reveals restrictions to the Brownian motion of water molecules caused by cell membranes. Oscillating gradient spin-echo (OGSE) sequences can sample this dependence with gradient shapes for which the power spectrum of the zeroth moment is focused at a target frequency. In order to maintain the total spectral power (ie the b-value), oscillating gradient amplitudes must grow with the frequency squared. For this reason, OGSE applications on clinical MRI scanners are limited to low frequencies, for which it is difficult to obtain a narrow frequency bandwidth of the gradient moment in a useful echo time. In particular, the commonly used pair of single-period trapezoidal-cosine pulses separated by a half-period produces significant side lobes away from the target frequency. To mitigate this effect, improved OGSE waveforms are proposed, which reduce the gap between the two gradient pulses to the minimum duration required for the refocusing RF pulse. Additionally, a slight deviation from the periodicity of the waveforms is proposed in order to permit using the maximum slew rate of the gradient system for all lobes of trapezoidal waveforms while maintaining advantageous spectral properties, which is not the case for the currently used OGSE sequences. Numerical calculations validate these changes, showing that both modifications significantly narrow the gradient moment power spectrum and increase the contribution of its main lobe to the b-value, thus improving the specificity of the measurement. The utility of the new shapes is demonstrated by diffusion tensor measurements of human white matter in vivo over the range of 30-75 Hz with a b-value of nearly 1000 s/mm2 , using a high-performance gradient insert. However, the improvement should increase the sampling precision of OGSE experiments for all gradient systems.

Effect of pulsation in TIG welding on the microstructure, residual stresses, tensile and impact properties of Ti-5Al-2.5Sn alloy

Pulsation of current is widely employed in Tungsten Inert Gas (TIG) welding of titanium alloys. In the current study, the influence of welding speed and welding currents (peak and background) on the properties pulsed TIG welded Ti-5Al-2.5Sn alloy were investigated. The weldment characteristics which were analyzed included weld zone size, microstructure, residual stresses, tensile and impact properties. Increasing the welding speed and gradient of pulsed current enhanced grain strengthening in the fusion zone (FZ) which was attributed to increased cooling rate. Such a welding condition was also favorable in achieving full penetration weldments with a reduced; heat affected zone width, grain size in FZ and residual stresses. However, a tensile strength superior to the base metal was observed. The impact toughness reduced by increasing the pulsed current gradient and a high value of both peak and background currents were required to improve the impact toughness of the weldments.

Gradient-based pulse sequences for benchtop NMR spectroscopy

Benchtop NMR spectroscopy has been on the rise for the last decade, by bringing high-resolution NMR in environments that are not easily compatible with high-field NMR. Benchtop spectrometers are accessible, low cost and show an impressive performance in terms of sensitivity with respect to the relatively low associated magnetic field (40–100 MHz). However, their application is limited by the strong and ubiquitous peak overlaps arising from the complex mixtures which are often targeted, often characterized by a great diversity of concentrations and by strong signals from non-deuterated solvents. Such limitations can be addressed by pulse sequences making clever use of magnetic field gradient pulses, capable of performing efficient coherence selection or encoding chemical shift or diffusion information. Gradients pulses are well-known ingredients of high-field pulse sequence recipes, but were only recently made available on benchtop spectrometers, thanks to the introduction of gradient coils in 2015. This article reviews the recent methodological advances making use of gradient pulses on benchtop spectrometers and the applications stemming from these developments. Particular focus is made on solvent suppression schemes, diffusion-encoded, and spatially-encoded experiments, while discussing both methodological advances and subsequent applications. We eventually discuss the exciting development and application perspectives that result from such advances.

New options for increasing the sensitivity, specificity and scope of synergistic contrast magnetic resonance imaging (scMRI) using Multiplied, Added, Subtracted and/or FiTted (MASTIR) pulse sequences.

This paper reviews magnetic resonance (MR) pulse sequences in which the same or different tissue properties (TPs) such as T1 and T2 are used to contribute synergistically to lesion contrast. It also shows how synergistic contrast can be created with Multiplied, Added, Subtracted and/or fiTted Inversion Recovery (MASTIR) sequences, and be used to improve the sensitivity, specificity and scope of clinical magnetic resonance imaging (MRI) protocols. Synergistic contrast can be created from: (i) the same TP, e.g., T1 used twice or more in a pulse sequence; (ii) different TPs such as ρm, T1, T2, and D* used once or more within a sequence, and (iii) additional suppression or reduction of signals from tissues and/or fluids such as fat, long T2 tissues and cerebrospinal fluid (CSF). The short inversion time (TI) inversion recovery (IR) (STIR) and double IR (DIR) sequences usually show synergistic positive contrast for lesions which have increases in both T1 and T2. The diffusion weighted pulsed gradient spin echo (PGSE) sequence shows synergistic contrast for lesions which have an increase in T2 and a decrease in D*; the sequence is both positively weighted for T2 and negatively weighted for D*. In the brain, when an IR sequence nulling white matter has subtracted from it an IR sequence nulling gray matter to form the subtracted IR (SIR) sequence, increases in the single TP T1 between the two nulling points of the original two sequences generate high synergistic positive contrast. In addition, the subtraction to produce the SIR sequence reduces fat and CSF signals. To provide high sensitivity to changes in TPs in disease the SIR sequence can be used (i) alone to provide synergistic T1 contrast as above; (ii) with T2-weighting to provide synergistic T1 and T2 contrast, and (iii) with T2- and D*-weighting to provide synergistic T1, T2, and D* contrast. The SIR sequence can also be used in reversed form (longer TI form minus shorter TI form) to produce very high positive synergistic T1 contrast for reductions in T1, and so increase the positive contrast enhancement produced by clinical gadolinium-based contrast agents (GBCAs) when they reduce T1. The specificity of MRI examinations can be improved by using the reversed SIR sequence with a long echo time (TE) gradient echo as well as echo subtraction to show synergistic high contrast from T1 and T2* shortening produced by organic iron. Other added and subtracted forms of the MASTIR sequence can be used synergistically to selectively show myelin, myelin water and fluids including blood and CSF. Protocols using MASTIR sequences to provide synergistic contrast in MRI of the brain, prostate and articular cartilage are included as illustrative examples, and the features of synergistic contrast MRI (scMRI) are compared to those of multiparametric MRI (mpMRI) and functional MRI (fMRI).

Development of fast multi‐slice apparent T1 mapping for improved arterial spin labeling MRI measurement of cerebral blood flow

To develop fast multi-slice apparent T1 (T1app ) mapping for accurate cerebral blood flow (CBF) quantification with arterial spin labeling (ASL) MRI. Fast multi-slice T1app was measured using a modified inversion recovery echo planar imaging (EPI) sequence with simultaneous application of ASL tagging radiofrequency (RF) and gradient pulses. The fast multi-slice T1app measurement was compared with the single-slice T1app imaging approach, repeated per slice. CBF was assessed in healthy adult Wistar rats (N = 5) and rats with acute stroke 24 hours after a transient middle cerebral artery occlusion (N = 5). The fast multi-slice T1app measurement was in good agreement with that of a single-slice T1app imaging approach (Lin's concordance correlation coefficient = 0.92). CBF calculated using T1app reasonably accounted for the finite labeling RF duration, whereas the routine T1 -normalized ASL MRI underestimated the CBF, particularly at short labeling durations. In acute stroke rats, the labeling time and the CBF difference (ΔCBF) between the contralateral normal area and the ischemic lesion were significantly correlated when using T1 -normalized perfusion calculation (R = 0.844, P = .035). In comparison, T1app -normalized ΔCBF had little labeling time dependence based on the linear regression equation of ΔCBF = -0.0247*τ + 1.579 mL/g/min (R = -0.352, P = .494). Our study found fast multi-slice T1app imaging improves the accuracy and reproducibility of CBF measurement.