Functional MRI using spin lock editing preparation pulses

Layer-specific BOLD effects in gradient and spin-echo acquisitions in somatosensory cortex

PURPOSE

Previous studies have shown varied BOLD signals with gradient echo (GE) across cortical depth. To interpret these variations, and understand the effects of vascular geometry and size, the magnitudes and layer distributions of GE and spin-echo (SE) BOLD functional MRI signals were compared in the somatosensory cortex of squirrel monkeys during tactile stimulation and in a resting state at high spatial resolution and high field.

METHODS

A block-design stimulation was used to identify tactile-evoked activation signals in somatosensory Areas 3b and 1. Layer-specific connectivities were calculated using resting-state data. Signal power spectra were compared by depth and pulse sequence. The measured ratios of transverse relaxation rate changes were compared with Anderson and Weiss's model.

RESULTS

SE signals showed a 26% lower percentage signal change during tactile stimulation compared with GE, along with a slower time course. SE signals remained consistent but weaker in lower layers, whereas GE signals decreased with cortical depth. This pattern extended to resting-state power spectra. Resting-state functional connectivity indicated larger connectivity between the top layers of Area 3b and Area 1 for GE, with minimal changes for SE. Comparisons with theory suggest vessel diameters ranging from 19.4 to 9 microns are responsible for BOLD effects across cortical layers at 9.4 T.

CONCLUSION

These results provide further evidence that at high field, SE BOLD signals are relatively free of contributions from sources other than microvascular changes in response to neural activity, whereas GE signals, even in the superficial layers, are not dominated by very large vessels.

Enabling brain-wide mapping of directed functional connectivity at 3T via layer-dependent fMRI with draining-vein suppression

Layer-dependent functional magnetic resonance imaging (fMRI) offers a compelling avenue for investigating directed functional connectivity (FC). To construct a comprehensive map of brain-wide directed FC, several technical criteria must be met, including sub-mm spatial resolution, adequate temporal resolution, functional sensitivity, global brain coverage, and high spatial specificity. Although gradient echo (GE)-based echo planar imaging (EPI) is commonly used for rapid fMRI acquisition, it faces significant challenges due to the draining-vein effect, particularly when utilizing blood oxygen level-dependent (BOLD) contrast. In this study, we mitigated this effect by incorporating velocity-nulling (VN) gradients into a GE-BOLD fMRI sequence, opting for a 3T magnetic field strength over 7T. We also integrated several advanced techniques, such as simultaneous multi-slice (SMS) acceleration and NORDIC denoising, to enhance temporal resolution, spatial coverage, and signal sensitivity. Collectively, the VN fMRI method exhibited notable spatial specificity, as evidenced by the identification of double-peak activation patterns within the primary motor cortex (M1) during a finger-tapping task. Additionally, the technique demonstrated BOLD sensitivity in the lateral geniculate nucleus (LGN). Furthermore, our VN fMRI technique displayed superior robustness when compared to conventional fMRI approaches across participants. Our findings of directed FC elucidate several layer-specific functional relationships between different brain regions and align closely with existing literature. Given the widespread availability of 3T scanners, this technical advancement has the potential for significant impact across multiple domains of neuroscience research.

Magnetic resonance myocardial T1ρ mapping : Technical overview, challenges, emerging developments, and clinical applications

The potential of cardiac magnetic resonance to improve cardiovascular care and patient management is considerable. Myocardial T1-rho (T1ρ) mapping, in particular, has emerged as a promising biomarker for quantifying myocardial injuries without exogenous contrast agents. Its potential as a contrast-agent-free ("needle-free") and cost-effective diagnostic marker promises high impact both in terms of clinical outcomes and patient comfort. However, myocardial T1ρ mapping is still at a nascent stage of development and the evidence supporting its diagnostic performance and clinical effectiveness is scant, though likely to change with technological improvements. The present review aims at providing a primer on the essentials of myocardial T1ρ mapping, and to describe the current range of clinical applications of the technique to detect and quantify myocardial injuries. We also delineate the important limitations and challenges for clinical deployment, including the urgent need for standardization, the evaluation of bias, and the critical importance of clinical testing. We conclude by outlining technical developments to be expected in the future. If needle-free myocardial T1ρ mapping is shown to improve patient diagnosis and prognosis, and can be effectively integrated in cardiovascular practice, it will fulfill its potential as an essential component of a cardiac magnetic resonance examination.

Tissue characterization using R1rho dispersion imaging at low locking fields

Measurements of the variations of spin-locking relaxation rates (R_1ρ) with locking field amplitude allow the derivation of quantitative parameters that describe different dynamic processes, such as slow molecular motions, chemical exchange and diffusion. In some samples, changes in R_1ρ values between locking frequency 0 and 200 Hz may be dominated mainly by diffusion of water in intrinsic field gradients, while those at higher locking fields are due to exchange processes. The exchange and diffusion effects act independently of each other, as confirmed by simulation and experimentally. In tissues, the relevant intrinsic field gradients may arise from the magnetic inhomogeneities caused by microvascular blood so that R_1ρ dispersion over weak locking field amplitudes (≤ 200 Hz) is affected by changes in capillary density and geometry. Here we first review the theoretical and experimental background to the interpretation of R_1ρ dispersions caused by intrinsic magnetic susceptibility variations within the tissue. We then provide new empirical results of R_1ρ dispersion imaging of the human brain and skeletal muscle at low locking field amplitudes for the first time and identify potential applications of R_1ρ dispersion imaging in clinical studies.

Imaging tumor acidosis: a survey of the available techniques for mapping in vivo tumor pH

Cancer cells are characterized by a metabolic shift in cellular energy production, orchestrated by the transcription factor HIF-1α, from mitochondrial oxidative phosphorylation to increased glycolysis, regardless of oxygen availability (Warburg effect). The constitutive upregulation of glycolysis leads to an overproduction of acidic metabolic products, resulting in enhanced acidification of the extracellular pH (pHe ~ 6.5), which is a salient feature of the tumor microenvironment. Despite the importance of pH and tumor acidosis, there is currently no established clinical tool available to image the spatial distribution of tumor pHe. The purpose of this review is to describe various imaging modalities for measuring intracellular and extracellular tumor pH. For each technique, we will discuss main advantages and limitations, pH accuracy and sensitivity of the applied pH-responsive probes and potential translatability to the clinic. Particular attention is devoted to methods that can provide pH measurements at high spatial resolution useful to address the task of tumor heterogeneity and to studies that explored tumor pH imaging for assessing treatment response to anticancer therapies.

Spin-lock imaging of intrinsic susceptibility gradients in tumors

PURPOSE

Previous studies have shown that diffusion of water through intrinsic susceptibility gradients produces a dispersion of the spin-lattice relaxation rate in the rotating frame (R_{1 ρ} ) over a low range of spin-locking amplitudes (0 < ω₁ < 100 Hz), whereas at higher ω₁ and high magnetic fields, a second dispersion arises due to chemical exchange. Here, we separated these different effects and evaluated their contributions in tumors.

METHODS

Maps of R_{1 ρ} and its changes with locking field were acquired on intracranial 9-L tumor models. The R_{1 ρ} changes due to diffusion ( R 1 ρ Diff ) were calculated by subtracting maps of R_{1 ρ} at 100 Hz (R_{1 ρ} [100 Hz]) from those at 0 Hz (R_{1 ρ} [0 Hz]). The R_{1 ρ} changes due to exchange ( R 1 ρ Ex ) were calculated by subtracting maps of R_{1 ρ} at 5620 Hz (R_{1 ρ} [5620 Hz]) from those of R_{1 ρ} at 100 Hz (R_{1 ρ} [100 Hz]). Measurements of vascular dimensions and spacing were performed ex vivo using 3D confocal microscopy.

RESULTS

The R_{1 ρ} changes at low ω₁ in tumors (5.24 ± 1.78 s^-1 ) are substantially (p = 3.7⁶ ) greater than those in normal tissues (1.36 ± 0.70 s^-1 ), which we suggest are due to greater contributions from diffusion through susceptibility gradients. Tumor vessels were larger and spaced less closely compared with normal brain, which may be 1 factor contributing the susceptibility within 9-L tumors. The contrast between tumor and normal tissues for R 1 ρ Diff is larger than for R 1 ρ Ex and for the apparent R_2w .

CONCLUSION

Images that are sensitive to the variations of spin-lock relaxation rates at low ω₁ provide a novel form of contrast that reflects the heterogeneous nature of intrinsic variations within tumors.

High-Resolution CBV-fMRI Allows Mapping of Laminar Activity and Connectivity of Cortical Input and Output in Human M1

Layer-dependent fMRI allows measurements of information flow in cortical circuits, as afferent and efferent connections terminate in different cortical layers. However, it is unknown to what level human fMRI is specific and sensitive enough to reveal directional functional activity across layers. To answer this question, we developed acquisition and analysis methods for blood-oxygen-level-dependent (BOLD) and cerebral-blood-volume (CBV)-based laminar fMRI and used these to discriminate four different tasks in the human motor cortex (M1). In agreement with anatomical data from animal studies, we found evidence for somatosensory and premotor input in superficial layers of M1 and for cortico-spinal motor output in deep layers. Laminar resting-state fMRI showed directional functional connectivity of M1 with somatosensory and premotor areas. Our findings demonstrate that CBV-fMRI can be used to investigate cortical activity in humans with unprecedented detail, allowing investigations of information flow between brain regions and outperforming conventional BOLD results that are often buried under vascular biases.

Establishing upper limits on neuronal activity-evoked pH changes with APT-CEST MRI at 7 T

Purpose

To detect neuronal activity–evoked pH changes by amide proton transfer–chemical exchange saturation transfer (APT‐CEST) MRI at 7 T.

Methods

Three healthy subjects participated in the study. A low‐power 3‐dimensional APT‐CEST sequence was optimized through the Bloch‐McConnell equations. pH sensitivity of the sequence was estimated both in phantoms and in vivo. The feasibility of pH–functional MRI was tested in Bloch‐McConnell‐simulated data using the optimized sequence. In healthy subjects, the visual stimuli were used to evoke transient pH changes in the visual cortex, and a 3‐dimensional APT‐CEST volume was acquired at the pH‐sensitive frequency offset of 3.5 ppm every 12.6 s.

Results

In theory, a three‐component general linear model was capable of separating the effects of blood oxygenation level–dependent contrast and pH. The Bloch‐McConnell equations indicated that a change in pH of 0.03 should be measurable at the experimentally determined temporal signal‐to‐noise ratio of 108. However, only a blood oxygenation level–dependent effect in the visual cortex could be discerned during the visual stimuli experiments performed in the healthy subjects.

Conclusions

The results of this study suggest that if indeed there are any transient brain pH changes in response to visual stimuli, those are under 0.03 units pH change, which is extremely difficult to detect using the existent techniques. Magn Reson Med 80:126–136, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

A rapid magnetization preparation for MRI measurements of sprays

The process of spray atomization, i.e., the transformation of a continuous liquid jet into μm-sub-mm sized droplets, is ubiquitous in industry yet quite complex to analyze theoretically and study experimentally. One of the main strengths of MRI is its ability to sensitize spatially-resolved NMR signal to a wide variety of physical and chemical parameters. However, standard preparation schemes are difficult to employ for studies of sprays due to sprays' fast speeds (>10-20m/s). In addition, sprays are usually low-density systems, leading to a poor SNR and a need for massive signal averaging and long acquisition time. In this paper, we reduced the interval between the preparation and the readout stages by performing SPI encoding on the rising gradients. This also enabled the use of 90-degree flip angles to maximize the spray signal and saturate the stationary water signal while avoiding unwanted slice-selection. The use of gradients during preparation stage was eliminated due to their time-consuming rise and stabilization times limiting possible preparation schemes to a combination of RF pulses and delays. The two preparation schemes presented here are Time-of-Flight (TOF) and T1ρ-weighting schemes. The total duration of the sequence (without TR) was 240-1100μs for the TOF and 410μs for T1ρ. The T1ρ prepared images of the near-atomization region (11 spin-locking frequencies, 0-15kHz) showed a strong signal attenuation at higher frequencies. In series of TOF images the clearly noticeable displacement of the liquid parcel can be utilized to measure spray speeds.

New insights into rotating frame relaxation at high field

Measurements of spin-lock relaxation rates in the rotating frame (R1ρ ) at high magnetic fields afford the ability to probe not only relatively slow molecular motions, but also other dynamic processes, such as chemical exchange and diffusion. In particular, measurements of the variation (or dispersion) of R1ρ with locking field allow the derivation of quantitative parameters that describe these processes. Measurements in deuterated solutions demonstrate the manner and degree to which exchange dominates relaxation at high fields (4.7 T, 7 T) in simple solutions, whereas temperature and pH are shown to be very influential factors affecting the rates of proton exchange. Simulations and experiments show that multiple exchanging pools of protons in realistic tissues can be assumed to behave independently of each other. R1ρ measurements can be combined to derive an exchange rate contrast (ERC) that produces images whose intensities emphasize protons with specific exchange rates rather than chemical shifts. In addition, water diffusion in the presence of intrinsic susceptibility gradients may produce significant effects on R1ρ dispersions at high fields. The exchange and diffusion effects act independently of each other, as confirmed by simulation and experimentally in studies of red blood cells at different levels of oxygenation. Collectively, R1ρ measurements provide an ability to quantify exchange processes, to provide images that depict protons with specific exchange rates and to describe the microstructure of tissues containing magnetic inhomogeneities. As such, they complement traditional T1 or T2 measurements and provide additional insights from measurements of R1ρ at a single locking field. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of diffusion in magnetically inhomogeneous media on rotating frame spin-lattice relaxation

In an aqueous medium containing magnetic inhomogeneities, diffusion amongst the intrinsic susceptibility gradients contributes to the relaxation rate R_1ρ of water protons to a degree that depends on the magnitude of the local field variations ΔB_z, the geometry of the perturbers inducing these fields, and the rate of diffusion of water, D. This contribution can be reduced by using stronger locking fields, leading to a dispersion in R_1ρ that can be analyzed to derive quantitative characteristics of the material. A theoretical expression was recently derived to describe these effects for the case of sinusoidal local field variations of a well-defined spatial frequency q. To evaluate the degree to which this dispersion may be extended to more realistic field patterns, finite difference Bloch-McConnell simulations were performed with a variety of three-dimensional structures to reveal how simple geometries affect the dispersion of spin-locking measurements. Dispersions were fit to the recently derived expression to obtain an estimate of the correlation time of the field variations experienced by the spins, and from this the mean squared gradient and an effective spatial frequency were obtained to describe the fields. This effective spatial frequency was shown to vary directly with the second moment of the spatial frequency power spectrum of the ΔB_z field, which is a measure of the average spatial dimension of the field variations. These results suggest the theory may be more generally applied to more complex media to derive useful descriptors of the nature of field inhomogeneities. The simulation results also confirm that such diffusion effects disperse over a range of locking fields of lower amplitude than typical chemical exchange effects, and should be detectable in a variety of magnetically inhomogeneous media including regions of dense microvasculature within biological tissues.