Comparison of automated and visual DWI ASPECTS in acute ischemic stroke.

BACKGROUND AND PURPOSE: To assess intra-and inter-rater agreement of the ASPECTS (Alberta Stroke Program Early CT Score) based on diffusion-weighted MRI and to compare it with fully - automated methods (eASPECTS). METHODS: DWI-ASPECTS of scans of 96 patients with acute ischemic stroke was rated by 2 experts. Automated methods based on thresholding the affected volumes of a coregistered atlas, and a regression tree learning method were established. Intra-rater, inter-rater and human-rater vs. automated methods agreements were investigated based on the intraclass correlation coefficients (ICC) and Bland Altman plots. RESULTS: Intra-rater agreement was good for both raters (ICC of 0.91 and 0.93). Inter-rater agreement was worse (ICC = 0.86) indicating a slight bias between both raters. Agreement with automated methods ranged from 0.81 to 0.87. Root-mean-squared deviation was 0.89 and 0.69 for the human raters and ranged from 0.95 to 1.24 for the automated methods. CONCLUSIONS: Agreement values are on the same order or higher compared to a literature review of CT-based ASPECTS. Automated methods perform slightly worse than human expert ratings, but they still have enough power to determine the DWI-ASPECTS with good precision in a clinical setting.

Do twisted laser beams evoke nuclear hyperpolarization?

The hyperpolarization of nuclear spins promises great advances in chemical analysis and medical diagnosis by substantially increasing the sensitivity of nuclear magnetic resonance (NMR). Current methods to produce a hyperpolarized sample, however, are arduous, time-consuming or costly and require elaborate equipment. Recently, a much simpler approach was introduced that holds the potential, if harnessed appropriately, to revolutionize the production of hyperpolarized spins. It was reported that high levels of hyperpolarization in nuclear spins can be created by irradiation with a laser beam carrying orbital angular momentum (twisted light). Aside from these initial reports however, no further experimental verification has been presented. In addition, this effect has so far evaded a critical theoretical examination. In this contribution, we present the first independent attempt to reproduce the effect. We exposed a sample of immersion oil or a fluorocarbon liquid that was placed within a low-field NMR spectrometer to Laguerre-Gaussian and Bessel laser beams at a wavelength of 514.5nm and various topological charges. We acquired (1)H and (19)F NMR free induction decay data, either during or alternating with the irradiation that was parallel to B0. We observed an irregular increase in NMR signal in experiments where the sample was exposed to beams with higher values of the topological charge. However, at no time did the effect reach statistical significance of 95%. Given the measured sensitivity of our setup, we estimate that a possible effect did not exceed a hyperpolarization (at 5mT) of 0.14-6%, depending on the assumed hyperpolarized volume. It should be noted though, that there were some differences between our setup and the previous implementation of the experiment, which may have inhibited the full incidence of this effect. To approach a theoretical description of this effect, we considered the interaction of an electron with a plane wave, which is known to be able to induce electronic (e.g. in rubidium) and subsequent nuclear hyperpolarization. Compared to the plane wave, the additional transitions caused by a twisted wave are of the order of 10(-3) less. This suggests that the twist of the laser is unlikely to be responsible for the hyperpolarization of nuclear spins, unless a new mechanism of momentum transfer is identified.

MesoFT: unifying diffusion modelling and fiber tracking.

One overarching challenge of clinical magnetic resonance imaging (MRI) is to quantify tissue structure at the cellular scale of micrometers, based on an MRI acquisition with a millimeter resolution. Diffusion MRI (dMRI) provides the strongest sensitivity to the cellular structure. However, interpreting dMRI measurements has remained a highly ill-posed inverse problem. Here we propose a framework that resolves the above challenge for human white matter fibers, by unifying intra-voxel mesoscopic modeling with global fiber tractography. Our algorithm is based on a Simulated Annealing approach which simultaneously optimizes diffusion parameters and fiber locations. Each fiber carries its by their individual set of diffusion parameters which allows to link them structural relationships.

Local and global fiber tractography in patients with epilepsy.

BACKGROUND AND PURPOSE: Fiber tractography is increasingly used in the preoperative evaluation of endangered fiber bundles. From a clinical point of view, an accurate and methodologically transparent procedure is desired. Our aim was to evaluate the recently described global tracking algorithm compared with other established methods, such as deterministic and probabilistic tractography. MATERIALS AND METHODS: Twenty patients, candidates for excision of epileptogenic lesions, were subjected to higher-angular resolution diffusion imaging-based fiber tractography. Seed points were created without manual bias, predominantly by FreeSurfer and voxel-based atlases. We focused on 2 important fiber bundles, namely the descending motor pathways and the optic radiation. Postoperatively, the accuracy of the predicted fiber route was controlled by structural MR imaging and by inflicted functional deficits. RESULTS: Among the 3 evaluated methods, global tracking was the only method capable of reconstructing the full extent of the descending motor pathways, including corticobulbar fibers from the area of face representation. Still, probabilistic tractography depicted the optic radiation better, especially the Meyer loop. The deterministic algorithm performed less adequately. CONCLUSIONS: The probabilistic method seems to be the best balance between computational time and effectiveness and seems to be the best choice in most cases, particularly for the optic radiation. If, however, a detailed depiction of the fiber anatomy is intended and tract crossings are implicated, then the computationally time-consuming global tracking should be preferred.

Global tracking in human gliomas: a comparison with established tracking methods.

PURPOSE: Global tracking (GT) is a recently published fibre tractography (FT) method that takes simultaneously all fibres into account during their reconstruction. The purpose of this study was to compare this new method with fibre assignment by continuous tracking (FACT) and probabilistic tractography (PT) for the detection of the corticospinal tract (CST) in patients with gliomas. METHODS: Tractography of the CST was performed in 17 patients with eight low grade and nine anaplastic astrocytomas located in the motor cortex or the corticospinal tract. Diffusions metrics as fractional anisotropy (FA), mean (MD), axial (AD) and radial diffusivity (RD) were obtained. The methods were additionally applied on a physical phantom to assess their accuracy. RESULTS: PT was successful in all (100 %), GT in 16 (94 %) and FACT in 15 patients (88 %). The case where GT and FACT, both, missed the CST showed the highest AD and RD, whereas the one where FACT algorithm, alone, was not successfully showed the lowest AD and RD of the group. FA was reduced on the pathologic side (FApath 0.35 ± 0.16 (mean ± SD) versus FAcontralateral 0.51 ± 0.15, pcorr < 0.03). RD was increased on the pathologic side (RDpath 0.67 ± 0.29 × 10(-3) mm(2)/s versus RDcontralateral 0.46 ± 0.08 × 10(-3) mm(2)/s, pcorr < 0.03). In the phantom measurement, only GT did not detect false positive fibres at fibre crossings. CONCLUSION: PT performed well even in areas of increased diffusivities indicating a severe oedema or disintegration of tissue. FACT was also susceptible to a decrease of diffusivities and to a susceptibility artefact, where GT was robust.

Extended phase graphs with anisotropic diffusion.

The extended phase graph (EPG) calculus gives an elegant pictorial description of magnetization response in multi-pulse MR sequences. The use of the EPG calculus enables a high computational efficiency for the quantitation of echo intensities even for complex sequences with multiple refocusing pulses with arbitrary flip angles. In this work, the EPG concept dealing with RF pulses with arbitrary flip angles and phases is extended to account for anisotropic diffusion in the presence of arbitrary varying gradients. The diffusion effect can be expressed by specific diffusion weightings of individual magnetization pathways. This can be represented as an action of a linear operator on the magnetization state. The algorithm allows easy integration of diffusion anisotropy effects. The formalism is validated on known examples from literature and used to calculate the effective diffusion weighting in multi-echo sequences with arbitrary refocusing flip angles.

Fully automated classification of HARDI in vivo data using a support vector machine.

The purpose of this study is the classification of high angular resolution diffusion imaging (HARDI) in vivo data using a model-free approach. This is achieved by using a Support Vector Machine (SVM) algorithm taken from the field of supervised statistical learning. Six classes of image components are determined: grey matter, parallel neuronal fibre bundles in white matter, crossing neuronal fibre bundles in white matter, partial volume between white and grey matter, background noise and cerebrospinal fluid. The SVM requires properties derived from the data as input, the so called feature vector, which should be rotation invariant. For our application we derive such a description from the spherical harmonic decomposition of the HARDI signal. With this information the SVM is trained in order to find the function for separating the classes. The SVM is systematically tested with simulated data and then applied to six in vivo data sets. This new approach is data-driven and enables fully automatic HARDI data segmentation without employing a T1 MPRAGE scan and subjective expert intervention. This was demonstrated on five test in vivo data sets giving robust results. The segmentation results could be used as a priori knowledge for increasing the performance of fibre tracking as well as for other clinical and diagnostic applications of diffusion weighted imaging (DWI).

Transverse NMR relaxation in magnetically heterogeneous media.

We consider the NMR signal from a permeable medium with a heterogeneous Larmor frequency component that varies on a scale comparable to the spin-carrier diffusion length. We focus on the mesoscopic part of the transverse relaxation, that occurs due to dispersion of precession phases of spins accumulated during diffusive motion. By relating the spectral lineshape to correlation functions of the spatially varying Larmor frequency, we demonstrate how the correlation length and the variance of the Larmor frequency distribution can be determined from the NMR spectrum. We corroborate our results by numerical simulations, and apply them to quantify human blood spectra.

Gibbs tracking: a novel approach for the reconstruction of neuronal pathways.

Reconstruction of neuronal fibers using diffusion-weighted (DW) MRI is an emerging method in biomedical research. Existing fiber-tracking algorithms are commonly based on the "walker principle." Fibers are reconstructed as trajectories of "walkers," which are guided according to local diffusion properties. In this study, a new method of fiber tracking is proposed that does not engage any "walking" algorithm. It resolves a number of inherent problems of the "walking" approach, in particular the reconstruction of crossing and spreading fibers. In the proposed method, the fibers are built with small line elements. Each line element contributes an anisotropic term to the simulated DW signal, which is adjusted to the measured signal. This method demonstrates good results for simulated fibers. A single in vivo result demonstrates the successful reconstruction of the dominant neuronal pathways. A comparison with the diffusion tensor imaging (DTI)-based fiber assignment with continuous tracking (FACT) method and the probabilistic index of connectivity (PICo) method based on a multitensor model is performed for the callosal fibers. The result shows a strong increase in the number of reconstructed fibers. These almost fill the total white matter (WM) volume and connect a large area of the cortex. The method is very computationally expensive. Possible ways to address this problem are discussed.

Connecting and merging fibres: pathway extraction by combining probability maps.

Probability mapping of connectivity is a powerful tool to determine the fibre structure of white matter in the brain. Probability maps are related to the degree of connectivity to a chosen seed area. In many applications, however, it is necessary to isolate a fibre bundle that connects two areas. A frequently suggested solution is to select curves, which pass only through two or more areas. This is very inefficient, especially for long-distance pathways and small areas. In this paper, a novel probability-based method is presented that is capable of extracting neuronal pathways defined by two seed points. A Monte Carlo simulation based tracking method, similar to the Probabilistic Index of Connectivity (PICo) approach, was extended to preserve the directional information of the main fibre bundles passing a voxel. By combining two of these extended visiting maps arising from different seed points, two independent parameters are determined for each voxel: the first quantifies the uncertainty that a voxel is connected to both seed points; the second represents the directional information and estimates the proportion of fibres running in the direction of the other seed point (connecting fibre) or face a third area (merging fibre). Both parameters are used to calculate the probability that a voxel is part of the bundle connecting both seed points. The performance and limitations of this DTI-based method are demonstrated using simulations as well as in vivo measurements.

Theoretical model of intravascular paramagnetic tracers effect on tissue relaxation.

The concentration of MRI tracers cannot be measured directly by MRI and is commonly evaluated indirectly using their relaxation effect. This study develops a comprehensive theoretical model to describe the transverse relaxation in perfused tissue caused by intravascular tracers. The model takes into account a number of individual compartments. The signal dephasing is simulated in a semianalytical way by embedding Monte Carlo simulations in the framework of analytical theory. This approach yields a tool for fast, realistic simulation of the change in the transverse relaxation. The results indicate that the relaxivity of intravascular contrast agents depends significantly on the host tissue. This agrees with experimental data by Johnson et al. (Magn Reson Med 2000;44:909). In particular, the present results suggest a several-fold increase in the relaxivity of Gd-based contrast agents in brain tissue compared with bulk blood. The enhancement of relaxation in tissue is due to the contrast in magnetic susceptibility between blood vessels and parenchyma induced by the presence of paramagnetic tracer. Beyond the perfusion measurements, the results can be applied to quantitation of functional MRI and to vessel size imaging.

Vessel size imaging in humans.

The relation of contrast-enhanced transverse relaxation rates R2* and R2 provides in vivo mapping of the mean caliber of cerebral vessels. This technique is referred to as vessel size imaging (VSI). Here a quantitative assessment of the vessel caliber in brain tumor patients is presented. The obtained mean vessel size shows sensitivity to the tumor type. A theoretical analysis is given to elucidate the morphological information content of VSI in the context of vessel architecture. The simplification of the theory underlying the data processing results in a systematic overestimation of the vessel caliber. An increase in the magnetic susceptibility of the contrast agent allows for quantitatively more accurate measurements. Quantitative VSI must include measurements of the regional diffusion coefficient and absolute determination of the regional cerebral blood volume.

Theory of susceptibility-induced transverse relaxation in the capillary network in the diffusion narrowing regime.

The transverse relaxation effect of deoxyhemoglobin compartmentalization in erythrocytes in the capillary network is investigated using an analytical approach. The capillaries are modeled as long arrays of paramagnetic spheres, simulating the individual red blood cells. Calculations are performed in the diffusion narrowing regime, which holds for the native blood paramagnetism at moderate fields up to about 1.5 T, for the free induction decay, the Hahn spin-echo, and the Carr-Purcell-Meiboom-Gill sequence. The commonly used model of capillaries as homogeneously magnetized cylinders is shown to underestimate the capillary contribution to the susceptibility-induced relaxation rate by up to 55%. This results in a noticeable change in the predicted deoxyhemoglobin concentration needed to cause the variation in the transverse relaxation rate observed in functional MRI and may affect subsequent quantification of physiological parameters derived from the BOLD signal. Furthermore, the model for the individual red blood cells (RBCs) represents a framework for investigating the effects of interspecies and intersubject variations in hematocrit, RBC deoxyhemoglobin concentration, and cell size on the relaxation rate. The results agree within their validity domain with previous Monte Carlo simulations.

Effect of magnetic field gradients induced by microvasculature on NMR measurements of molecular self-diffusion in biological tissues.

Presence of induced mesoscopic gradients of magnetic field in magnetically heterogeneous samples affects the measured value of apparent diffusion coefficient. This effect is investigated theoretically in the context of diffusion measurements in perfused biological tissues with blood as the paramagnetic compartment. It is shown that the apparent diffusion coefficient is sensitive to mutual correlations in vessel positions. Neglect of these correlations results in a failure of the commonly used model of microvasculature in which vessels are described as independently placed cylinders. The model is modified to account for intervessel correlations. The results indicate an underestimation of apparent diffusion coefficient in proportion to the magnetic susceptibility of intravascular compartment in agreement with published experimental data. The proportionality coefficient depends on the microvascular architecture. Comparison with experimental data yields a numerical value for a new model parameter that characterises the correlation in mutual positions of blood vessels.

Calculation of diffusion effect for arbitrary pulse sequences.

A method is presented for calculating the nuclear spin magnetization created by an arbitrary number of short radio frequency pulses and of piecewise constant gradient applied in a selected direction. The isotropic diffusion, the transverse and longitudinal relaxations as well as the global transport are taken into account. A thorough analysis of the magnetization density evolution results in an algorithm for the analytical calculation of final NMR signal. Computationally, it requires only accumulating numerical coefficients in the found analytical structure. For arbitrary sequences this is done with a computer program. This approach, which can be classified as symbolical computations, results in a high performance and in a practically unlimited accuracy. Results for sample pulse sequences are presented.

On the theoretical basis of perfusion measurements by dynamic susceptibility contrast MRI.

A quantitative analysis was undertaken to calibrate the perfusion quantification technique based on tracking the first pass of a bolus of a blood pool contrast agent. A complete simulation of the bolus passage, of the associated changes in the T2 and T2* signals, and of the data processing was performed using the tracer dilution theory, an analytical theory of the MR signal from living tissues and numerical simulations. The noise was excluded in the simulation in order to analyze the ultimate accuracy of the method. It is demonstrated that the relationship between the contrast agent concentration and the associated changes in the transverse relaxation rate shows essentially different forms in studied tissue and in the reference artery. This effect results in systematic deviations of the measured blood flow, blood volume, and the residue function obtained with conventional processing from their true values. The error depends on the microvascular composition, the properties of the contrast agent, and the weights of the various compartments in the total signal. The results show that dynamic susceptibility contrast MRI can reach the goal of absolute perfusion quantification only with additional input from measurements of the microvascular architecture. Alternatively, the method can be used to provide such information if the perfusion is quantified by another modality.

Effect of graded hypo- and hypercapnia on fMRI contrast in visual cortex: quantification of T(*)(2) changes by multiecho EPI.

The sensitivity of functional magnetic resonance imaging (fMRI) in visual cortex to graded hypo- and hypercapnia was quantified in 10 normal subjects using single-shot multiecho echo-planar imaging (Turbo-PEPSI) with eight equidistant echo times (TEs) between 12 and 140 ms. Visual stimulation was combined with controlled hyperventilation and carbon dioxide inhalation to perform fMRI at six levels of end-expiratory pCO(2) (PETCO(2)) between 20 and 70 mm Hg. T(*)(2) in visual cortex during baseline conditions (light off) increased nonlinearly from 20 to 70 mm Hg, from 61.1 +/- 4.2 ms to 72.0 +/- 4.6 ms. Changes in T(*)(2) due to visual stimulation increased 2.1-fold, from 1.2 +/- 0.6 ms at 20 mm Hg to 2.5 +/- 0.7 ms at 50 mm Hg. An almost complete loss of functional contrast was measured at 70 mm Hg. The model of MR signal dephasing by Yablonskiy and Haacke (Mag Reson Med 1994;32:749-763) was used to predict changes in cerebral blood flow (CBF), which were found to be consistent with results from previous positron emission tomography (PET) studies. This study further emphasizes that global CBF changes (due to PETCO(2) changes even in the physiological range) strongly influence fMRI contrast and need to be controlled for.

Enhancement of BOLD-contrast sensitivity by single-shot multi-echo functional MR imaging.

Improved data acquisition and processing strategies for blood oxygenation level-dependent (BOLD)-contrast functional magnetic resonance imaging (fMRI), which enhance the functional contrast-to-noise ratio (CNR) by sampling multiple echo times in a single shot, are described. The dependence of the CNR on T2*, the image encoding time, and the number of sampled echo times are investigated for exponential fitting, echo summation, weighted echo summation, and averaging of correlation maps obtained at different echo times. The method is validated in vivo using visual stimulation and turbo proton echoplanar spectroscopic imaging (turbo-PEPSI), a new single-shot multi-slice MR spectroscopic imaging technique, which acquires up to 12 consecutive echoplanar images with echo times ranging from 12 to 213 msec. Quantitative T2*-mapping significantly increases the measured extent of activation and the mean correlation coefficient compared with conventional echoplanar imaging. The sensitivity gain with echo summation, which is computationally efficient provides similar sensitivity as fitting. For all data processing methods sensitivity is optimum when echo times up to 3.2 T2* are sampled. This methodology has implications for comparing functional sensitivity at different magnetic field strengths and between brain regions with different magnetic field inhomogeneities.

Analytical model of susceptibility-induced MR signal dephasing: effect of diffusion in a microvascular network.

A deterministic analytical model that describes the time course of magnetic resonance signal relaxation due to magnetic field inhomogeneity induced by a vascular network is developed. Both static and diffusion dephasing are taken into account. The contribution of the diffusion dephasing is calculated for relatively large vessels (R>10 microm) or short measurement times when the diffusion length is smaller than the vessel radius. The signal is found to possess the following features: a) an initial deviation from the monoexponential relaxation which is more pronounced for the imaginary part of the signal; b) a deviation from monoexponential relaxation at short echo times for the spin-echo (SE) signal measured as a function of the echo time; c) the echo maximum of the SE signal shifted from the nominal echo time to a shorter time; and d) a diffusion effect much stronger for the SE than for the free induction decay experiment. The model presented agrees within its validity domain with a known Monte Carlo simulation.

[Organization and volume of work in the department of anesthesiology and resuscitation in a central district hospital]. / Organizatsiia i ob"em raboty otdeleniia anesteziologii i reanimatsii tsentral'noi raionnoi bol'nitsy.

The experience accumulated during organization and functioning of an anesthesiology and intensive care unit in a central district hospital is described. The efficacy of anesthesiological aid and intensive care has been demonstrated in conditions of a rural region. The authors believe advisable the establishment of anesthesiology and intensive care units in large central district hospitals.