| 1. |
- Ahlgren, André, et al.
(författare)
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Quantification of microcirculatory parameters by joint analysis of flow-compensated and non-flow-compensated intravoxel incoherent motion (IVIM) data.
- 2016
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480 .- 1099-1492. ; 29:5, s. 640-649
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to improve the accuracy and precision of perfusion fraction and blood velocity dispersion estimates in intravoxel incoherent motion (IVIM) imaging, using joint analysis of flow-compensated and non-flow-compensated motion-encoded MRI data. A double diffusion encoding sequence capable of switching between flow-compensated and non-flow-compensated encoding modes was implemented. In vivo brain data were collected in eight healthy volunteers and processed using the joint analysis. Simulations were used to compare the performance of the proposed analysis method with conventional IVIM analysis. With flow compensation, strong rephasing was observed for the in vivo data, approximately cancelling the IVIM effect. The joint analysis yielded physiologically reasonable perfusion fraction maps. Estimated perfusion fractions were 2.43 ± 0.81% in gray matter, 1.81 ± 0.90% in deep gray matter, and 1.64 ± 0.72% in white matter (mean ± SD, n = 8). Simulations showed improved accuracy and precision when using joint analysis of flow-compensated and non-flow-compensated data, compared with conventional IVIM analysis. Double diffusion encoding with flow compensation was feasible for in vivo imaging of the perfusion fraction in the brain. The strong rephasing implied that blood flowing through the cerebral microvascular system was closer to the ballistic limit than the diffusive limit. © 2016 The Authors NMR in Biomedicine published by John Wiley & Sons Ltd.
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| 2. |
- Lätt, Jimmy, et al.
(författare)
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Diffusion-weighted MRI measurements on stroke patients reveal water-exchange mechanisms in sub-acute ischaemic lesions.
- 2009
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480 .- 1099-1492. ; 22:6, s. 619-628
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to investigate the diffusion time dependence of signal-versus-b curves obtained from diffusion-weighted magnetic resonance imaging (DW-MRI) of sub-acute ischaemic lesions in stroke patients. In this case series study, 16 patients with sub-acute ischaemic stroke were examined with DW-MRI using two different diffusion times (60 and 260 ms). Nine of these patients showed sufficiently large lesions without artefacts to merit further analysis. The signal-versus-b curves from the lesions were plotted and analysed using a two-compartment model including compartmental exchange. To validate the model and to aid the interpretation of the estimated model parameters, Monte Carlo simulations were performed. In eight cases, the plotted signal-versus-b curves, obtained from the lesions, showed a signal-curve split-up when data for the two diffusion times were compared, revealing effects of compartmental water exchange. For one of the patients, parametric maps were generated based on the extracted model parameters. These novel observations suggest that water exchange between different water pools is measurable and thus potentially useful for clinical assessment. The information can improve the understanding of the relationship between the DW-MRI signal intensity and the microstructural properties of the lesions. Copyright (c) 2009 John Wiley & Sons, Ltd.
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| 3. |
- Ahlgren, André, et al.
(författare)
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Partial volume correction of brain perfusion estimates using the inherent signal data of time-resolved arterial spin labeling.
- 2014
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480. ; 27:9, s. 1112-1122
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Tidskriftsartikel (refereegranskat)abstract
- Quantitative perfusion MRI based on arterial spin labeling (ASL) is hampered by partial volume effects (PVEs), arising due to voxel signal cross-contamination between different compartments. To address this issue, several partial volume correction (PVC) methods have been presented. Most previous methods rely on segmentation of a high-resolution T1 -weighted morphological image volume that is coregistered to the low-resolution ASL data, making the result sensitive to errors in the segmentation and coregistration. In this work, we present a methodology for partial volume estimation and correction, using only low-resolution ASL data acquired with the QUASAR sequence. The methodology consists of a T1 -based segmentation method, with no spatial priors, and a modified PVC method based on linear regression. The presented approach thus avoids prior assumptions about the spatial distribution of brain compartments, while also avoiding coregistration between different image volumes. Simulations based on a digital phantom as well as in vivo measurements in 10 volunteers were used to assess the performance of the proposed segmentation approach. The simulation results indicated that QUASAR data can be used for robust partial volume estimation, and this was confirmed by the in vivo experiments. The proposed PVC method yielded probable perfusion maps, comparable to a reference method based on segmentation of a high-resolution morphological scan. Corrected gray matter (GM) perfusion was 47% higher than uncorrected values, suggesting a significant amount of PVEs in the data. Whereas the reference method failed to completely eliminate the dependence of perfusion estimates on the volume fraction, the novel approach produced GM perfusion values independent of GM volume fraction. The intra-subject coefficient of variation of corrected perfusion values was lowest for the proposed PVC method. As shown in this work, low-resolution partial volume estimation in connection with ASL perfusion estimation is feasible, and provides a promising tool for decoupling perfusion and tissue volume. Copyright © 2014 John Wiley & Sons, Ltd.
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| 4. |
- Bibic, Adnan, et al.
(författare)
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Measurement of vascular water transport in human subjects using time-resolved pulsed arterial spin labelling.
- 2015
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480. ; 28:8, s. 1059-1068
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Tidskriftsartikel (refereegranskat)abstract
- Most approaches to arterial spin labelling (ASL) data analysis aim to provide a quantitative measure of the cerebral blood flow (CBF). This study, however, focuses on the measurement of the transfer time of blood water through the capillaries to the parenchyma (referred to as the capillary transfer time, CTT) as an alternative parameter to characterise the haemodynamics of the system. The method employed is based on a non-compartmental model, and no measurements need to be added to a common time-resolved ASL experiment. Brownian motion of labelled spins in a potential was described by a one-dimensional general Langevin equation as the starting point, and as a Fokker-Planck differential equation for the averaged distribution of labelled spins at the end point, which takes into account the effects of flow and dispersion of labelled water by the pseudorandom nature of the microvasculature and the transcapillary permeability. Multi-inversion time (multi-TI) ASL data were acquired in 14 healthy subjects on two occasions in a test-retest design, using a pulsed ASL sequence and three-dimensional gradient and spin echo (3D-GRASE) readout. Based on an error analysis to predict the size of a region of interest (ROI) required to obtain reasonably precise parameter estimates, data were analysed in two relatively large ROIs, i.e. the occipital lobe (OC) and the insular cortex (IC). The average values of CTT in OC were 260 ± 60 ms in the first experiment and 270 ± 60 ms in the second experiment. The corresponding IC values were 460 ± 130 ms and 420 ± 139 ms, respectively. Information related to the water transfer time may be important for diagnostics and follow-up of cerebral conditions or diseases characterised by a disrupted blood-brain barrier or disturbed capillary blood flow. Copyright © 2015 John Wiley & Sons, Ltd.
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| 5. |
- Knutsson, Linda, et al.
(författare)
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Combined diffusion weighting and CSF suppression in functional MRI.
- 2002
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480. ; 15:3, s. 235-240
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Tidskriftsartikel (refereegranskat)abstract
- In this study, EPI pulse sequences with diffusion weighting for reduction of contributions from large vessels and inversion pulses in order to minimize the effects of CSF pulsations and CSF partial volume effects were developed for BOLD contrast investigations in functional MR imaging. One inversion recovery echo-planar imaging (IR-EPI) pulse sequence and one IR-EPI with additional diffusion weighting (DW-IR-EPI) were developed and compared to a standard gradient-echo EPI sequence in a cortical stimulation experiment in nine healthy volunteers. Stimulation of motor cortex was performed using a semi-complex finger-tapping paradigm in seven periods of alternating rest and stimulation. Comparison between the three pulse sequences was made by measuring the activated volume in each subject, as well as by calculating the relative signal increase during stimulation. Due to different baseline signal-to-noise levels in the images generated by the three pulse sequences, artificial noise was added so that the comparative investigation could be performed independently of the noise level. The activated volume was 128 +/- 73 pixels (mean +/- SD) using the standard EPI pulse sequence, 31 +/- 12 pixels using IR-EPI and 15 +/- 13 pixels when DW-IR-EPI was employed. The relative signal increase was 5.7 +/- 1.1% using standard EPI, 11.5 +/- 3.1% using IR-EPI and 9.9 +/- 2.4% using DW-IR-EPI. The activated volume obtained with the addition of extra noise, i.e. at equal S/N, was 70 +/- 50 pixels using the standard EPI, and when using IR-EPI, the activated volume was 28 +/- 13 pixels. At equal S/N, the signal increase was 7.3 +/- 1.4% using standard EPI and 12.0 +/- 3.6% using IR-EPI. In BOLD contrast imaging, a combination of diffusion weighting and inversion recovery appeared to reduce false activation caused by CSF pulsation and blood flow in large vessels.
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| 6. |
- Moser, Ewald, et al.
(författare)
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7-T MR-from research to clinical applications?
- 2012
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480. ; 25:5, s. 695-716
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Forskningsöversikt (refereegranskat)abstract
- Over 20?000 MR systems are currently installed worldwide and, although the majority operate at magnetic fields of 1.5?T and below (i.e. about 70%), experience with 3-T (in high-field clinical diagnostic imaging and research) and 7-T (research only) human MR scanners points to a future in functional and metabolic MR diagnostics. Complementary to previous studies, this review attempts to provide an overview of ultrahigh-field MR research with special emphasis on emerging clinical applications at 7?T. We provide a short summary of the technical development and the current status of installed MR systems. The advantages and challenges of ultrahigh-field MRI and MRS are discussed with special emphasis on radiofrequency inhomogeneity, relaxation times, signal-to-noise improvements, susceptibility effects, chemical shifts, specific absorption rate and other safety issues. In terms of applications, we focus on the topics most likely to gain significantly from 7-T MR, i.e. brain imaging and spectroscopy and musculoskeletal imaging, but also body imaging, which is particularly challenging. Examples are given to demonstrate the advantages of susceptibility-weighted imaging, time-of-flight MR angiography, high-resolution functional MRI, 1H and 31P MRSI in the human brain, sodium and functional imaging of cartilage and the first results (and artefacts) using an eight-channel body array, suggesting future areas of research that should be intensified in order to fully explore the potential of 7-T MR systems for use in clinical diagnosis. Copyright (C) 2011 John Wiley & Sons, Ltd.
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| 7. |
- Nilsson, Markus, et al.
(författare)
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The importance of axonal undulation in diffusion MR measurements: a Monte Carlo simulation study.
- 2012
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Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480. ; 25, s. 795-805
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Tidskriftsartikel (refereegranskat)abstract
- Many axons follow wave-like undulating courses. This is a general feature of extracranial nerve segments, but is also found in some intracranial nervous tissue. The importance of axonal undulation has previously been considered, for example, in the context of biomechanics, where it has been shown that posture affects undulation properties. However, the importance of axonal undulation in the context of diffusion MR measurements has not been investigated. Using an analytical model and Monte Carlo simulations of water diffusion, this study compared undulating and straight axons in terms of diffusion propagators, diffusion-weighted signal intensities and parameters derived from diffusion tensor imaging, such as the mean diffusivity (MD), the eigenvalues and the fractional anisotropy (FA). All parameters were strongly affected by the presence of undulation. The diffusivity perpendicular to the undulating axons increased with the undulation amplitude, thus resembling that of straight axons with larger diameters. Consequently, models assuming straight axons for the estimation of the axon diameter from diffusion MR measurements might overestimate the diameter if undulation is present. FA decreased from approximately 0.7 to 0.5 when axonal undulation was introduced into the simulation model structure. Our results indicate that axonal undulation may play a role in diffusion measurements when investigating, for example, the optic and sciatic nerves and the spinal cord. The simulations also demonstrate that the stretching or compression of neuronal tissue comprising undulating axons alters the observed water diffusivity, suggesting that posture may be of importance for the outcome of diffusion MRI measurements. Copyright © 2011 John Wiley & Sons, Ltd.
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