| 1. |
- Bibic, Adnan, et al.
(författare)
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Effects of red blood cells with reduced deformability on cerebral blood flow and vascular water transport: measurements in rats using time-resolved pulsed arterial spin labelling at 9.4 T
- 2021
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Ingår i: European Radiology Experimental. - : Springer Science and Business Media LLC. - 2509-9280. ; 5, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- BackgroundOur aim was to introduce damaged red blood cells (RBCs) as a tool for haemodynamic provocation in rats, hypothesised to cause decreased cerebral blood flow (CBF) and prolonged water capillary transfer time (CTT), and to investigate whether expected changes in CBF could be observed and if haemodynamic alterations were reflected by the CTT metric.MethodsDamaged RBCs exhibiting a mildly reduced deformability were injected to cause aggregation of RBCs. Arterial spin labelling (ASL) magnetic resonance imaging experiments were performed at 9.4 T. Six datasets (baseline plus five datasets after injection) were acquired for each animal in a study group and a control group (13 and 10 female adult Wistar rats, respectively). For each dataset, ASL images at ten different inversion times were acquired. The CTT model was adapted to the use of a measured arterial input function, implying the use of a realistic labelling profile. Repeated measures ANOVA was used (alpha error = 0.05).ResultsAfter injection, significant differences between the study group and control group were observed for relative CBF in white matter (up to 20 percentage points) and putamen (up to 18–20 percentage points) and for relative CTT in putamen (up to 35–40 percentage points).ConclusionsHaemodynamic changes caused by injection of damaged RBCs were observed by ASL-based CBF and CTT measurements. Damaged RBCs can be used as a tool for test and validation of perfusion imaging modalities. CTT model fitting was challenging to stabilise at experimental signal-to-noise ratio levels, and the number of free parameters was minimised.
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| 2. |
- Chakwizira, Arthur, et al.
(författare)
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Non-parametric deconvolution using Bézier curves for quantification of cerebral perfusion in dynamic susceptibility contrast MRI
- 2022
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Ingår i: Magnetic Resonance Materials in Physics, Biology, and Medicine. - : Springer Science and Business Media LLC. - 1352-8661. ; 35:5, s. 791-804
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Tidskriftsartikel (refereegranskat)abstract
- Objective Deconvolution is an ill-posed inverse problem that tends to yield non-physiological residue functions R(t) indynamic susceptibility contrast magnetic resonance imaging (DSC-MRI). In this study, the use of Bézier curves is proposedfor obtaining physiologically reasonable residue functions in perfusion MRI.Materials and methods Cubic Bézier curves were employed, ensuring R(0)=1, bounded-input, bounded-output stability anda non-negative monotonically decreasing solution, resulting in 5 parameters to be optimized. Bézier deconvolution (BzD),implemented in a Bayesian framework, was tested by simulation under realistic conditions, including efects of arterial delayand dispersion. BzD was also applied to DSC-MRI data from a healthy volunteer.Results Bézier deconvolution showed robustness to diferent underlying residue function shapes. Accurate perfusion estimates were observed, except for boxcar residue functions at low signal-to-noise ratio. BzD involving corrections for delay,dispersion, and delay with dispersion generally returned accurate results, except for some degree of cerebral blood fow(CBF) overestimation at low levels of each efect. Maps of mean transit time and delay were markedly diferent betweenBzD and block-circulant singular value decomposition (oSVD) deconvolution.Discussion A novel DSC-MRI deconvolution method based on Bézier curves was implemented and evaluated. BzD produced physiologically plausible impulse response, without spurious oscillations, with generally less CBF underestimationthan oSVD.
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| 3. |
- Holmgren, Madelene, 1992-
(författare)
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4D flow MRI and modelling to assess cerebral arterial hemodynamics : method development and evaluation, with implementation in patients with symptomatic carotid stenosis
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Blood flow measurements are important for understanding the development of cerebrovascular diseases. With 4D flow magnetic resonance imaging (4D flow MRI), simultaneous velocity measurements are obtained in all cerebral arteries in a scan of about ten minutes. However, 4D flow MRI is a relatively new technique. For usefulness in both clinics and research, detailed knowledge is needed about its accuracy and precision for flow quantification. In patients with stroke or transient ischemic attack (TIA) from a symptomatic carotid stenosis, the stenosis may generate a difference in blood pressure and flow between the left and right cerebral hemispheres. Such a hemispheric pressure difference could be an early marker of to what extent a stenosis is affecting cerebral hemodynamics, which could be useful in the planning of carotid surgery. The overall aim of the thesis was to determine the accuracy of 4D flow MRI to measure cerebral arterial blood flow, and to develop and evaluate an approach combining 4D flow MRI and computational fluid dynamics (CFD) to characterize the cerebral arterial hemodynamics, with implementation in patients with symptomatic carotid stenosis. The thesis is based on four papers, investigating two cohorts.The first cohort consisted of 35 elderly volunteers (mean age 79 years) and was studied in paper I-II. Blood flow rates were measured in nine cerebral arteries with 4D flow MRI and 2D phase-contrast MRI as reference. Three different flow quantification methods for 4D flow MRI were evaluated and optimized: one clustering approach and two threshold-based methods. The proposed new method, based on a locally adapted threshold, outperformed the previously suggested methods in flow rate quantification. For the clustering method, flow rates were systematically underestimated. 4D flow MRI was also evaluated to assess different arterial pulsatility measures, and a Windkessel model was used to estimate reference values for cerebrovascular resistance and cerebral arterial compliance in elderly.The second cohort consisted of 28 stroke and TIA patients (mean age 73 years) with symptomatic carotid stenosis and was studied in paper III-IV. With 4D flow MRI and CFD, the preoperative hemispheric pressure laterality was quantified in the patients. The pressure laterality was compared to hemispheric flow lateralities. Estimating the hemispheric pressure laterality was a promising physiological biomarker for grading the cerebral arterial hemodynamic disturbances in patients with symptomatic carotid stenosis. A CFD model was also developed to predict carotid stump pressure, i.e., the important pressure measured in the clamped carotid artery during surgical removal of the stenosis. The predicted stump pressures were correlated with the pressures measured during surgery. Stump pressure prediction was promising and could be a potential tool in the preoperative planning in order to avoid hypoperfusion during surgery. In summary, post-processing methods were successfully developed and evaluated for accurate assessment of mean and pulsatile cerebral blood flow rates with 4D flow MRI. Thereby, this thesis provided knowledge about possibilities and limitations of how 4D flow MRI can be used with respect to cerebral arterial blood flow rate assessment. By contributing with models combining 4D flow MRI and CFD, specifically developed for analysis of pressure distributions in cerebral arteries, novel methods were proposed for assessing patients with symptomatic carotid stenosis in the planning of carotid surgery.
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| 4. |
- Knutsson, Linda, et al.
(författare)
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Arterial Input Function : A Friend or Foe?
- 2023. - 1
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Ingår i: Quantitative Perfusion MRI : Techniques, Applications and Practical Considerations - Techniques, Applications and Practical Considerations. - 9780323952101 - 9780323952095 ; 11, s. 171-196
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Bokkapitel (refereegranskat)
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| 5. |
- Lehmann, Patrick M, et al.
(författare)
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A numerical human brain phantom for dynamic glucose-enhanced (DGE) MRI : On the influence of head motion at 3T
- 2023
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 89:5, s. 1871-1887
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: Dynamic glucose-enhanced (DGE) MRI relates to a group of exchange-based MRI techniques where the uptake of glucose analogues is studied dynamically. However, motion artifacts can be mistaken for true DGE effects, while motion correction may alter true signal effects. The aim was to design a numerical human brain phantom to simulate a realistic DGE MRI protocol at 3T that can be used to assess the influence of head movement on the signal before and after retrospective motion correction.METHODS: MPRAGE data from a tumor patient were used to simulate dynamic Z-spectra under the influence of motion. The DGE responses for different tissue types were simulated, creating a ground truth. Rigid head movement patterns were applied as well as physiological dilatation and pulsation of the lateral ventricles and head-motion-induced B 0 -changes in presence of first-order shimming. The effect of retrospective motion correction was evaluated. RESULTS: Motion artifacts similar to those previously reported for in vivo DGE data could be reproduced. Head movement of 1 mm translation and 1.5 degrees rotation led to a pseudo-DGE effect on the order of 1% signal change. B 0 effects due to head motion altered DGE changes due to a shift in the water saturation spectrum. Pseudo DGE effects were partly reduced or enhanced by rigid motion correction depending on tissue location. CONCLUSION: DGE MRI studies can be corrupted by motion artifacts. Designing post-processing methods using retrospective motion correction including B 0 correction will be crucial for clinical implementation. The proposed phantom should be useful for evaluation and optimization of such techniques.
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| 6. |
- Lind, Emelie, et al.
(författare)
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Dynamic contrast-enhanced QSM for perfusion imaging : a systematic comparison of ΔR2*- and QSM-based contrast agent concentration time curves in blood and tissue
- 2020
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Ingår i: Magnetic Resonance Materials in Physics, Biology, and Medicine. - : Springer Science and Business Media LLC. - 1352-8661 .- 0968-5243. ; 33:5, s. 663-676
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVE: In dynamic susceptibility contrast MRI (DSC-MRI), an arterial input function (AIF) is required to quantify perfusion. However, estimation of the concentration of contrast agent (CA) from magnitude MRI signal data is challenging. A reasonable alternative would be to quantify CA concentration using quantitative susceptibility mapping (QSM), as the CA alters the magnetic susceptibility in proportion to its concentration.MATERIAL AND METHODS: AIFs with reasonable appearance, selected on the basis of conventional criteria related to timing, shape, and peak concentration, were registered from both ΔR2* and QSM images and mutually compared by visual inspection. Both ΔR2*- and QSM-based AIFs were used for perfusion calculations based on tissue concentration data from ΔR2*as well as QSM images.RESULTS: AIFs based on ΔR2* and QSM data showed very similar shapes and the estimated cerebral blood flow values and mean transit times were similar. Analysis of corresponding ΔR2* versus QSM-based concentration estimates yielded a transverse relaxivity estimate of 89 s-1 mM-1, for voxels identified as useful AIF candidate in ΔR2* images according to the conventional criteria.DISCUSSION: Interestingly, arterial concentration time curves based on ΔR2* versus QSM data, for a standard DSC-MRI experiment, were generally very similar in shape, and the relaxivity obtained in voxels representing blood was similar to tissue relaxivity obtained in previous studies.
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| 7. |
- Lundberg, Anna, et al.
(författare)
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Comparison of MRI methods for measuring whole-brain oxygen extraction fraction under different geometric conditions at 7T
- 2022
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Ingår i: Journal of Neuroimaging. - : Wiley. - 1051-2284 .- 1552-6569. ; 32:3, s. 442-458
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Tidskriftsartikel (refereegranskat)abstract
- Background and Purpose: Cerebral tissue oxygenation is a critical brain viability parameter, and the magnetic properties of hemoglobin offer the opportunity to noninvasively quantify oxygen extraction fraction (OEF) by magnetic resonance imaging (MRI). Ultrahigh-field MRI shows advantages such as increased sensitivity to magnetic susceptibility differences and improved signal-to-noise ratio that can be translated into smaller voxel size, but also increased sensitivity to static and B1 field inhomogeneities. The aim was to produce a systematic comparison of three MRI-based methods for estimation of OEF.Methods: OEF estimates in 16 healthy subjects were obtained at 7T utilizingsusceptometry-based oximetry (SBO), quantitative susceptibility mapping (QSM), and transverse relaxation rate (R2*). Two major draining veins, that is, the superior sagittal sinus (SSS) and the straight sinus (SS), were investigated, including mutual agreement between the methods in each of the two different vessels, agreement between vessels as well as potential vessel angle and vessel size dependences.Results: Very good correlation (r = .88) was found between SBO-based and QSM-based OEF estimates in SSS. Only QSM showed a moderate correlation (r = .61) between corresponding OEF estimates in SSS and SS. For SBO, a trend of increasing OEF estimates was observed as the SS vessel angle relative to the main magnetic field increased. No obvious size dependence could be established for anymethod. TheR2*-basedOEFestimates were reasonable (35%-36%), but the observed range was somewhat low.Conclusion: The results indicate thatQSMis a promising candidate for assessment of OEF estimates, for example, providing reasonably robust estimates across awide range of vessel orientations.
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| 8. |
- Lundberg, Anna, et al.
(författare)
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Notes on quantitative susceptibility mapping reconstruction accuracy under challenging conditions: Phantom measurements and simulations
- 2022
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Ingår i: Journal of Medical and Scientific Research. ; 10:3, s. 111-117
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic susceptibility can be assessed by quantitative susceptibility mapping (QSM), based on measured magnetic resonance imaging (MRI) phase data. The QSM reconstruction process is, however, mathematically challenging and still not fully robust. A signal-generating holmium [Ho(III)] aqueous solution with air-equivalent magnetic susceptibility was prepared, and used as a surrounding medium in a water phantom with tubes filled with a solution of gadolinium contrast agent at various concentrations. Extended analyses under controlled conditions were accomplished by simulations of the phantom construction. Without surrounding holmium solution, a gadolinium tube positioned centrally, parallel with B0, showed a susceptibility difference that agreed well with theoretical values, whereas a peripheral parallel tube position showed larger deviation. Orientation perpendicular to B0 resulted in less variation between the internal tube positions. Air-equivalent magnetic susceptibility corresponded to 16.5 mM Ho(III) solution. With surrounding holmium solution, several post-processing steps became challenging. Simulations indicated higher degree of underestimation when the theoretical susceptibility difference increased. Details in the mathematical implementation, for example, background field removal can strongly influence the result. Simulated results were, in part, unexpected, and provided awareness of limitations in the reconstruction technique, mainly related to conditions with large susceptibility differences between compartments.
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| 9. |
- Mohammed Ali, Sajad, et al.
(författare)
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Deep learning-based Lorentzian fitting of water saturation shift referencing spectra in MRI
- 2023
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Ingår i: Magnetic Resonance in Medicine. - 1522-2594. ; 90:4, s. 1610-1624
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: Water saturation shift referencing (WASSR) Z-spectra are used commonly for field referencing in chemical exchange saturation transfer (CEST) MRI. However, their analysis using least-squares (LS) Lorentzian fitting is time-consuming and prone to errors because of the unavoidable noise in vivo. A deep learning-based single Lorentzian Fitting Network (sLoFNet) is proposed to overcome these shortcomings.METHODS: A neural network architecture was constructed and its hyperparameters optimized. Training was conducted on a simulated and in vivo-paired data sets of discrete signal values and their corresponding Lorentzian shape parameters. The sLoFNet performance was compared with LS on several WASSR data sets (both simulated and in vivo 3T brain scans). Prediction errors, robustness against noise, effects of sampling density, and time consumption were compared.RESULTS: LS and sLoFNet performed comparably in terms of RMS error and mean absolute error on all in vivo data with no statistically significant difference. Although the LS method fitted well on samples with low noise, its error increased rapidly when increasing sample noise up to 4.5%, whereas the error of sLoFNet increased only marginally. With the reduction of Z-spectral sampling density, prediction errors increased for both methods, but the increase occurred earlier (at 25 vs. 15 frequency points) and was more pronounced for LS. Furthermore, sLoFNet performed, on average, 70 times faster than the LS-method.CONCLUSION: Comparisons between LS and sLoFNet on simulated and in vivo WASSR MRI Z-spectra in terms of robustness against noise and decreased sample resolution, as well as time consumption, showed significant advantages for sLoFNet.
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| 10. |
- Olsson, Hampus, et al.
(författare)
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Mapping magnetization transfer saturation (MTsat) in human brain at 7T : Protocol optimization under specific absorption rate constraints
- 2021
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194.
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To optimize a whole-brain magnetization transfer saturation (MTsat) protocolat 7T, focusing on maximizing obtainable MTsat under the constraints of specificabsorption rate (SAR) and transmit field inhomogeneity, while avoiding bias and keeping scan time short.Theory and Methods: MTsat is a semi-quantitative metric, obtained by spoiled gradient-echo MRI in the imaging steady-state. Optimization was based on an established 7T dual flip angle protocol, and focused on MT pulse, readout flip angle, repetition time (TR), offset frequency (Δ), and correction of residual effects from transmit field inhomogeneities by separate flip angle mapping.Results: A 100% SAR level was reached at a 180° MT pulse flip angle, using a compact sinc main lobe (4 ms duration) and minimum TR = 26.5 ms. The use of Δ = +2.0 kHz caused no discernible direct saturation, while Δ = −2.0 kHz resulted in 45% higher MTsat in white matter (WM) compared to Δ = +2.0 kHz. A 4° readout flip angle eliminated bias while yielding a good signal-to-noise ratio. Increased TR yielded only a little increase in MTsat, and TR = 26.5 ms (scan time 04:58 min) was thus selected. Post hoc transmit field correction clearly improved homogeneity, especially in WM.Conclusions: The range of MTsat is limited at 7T, and this can partly be overcome by the exploitation of the asymmetry of the macromolecular lineshape through the sign of Δ. To reduce scan time, a compact MT pulse with a sufficiently narrow frequency response should be used. TR and readout flip angle should be kept short/small. Transmit field correction through separate flip angle mapping is required.
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