| 1. |
- Bibic, Adnan, et al.
(författare)
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Denoising of arterial spin labeling data: wavelet-domain filtering compared with Gaussian smoothing.
- 2010
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Ingår i: Magma. - : Springer Science and Business Media LLC. - 1352-8661. ; 23:3, s. 125-137
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: To investigate a wavelet-based filtering scheme for denoising of arterial spin labeling (ASL) data, potentially enabling reduction of the required number of averages and the acquisition time. METHODS: ASL magnetic resonance imaging (MRI) provides quantitative perfusion maps by using arterial water as an endogenous tracer. The signal difference between a labeled image, where inflowing arterial spins are inverted, and a control image is proportional to blood perfusion. ASL perfusion maps suffer from low SNR, and the experiment must be repeated a number of times (typically more than 40) to achieve adequate image quality. In this study, systematic errors introduced by the proposed wavelet-domain filtering approach were investigated in simulated and experimental image datasets and compared with conventional Gaussian smoothing. RESULTS: Application of the proposed method enabled a reduction of the number of averages and the acquisition time by at least 50% with retained standard deviation, but with effects on absolute CBF values close to borders and edges. CONCLUSIONS: When the ASL perfusion maps showed moderate-to-high SNRs, wavelet-domain filtering was superior to Gaussian smoothing in the vicinity of borders between gray and white matter, while Gaussian smoothing was a better choice for larger homogeneous areas, irrespective of SNR.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- Bibic, Adnan
(författare)
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WAVELET NOISE REDUCTION AND VASCULAR WATER TRANSPORT MODELLING : APPLICATIONS TO DIFFUSION AND PERFUSION MRI
- 2018. - 1
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetic resonance imaging (MRI) is a powerful medical imaging technique, used to detect and characterise a range of diseases and conditions. It is based on the use of a strong static magnetic field in combination with magnetic field gradients and pulsed radiofrequency electromagnetic fields to visualise various organs and structures in the body according to their morphology or function.Diffusion and perfusion MRI are established methods for quantitative measurements, often used in neurological and neurovascular clinical applications. Although these techniques are often used separately to investigate a number of diseases, combined diffusion and perfusion information can provide unique information, e.g. for assessment of whether stroke patients in the acute stage are likely to benefit from reperfusion therapy. This may be accomplished by identification of the so-called ischemic penumbra (i.e. the area surrounding the core of an infarct, exhibiting disturbed microcirculation but still viable and salvageable if the local blood supply is efficiently restored). This identification concept is often referred to as the diffusion–perfusion mismatch. In oncological applications, a combination of diffusion and perfusion MRI is sometimes used in tumour characterisation and in attempts to monitor early treatment response.Quantitative diffusion MRI may be hampered by a bias induced by the so-called rectified noise floor in areas with low signal-to-noise ratio (SNR), and to address this issue, a wavelet-based filtering method was presented and used for noise reduction in diffusion MRI.Perfusion images acquired by arterial spin labelling (ASL), which is the technique investigated in the present work, suffer from inherently low SNR, and this is commonly addressed by averaging multiple repetitions, which leads to a prolonged acquisition time. As an alternative approach, wavelet-domain filtering for noise reduction was applied to ASL data, and the performance of the proposed filtering technique was investigated (in terms of accuracy, precision and structural degradation), and a comparison with conventional Gaussian smoothing was also included. Additionally, a quantitative non-compartment modelling approach for assessment of blood water transit time through the microvasculature and the blood–brain barrier (BBB) was investigated. In one study, the model was adapted to a clinical setup and applied to test–retest data from healthy volunteers, and the effects of noise on the model were examined by simulations. In an animal study, the model was further developed by introducing a bolus- tracking ASL solution that included a measured arterial input function (AIF) instead of a theoretical rectangular input function. Furthermore, it was explored whether effects of mildly damaged red blood cells on microvascular parameters were detectable using the proposed modelling approach and by ASL-based CBF quantification.The extracted water transit time parameters can be used separately or in combination with conventional perfusion and diffusion estimates. Changes in the blood water transit time in the microvasculature may be related to alterations in capillary water permeability, and may thus be useful in the assessment of BBB integrity. Disturbed BBB permeability has been attributed to a number of disease states, and may be relevant to, for example, early diagnosis of Alzheimer's disease, inflammation, tumour grading and ischaemic stroke.
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| 5. |
- Jungner, Åsa, et al.
(författare)
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White Matter Brain Development after Exposure to Circulating Cell-Free Hemoglobin and Hyperoxia in a Rat Pup Model
- 2020
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Ingår i: Developmental Neuroscience. - : S. Karger AG. - 0378-5866 .- 1421-9859. ; 41:3-4, s. 234-246
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Tidskriftsartikel (refereegranskat)abstract
- Neonates born with critical congenital heart defects are at risk of diffuse white matter injuries and neurodevelopmental impairments. This study aimed to determine the impact of circulating cell-free hemoglobin and hyperoxia, both present during cardiopulmonary bypass circulation, on white matter brain development. Postnatal day 6 rat pups were injected intraperitoneally with cell-free Hb or vehicle and exposed to hyperoxia (fiO2 = 0.8) or normoxia (fiO2 = 0.21) for 24 h. We evaluated apoptosis, myelination, and oligodendrocyte maturation with immunohistochemistry, gene and protein analyses, and in vivo diffusion tensor magnetic resonance imaging (MRI). Consistent with previous studies, we found an increase in apoptosis of oligodendrocytes as determined by TUNEL+ staining in Olig2+ cells in white matter, cortex, thalamus, and hippocampus following exposure to hyperoxia with no additional effect of cell-free Hb. A transient increase in the mRNA expression of intercellular adhesion molecule 1 at 6 h was observed following combined exposure to cell-free Hb and hyperoxia. No indications of oligodendrocyte maturational delay or hypomyelination were observed after either insult, delivered separately or combined, as determined by immunohistochemistry, Western blot, and diffusion tensor MRI. In our model, exposure to circulatory cell-free Hb, with or without concomitant hyperoxia, did not significantly alter brain white matter development.
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| 6. |
- Madru, Renata, et al.
(författare)
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(68)Ga-labeled superparamagnetic iron oxide nanoparticles (SPIONs) for multi-modality PET/MR/Cherenkov luminescence imaging of sentinel lymph nodes.
- 2014
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Ingår i: American journal of nuclear medicine and molecular imaging. - 2160-8407. ; 4:1, s. 60-69
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to develop (68)Ga-SPIONs for use as a single contrast agent for dynamic, quantitative and high resolution PET/MR imaging of Sentinel Lymph Node (SLN). In addition (68)Ga enables Cherenkov light emission which can be used for optical guidance during resection of SLN. SPIONs were labeled with (68)Ga in ammonium acetate buffer, pH 5.5. The labeling yield and stability in human serum were determined using instant thin layer chromatography. An amount of 0.07-0.1 mL (~5-10 MBq, 0.13 mg Fe) of (68)Ga-SPIONs was subcutaneously injected in the hind paw of rats. The animals were imaged at 0-3 h and 25 h post injection with PET/CT, 9.4 T MR and CCDbased Cherenkov optical systems. A biodistribution study was performed by dissecting and measuring the radioactivity in lymph nodes, kidneys, spleen, liver and the injection site. The labeling yield was 97.3 ± 0.05% after 15 min and the (68)Ga-SPIONs were stable in human serum. PET, MR and Cherenkov luminescence imaging clearly visualized the SLN. Biodistribution confirmed a high uptake of the (68)Ga-SPIONs within the SLN. We conclude that generator produced (68)Ga can be labeled to SPIONs. Subcutaneously injected (68)Ga-SPIONs can enhance the identification of the SLNs by combining sensitive PET and high resolution MR imaging. Clinically, hybrid PET/MR cameras are already in use and (68)Ga-SPIONs have a great potential as a single-dose, tri-modality agent for diagnostic imaging and potential Cherenkov luminescent guided resection of SLN.
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| 7. |
- Sanchez, Juan-Diego, et al.
(författare)
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Radiative MRI coil design using parasitic scatterers: MRI Yagi
- 2018
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Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 66:3, s. 1570-1575
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Tidskriftsartikel (refereegranskat)abstract
- Conventionally, radiofrequency (RF) coils used for magnetic resonance imaging (MRI) are electrically small and designed for nearfield operation. Therefore, existing antenna design techniques are mostly irrelevant for RF coils. However, the use of higher frequencies in ultrahigh field (UHF) MRI allows for antenna design techniques to be adapted to RF coil designs. This study proposes the use of parasitic scatterers to improve the performance of an existing 7T MRI coil called the single-sided adapted dipole (SSAD) antenna. The results reveal that scatterers arranged in a Yagi fashion can be applied to reduce local specific absorption rate (SAR) maxima of a reference SSAD by 40% with only a 6% decrease in the propagated B1+ field at the tissue depth of 15 cm. The higher directivity of the proposed design also decreasing the coupling with additional elements, making this antenna suitable for use in high density arrays. These findings show the potential of parasitic scatterers as an effective method to improve the performance of existing radiative MRI coils.
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| 8. |
- Truong, My, et al.
(författare)
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Classifications of atherosclerotic plaque components with T1 and T2* mapping in 11.7 T MRI
- 2021
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Ingår i: European Journal of Radiology Open. - : Elsevier BV. - 2352-0477. ; 8, s. 100323-100323
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Tidskriftsartikel (refereegranskat)abstract
- Background and aims: Histopathology is the gold standard for analysis of atherosclerotic plaques but has drawbacks due to the destructive nature of the method. Ex vivo MRI is a non-destructive method to image whole plaques. Our aim was to use quantitative high field ex vivo MRI to classify plaque components, with histology as gold standard.Methods: Surgically resected carotid plaques from 12 patients with recent TIA or stroke were imaged at 11.7 T MRI. Quantitative T1/T2* mapping sequences and qualitative T1/T2* gradient echo sequences with voxel size of 30 × 30 × 60 μm3 were obtained prior to histological preparation, sectioning and staining for lipids, inflammation, hemorrhage, and fibrous tissue. Regions of interest (ROI) were selected based on the histological staining at multiple levels matched between histology and MRI. The MRI parameters of each ROI were then analyzed with quadratic discriminant analysis (QDA) for classification.Results: A total of 965 ROIs, at 70 levels matched between histology and MRI, were registered based on histological staining. In the nine plaques where three or more plaque components were possible to co-localize with MRI, the mean degree of misclassification by QDA was 16.5 %. One of the plaques contained mostly fibrous tissue and lipids and had no misclassifications, and two plaques mostly contained fibrous tissue. QDA generally showed good classification for fibrous tissue and lipids, whereas plaques with hemorrhage and inflammation had more misclassifications.Conclusion: 11.7 T ex vivo high field MRI shows good visual agreement with histology in carotid plaques. T1/T2* maps analyzed with QDA is a promising non-destructive method to classify plaque components, but with a higher degree of misclassifications in plaques with hemorrhage or inflammation.
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| 9. |
- Wirestam, Ronnie, et al.
(författare)
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Denoising of complex MRI data by wavelet-domain filtering: Application to high-b-value diffusion-weighted imaging.
- 2006
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Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 56:5, s. 1114-1120
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Tidskriftsartikel (refereegranskat)abstract
- The Rician distribution of noise in magnitude magnetic resonance (MR) images is particularly problematic in low signal-to-noise ratio (SNR) regions. The Rician noise distribution causes a nonzero minimum signal in the image, which is often referred to as the rectified noise floor. True low signal is likely to be concealed in the noise, and quantification is severely hampered in low-SNR regions. To address this problem we performed noise reduction (or denoising) by Wiener-like filtering in the wavelet domain. The filtering was applied to complex MRI data before construction of the magnitude image. The noise-reduction algorithm was applied to simulated and experimental diffusion-weighted (DW) images. Denoising considerably reduced the signal standard deviation (SD, by up to 87% in simulated images) and decreased the background noise floor (by approximately a factor of 6 in simulated and experimental images).
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