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| 3. |
- 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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| 4. |
- Carlsson, Åsa, 1978, et al.
(författare)
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Degraded water suppression in small volume ¹H MRS due to localised shimming.
- 2011
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Ingår i: Magma (New York, N.Y.). - : Springer Science and Business Media LLC. - 0968-5243. ; 24:2, s. 97-107
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Tidskriftsartikel (refereegranskat)abstract
- High quality, artifact free, ¹H MRS require a homogeneous magnetic field over the volume of interest (VOI) and an effective global water suppression (WS). However, the shim coils act globally and, hence, affect the resonance frequency over the entire object, making the frequency selective WS spatially selective. Unsuppressed water signal may cause spurious echo artifacts. Aim: Study and visualise the effect localised shimming has on WS.
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| 5. |
- Garpebring, Anders, et al.
(författare)
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Phase-based arterial input functions in humans applied to dynamic contrast-enhanced MRI: potential usefulness and limitations
- 2011
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Ingår i: Magma. - : Springer Science and Business Media LLC. - 1352-8661 .- 0968-5243. ; 24:4, s. 233-245
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Tidskriftsartikel (refereegranskat)abstract
- Object Phase-based arterial input functions (AIFs) provide a promising alternative to standard magnitude-based AIFs, for example, because inflow effects are avoided. The usefulness of phase-based AIFs in clinical dynamic contrast-enhanced MRI (DCE-MRI) was investigated, and relevant pitfalls and sources of uncertainty were identified. Materials and methods AIFs were registered from eight human subjects on, in total, 21 occasions. AIF quality was evaluated by comparing AIFs from right and left internal carotid arteries and by assessing the reliability of blood plasma volume estimates. Results Phase-based AIFs yielded an average bolus peak of 3.9 mM and a residual concentration of 0.37 mM after 3 min, (0.033 mmol/kg contrast agent injection). The average blood plasma volume was 2.7% when using the AIF peak in the estimation, but was significantly different (p < 0.0001) and less physiologically reasonable when based on the AIF tail concentration. Motion-induced phase shifts and accumulation of contrast agent in background tissue regions were identified as main sources of uncertainty. Conclusion Phase-based AIFs are a feasible alternative to magnitude AIFs, but sources of errors exist, making quantification difficult, especially of the AIF tail. Improvement of the technique is feasible and also required for the phase-based AIF approach to reach its full potential.
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- Knutsson, Linda, et al.
(författare)
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Absolute quantification of perfusion using dynamic susceptibility contrast MRI: pitfalls and possibilities.
- 2010
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Ingår i: Magma. - : Springer Science and Business Media LLC. - 1352-8661. ; 23, s. 1-21
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Forskningsöversikt (refereegranskat)abstract
- Absolute quantification of cerebral blood flow, cerebral blood volume and mean transit time is desirable in the determination of tissue viability thresholds and tissue at risk in acute ischaemic stroke, as well as in cases where a global reduction in cerebral blood flow is expected, for example, in patients with dementia or depressive disorders. Absolute values are also useful when comparing sequential examinations of tissue perfusion parameters, for example, in the monitoring and follow-up of various kinds of therapy. Regardless of the method employed, a number of assumptions and approximations must be made to obtain absolute measures of perfusion. Furthermore, the different stages of data acquisition and processing are associated with various degrees of uncertainty. In this review, the problems of particular relevance to absolute quantification of cerebral perfusion parameters using dynamic susceptibility contrast magnetic resonance imaging are discussed, and possible solutions are outlined.
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| 7. |
- Larsson, Anne, et al.
(författare)
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Evaluation of an attenuation correction method for PET/MR imaging of the head based on substitute CT images
- 2013
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Ingår i: Magnetic Resonance Materials in Physics, Biology and Medicine. - : Springer Science and Business Media LLC. - 0968-5243 .- 1352-8661. ; 26:1, s. 127-136
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to evaluate MR-based attenuation correction of PET emission data of the head, based on a previously described technique that calculates substitute CT (sCT) images from a set of MR images. Images from eight patients, examined with F-18-FLT PET/CT and MRI, were included. sCT images were calculated and co-registered to the corresponding CT images, and transferred to the PET/CT scanner for reconstruction. The new reconstructions were then compared with the originals. The effect of replacing bone with soft tissue in the sCT-images was also evaluated. The average relative difference between the sCT-corrected PET images and the CT-corrected PET images was 1.6 % for the head and 1.9 % for the brain. The average standard deviations of the relative differences within the head were relatively high, at 13.2 %, primarily because of large differences in the nasal septa region. For the brain, the average standard deviation was lower, 4.1 %. The global average difference in the head when replacing bone with soft tissue was 11 %. The method presented here has a high rate of accuracy, but high-precision quantitative imaging of the nasal septa region is not possible at the moment.
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| 9. |
- Mannfolk, Peter, et al.
(författare)
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Dimensionality reduction of fMRI time series data using locally linear embedding.
- 2010
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Ingår i: Magma. - : Springer Science and Business Media LLC. - 1352-8661. ; 23:5-6, s. 327-338
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVE: Data-driven methods for fMRI analysis are useful, for example, when an a priori model of signal variations is unavailable. However, activation sources are typically assumed to be linearly mixed, although non-linear properties of fMRI data, including resting-state data, have been observed. In this work, the non-linear locally linear embedding (LLE) algorithm is introduced for dimensionality reduction of fMRI time series data. MATERIALS AND METHODS: LLE performance was optimised and tested using simulated and volunteer data for task-evoked responses. LLE was compared with principal component analysis (PCA) as a preprocessing step to independent component analysis (ICA). Using an example data set with known non-linear properties, LLE-ICA was compared with PCA-ICA and non-linear PCA-ICA. A resting-state data set was analysed to compare LLE-ICA and PCA-ICA with respect to identifying resting-state networks. RESULTS: LLE consistently found task-related components as well as known resting-state networks, and the algorithm compared well to PCA. The non-linear example data set demonstrated that LLE, unlike PCA, can separate non-linearly modulated sources in a low-dimensional subspace. Given the same target dimensionality, LLE also performed better than non-linear PCA. CONCLUSION: LLE is promising for fMRI data analysis and has potential advantages compared with PCA in terms of its ability to find non-linear relationships.
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| 10. |
- Mårtensson, Johanna, et al.
(författare)
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Spatial analysis of diffusion tensor tractography statistics along the inferior fronto-occipital fasciculus with application in progressive supranuclear palsy
- 2013
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Ingår i: Magnetic Resonance Materials in Physics, Biology and Medicine. - : Springer Science and Business Media LLC. - 0968-5243 .- 1352-8661. ; 26:6, s. 527-537
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of the study was to develop a method for analysis of diffusion parameters along white matter (WM) tracts, using spatial normalization based on anatomical landmarks, and to introduce the apparent area coefficient (AAC). The method's applicability was tested in the inferior fronto-occipital fasciculus (IFO) in patients with progressive supranuclear palsy (PSP) and healthy controls (HCs). A framework for analysis of diffusion parameters was developed. Spatial normalization of the tracts was performed using anatomical landmarks, to avoid deformations caused by cerebral atrophy. Initially, 38 HCs were used to optimize a threshold for the minimal size of regions that differ between groups. The fractional anisotropy, mean diffusivity, AAC, and the hemispheric asymmetry index (AI), were compared between 11 PSP patients and 15 HCs. The method was feasible for analysis of PSP patients and HCs. The AI showed that the observed hemispheric asymmetry of AAC was significantly larger in PSP patients compared with HCs in small regions of the IFO. The method was successfully employed for analysis of diffusion parameters along the IFO in a patient group. This method can be potentially useful in studies of WM diseases, with or without cerebral atrophy.
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