| 1. |
- Lätt, Jimmy, et al.
(författare)
-
Accuracy of q-space related parameters in MRI: Simulations and phantom measurements
- 2007
-
Ingår i: IEEE Transactions on Medical Imaging. - 1558-254X. ; 26:11, s. 1437-1447
-
Tidskriftsartikel (refereegranskat)abstract
- The accuracy of q-space measurements was evaluated at a 3.0-T clinical magnetic resonance imaging (MRI) scanner, as compared with a 4.7-T nuclear magnetic resonance (NMR) spectrometer. Measurements were performed using a stimulated-echo pulse-sequence on n-decane as well as on polyethylene glycol (PEG) mixed with different concentrations of water, in order to obtain bi-exponential signal decay curves. The diffusion coefficients as well as the modelled diffusional kurtosis K-fit, were obtained from the signal decay curve, while the full-width at half-maximum (FWHM) and the diffusional kurtosis K were obtained from the displacement distribution. Simulations of restricted diffusion, under conditions similar to those obtainable with a clinical MRI scanner, were carried out assuming various degrees of violation of the short gradient pulse (SGP) condition and of the long diffusion time limit., The results indicated that an MRI system can not be used for quantification of structural sizes less than about 10 mu m by means of FWHM since the parameter underestimates the confinements due to violation of the SGP condition. However, FWHM can still be used as an important contrast parameter. The obtained kurtosis values were lower than expected from theory and the results showed that care must be taken when interpreting a kurtosis estimate deviating from zero.
|
|
| 2. |
- Lätt, Jimmy, et al.
(författare)
-
Diffusion-weighted MRI measurements on stroke patients reveal water-exchange mechanisms in sub-acute ischaemic lesions.
- 2009
-
Ingår i: NMR in Biomedicine. - : Wiley. - 0952-3480 .- 1099-1492. ; 22:6, s. 619-628
-
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.
|
|
| 3. |
- Lätt, Jimmy, et al.
(författare)
-
Effects of restricted diffusion in a biological phantom: a q-space diffusion MRI study of asparagus stems at a 3T clinical scanner
- 2007
-
Ingår i: Magma. - : Springer Science and Business Media LLC. - 1352-8661. ; 20:4, s. 213-222
-
Tidskriftsartikel (refereegranskat)abstract
- Introduction The aim of this work was to study the effects of restricted diffusion in a biological phantom consisting of green asparagus stems using q-space MRI at a clinical scanner. Method Measurements of the full width at half maximum (FWHM) of the displacement distribution were performed with varied diffusion time (T d). The accuracy of the measurements was investigated with respect to the degree of violation of the short gradient pulse (SGP) condition, partial volume effects and a FWHM-based tensor model. Results The measurements showed a reasonably constant FWHM perpendicular to the capillaries in the vascular bundles and an increased FWHM parallel with the bundles when the T d was increased. A 15% decrease in FWHM perpendicular to the bundles was observed when the diffusion encoding duration was prolonged from 24 to 74 ms, owing to the violation of the SGP condition. For a population of different confinement sizes, simulations indicated that the FWHM reflects the smaller sizes rather then the mean size of the confinements. Conclusion A new method allowing tensor analysis of FWHM was derived and yielded accurate results. In conclusion, we found it possible to measure the effects of restricted diffusion with q-space MRI using a clinical MRI scanner.
|
|
| 4. |
- Lätt, Jimmy, et al.
(författare)
-
In vivo visualization of displacement-distribution-derived parameters in q-space imaging.
- 2008
-
Ingår i: Magnetic Resonance Imaging. - : Elsevier BV. - 1873-5894 .- 0730-725X. ; 26:1, s. 77-87
-
Tidskriftsartikel (refereegranskat)abstract
- Objective This study aimed to explore the potential of in vivo q-space imaging in the differentiation between different cerebral water components. Materials and Methods Diffusion-weighted imaging was performed in six directions with 32 equally spaced q values and a maximum b value of 6600 s/mm2. The shape of the signal-attenuation curve and the displacement propagator were examined and compared with a normal distribution using the kurtosis parameter. Maps displaying kurtosis, fast and slow components of the apparent diffusion coefficients, fractional anisotropy and directional diffusion were calculated. The displacement propagator was further described by the full width at half and at tenth maximum and by the probability density of zero displacement P(0). Three healthy volunteers and three patients with previously diagnosed multiple sclerosis (MS) were examined. Results Simulations indicated that the kurtosis of a signal-attenuation curve can determine if more than one water component is present and that care must be taken to select an appropriate threshold. It was possible to distinguish MS plaques in both signal and diffusional kurtosis maps, and in one patient, plaques of different degree of demyelinization showed different behavior. Discussion Our results indicate that in vivo q-space analysis is a potential tool for the assessment of different cerebral water components, and it might extend the diagnostic interpretation of data from diffusion magnetic resonance imaging.
|
|
| 5. |
- Malmborg, Carin, et al.
(författare)
-
Mapping the intracellular fraction of water by varying the gradient pulse length in q-space diffusion MRI
- 2006
-
Ingår i: Journal of Magnetic Resonance. - : Elsevier BV. - 1096-0856 .- 1090-7807. ; 180:2, s. 280-285
-
Tidskriftsartikel (refereegranskat)abstract
- Finite gradient pulse lengths are traditionally considered a nuisance in q-space diffusion NMR and MRI, since the simple Fourier relation between the acquired signal and the displacement probability is invalidated. Increasing the value of the pulse length leads to an apparently smaller value of the estimated compartment size. We propose that q-space data at different gradient pulse lengths, but with the same effective diffusion time, can be used to identify and quantify components with free or restricted diffusion from multi-exponential echo decay curves obtained on cellular systems. The method is demonstrated with experiments on excised human brain white matter and a series of model systems with well-defined free, restricted, and combined free and restricted diffusion behavior. Time-resolved diffusion MRI experiments are used to map the spatial distribution of the intracellular fraction in a yeast cell suspension during sedimentation, and observe the disappearance of this fraction after a heat treatment. (c) 2006 Elsevier Inc. All rights reserved.
|
|
| 6. |
- Nilsson, Christer, et al.
(författare)
-
Tracking the neurodegeneration of parkinsonian disorders - A pilot study
- 2007
-
Ingår i: Neuroradiology. - : Springer Science and Business Media LLC. - 1432-1920 .- 0028-3940. ; 49:2, s. 111-119
-
Tidskriftsartikel (refereegranskat)abstract
- The purpose of the study was to explore the possibilities of using diffusion tensor imaging (DTI) and tractography (DTT) for the differential diagnosis and monitoring of disease progression in idiopathic Parkinson's disease (IPD), compared with the atypical parkinsonian disorders multiple system atrophy (MSA) and progressive supranuclear palsy (PSP). A 3.0-T MR scanner was used. DTI was acquired using a single-shot EPI sequence with diffusion encoding in 32 directions and a voxel size of 2×2×2 mm3. DTI data were analysed and DTT was performed using the PRIDE fibre tracking tool supplied by the manufacturer. The fractional anisotropy (FA) and apparent diffusion coefficient (ADC) within each tract were determined. DTI and DTT images in patients with moderate to advanced MSA demonstrated degeneration of the middle cerebellar peduncles and pontine crossing tracts, with decreased FA and increased ADC. This accounted for most of the pontine and cerebellar atrophy characteristic of this disease. In contrast, patients with PSP showed a selective degeneration of the superior cerebellar peduncle. Three-dimensional images of whole-brain white matter tracts demonstrated a reduction of cortical projection fibres in all patients with PSP. Visualization of the selective degeneration of individual fibre tracts, using DTI and DTT, adds qualitative data facilitating the differential diagnosis of parkinsonian disorders. Repeated measurements of FA and ADC values in a whole fibre tract might be used for monitoring disease progression and studying the effect of treatment in neuroprotective trials. The results are preliminary considering the small number of subjects in the study. © Springer-Verlag 2007.
|
|
| 7. |
- Nilsson, Markus, et al.
(författare)
-
On the effects of a varied diffusion time in vivo: is the diffusion in white matter restricted?
- 2009
-
Ingår i: Magnetic Resonance Imaging. - : Elsevier BV. - 1873-5894 .- 0730-725X. ; 27:2008 Jul 25, s. 176-187
-
Tidskriftsartikel (refereegranskat)abstract
- The aim of this work was to study the diffusion-related signal attenuation curves (signal-vs.-b curves) measured perpendicular and parallel to the neuronal fibers of the corticospinal tract in vivo and to determine whether effects of restricted diffusion could be observed when varying the diffusion time (T(D)). A biexponential model and a two-compartment model including exchange according to the Kärger formalism were employed to analyze the signal-vs.-b curves. To validate the two-compartment model, restricted diffusion with exchange was simulated for uniformly sized cylinders, using different diameters and exchange times. The model was shown to retrieve the simulated parameters well, also when the short gradient pulse approximation was not met. The in vivo measurements performed perpendicular to the tracts, using b values up to 28000 s/mm(2) and T(D) values between 64 and 256 ms, did not show the effects of restricted diffusion as expected from previous ex vivo studies. The applied two-compartment model yielded an average axonal diameter of about 4 mum and an intracellular exchange time of about 300 ms, but did not fit statistically well to the data. In conclusion, this study indicates that if the diffusion is modeled as two compartments, of which one is restricted, exchange must be included in the model.
|
|
| 8. |
|
|
| 9. |
- van Westen, Danielle, et al.
(författare)
-
Tumor extension in high-grade gliomas assessed with diffusion magnetic resonance imaging: values and lesion-to-brain ratios of apparent diffusion coefficient and fractional anisotropy.
- 2006
-
Ingår i: Acta Radiologica. - : SAGE Publications. - 1600-0455 .- 0284-1851. ; 47:3, s. 311-319
-
Tidskriftsartikel (refereegranskat)abstract
- Purpose: To determine whether the apparent diffusion coefficient (ADC) and fractional anisotropy (FA) can distinguish tumor-infiltrated edema in gliomas from pure edema in meningiomas and metastases. Material and Methods: Thirty patients were studied: 18 WHO grade III or IV gliomas, 7 meningiomas, and 5 metastatic lesions. ADC and FA were determined from ROIs placed in peritumoral areas with T2-signal changes, adjacent normal appearing white matter (NAWM), and corresponding areas in the contralateral healthy brain. Values and lesion-to-brain ratios from gliomas were compared to those from meningiomas and metastases. Results: Values and lesion-to-brain ratios of ADC and FA in peritumoral areas with T2-signal changes did not differ between gliomas, meningiomas, and metastases (P = 0.40, P = 0.40, P = 0.61, P = 0.34). Values of ADC and FA and the lesion-to-brain ratio of FA in the adjacent NAWM did not differ between tumor types (P = 0.74, P = 0.25, and P = 0.31). The lesion-to-brain ratio of ADC in the adjacent NAWM was higher in gliomas than in meningiomas and metastases (P = 0.004), but overlapped between tumor types. Conclusion: Values and lesion-to-brain ratios of ADC and FA in areas with T2-signal changes surrounding intracranial tumors and adjacent NAWM were not helpful for distinguishing pure edema from tumor-infiltrated edema when data from gliomas, meningiomas, and metastases were compared.
|
|
| 10. |
- Wirestam, Ronnie, et al.
(författare)
-
Denoising of complex MRI data by wavelet-domain filtering: Application to high-b-value diffusion-weighted imaging.
- 2006
-
Ingår i: Magnetic Resonance in Medicine. - : Wiley. - 1522-2594 .- 0740-3194. ; 56:5, s. 1114-1120
-
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).
|
|
