1. |
- Brockstedt, Sara
(författare)
-
Nuclear Magnetic Resonance for Diffusion Imaging Development and evaluation of spin-echo-based pulse sequences for in vivo studies of water self-diffusion
- 1998
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interest in diffusion imaging with nuclear magnetic resonance techniques has increased during the last few years. The main reason is the possibility of early diagnosis of acute ischaemic stroke, at a time when conventional MR techniques and computerised tomography fail. The purpose of this study was to develop and evaluate methods for the measurement of water self-diffusion in vivo using magnetic resonance imaging (MRI). The main objective was to design pulse sequences that allow quantification of the diffusion coefficient, and provide diffusion-weighted (DW) images of high quality. DW-MRI is highly sensitive to macroscopic motion, which causes severe image artifacts and makes quantification difficult. This problem was addressed through pulse sequence design and image post-processing techniques. The use of first-order motion-compensating diffusion-encoding gradients in combination with DW spin-echo (SE) imaging proved to increase image quality and accuracy in quantitative data. Further more, quantitative diffusion imaging was found to be possible with fast-SEs, a method that does not require any high-performance gradient systems. Using a segmented echo-planar imaging (SEPI) technique high-resolution DW-images and ADC maps were obtained, showing no proof of susceptibility artifacts or geometrical distortions, problems normally associated with EPI. Navigator-echo phase-corrections were found to reduce motion artifacts present in images obtained with SE, FSE and SEPI techniques. The use of electro-cardiographic triggering for DW-EPI was also evaluated, and found to be relevant in quantitative measurements. Assessment of the perfusion fraction in vivo from DW-images might be helpful in diagnosis and characterisation of acute stroke and can be performed whenever enough data are available. Finally, the performance of DW-SE and DW-EPI for diagnosis of ischaemic stroke in the acute phase was evaluated. DW-EPI and DW-SE in combination with ADC maps were compared with respect to diagnostic accuracy. It was found that although DW-SE methods suffer from limited diffusion sensitivity and limited numbers of slices, it can be used for diagnosis of acute stroke, but ADC maps are essential in discriminating between acute and chronic infarcts.
|
|
2. |
- Brockstedt, Sara, et al.
(författare)
-
Quantitative diffusion coefficient maps using fast spin-echo MRI
- 1998
-
Ingår i: Magnetic Resonance Imaging. - 1873-5894. ; 16:8, s. 877-886
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we have evaluated the performance of a diffusion-sensitive fast spin-echo (FSE) pulse sequence. The proposed pulse sequence utilises velocity-compensating diffusion-encoding gradients and includes the collection of navigator echoes. Spoiler gradients were inserted in the slice-selecting direction to minimise effects from stimulated echoes. Calculations of the b values showed that cross-terms between imaging gradients and diffusion gradients only led to a marginal increase of b values. Pixel-wise calculation of apparent diffusion coefficient (ADC) maps was performed numerically, considering cross-terms between diffusion-encoding and imaging gradients. The sequences investigated used echo train lengths of 16, 8 and 4 echoes and were encoded in either the slice-, frequency- or phase-encoding direction. In order to allow for higher b values a pulse-sequence version using non-motion compensating diffusion-encoding gradients was written. Phantom measurements were performed and the diffusion coefficients of water and acetone were reasonable. Seven healthy volunteers (age 28-50 years) were examined and apparent diffusion coefficient values agreed well with expected values. Diffusion-weighted images, apparent diffusion coefficient maps and images corresponding to the trace of the diffusion tensor of good quality were retrieved in vivo.
|
|
3. |
- Olsrud, Johan, et al.
(författare)
-
MRI thermometry in phantoms by use of the proton resonance frequency shift method: application to interstitial laser thermotherapy
- 1998
-
Ingår i: Physics in Medicine and Biology. - 1361-6560. ; 43:9, s. 2597-2613
-
Tidskriftsartikel (refereegranskat)abstract
- In this work the temperature dependence of the proton resonance frequency was assessed in agarose gel with a high melting temperature (95 degrees C) and in porcine liver in vitro at temperatures relevant to thermotherapy (25-80 degrees C). Furthermore, an optically tissue-like agarose gel phantom was developed and evaluated for use in MRI. The phantom was used to visualize temperature distributions from a diffusing laser fibre by means of the proton resonance frequency shift method. An approximately linear relationship (0.0085 ppm degrees C(-1)) between proton resonance frequency shift and temperature change was found for agarose gel, whereas deviations from a linear relationship were observed for porcine liver. The optically tissue-like agarose gel allowed reliable MRI temperature monitoring, and the MR relaxation times (T1 and T2) and the optical properties were found to be independently alterable. Temperature distributions around a diffusing laser fibre, during irradiation and subsequent cooling, were assessed with high spatial resolution (voxel size = 4.3 mm3) and with random uncertainties ranging from 0.3 degrees C to 1.4 degrees C (1 SD) with a 40 s scan time.
|
|