| 1. |
- Björkman-Burtscher, Isabella, et al.
(författare)
-
Aneurysm clip MR artifacts. Titanium versus stainless steel and influence of imaging parameters
- 1998
-
Ingår i: Acta Radiologica. - 1600-0455. ; 39:1, s. 70-76
-
Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: The aim of this study was to evaluate the extent to which titanium aneurysm clips could improve the quality of MR imaging compared with stainless steel clips, and to determine whether the clip artifacts could be reduced by controlling certain MR imaging parameters in frequently used pulse sequences. MATERIAL AND METHODS: The metal artifacts induced by 3 aneurysm clips were compared in 3 pulse sequences. The clips were: a Yasargil titanium aneurysm clip FT 752 T; a Yasargil standard aneurysm clip FE 752 K; and, for comparison, a ferromagnetic Scoville aneurysm clip En-58J. The pulse sequences were: spin echo (SE); gradient echo (GE); and fast SE. An evaluation was made of 3 imaging parameters with regard to their influence on the size of the metal artifacts. The parameters were: bandwidth; echo time (TE); and echo-train length. RESULTS: The titanium clip showed artifacts that were about 60% smaller than those from the stainless steel clip. The only parameter that influenced artifact size to any major degree was bandwidth in the SE sequences but not in the GE sequences. GE sequences induced larger artifacts than SE sequences and showed larger artifacts with longer TE. CONCLUSION: Titanium aneurysm clips reduced MR artifacts by approximately 60% compared to stainless steel clips. Artifacts were further reduced by using SE-based sequences with a high bandwidth or, if necessary, GE sequences with a low TE.
|
|
| 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. |
- Eriksson, Joakim, et al.
(författare)
-
Measurements of Kinematic Properties of the Cervical Spine Using Magnetic Resonance Imaging
- 1998
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents kinematic data on the cervical and upper thoracic spine, based on measurements made on 20 Scandinavian healthy, female volunteers, aged 22-58 years (mean age 40.4). The aim was to provide anatomical in vivo data, primarily intended as data for biomechanical modelling of the upper spine. Together with the measurements of standard anthropometric body dimensions, magnetic resonance imaging (MRI) was used to capture the inner anatomy for each subject. A rigid linkage system is described for the vertebrae C1 –Tvi, with one link per vertebra. Measurements include link lengths, link rotations, and antero-posterior endpoints of the spinous process. Furthermore, correlation coefficients are calculated between link lengths and anthropometric measurements. Also presented are regression equations for each link length, with stature as a predictor. Using additional images of lower accuracy, a sub-study (N=15) investigated possible differences in link length and link rotation between non-flexion and maximum-flexion of the neck. The differences in link lengths were significant (p>0.05) for only 1 of 16 measured links (Cii-Tx). Regarding link rotation, differences were significant for 4 links (Cv–T1). Finally, the precision of the results was evaluated using two methods: by using a phantom for determining the geometrical uncertainties caused by the scanner; and by comparing results between two repeated measurement rounds. The phantom test revealed that the pixel resolution and magnetic field inhomogenities had only a minor influence on the results. The comparisons of repeated measurements revealed a significant difference for the links Ci and Cii, indicating that the landmarks for determining the occipital and Ci/Cii joints were the most difficult to identify on the images.
|
|
| 4. |
|
|
| 5. |
- 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.
|
|
