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Sökning: WFRF:(Brockstedt Sara)

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1.
  • Brockstedt, Sara, et al. (författare)
  • BE AWARE OF NEUTRONS OUTSIDE SHORT MAZES FROM 10-MV LINEAR ACCELERATORS X-RAYS IN RADIOTHERAPY FACILITIES.
  • 2015
  • Ingår i: Radiation Protection Dosimetry. - Nuclear Technology Publishing. - 1742-3406. ; 165:1-4, s. 464-467
  • Tidskriftsartikel (refereegranskat)abstract
    • During the radiation survey of a reinstalled 10-MV linear accelerator in an old radiation treatment facility, high dose rates of neutrons were observed. The area outside the maze entrance is used as a waiting room where patients, their relatives and staff other than those involved in the actual treatment can freely pass. High fluence rates of neutrons would cause an unnecessary high effective dose to the staff working in the vicinity of such a system, and it can be several orders higher than the doses received due to X-rays at the same location. However, the common knowledge appears to have been that the effect of neutrons at 10-MV X-ray linear accelerator facilities is negligible and shielding calculations models seldom mention neutrons for this operating energy level. Although data are scarce, reports regarding this phenomenon are now emerging. For the future, it is advocated that contributions from neutrons are considered already during the planning stage of new or modified facilities aimed for 10 MV and that estimated dose levels are verified.
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2.
  • Brockstedt, Sara, et al. (författare)
  • High-resolution diffusion imaging using phase-corrected segmented echo-planar imaging
  • 2000
  • Ingår i: Magnetic Resonance Imaging. - Elsevier. - 1873-5894. ; 18:6, s. 649-657
  • Tidskriftsartikel (refereegranskat)abstract
    • Diffusion magnetic resonance imaging (MRI) was performed with a high-resolution segmented echo-planar imaging technique, which provided images with substantially less susceptibility artifacts than images obtained with single-shot echo-planar imaging (EPI). Diffusion imaging performed with any multishot pulse sequence is inherently sensitive to motion artifacts and in order to reduce motion artifacts, the presented method utilizes navigator echo phase corrections, performed after a one-dimensional Fourier transform along the frequency-encoding direction. Navigator echo phases were fitted to a straight line prior to phase correction to avoid errors from internal motion. In vivo imaging was performed using electro cardiographic (ECG) triggering. Apparent diffusion coefficient (ADC) maps were calculated on a pixel-by-pixel basis using up to seven diffusion sensitivities, ranging from b = 0 to 1129 x 10(6) s/m(2).
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3.
  • 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)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 <i>in vivo</i> 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 <i>in vivo</i> 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.
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4.
  • Brockstedt, Sara, et al. (författare)
  • Quantitative diffusion coefficient maps using fast spin-echo MRI
  • 1998
  • Ingår i: Magnetic Resonance Imaging. - Elsevier. - 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.
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5.
  • Brockstedt, Sara, et al. (författare)
  • SchnelleMagnetresonanz-Bildgebung
  • 2002
  • Ingår i: Radiologie Up2Date. - Georg Thieme Verlag KG. - 1616-0681. ; :4, s. 413-438
  • Tidskriftsartikel (refereegranskat)abstract
    • Abstract in German Die Verkürzung der Untersuchungszeiten bei der klinischen Magnetresonanztomographie (MRT) bietet für den Patienten den Vorteil einer angenehmeren und schnelleren Untersuchung und erhöht gleichzeitig den Patientendurchsatz. Dies erfordert schnelle und robuste Aufnahmetechniken, welche beispielsweise auch bei der Aufnahme bewegter Organe oder für atemangehaltene Messungen eingesetzt werden können. Durch die Entwicklung leistungsfähiger Gradientensysteme konnten in den letzten Jahren neue und schnelle Pulssequenzen entwickelt werden. Die rasante Entwicklung neuer Aufnahmemethoden und Rekonstruktionstechniken macht es für den Anwender immer wichtiger, die grundlegenden Prinzipien schneller Aufnahmetechniken zu verstehen. In diesem Übersichtsartikel wird versucht, die Grundlagen der schnellen Pulssequenzen und deren klinische Anwendungsbereiche darzustellen. Dies beinhaltet die schnelle Spin-Echo-Technik und deren Kombination mit selektiven Präparationstechniken, die schnelle und ultraschnelle Gradienten-Echo-Technik sowie die echoplanare Bildgebung. Abschließend werden kurz neueste Entwicklungen wie die parallele Bildgebung beschrieben.Zusammenfassung auf S. 434
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6.
  • Brockstedt, Sara, et al. (författare)
  • Triggering in quantitative diffusion imaging with single-shot EPI
  • 1999
  • Ingår i: Acta Radiologica. - SAGE Publications. - 1600-0455. ; 40:3, s. 263-269
  • Tidskriftsartikel (refereegranskat)abstract
    • PURPOSE: The aim of this study was to evaluate any possible effects of brain motion, CSF pulsations and other possible sources of physiological motion in electrocardiographic (ECG) triggered and non-triggered single-shot echo-planar imaging (EPI) measurements of diffusion. MATERIAL AND METHODS: Three different triggering protocols were evaluated in 6 healthy volunteers: 1) ECG triggering with time delay (TD) 100 ms; 2) ECG triggering with TD 400 ms; and 3) no triggering at all. RESULTS: The results obtained showed that white matter mean apparent diffusion coefficient (ADC) values were similar for all triggering protocols and that the reproducibility in ADC measurements using diffusion-weighted (DW) EPI was good. The average standard deviation of the ADC values was, however, higher for ADC maps obtained without ECG triggering. CONCLUSION: The use of prospective diastolic ECG triggering significantly improves the accuracy of quantitative diffusion measurements but for routine clinical diffusion imaging, where quantitative data is of less importance, the accuracy obtained without ECG triggering can be considered adequate.
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7.
  • Brockstedt, Sara, et al. (författare)
  • Use of an enhanced gradient system for diffusion MR imaging with motion-artifact reduction
  • 1995
  • Ingår i: Acta Radiologica. - SAGE Publications. - 1600-0455. ; 36:6, s. 662-670
  • Tidskriftsartikel (refereegranskat)abstract
    • PURPOSE: A spin-echo diffusion-sensitized pulse sequence using high gradients (23 mT/m) is introduced. MATERIAL AND METHODS: In order to minimize motion artefacts, velocity-compensating gradients, ECG-triggering and post-processing with phase correction and raw data averaging using navigator echoes was performed. The in vitro ratio of diffusion coefficients for water and acetone was determined and the water self-diffusion coefficient at different temperatures was evaluated. The pulse sequence was tested in 7 healthy volunteers and in 2 tumour patients with astrocytomas of grades I-II and III-IV. Both single-slice and multi-slice techniques were used. RESULTS: The incorporation of phase correction clearly improved the quality of both diffusion-encoded images and the calculated diffusion maps. Mean values of the diffusion coefficients in vivo were for CSF 2.66 x 10(-9) m2/s and for white and grey matter 0.69 x 10(-9) m2/s and 0.87 x 10(-9) m2/s, respectively. CONCLUSION: Velocity-compensating gradients in combination with a high gradient strength were shown to be useful for in vivo diffusion MR imaging.
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8.
  • Brockstedt, Sara, et al. (författare)
  • Vertical field MR imaging of upper thorax and spine in small children. Evaluation of a new surface coil
  • 1993
  • Ingår i: Acta Radiologica. - SAGE Publications. - 1600-0455. ; 34:6, s. 549-553
  • Tidskriftsartikel (refereegranskat)abstract
    • To improve image quality in a vertical field MR imaging unit, operating at low field strength (0.3 T), we have designed a half-elliptical coil for use in the upper thoracic region of small children. Our intention was also to shorten the examination time, which until now has been long, because several scans with different coils have been necessary to cover the thoracic region. The experimental coil is designed so that a child's shoulders fit into the central region. The coil consists of 2 serially connected cable-loops, mounted on a foam rubber vest. The coil performance was tested in a phantom and improvements relative to standard coils were demonstrated in in vivo studies. The results indicate that by using the half-elliptical coil, the signal-to-noise (S/N) ratio can be improved by a factor of 2 to 3 in the thoracic region of a child.
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9.
  • Engellau, Lena, et al. (författare)
  • MR evaluation ex vivo and in vivo of a covered stent-graft for abdominal aortic aneurysms: ferromagnetism, heating, artifacts, and velocity mapping
  • 2000
  • Ingår i: Journal of Magnetic Resonance Imaging. - John Wiley and Sons Inc.. - 1522-2586. ; 12:1, s. 112-121
  • Tidskriftsartikel (refereegranskat)abstract
    • Magnetic resonance imaging (MRI) safety was evaluated at 1.5 T in a covered nickel titanium stent-graft (Vanguard) used for endovascular treatment of abdominal aortic aneurysms (AAAs). Imaging artifacts were assessed on MRI with contrast-enhanced (CE) three-dimensional (3D) MR angiography (MRA) and spiral computed tomography (CT) in 10 patients as well as ex vivo. Velocity mapping was performed in the suprarenal aorta and femoral arteries in 14 patients before and after stent-graft placement. For comparison it was also performed in six healthy volunteers. No ferromagnetism or heating was detected. Metal artifacts caused minimal image distortion on MRI/MRA. The artifacts disturbed image evaluation on CT at the graft bifurcation and graft limb junction. No significant differences in mean flow were found in patients before and after stent-graft placement. Our study indicates that MRI at 1.5 T may be performed safely in patients with the (Vanguard) stent-graft. MRI/MRA provides diagnostic image information. Velocity mapping is not included in our routine protocol.
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10.
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  • Resultat 1-10 av 39
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