Browsing by Subject "DISTORTION"

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  • Kavaluus, Henna; Nousiainen, Katri; Kaijaluoto, Sampsa; Seppälä, Tiina; Saarilahti, Kauko; Tenhunen, Mikko (2021)
    Background and Purpose: Magnetic resonance imaging is increasingly used in radiotherapy planning; yet, the performance of the utilized scanners is rarely regulated by any authority. The aim of this study was to determine the geometric accuracy of several magnetic resonance imaging scanners used for radiotherapy planning, and to establish acceptance criteria for such scanners. Materials and Methods: The geometric accuracy of five different scanners was measured with three sequences using a commercial large-field-of-view phantom. The distortion magnitudes were determined in spherical volumes around the scanner isocenter and in cylindrical volumes along scanner z-axis. The repeatability of the measurements was determined on a single scanner with two quality assurance sequences with three single-setup and seven repeated-setup measurements. Results: For all scanners and sequences except one, the mean and median distortion magnitude was Conclusions: All tested scanners were geometrically accurate for their current use in radiotherapy planning. The acceptance criteria of geometric accuracy for regulatory inspections of a supervising authority could be set according to these results.
  • Iwaniec, Tadeusz; Onninen, Jani (2017)
    Let X; Y subset of R-2 be topologically equivalent bounded Lipschitz domains. We prove that weak and strong limits of homeomorphisms h: X (onto)-> Y in the Sobolev space W-1,W-p (X, R-2), p >= 2; are the same. As an application, we establish the existence of 2D-traction free minimal deformations for fairly general energy integrals.
  • Koivula, Kimmo; Eskola, Markku; Viikilä, Juho; Lilleberg, Jyrki; Huhtala, Heini; Birnbaum, Yochai; Nikus, Kjell (2018)
    Background: Grade 3 ischemia (G3I) in the 12 lead electrocardiogram (ECG) predicts poor outcome in patients with ST-elevation myocardial infarction (STEMI). The outcome of G3I in "real-life" patient cohorts is unclear. Methods: The aim of the study was to establish the prognostic significance of grade 2 ischemia (G2I), G3I and the STEMI patients excluded from ischemia grading (No grade of ischemia, NG) in a real-life patient population. We assessed in-hospital, 30-day and 1-year mortality as well as other endpoints. Results: The NG patients had more comorbidities and longer treatment delays than the two other groups. Shortterm and 1-year mortality were highest in patients with NG and lowest in patients with G2I. Maximum troponin level was highest in G3I, followed by NG and G2I. In logistic regression multivariable analysis, NG was independently associated with 1-year mortality. Conclusions: NG predicted poor outcome in STEMI patients. G2I predicted relatively favorable outcome. (C) 2018 Elsevier Inc. All rights reserved.
  • Brandstack, Nina; Kurki, T.; Laalo, J.; Kauko, T.; Tenovuo, O. (2016)
    Reproducibility of two different methods for quantifying fiber tracts by using a diffusion tensor imaging (DTI) sequence suitable for clinical magnetic resonance imaging (MRI) protocols was evaluated. DTI of 15 subjects was used to analyze intra-rater and inter-rater reproducibility. Another 10 subjects underwent MRI twice for assessment of between-scan reliability. Ten long association tracts were defined by fiber tracking using inclusion and exclusion regions of interest (ROIs). Whole-tract analysis and tractography-based core analysis were performed, and the effect of fractional anisotropy (FA 0.15/0.30) and turning angle threshold (27A degrees/60A degrees) on reproducibility was evaluated. Additionally, ROI measurements were performed in the core of the tracts. For the tract-based methods, intra-rater and inter-rater reliabilities of FA and mean diffusivity (MD) measurements were excellent. Between-scan reproducibility was good or excellent in 127 of 130 of the measurements. There was no systematic difference in the reproducibility of the FA, MD, and volume measurements depending on the FA or turning angle threshold. For the cross-sectional ROI measurements, reliability showed large variation from poor to excellent depending on the tract. Compared with the commonly used cross-sectional core ROI method, the tract-based analyses seem to be a more robust way to identify and measure white matter tracts of interest, and provide a novel reproducible tool to perform core analysis.
  • PIPARI Study Grp; Lahti, Katri; Parkkola, Riitta; Jääsaari, Päivi; Haataja, Leena; Saunavaara, Virva (2021)
    Background Diffusion tensor imaging is a widely used imaging method of brain white matter, but it is prone to imaging artifacts. The data corrections can affect the measured values. Objective To explore the impact of susceptibility correction on diffusion metrics. Materials and methods A cohort of 27 healthy adolescents (18 boys, 9 girls, mean age 12.7 years) underwent 3-T MRI, and we collected two diffusion data sets (anterior-posterior). The data were processed both with and without susceptibility artifact correction. We derived fractional anisotropy, mean diffusivity and histogram data of fiber length distribution from both the corrected and uncorrected data, which were collected from the corpus callosum, corticospinal tract and cingulum bilaterally. Results Fractional anisotropy and mean diffusivity values significantly differed when comparing the pathways in all measured tracts. The fractional anisotropy values were lower and the mean diffusivity values higher in the susceptibility-corrected data than in the uncorrected data. We found a significant difference in total tract length in the corpus callosum and the corticospinal tract. Conclusion This study indicates that susceptibility correction has a significant effect on measured fractional anisotropy, and on mean diffusivity values and tract lengths. To receive reliable and comparable results, the correction should be used systematically.