74 research outputs found
Quantitative Effect of Metal Artefact Reduction on CT-based attenuation correction in FDG PET/CT in patients with hip prosthesis
No full textAbstract Background Metal artefact reduction (MAR) techniques still are in limited use in positron emission tomography/computed tomography (PET/CT). This study aimed to investigate the effect of Smart MAR on quantitative PET analysis in the vicinity of hip prostheses. Materials and methods Activities were measured on PET/CT images in 6 sources with tenfold activity concentration contrast to background, attached to the head, neck and the major trochanter of a human cadaveric femur, and in the same sources in similar locations after a hip prosthesis (titanium cup, ceramic head, chrome-cobalt stem) had been inserted into the femur. Measurements were compared between PET attenuation corrected using either conventional or MAR CT. In 38 patients harbouring 49 hip prostheses, standardized uptake values (SUV) in 6 periprosthetic regions and the bladder were compared between PET attenuation corrected with either conventional or MAR CT. Results Using conventional CT, measured activity decreased with 2 to 13% when the prosthesis was inserted. Use of MAR CT increased measured activity by up to 11% compared with conventional CT and reduced the relative difference with the reference values to under 5% in all sources. In all regions, to the exception of the prosthesis shaft, SUVmean increased significantly (p < 0.001) by use of MAR CT. Median (interquartile range) percentual increases of SUVmean were 1.4 (0.0–4.2), 4.0 (1.8–7.8), 7.8 (4.1–12.4), 1.5 (0.0–3.2), 1.4 (0.8–2.8) in acetabulum, lateral neck, medial neck, lateral diaphysis and medial diaphysis, respectively. Except for the shaft, the coefficient of variation did not increase significantly. Except for the erratic changes in the prosthesis shaft, decreases in SUVmean were rare and small. Bladder SUVmean increased by 0.9% in patients with unilateral prosthesis and by 4.1% in patients with bilateral prosthesis. Conclusions In a realistic hip prosthesis phantom, Smart MAR restores quantitative accuracy by recovering counts in underestimated sources. In patient studies, Smart MAR increases SUV in all areas surrounding the prosthesis, most markedly in the femoral neck region. This proves that underestimation of activity in the PET image is the most prevalent effect due to metal artefacts in the CT image in patients with hip prostheses. Smart MAR increases SUV in the urinary bladder, indicating effects at a distance from the prosthesis
Structuurplan westelijke waddenzee
No full textBestuudeerd wordt in het hydraulisch deel: Wijziging in hoogte, freqentie van stormvloeden in het betreffende waddengebied en de afleiding voor dit onderzoek noodzakelijke formules. Uitvoering van de gedtijberekeningen. In the technisch deel: het tracee en profiel van de dam in het Eijerlandse gat, en de scheidingsdam, de sluitgatern en een kostenvergelijking. Beschouwing over de wijze van uitvoeringHydraulic EngineeringCivil Engineering and Geoscience
Effective weld length of beam to column connections with and without stiffeners (twee versies)
No full textCivil Engineering and GeosciencesStructural Engineerin
Simulating the variability of TMS responses in a speech mapping case study
No full textWhen delivered over a specific cortical site, TMS can temporarily disrupt the ongoing process in that area. This allows for mapping motor and speech-related cortical areas for preoperative evaluation with recent promising clinical outcomes [1,2].
Speech corresponds to an extended, complex, highly individualized neural network [3]. We aimed to numerically explain the observed variability of TMS responses during a speech mapping experiment, performed on a healthy, right-handed male subject following Lioumis’ approach [4]. We selected four study cases with very small differences in coil position and orientation. In one case (E) a naming error occurred, while in the other three cases (NEa,b,c) the subject appointed the object images as smoothly as without TMS. T1-weighted and diffusion-weighted MRI were acquired from the subject and post-processed to construct a realistic 2-mm resolution anisotropic head model. The induced electric field distributions were computed, with the coil configuration parameters retrieved from the neuronavigation system, using the anisotropic independent impedance method [5]. Whole brain tractography was performed using the graphical toolbox ExploreDTI [6]. 35 relevant tracts were identified in a region of interest obtained from the electric field distribution. Finally, the spatio-temporal variation of the membrane potentials along these tracts was computed for all four case using the compartmental cable equation [7], with passive and active neural components.
One tract is activated for all coil positions. Another tract is only triggered for case E. NEa induced action potentials in 13 tracts, while NEb stimulated 11 tracts and NEc only one. The calculated results are certainly sensitive to the coil specifications, confirming the observed variability in this speech mapping study. However, even though one neural tract only appears to be triggered for the error case, we do not want to draw strong conclusions from this. Further research is needed on the location and functional meaning of this tract in terms of the speech-related network and on refining the neural model with synapses and network connections.
We believe case- and subject-specific modelling is necessary to accurately capture the electromagnetic and neurophysiologic phenomena triggered by TMS, certainly when the stimulation interacts with complex neural networks that can differ significantly from person to person.
References:
[1] Krieg, S.M. et al. (2012) J. Neurosurgery, 116:994-1001.
[2] Picht, T. et al. (2013) Neurosurgery, 72:808–819.
[3] Catani, M. et al. (2005) Ann. Neurol., 57:8–16.
[4] Lioumis, P. et al. (2012) J. Neurosci. Meth., 204:349–354.
[5] De Geeter, N. et al. (2012) Phys. Med. Biol., 57:2169-2188.
[6] Leemans, A. et al. (2009) Proceedings of 17th Annual Meeting of Intl. Soc. Mag. Reson. Med.
[7] Salvador, R. (2009) Numerical modelling in transcranial magnetic stimulation. PhD thesis
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