| 1. |
- Karlsson, Anette, 1986-
(författare)
-
Quantitative Muscle Composition Analysis Using Magnetic Resonance Imaging
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Changes in muscle tissue composition, e.g. decrease in volume and/or increase of fat infiltration, are related to adverse health conditions such as sarcopenia, inflammation, muscular dystrophy, and chronic pain. However, the onset and progression of disease and the effect of potential intervention effects are not fully understood, partly due to insufficient measurement tools. For advanced knowledge regarding these diseases, an accurate and precise measurement tool for detecting changes in muscle composition is needed. The tool must be able to detect both local changes on specific muscles for investigating local injuries and generalized muscle composition changes on a whole-body level. Magnetic resonance imaging is an excellent tool due to its superior soft tissue contrast but is normally not quantitative, making it challenging to produce reproducible results. Furthermore, manual analysis of the vast amount of images produced is extremely time consuming and therefore expensive. The aim of this thesis was to develop and validate a new magnetic resonance imaging method for muscle volume quantification and fat infiltration estimation that would have the potential to be used in both large-scale studies and for analyzing small individual muscles.The method development was divided into four main steps: 1) Rapid acquisition and reconstruction of data with sufficient resolution and calibration giving quantitative images where the relative fat content of each voxel (related to pure fat voxels) is attainable; 2) Automated muscle tissue classification based on non-rigid multi-atlas segmentation followed by probability voting to acquire the region of interest for each muscle; 3) Quantification of muscle tissue volume and fat infiltration from the classification step and the local fat signal; 4) Evaluation of the potential of the method in clinical studies.In Paper I, a method for automatic muscle volume quantification of both whole-body and regional muscles, i.e. involving steps 1–3, is presented. The automated method showed good agreement compared to manual segmentation. It was robust to an 8-fold resolution difference using two different scanner field strengths. Papers II and III evaluated the clinical relevance and the need for developing methods with high-resolution images to answer the research questions regarding the effect of a whiplash trauma on the multifidus muscles. This involved steps 1–4. The method enabled acquisition of high-resolution data to distinguish the small multifidus muscles (Paper II). The paper also showed a higher fat infiltration in the multifidus muscles in individuals with severe self-reported disability compared to individuals with milder symptoms and to healthy controls. Furthermore, the local fat infiltration was also related to widespread muscle fat infiltration (Paper III). However, the difference in widespread muscle fat infiltration could not alone distinguish between the three different groups. Paper IV showed the robustness of fat infiltration estimation when changing flip angle, and thereby the T1 weighting, of the acquired images (steps 1–3). The higher flip angle also provided better noise characteristics. Therefore, this quantitative method can be used with higher flip angle, and thus a potentially better anatomical contrast, without losing accuracy or precision.To conclude, this thesis presents a method that quantifies muscle volume and estimates fat infiltration robustly and reproducibly. The versatility of the method allows for both high-resolution images of small muscles and rapid acquisition of whole-body data. The method can be a useful tool in clinical studies regarding small individual muscles. Furthermore, the combination of being quantitative and automatic means that the method has potential to be used in longitudinal, multi-center, and large-scale studies for advanced understanding of muscular diseases.
|
|
| 2. |
- Karlsson, Jerker, 1967-
(författare)
-
Abdominal Aortic Wall Mechanics - Stress, Strain and Stiffness in A Medical Perspective : An Experimental Study in Man
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Background: The stiffness of the abdominal aorta is considered a significant factor affecting the morbidity and mortality of cardiovascular disease. Estimating vascular stiffness is an integral part in cardiovascular risk assessment. Wall stress of the abdominal aorta appears to be a crucial factor in the remodeling of the arterial wall and the growth of aneurysms. Consequently, arterial mechanics plays a vital role in the function of the cardiovascular system. Therefore, there is a need for comprehensive studies of mechanical forces in the vessel wall to better understand the mechanisms behind normal and pathological changes that are significant for hypertension, atherosclerosis, and the development of arterial aneurysms. The aim of this study was to explore the blood pressure-induced forces in the aortic wall using a computational mechanical model, with particular attention to the effects of age, sex, and blood pressure on the remodeling process of the vessel wall. Methods: A computational model, comprising a solid mechanical model and a parameter identification process known as the Parameter Identification Method for Mechanical Parameters (PIMMP), was used to investigate the mechanical properties of the abdominal aortic vessel wall. Data for the model were obtained from the human abdominal aorta of volunteers: 30 healthy individuals, females (n=15) and males, divided into three age groups with an equal number of females and males (n=5 in each age group). Invasive blood pressure, measured via catheter, and diameter variation in the abdominal aorta, measured via ultrasound, were acquired to be used as input data for PIMMP. This dataset was utilized in Papers I, III, and IV. In Paper II, 24 datasets were generated, based on model parameters presented in the scientific literature. Results: Paper I reveals that elderly males exhibit both higher aortic wall stress and higher isotropic stress component, than females. With age, males show an increase in isotropic load-bearing fraction and a decrease in anisotropic load-bearing fraction, a trend not observed in females. Paper II validates an in silico aortic model against a computerized membrane model of an abdominal aorta. The membrane model accurately predicts stress states as well as the load-bearing fraction of anisotropic material across all blood pressure levels, independent of the transmural stress gradient. However, the model’s accuracy is limited due to insufficient in vivo axial loading information. Paper III demonstrates that changes in circumferential stretch have a more pronounced effect on longitudinal stress than the other way around. Both circumferential and longitudinal stiffnesses increase with age, irrespective of sex. However, sex-based differences in stiffness are observed when comparing younger and older groups. Paper IV investigates pulse wave velocity (PWV) calculations using the Moens-Korteweg equation and the Bramwell-Hill equation. PWV shows a positive association with both isotropic and anisotropic material properties, with a transition zone observed between diastolic and systolic blood pressures, to a positive association with anisotropic properties at systolic blood pressure. Furthermore, an increase in PWV with age, with no significant difference between sexes, is observed. The Extra Material suggests a deficiency in age-related wall stress regulation in males, potentially due to insufficient stiffness of anisotropic materials such as collagen. In contrast, females show an age-related increase in abdominal aortic wall thickness and anisotropic material stiffness, indicating adequate wall stress regulation. Conclusions: This doctoral dissertation focused on the effects of age and sex on the abdominal aortic wall. Overall, the findings suggest potential alterations in the collagen and elastin content during the remodeling of the abdominal aorta, which may differ between sexes. These alterations could be induced chemically or mechanically. The model has shown potential in identifying healthy individuals within a population. These insights may contribute to the understanding of cardiovascular health and disease progression.
|
|
| 3. |
- Karlsson, Magnus, 1967
(författare)
-
Nonlinear Propagation of Optical Pulses and Beams
- 1994
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The propagation of optical beams and pulses under the influence of nonlinear effects is characterized by a rich variety of phenomena and many potentially important applications. We analyse two main topics in this context: nonlinear beam propagation, and nonlinear pulse propagation in optical fibers. The propagation of an optical beam is characterized by diffractive broadening. For Kerr-media, in which the refractive index increases with beam intensity, at a certain intensity, the beam may induce its own waveguide and propagate without broadening. For higher intensities, the nonlinearly induced refractive index causes self-focusing and in some cases even collapsing singularities. We demonstrate in this work that an analytical variational approach describes the dynamics of nonlinear beam propagation very well, in particular with respect to the phase modulation dynamics, which previous approaches are found to describe erroneously. The collapse can be removed by allowing the refractive index to saturate. Beam dynamics in saturable nonlinear media is therefore an important issue. Using the variational method, we manage to reproduce the essential features from numerical simulations, and to give a complete picture of optical beam dynamics in saturable nonlinear media. Another important effect in nolinear media is the modulational instability, which is well-known to break up broad beams into filaments. However, this instability can occur only in nonlinear focusing media. Considering a pulsed beam with a defocusing-in-time and focusing-in-space nonlinearity, we show that temporal breakup is possible due to the spatial instability, despite the fact that a purely temporal modulation is stable.
|
|
