Search

We provide a mathematical description of the imaging concept of the Spectrometer/Telescope for Imaging X-rays (STIX) onboard Solar Orbiter. Specifically, we describe the STIX indirect-imaging technique, which is based on spatial modulation of the X-ray photon flux by means of tungsten grids, and we show that each of 30 STIX imaging sub-collimators measures a Fourier component of the flaring X-ray source corresponding to a specific two-dimensional angular frequency. We also provide details about the count-distribution model, which describes the relationship between the photon flux and the measured pixel counts. The derived imaging model is the fundamental starting point both for the interpretation of STIX data and for the description of the data-calibration process. Finally, we provide an overview of the algorithms implemented for the solution of the imaging problem and a comparison of the results obtained with these different methods in the case of the SOL2022-03-31T18 flaring event. [ABSTRACT FROM AUTHOR]

The Spectrometer/Telescope for Imaging X-rays (STIX) is one of six remote sensing instruments on-board Solar Orbiter. The telescope applies an indirect imaging technique that uses the measurement of 30 visibilities, i.e., angular Fourier components of the solar flare X-ray source. Hence, the imaging problem for STIX consists of the Fourier inversion of the data measured by the instrument. In this work, we show that the visibility amplitude and phase calibration of 24 out of 30 STIX sub-collimators has reached a satisfactory level for scientific data exploitation and that a set of imaging methods is able to provide the first hard X-ray images of solar flares from Solar Orbiter. Four visibility-based image reconstruction methods and one count-based are applied to calibrated STIX observations of six events with GOES class between C4 and M4 that occurred in May 2021. The resulting reconstructions are compared to those provided by an optimization algorithm used for fitting the amplitudes of STIX visibilities. We show that the five imaging methods produce results morphologically consistent with the ones provided by the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (SDO/AIA) in UV wavelengths. The χ 2 values and the parameters of the reconstructed sources are comparable between methods, thus confirming their robustness. [ABSTRACT FROM AUTHOR]

Purpose: Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease leading to neuromuscular palsy and death. We propose a computational approach to [18F]-fluorodeoxyglucose (FDG) PET/CT images to analyze the structure and metabolic pattern of skeletal muscle in ALS and its relationship with disease aggressiveness. Materials and methods: A computational 3D method was used to extract whole psoas muscle's volumes and average attenuation coefficient (AAC) from CT images obtained by FDG PET/CT performed in 62 ALS patients and healthy controls. Psoas average standardized uptake value (normalized on the liver, N-SUV) and its distribution heterogeneity (defined as N-SUV variation coefficient, VC-SUV) were also extracted. Spinal cord and brain motor cortex FDG uptake were also estimated. Results: As previously described, FDG uptake was significantly higher in the spinal cord and lower in the brain motor cortex, in ALS compared to controls. While psoas AAC was similar in patients and controls, in ALS a significant reduction in psoas volume (3.6 ± 1.02 vs 4.12 ± 1.33 mL/kg; p < 0.01) and increase in psoas N-SUV (0.45 ± 0.19 vs 0.29 ± 0.09; p < 0.001) were observed. Higher heterogeneity of psoas FDG uptake was also documented in ALS (VC-SUV 8 ± 4%, vs 5 ± 2%, respectively, p < 0.001) and significantly predicted overall survival at Kaplan–Meier analysis. VC-SUV prognostic power was confirmed by univariate analysis, while the multivariate Cox regression model identified the spinal cord metabolic activation as the only independent prognostic biomarker. Conclusion: The present data suggest the existence of a common mechanism contributing to disease progression through the metabolic impairment of both second motor neuron and its effector. [ABSTRACT FROM AUTHOR]

Purpose: In amyotrophic lateral sclerosis, functional alterations within the brain have been intensively assessed, while progression of lower motor neuron damage has scarcely been defined. The aim of the present study was to develop a computational method to systematically evaluate spinal cord metabolism as a tool to monitor disease mechanisms. Methods: A new computational three-dimensional method to extract the spinal cord from F-FDG PET/CT images was evaluated in 30 patients with spinal onset amyotrophic lateral sclerosis and 30 controls. The algorithm identified the skeleton on the CT images by using an extension of the Hough transform and then extracted the spinal canal and the spinal cord. In these regions, F-FDG standardized uptake values were measured to estimate the metabolic activity of the spinal canal and cord. Measurements were performed in the cervical and dorsal spine and normalized to the corresponding value in the liver. Results: Uptake of F-FDG in the spinal cord was significantly higher in patients than in controls ( p < 0.05). By contrast, no significant differences were observed in spinal cord and spinal canal volumes between the two groups. F-FDG uptake was completely independent of age, gender, degree of functional impairment, disease duration and riluzole treatment. Kaplan-Meier analysis showed a higher mortality rate in patients with standardized uptake values above the fifth decile at the 3-year follow-up evaluation (log-rank test, p < 0.01). The independence of this value was confirmed by multivariate Cox analysis. Conclusion: Our computational three-dimensional method enabled the evaluation of spinal cord metabolism and volume and might represent a potential new window onto the pathophysiology of amyotrophic lateral sclerosis. [ABSTRACT FROM AUTHOR]

The Hough transform is a standard pattern recognition technique introduced between the 1960s and the 1970s for the detection of straight lines, circles, and ellipses with several applications including the detection of symmetries in images. Recently, based on algebraic geometry arguments, the procedure has been extended to the automated recognition of special classes of algebraic plane curves. This allows us to detect curves of symmetry present in images, that is, curves that recognize midpoints maps of various shapes extracted by an ad hoc symmetry algorithm, here proposed. Further, in the case of straight lines, the detection of lines of symmetry allows us, by a pre-processing step of the image, to improve the efficiency of the recognition algorithm on which the Hough transform technique is founded, without loss of generality and additional computational costs. [ABSTRACT FROM AUTHOR]

We develop a formal procedure for the automated recognition of rational and elliptic curves in medical and astronomical images. The procedure is based on the extension of the Hough transform concept to the definition of Hough transform of special classes of algebraic curves. We first introduce a catalogue of curves that satisfy the conditions to be automatically extracted from an image and the recognition algorithm, then we illustrate the power of this method to identify skeleton profiles in clinical X-ray tomography maps and front ends of solar eruptions in astronomical images provided by the NASA solar dynamics observatory satellite. [ABSTRACT FROM AUTHOR]

Purpose: Despite their relevance in clinical medicine, the extension and activity of the bone marrow (BM) cannot be directly evaluated in vivo. We propose a new method to estimate these variables by combining structural and functional maps provided by CT and PET. Methods: BM extension and glucose uptake were estimated in 102 patients undergoing whole-body PET/CT because of a history of nonmetastatic melanoma. Image analysis assumed that the BM is surrounded by compact bone. An iterative optimization scheme was applied to each CT slice to identify the external border of the bone. To identify compact bone, the algorithm measured the average Hounsfield coefficient within a two-pixel ring located just inside the bone contour. All intraosseous pixels with an attenuation coefficient lower than this cut-off were flagged as 1, while the remaining pixels were set at 0. Binary masks created from all CT slices were thus applied to the PET data to determine the metabolic activity of the intraosseous volume (IBV). Results: Estimated whole-body IBV was 1,632 ± 587 cm and was higher in men than in women (2,004 ± 498 cm vs. 1,203 ± 354 cm, P < 0.001). Overall, it was strictly correlated with ideal body weight ( r = 0.81, P = 0.001) but only loosely with measured body weight ( r = 0.43, P = 0.01). The average FDG standardized uptake value (SUV) in the thoracic and lumbar vertebrae was 2.01 ± 0.36, Accordingly, intraosseous voxels with SUV ≥1.11 (mean spine SUV − 2.5 × SD) were considered as active 'red' BM and those with SUV <1.11 as 'yellow' BM. Estimated red BM volume was 541 ± 195 ml, with a higher prevalence in the axial than in the appendicular skeleton (87 ± 8 % vs. 10 ± 8 %, P < 0.001). Again, red BM volume was higher in men than in women (7.8 ± 2.2 vs. 6.7 ± 2.1 ml/kg body weight, P < 0.05), but in women it occupied a greater fraction of the IBV (32 ± 7 % vs. 36 ± 10 %, P < 0.05). Patient age modestly predicted red BM SUV, while it was robustly and inversely correlated with red BM volume. Conclusion: Our computational analysis of PET/CT images provides a first estimation of the extension and metabolism of the BM in a population of adult patients without haematooncological disorders. This information might represent a new window to explore pathophysiology the BM and the response of BM diseases to chemotherapy. [ABSTRACT FROM AUTHOR]

Kontaret al.(2004) have shown how to recover mean source electron spectrain solar flares through a physical constraint regularization analysis of the bremsstrahlung photon spectraI(e) that they produce. They emphasize the use of non-square inversion techniques, and preconditioning combined with physical properties of the spectra to achieve the most meaningful solution to the problem. Higher-order regularization techniques may be used to generateforms with certain desirable properties (e.g., higher-order derivatives). They further note that such analysis may be used to infer properties of the electron energy spectra at energies well above the maximum photon energy observed. In this paper we apply these techniques to data from a solar flare observed by RHESSI on 26 February, 2002. Results using different orders of regularization are presented and compared for various time intervals. Clear evidence is presented for a change in the value of the high-energy cutoff in the mean source electron spectrum with time. We also show how the construction of theinjectedelectron spectrumF0(E0) (assuming that Coulomb collisions in a cold target dominate the electron transport) is facilitated by the use of higher-order regularization methods. [ABSTRACT FROM AUTHOR]

Hard X-ray spectra in solar flares provide knowledge of the electron spectrum that results from acceleration and propagation in the solar atmosphere. However, the inference of the electron spectra from solar X-ray spectra is an ill-posed inverse problem. Here, we develop and apply an enhanced regularization algorithm for this process making use of physical constraints on the form of the electron spectrum. The algorithm incorporates various features not heretofore employed in the solar flare context: Generalized Singular Value Decomposition (GSVD) to deal with different orders of constraints; rectangular form of the cross-section matrix to extend the solution energy range; regularization with various forms of the smoothing operator; and “preconditioning” of the problem. We show by simulations that this technique yields electron spectra with considerably more information and higher quality than previous algorithms. [ABSTRACT FROM AUTHOR]