Signatures of the neutrino thermal history in the spectrum of primordial gravitational waves

In this paper we study the effect of the anisotropic stress generated by neutrinos on the propagation of primordial cosmological gravitational waves. The presence of anisotropic stress, like the one generated by free-streaming neutrinos, partially absorbs the gravitational waves (GWs) propagating across the Universe. We find that in the standard case of three neutrino families, 22% of the intensity of the wave is absorbed, in fair agreement with previous studies. We have also calculated the maximum possible amount of damping, corresponding to the case of a flat Universe completely dominated by ultrarelativistic collisionless particles. In this case 43% of the intensity of the wave is absorbed. Finally, we have taken into account the effect of collisions, using a simple form for the collision term parameterized by the mean time between interactions, that allows to go smoothly from the case of a tigthly-coupled fluid to that of a collisionless gas. The dependence of the absorption on the neutrino energy density and on the effectiveness of the interactions opens the interesting possibility of observing spectral features related to particular events in the thermal history of the Universe, like neutrino decoupling and electron-positron annihilation, both occurring at T~1 MeV. GWs entering the horizon at that time will have today a frequency \nu\sim 10^{-9} \Hz, a region that is going to be probed by Pulsar Timing Arrays.Comment: V1: 14 pages, 2 figures. To appear in Gen. Rel. Grav. V2: References Adde

Optimized Quantification of Spin Relaxation Times in the Hybrid State

Purpose: The analysis of optimized spin ensemble trajectories for relaxometry in the hybrid state. Methods: First, we constructed visual representations to elucidate the differential equation that governs spin dynamics in hybrid state. Subsequently, numerical optimizations were performed to find spin ensemble trajectories that minimize the Cram\'er-Rao bound for $T_1$-encoding, $T_2$-encoding, and their weighted sum, respectively, followed by a comparison of the Cram\'er-Rao bounds obtained with our optimized spin-trajectories, as well as Look-Locker and multi-spin-echo methods. Finally, we experimentally tested our optimized spin trajectories with in vivo scans of the human brain. Results: After a nonrecurring inversion segment on the southern hemisphere of the Bloch sphere, all optimized spin trajectories pursue repetitive loops on the northern half of the sphere in which the beginning of the first and the end of the last loop deviate from the others. The numerical results obtained in this work align well with intuitive insights gleaned directly from the governing equation. Our results suggest that hybrid-state sequences outperform traditional methods. Moreover, hybrid-state sequences that balance $T_1$- and $T_2$-encoding still result in near optimal signal-to-noise efficiency. Thus, the second parameter can be encoded at virtually no extra cost. Conclusion: We provide insights regarding the optimal encoding processes of spin relaxation times in order to guide the design of robust and efficient pulse sequences. We find that joint acquisitions of $T_1$ and $T_2$ in the hybrid state are substantially more efficient than sequential encoding techniques.Comment: 10 pages, 5 figure

Tools for hardware design and validation in magnetic resonance imaging

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2008.Vita.Includes bibliographical references (p. 147-155).Parallel MRI techniques allow acceleration of MR imaging beyond traditional speed limits. In parallel MRI, radiofrequency (RF) detector coil arrays are used to perform some degree of spatial encoding which complements traditional encoding using magnetic field gradients. As the acceleration factor increases, coil design becomes critical to the overall image quality. The quality of a design is commonly judged on how it compares with other coil configurations. A procedure to evaluate the absolute performance of RF coil arrays is proposed. Electromagnetic calculations to compute the ultimate intrinsic signal-to-noise ratio (SNR) available for any physically realizable coil array are shown, and coil performance maps are generated based on the ratio of experimentally measured SNR to this ultimate intrinsic SNR. Parallel excitation, which involves independent transmission with multiple RF coils distributed around the body, can be used to improve the homogeneity of RF excitations and minimize the RF energy deposited in tissues - both critical issues for MRI at high magnetic field strength. As its use is explored further, it will be important to investigate the intrinsic constraints of the technique. We studied the trade-off between transmit homogeneity and specific absorption rate (SAR) reduction with respect to main magnetic field strength, object size and acceleration. We introduced the concept of ultimate intrinsic SAR, the theoretical smallest RF energy deposition for a target flip angle distribution, and we calculated the corresponding ideal current patterns. Knowledge of these optimal current patterns will serve as an important guide for future high-field coil designs.by Riccardo Lattanzi.Ph.D

Hybrid-State Free Precession in Nuclear Magnetic Resonance

The dynamics of large spin-1/2 ensembles in the presence of a varying magnetic field are commonly described by the Bloch equation. Most magnetic field variations result in unintuitive spin dynamics, which are sensitive to small deviations in the driving field. Although simplistic field variations can produce robust dynamics, the captured information content is impoverished. Here, we identify adiabaticity conditions that span a rich experiment design space with tractable dynamics. These adiabaticity conditions trap the spin dynamics in a one-dimensional subspace. Namely, the dynamics is captured by the absolute value of the magnetization, which is in a transient state, while its direction adiabatically follows the steady state. We define the hybrid state as the co-existence of these two states and identify the polar angle as the effective driving force of the spin dynamics. As an example, we optimize this drive for robust and efficient quantification of spin relaxation times and utilize it for magnetic resonance imaging of the human brain

Čimbenici rizika i poslijeoperacijski prediktori za ponavljajuću herniju lumbalnog diska: dugoročno praćenje

The purpose of this study is to identify some risk factors and post-operative predictors for recurrent lumbar disc hernia (rLDH) during a long-term follow-up in patients treated with microdiscectomy. Aim of the paper: This study analyzes some risk factors and postoperative predictors for recurrent lumbar disc hernia (rLDH) during a long-term follow-up in patients treated with microdiscectomy. Material and methods. We analyzed retrospectively a consecutive series of patients who underwent lumbar spinal microdiscectomy for lumbar disc herniation (LDH) from January 2013 to June 2018 at our Institute. The rate of rLDH during long-term follow-up was analyzed and correlated with baseline and post-operative data. Results. A total of 263 patients were included with a median follow-up time of 24 months (from 13 to 43 months). Most of the patients had rLDH within the first 36 months after surgery. At multivariate analysis, recurrence of LDH was associated with higher pre-operative body mass index (BMI) and higher post-operative Oswentry disability index (ODI) with statistical significance. Conclusions. Baseline BMI and post-surgery ODI could predict rLDH after surgery during a long-term follow-up.Cilj. Ova studija analizira određene čimbenike rizika i poslijeoperacijske prediktore za ponavljajuću herniju lumbalnog diska (engl. recurrent lumbar disc hernia – rLDH) tijekom dugotrajnog praćenjaa bolesnika liječenih mikrodiskektomijom. Metode. Retrospektivno smo analizirali niz uzastopnih serija pacijenata koji su podvrgnuti lumbalnoj spinalnoj mikrodiskektomiji zbog hernije lumbalnog diska (engl. lumbar disc hernia – LDH) u razdoblju od siječnja 2013. do lipnja 2018. u našem Institutu. Stopa ponavljajuće hernije lumbalnog diska tijekom dugotrajnog praćenja analizirana je i korelirana s početnim anamnestičkim podacima i poslijeoperacijskim podacima. Rezultati. U studiju je uključeno ukupno 263 pacijenta s prosječnim vremenom praćenja od 24 mjeseca (od 13 do 43 mjeseca). Većina pacijenata imala je ponavljajuću herniju lumbalnog diska u prvih 36 mjeseci nakon operacije. Pri multivarijantnoj analizi, recidiv hernije lumbalnog diska povezan je s višim indeksom tjelesne mase (engl. body mass index – BMI) prije operacije i višim Oswestry indeksom invaliditeta nakon operacije (engl. Oswestry disability index – ODI) sa statističkom značajnošću. Zaključak. Početni indeks tjelesne mase i poslijeoperacijski Oswestry indeks invaliditeta mogu poslužiti kao prediktori ponavljajuće hernije lumbalnog diska nakon operacije tijekom dugotrajnog praćenja

A radiomics approach to the diagnosis of femoroacetabular impingement

IntroductionFemoroacetabular Impingement (FAI) is a hip pathology characterized by impingement of the femoral head-neck junction against the acetabular rim, due to abnormalities in bone morphology. FAI is normally diagnosed by manual evaluation of morphologic features on magnetic resonance imaging (MRI). In this study, we assess, for the first time, the feasibility of using radiomics to detect FAI by automatically extracting quantitative features from images.Material and methods17 patients diagnosed with monolateral FAI underwent pre-surgical MR imaging, including a 3D Dixon sequence of the pelvis. An expert radiologist drew regions of interest on the water-only Dixon images outlining femur and acetabulum in both impingement (IJ) and healthy joints (HJ). 182 radiomic features were extracted for each hip. The dataset numerosity was increased by 60 times with an ad-hoc data augmentation tool. Features were subdivided by type and region in 24 subsets. For each, a univariate ANOVA F-value analysis was applied to find the 5 features most correlated with IJ based on p-value, for a total of 48 subsets. For each subset, a K-nearest neighbor model was trained to differentiate between IJ and HJ using the values of the radiomic features in the subset as input. The training was repeated 100 times, randomly subdividing the data with 75%/25% training/testing.ResultsThe texture-based gray level features yielded the highest prediction max accuracy (0.972) with the smallest subset of features. This suggests that the gray image values are more homogeneously distributed in the HJ in comparison to IJ, which could be due to stress-related inflammation resulting from impingement.ConclusionsWe showed that radiomics can automatically distinguish IJ from HJ using water-only Dixon MRI. To our knowledge, this is the first application of radiomics for FAI diagnosis. We reported an accuracy greater than 97%, which is higher than the 90% accuracy for detecting FAI reported for standard diagnostic tests (90%). Our proposed radiomic analysis could be combined with methods for automated joint segmentation to rapidly identify patients with FAI, avoiding time-consuming radiological measurements of bone morphology

Short-Term, Voluntary Exercise Affects Morpho-Functional Maturation of Adult-Generated Neurons in Rat Hippocampus

none5noPhysical exercise is a well-proven neurogenic stimulus, promoting neuronal progenitor proliferation and affecting newborn cell survival. Besides, it has beneficial effects on brain health and cognition. Previously, we found that three days of physical activity in a very precocious period of adult-generated granule cell life is able to antedate the appearance of the first GABAergic synaptic contacts and increase T-type Ca2+ channel expression. Considering the role of GABA and Ca2+ in fostering neuronal maturation, in this study, we used short-term, voluntary exercise on a running wheel to investigate if it is able to induce long-term morphological and synaptic changes in newborn neurons. Using adult male rats, we found that: (i) three days of voluntary physical exercise can definitively influence the morpho-functional maturation process of newborn granule neurons when applied very early during their development; (ii) a significant percentage of new neurons show more mature morphological characteristics far from the end of exercise protocol; (iii) the long-term morphological effects result in enhanced synaptic plasticity. Present findings demonstrate that the morpho-functional changes induced by exercise on very immature adult-generated neurons are permanent, affecting the neuron maturation and integration in hippocampal circuitry. Our data contribute to underpinning the beneficial potential of physical activity on brain health, also performed for short times.Davide Lattanzi, David Savelli, Marica Pagliarini, Riccardo Cuppini, Patrizia AmbroginiLattanzi, Davide; Savelli, David; Pagliarini, Marica; Cuppini, Riccardo; Ambrogini, Patrizi

Compression of volume-surface integral equation matrices via Tucker decomposition for magnetic resonance applications

In this work, we propose a method for the compression of the coupling matrix in volume\hyp surface integral equation (VSIE) formulations. VSIE methods are used for electromagnetic analysis in magnetic resonance imaging (MRI) applications, for which the coupling matrix models the interactions between the coil and the body. We showed that these effects can be represented as independent interactions between remote elements in 3D tensor formats, and subsequently decomposed with the Tucker model. Our method can work in tandem with the adaptive cross approximation technique to provide fast solutions of VSIE problems. We demonstrated that our compression approaches can enable the use of VSIE matrices of prohibitive memory requirements, by allowing the effective use of modern graphical processing units (GPUs) to accelerate the arising matrix\hyp vector products. This is critical to enable numerical MRI simulations at clinical voxel resolutions in a feasible computation time. In this paper, we demonstrate that the VSIE matrix\hyp vector products needed to calculate the electromagnetic field produced by an MRI coil inside a numerical body model with $1$ mm$^3$ voxel resolution, could be performed in $\sim 33$ seconds in a GPU, after compressing the associated coupling matrix from $\sim 80$ TB to $\sim 43$ MB.Comment: 13 pages, 11 figure