113 research outputs found
A two-phase gradient method for quadratic programming problems with a single linear constraint and bounds on the variables
We propose a gradient-based method for quadratic programming problems with a
single linear constraint and bounds on the variables. Inspired by the GPCG
algorithm for bound-constrained convex quadratic programming [J.J. Mor\'e and
G. Toraldo, SIAM J. Optim. 1, 1991], our approach alternates between two phases
until convergence: an identification phase, which performs gradient projection
iterations until either a candidate active set is identified or no reasonable
progress is made, and an unconstrained minimization phase, which reduces the
objective function in a suitable space defined by the identification phase, by
applying either the conjugate gradient method or a recently proposed spectral
gradient method. However, the algorithm differs from GPCG not only because it
deals with a more general class of problems, but mainly for the way it stops
the minimization phase. This is based on a comparison between a measure of
optimality in the reduced space and a measure of bindingness of the variables
that are on the bounds, defined by extending the concept of proportioning,
which was proposed by some authors for box-constrained problems. If the
objective function is bounded, the algorithm converges to a stationary point
thanks to a suitable application of the gradient projection method in the
identification phase. For strictly convex problems, the algorithm converges to
the optimal solution in a finite number of steps even in case of degeneracy.
Extensive numerical experiments show the effectiveness of the proposed
approach.Comment: 30 pages, 17 figure
Directional TGV-based image restoration under Poisson noise
We are interested in the restoration of noisy and blurry images where the
texture mainly follows a single direction (i.e., directional images). Problems
of this type arise, for example, in microscopy or computed tomography for
carbon or glass fibres. In order to deal with these problems, the Directional
Total Generalized Variation (DTGV) was developed by Kongskov et al. in 2017 and
2019, in the case of impulse and Gaussian noise. In this article we focus on
images corrupted by Poisson noise, extending the DTGV regularization to image
restoration models where the data fitting term is the generalized
Kullback-Leibler divergence. We also propose a technique for the identification
of the main texture direction, which improves upon the techniques used in the
aforementioned work about DTGV. We solve the problem by an ADMM algorithm with
proven convergence and subproblems that can be solved exactly at a low
computational cost. Numerical results on both phantom and real images
demonstrate the effectiveness of our approach.Comment: 20 pages, 1 table, 13 figure
Using gradient directions to get global convergence of Newton-type methods
The renewed interest in Steepest Descent (SD) methods following the work of
Barzilai and Borwein [IMA Journal of Numerical Analysis, 8 (1988)] has driven
us to consider a globalization strategy based on SD, which is applicable to any
line-search method. In particular, we combine Newton-type directions with
scaled SD steps to have suitable descent directions. Scaling the SD directions
with a suitable step length makes a significant difference with respect to
similar globalization approaches, in terms of both theoretical features and
computational behavior. We apply our strategy to Newton's method and the BFGS
method, with computational results that appear interesting compared with the
results of well-established globalization strategies devised ad hoc for those
methods.Comment: 22 pages, 11 Figure
ACQUIRE: an inexact iteratively reweighted norm approach for TV-based Poisson image restoration
We propose a method, called ACQUIRE, for the solution of constrained
optimization problems modeling the restoration of images corrupted by Poisson
noise. The objective function is the sum of a generalized Kullback-Leibler
divergence term and a TV regularizer, subject to nonnegativity and possibly
other constraints, such as flux conservation. ACQUIRE is a line-search method
that considers a smoothed version of TV, based on a Huber-like function, and
computes the search directions by minimizing quadratic approximations of the
problem, built by exploiting some second-order information. A classical
second-order Taylor approximation is used for the Kullback-Leibler term and an
iteratively reweighted norm approach for the smoothed TV term. We prove that
the sequence generated by the method has a subsequence converging to a
minimizer of the smoothed problem and any limit point is a minimizer.
Furthermore, if the problem is strictly convex, the whole sequence is
convergent. We note that convergence is achieved without requiring the exact
minimization of the quadratic subproblems; low accuracy in this minimization
can be used in practice, as shown by numerical results. Experiments on
reference test problems show that our method is competitive with
well-established methods for TV-based Poisson image restoration, in terms of
both computational efficiency and image quality.Comment: 37 pages, 13 figure
LSOS: Line-search Second-Order Stochastic optimization methods for nonconvex finite sums
We develop a line-search second-order algorithmic framework for minimizing
finite sums. We do not make any convexity assumptions, but require the terms of
the sum to be continuously differentiable and have Lipschitz-continuous
gradients. The methods fitting into this framework combine line searches and
suitably decaying step lengths. A key issue is a two-step sampling at each
iteration, which allows us to control the error present in the line-search
procedure. Stationarity of limit points is proved in the almost-sure sense,
while almost-sure convergence of the sequence of approximations to the solution
holds with the additional hypothesis that the functions are strongly convex.
Numerical experiments, including comparisons with state-of-the art stochastic
optimization methods, show the efficiency of our approach.Comment: 22 pages, 4 figure
Post vaccinal temporary sensorineural hearing loss
In our systematic research we identified four studies concerning the onset of neurological adverse events following vaccination and two excluding this association. A 33-year-old Italian man, belonging to the Italian Army was hospitalized because he suffered from vertigo, nausea and sudden right hearing loss not classified (NDD), that set in 24 h after the administration of tetanus-diphtheria and meningococcal vaccines. Some neurological events arising after vaccination are very difficult to treat. In our case, the functional recovery on low and medium frequencies was possible about 6 months after the morbid event
Successful endovascular embolization of a giant splenic artery pseudoaneurysm secondary to a huge pancreatic pseudocyst with concomitant spleen invasion
Pseudoaneurysms of the pancreatic and peripancreatic arteries is a well-known complication of chronic or necrotizing pancreatitis due to proteolytic enzymatic digestion of the arterial wall. A major part of peripancreatic pseudoaneurysms involve the splenic artery, but any peripancreatic artery may be involved and bleed. They are potentially life threatening for patients, due to spontaneous intraperitoneal rupture, rupture and fistulization into the surrounding organs, or fistulization into the pancreatic duct. Small ones are usually asymptomatic and are often diagnosed incidentally, while giant (> 5 cm) aneurysms and pseudoaneurysms are symptomatic and may be detected as a pulsatile mass in the upper-left quadrant or epigastrium. Imaging plays a key role in the identification of splenic artery aneurysms and pseudoaneurysms, while angiography still represents the gold standard for the diagnosis, although nowadays it plays a prominent role in treatment. Treatment of splenic artery pseudoaneurysms is mandatory because of the high probability of rupture, with a mortality rate of up to 90%. The gold standard treatment is represented by surgery, with a mortality rate between 16% and 50%. In recent years the endovascular approach has proven to be an effective alternative treatment for splenic artery pseudoaneurysms, and it is currently the method of choice. In this article, we present the case of a ant pseudoaneurysm of the splenic artery due to huge pseudocysts in a young alcoholic patient with recurrent and chronic pancreatitis, complicated by fistulization and invasion of spleen parenchyma and arteriovenous fistula
Faecal calprotectin and ultrasonography as non-invasive screening tools for detecting colorectal polyps in children with sporadic rectal bleeding. a prospective study
Background: Colorectal polyps are reported in 6,1% of paediatric colonoscopies and in 12% of those performed for
lower gastrointestinal bleeding. Although colonoscopy is widely used in paediatric patients, it requires bowel
preparation and general anaesthesia or deep sedation, and in rare cases, it can cause complications. Non-invasive
screening techniques able to predict polyps in children with isolated and sporadic rectal bleeding may play a key
role in the selection of patients needing colonoscopy.
Methods: We enrolled all children undergoing colonoscopy for isolated and sporadic rectal bleeding to determine
the diagnostic accuracy of faecal calprotectin, ultrasonography (US) and digital rectal examination as diagnostic
methods for screening colorectal polyps.
Results: A total of 26 of 59 enrolled patients (44.1%) had colonic polyps, one patient had multiple polyps, and 23%
of children had polyps proximal to the splenic flexure. The diagnostic accuracy of faecal calprotectin for detecting
colorectal polyps was 96.6%, with a sensitivity of 100%. False-positive faecal calprotectin was shown in 2 patients
with non-steroidal anti-inflammatory drug-related lesions. The diagnostic accuracy of ultrasound was 77.9%. Polyps
not seen with ultrasound tended to be relatively smaller (1.5 vs 2.3, p = 0.001) and located in the rectum. The
combined use of FC, US and digital rectal examination obtained a specificity and PPV of 100%.
Conclusions: FC combined with US and digital rectal examination is a good and promising non-invasive screening
test for detecting colorectal polyps in children with isolated and sporadic rectal bleeding
Pelvic pain in reproductive age: US findings
Pelvic pain in reproductive age often represents a diagnostic challenge due to the variety of potential causes characterized by overlapping clinical symptoms, including gynecological and other disorders (e.g., entero-colic or urological). It is also necessary to determine if there is a possibility of pregnancy to rule out any related complications, such as ectopic pregnancy. Although ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI) are strongly integrated, the choice of which is the ideal diagnostic tool should be guided both by clinical suspicion (gynecological vs. non-gynecological cause) and by the risk ratio-benefit (ionizing radiation and instrumental costs), too. The didactic objective proposed by this review consists in the diagnosis of the cause and differential of pelvic pain in reproductive age by describing and critically analyzing the US diagnostic clues of the most frequent adnexal, uterine, and vascular causes
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