241 research outputs found
Enhanced imaging of microcalcifications in digital breast tomosynthesis through improved image-reconstruction algorithms
PURPOSE: We develop a practical, iterative algorithm for image-reconstruction
in under-sampled tomographic systems, such as digital breast tomosynthesis
(DBT).
METHOD: The algorithm controls image regularity by minimizing the image total
-variation (TpV), a function that reduces to the total variation when
or the image roughness when . Constraints on the image, such as
image positivity and estimated projection-data tolerance, are enforced by
projection onto convex sets (POCS). The fact that the tomographic system is
under-sampled translates to the mathematical property that many widely varied
resultant volumes may correspond to a given data tolerance. Thus the
application of image regularity serves two purposes: (1) reduction of the
number of resultant volumes out of those allowed by fixing the data tolerance,
finding the minimum image TpV for fixed data tolerance, and (2) traditional
regularization, sacrificing data fidelity for higher image regularity. The
present algorithm allows for this dual role of image regularity in
under-sampled tomography.
RESULTS: The proposed image-reconstruction algorithm is applied to three
clinical DBT data sets. The DBT cases include one with microcalcifications and
two with masses.
CONCLUSION: Results indicate that there may be a substantial advantage in
using the present image-reconstruction algorithm for microcalcification
imaging.Comment: Submitted to Medical Physic
Quantifying admissible undersampling for sparsity-exploiting iterative image reconstruction in X-ray CT
Iterative image reconstruction (IIR) with sparsity-exploiting methods, such
as total variation (TV) minimization, investigated in compressive sensing (CS)
claim potentially large reductions in sampling requirements. Quantifying this
claim for computed tomography (CT) is non-trivial, because both full sampling
in the discrete-to-discrete imaging model and the reduction in sampling
admitted by sparsity-exploiting methods are ill-defined. The present article
proposes definitions of full sampling by introducing four sufficient-sampling
conditions (SSCs). The SSCs are based on the condition number of the system
matrix of a linear imaging model and address invertibility and stability. In
the example application of breast CT, the SSCs are used as reference points of
full sampling for quantifying the undersampling admitted by reconstruction
through TV-minimization. In numerical simulations, factors affecting admissible
undersampling are studied. Differences between few-view and few-detector bin
reconstruction as well as a relation between object sparsity and admitted
undersampling are quantified.Comment: Revised version that was submitted to IEEE Transactions on Medical
Imaging on 8/16/201
Convex optimization problem prototyping for image reconstruction in computed tomography with the Chambolle-Pock algorithm
The primal-dual optimization algorithm developed in Chambolle and Pock (CP),
2011 is applied to various convex optimization problems of interest in computed
tomography (CT) image reconstruction. This algorithm allows for rapid
prototyping of optimization problems for the purpose of designing iterative
image reconstruction algorithms for CT. The primal-dual algorithm is briefly
summarized in the article, and its potential for prototyping is demonstrated by
explicitly deriving CP algorithm instances for many optimization problems
relevant to CT. An example application modeling breast CT with low-intensity
X-ray illumination is presented.Comment: Resubmitted to Physics in Medicine and Biology. Text has been
modified according to referee comments, and typos in the equations have been
correcte
Toward optimal X-ray flux utilization in breast CT
A realistic computer-simulation of a breast computed tomography (CT) system
and subject is constructed. The model is used to investigate the optimal number
of views for the scan given a fixed total X-ray fluence. The reconstruction
algorithm is based on accurate solution to a constrained, TV-minimization
problem, which has received much interest recently for sparse-view CT data.Comment: accepted to the 11th International Meeting on Fully Three-Dimensional
Image Reconstruction in Radiology and Nuclear Medicine 201
Ensuring convergence in total-variation-based reconstruction for accurate microcalcification imaging in breast X-ray CT
Breast X-ray CT imaging is being considered in screening as an extension to
mammography. As a large fraction of the population will be exposed to
radiation, low-dose imaging is essential. Iterative image reconstruction based
on solving an optimization problem, such as Total-Variation minimization, shows
potential for reconstruction from sparse-view data. For iterative methods it is
important to ensure convergence to an accurate solution, since important image
features, such as presence of microcalcifications indicating breast cancer, may
not be visible in a non-converged reconstruction, and this can have clinical
significance. To prevent excessively long computational times, which is a
practical concern for the large image arrays in CT, it is desirable to keep the
number of iterations low, while still ensuring a sufficiently accurate
reconstruction for the specific imaging task. This motivates the study of
accurate convergence criteria for iterative image reconstruction. In simulation
studies with a realistic breast phantom with microcalcifications we compare
different convergence criteria for reliable reconstruction. Our results show
that it can be challenging to ensure a sufficiently accurate microcalcification
reconstruction, when using standard convergence criteria. In particular, the
gray level of the small microcalcifications may not have converged long after
the background tissue is reconstructed uniformly. We propose the use of the
individual objective function gradient components to better monitor possible
regions of non-converged variables. For microcalcifications we find empirically
a large correlation between nonzero gradient components and non-converged
variables, which occur precisely within the microcalcifications. This supports
our claim that gradient components can be used to ensure convergence to a
sufficiently accurate reconstruction.Comment: 5 pages, 4 figures, extended version of conference paper for 2011
IEEE Nuclear Science Symposium and Medical Imaging Conferenc
GPU-based Iterative Cone Beam CT Reconstruction Using Tight Frame Regularization
X-ray imaging dose from serial cone-beam CT (CBCT) scans raises a clinical
concern in most image guided radiation therapy procedures. It is the goal of
this paper to develop a fast GPU-based algorithm to reconstruct high quality
CBCT images from undersampled and noisy projection data so as to lower the
imaging dose. For this purpose, we have developed an iterative tight frame (TF)
based CBCT reconstruction algorithm. A condition that a real CBCT image has a
sparse representation under a TF basis is imposed in the iteration process as
regularization to the solution. To speed up the computation, a multi-grid
method is employed. Our GPU implementation has achieved high computational
efficiency and a CBCT image of resolution 512\times512\times70 can be
reconstructed in ~5 min. We have tested our algorithm on a digital NCAT phantom
and a physical Catphan phantom. It is found that our TF-based algorithm is able
to reconstrct CBCT in the context of undersampling and low mAs levels. We have
also quantitatively analyzed the reconstructed CBCT image quality in terms of
modulation-transfer-function and contrast-to-noise ratio under various scanning
conditions. The results confirm the high CBCT image quality obtained from our
TF algorithm. Moreover, our algorithm has also been validated in a real
clinical context using a head-and-neck patient case. Comparisons of the
developed TF algorithm and the current state-of-the-art TV algorithm have also
been made in various cases studied in terms of reconstructed image quality and
computation efficiency.Comment: 24 pages, 8 figures, accepted by Phys. Med. Bio
A comprehensive study on the relationship between image quality and imaging dose in low-dose cone beam CT
While compressed sensing (CS) based reconstructions have been developed for
low-dose CBCT, a clear understanding on the relationship between the image
quality and imaging dose at low dose levels is needed. In this paper, we
qualitatively investigate this subject in a comprehensive manner with extensive
experimental and simulation studies. The basic idea is to plot image quality
and imaging dose together as functions of number of projections and mAs per
projection over the whole clinically relevant range. A clear understanding on
the tradeoff between image quality and dose can be achieved and optimal
low-dose CBCT scan protocols can be developed for various imaging tasks in
IGRT. Main findings of this work include: 1) Under the CS framework, image
quality has little degradation over a large dose range, and the degradation
becomes evident when the dose < 100 total mAs. A dose < 40 total mAs leads to a
dramatic image degradation. Optimal low-dose CBCT scan protocols likely fall in
the dose range of 40-100 total mAs, depending on the specific IGRT
applications. 2) Among different scan protocols at a constant low-dose level,
the super sparse-view reconstruction with projection number less than 50 is the
most challenging case, even with strong regularization. Better image quality
can be acquired with other low mAs protocols. 3) The optimal scan protocol is
the combination of a medium number of projections and a medium level of
mAs/view. This is more evident when the dose is around 72.8 total mAs or below
and when the ROI is a low-contrast or high-resolution object. Based on our
results, the optimal number of projections is around 90 to 120. 4) The
clinically acceptable lowest dose level is task dependent. In our study,
72.8mAs is a safe dose level for visualizing low-contrast objects, while 12.2
total mAs is sufficient for detecting high-contrast objects of diameter greater
than 3 mm.Comment: 19 pages, 12 figures, submitted to Physics in Medicine and Biolog
Data quality considerations for evaluating COVID-19 treatments using real world data: learnings from the National COVID Cohort Collaborative (N3C)
Background: Multi-institution electronic health records (EHR) are a rich source of real world data (RWD) for generating real world evidence (RWE) regarding the utilization, benefits and harms of medical interventions. They provide access to clinical data from large pooled patient populations in addition to laboratory measurements unavailable in insurance claims-based data. However, secondary use of these data for research requires specialized knowledge and careful evaluation of data quality and completeness. We discuss data quality assessments undertaken during the conduct of prep-to-research, focusing on the investigation of treatment safety and effectiveness. Methods: Using the National COVID Cohort Collaborative (N3C) enclave, we defined a patient population using criteria typical in non-interventional inpatient drug effectiveness studies. We present the challenges encountered when constructing this dataset, beginning with an examination of data quality across data partners. We then discuss the methods and best practices used to operationalize several important study elements: exposure to treatment, baseline health comorbidities, and key outcomes of interest. Results: We share our experiences and lessons learned when working with heterogeneous EHR data from over 65 healthcare institutions and 4 common data models. We discuss six key areas of data variability and quality. (1) The specific EHR data elements captured from a site can vary depending on source data model and practice. (2) Data missingness remains a significant issue. (3) Drug exposures can be recorded at different levels and may not contain route of administration or dosage information. (4) Reconstruction of continuous drug exposure intervals may not always be possible. (5) EHR discontinuity is a major concern for capturing history of prior treatment and comorbidities. Lastly, (6) access to EHR data alone limits the potential outcomes which can be used in studies. Conclusions: The creation of large scale centralized multi-site EHR databases such as N3C enables a wide range of research aimed at better understanding treatments and health impacts of many conditions including COVID-19. As with all observational research, it is important that research teams engage with appropriate domain experts to understand the data in order to define research questions that are both clinically important and feasible to address using these real world data
Prevalence of Chlamydia trachomatis infection among women in a Middle Eastern community
BACKGROUND: Common vaginal infections that manifest in women are usually easily diagnosed. However, Chlamydia infection is often asymptomatic, leading to infertility before it is detected. If it occurs in pregnancy, it could lead to significant neonatal morbidity. It may also play a role with other viral infections for e.g. Human Papilloma Virus in the development of cervical cancer. The objective of this study was to determine the prevalence of Chlamydia infection in women undergoing screening for cervical abnormalities as a part of a research project in primary and secondary care institutions in the United Arab Emirates. METHODS: In this cross sectional study married women attending primary and secondary care participating in a large nationwide cervical abnormalities screening survey were offered Chlamydia testing using a commercially available test kit. This kit uses a rapid immunoassay for the direct detection of Chlamydia trachomatis antigen in endocervical swab specimens. As this study was performed in a traditional Islamic country, unmarried women were excluded from testing, as the management of any positive cases would create legal and social problems. All married women consenting to take part in the study were included irrespective of age. RESULTS: Of 1039 women approached over a period of eight months 919 (88.5%) agreed to participate. The number of women in the 16 to 19 years was small (0.01%) and 30% were aged over 40 years. The prevalence of Chlamydia infection in this study was 2.6% (95% confidence interval 1.2–3.3%), which was marginally higher in women screened in secondary care (p = 0.05). CONCLUSION: This is one of the few reports on the prevalence of Chlamydia infection in women from the Middle East. Due to cultural and social constraints this study excluded a large proportion of women aged less than 19 years of age. Hence no direct comparisons on prevalence could be made with studies from the West, which all included younger women at high risk of Chlamydia. However this study emphasizes the importance of cultural factors while interpreting results of studies from different cultures and communities
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