278 research outputs found
Imaging intracranial arterial patency and intravenous thrombolysis in acute ischaemic stroke
Among patients presenting acutely with ischaemic stroke who are being considered
for intravenous thrombolysis, prompt brain imaging is used to exclude contraindications
to treatment (chiefly haemorrhagic stroke or other conditions mimicking
stroke) rather than to identify which patients are more or less likely to benefit from
thrombolysis. For example, it is unclear whether the presence or absence of arterial
obstruction on imaging should be used to guide thrombolysis treatment decisions.
In this thesis I explore methods of imaging arterial patency among patients
presenting acutely with ischaemic stroke and look for associations between these
early imaging findings, response to intravenous thrombolysis and functional outcome
six-months after stroke onset. I primarily use data from the Third International Stroke
Trial (IST-3), the largest ever randomised-controlled trial testing the use of
intravenous alteplase for the acute treatment of ischaemic stroke.
I begin by summarising the main features of stroke, covering techniques for imaging
the brain and for imaging arterial patency, and post-stroke outcomes. Next I describe
two literature reviews which I compiled to increase my understanding of the topic
with particular reference to imaging arterial patency. This is followed by a summary
of IST-3. Then I describe the general methods I used to address my thesis aims
exploring relationships between imaging characteristics of arterial patency, treatment
with intravenous alteplase and functional outcome after ischaemic stroke.
Specifically, I investigated the following imaging features:
- The hyperattenuating artery sign (HAS), which is a non-contrast enhanced
CT finding thought to be indicative of acute arterial obstruction by thrombus
or embolus
- Arterial patency or obstruction as demonstrated using contrast enhanced CT
and MR angiographic imaging.
In addition to providing better characterisation of the HAS and a better
understanding of how angiography helps to assess ischaemic stroke patients, I found
that arterial obstruction (however this is identified on imaging) is associated with more severe stroke at baseline and worse functional outcome six months after stroke.
I also prove that intravenous alteplase is effective in the presence of arterial
obstruction, counter to a widely held concern that it may not be effective in this
context. Most of my work has been published in peer reviewed journals.
My work should give front line clinicians greater confidence to use intravenous
alteplase for the treatment of ischaemic stroke associated with arterial obstruction on
imaging, but more work is needed to better understand the implications of apparently
normal arterial patency on imaging among patients with ischaemic stroke
ACAT: Adversarial Counterfactual Attention for Classification and Detection in Medical Imaging
In some medical imaging tasks and other settings where only small parts of
the image are informative for the classification task, traditional CNNs can
sometimes struggle to generalise. Manually annotated Regions of Interest (ROI)
are sometimes used to isolate the most informative parts of the image. However,
these are expensive to collect and may vary significantly across annotators. To
overcome these issues, we propose a framework that employs saliency maps to
obtain soft spatial attention masks that modulate the image features at
different scales. We refer to our method as Adversarial Counterfactual
Attention (ACAT). ACAT increases the baseline classification accuracy of
lesions in brain CT scans from 71.39% to 72.55% and of COVID-19 related
findings in lung CT scans from 67.71% to 70.84% and exceeds the performance of
competing methods. We investigate the best way to generate the saliency maps
employed in our architecture and propose a way to obtain them from
adversarially generated counterfactual images. They are able to isolate the
area of interest in brain and lung CT scans without using any manual
annotations. In the task of localising the lesion location out of 6 possible
regions, they obtain a score of 65.05% on brain CT scans, improving the score
of 61.29% obtained with the best competing method.Comment: 17 pages, 7 figure
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Apparatus for centrifuge modelling of twin-tunnel construction
In urban areas it is common for pairs of tunnels to be used as a method for building rapid transit systems. Driven by
an increasing population and demand for services, tunnels are more widespread in their use than at any previous time. Construction of any form of tunnel causes ground movements which have the potential to damage existing surface and
sub-surface structures. Modern tunnelling practice aims to reduce these movements to a minimum but there is still a requirement for accurate assessments of possible damage to structures resulting from settlements. For tunnels driven in clay, superposition of settlement predictions made by considering a single tunnel is an accepted method used to estimate movements around pairs of tunnels. Previous research, particularly numerical studies, has indicated that this may not necessarily be sufficient. In this paper a series of centrifuge model tests designed to investigate settlements related to twin-tunnel construction are described. The development of the experimental apparatus for sequential twin-tunnel construction with variable centre-to-centre spacing and volume loss is described in detail
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