8 research outputs found
Deterministic Digital Clustering of Wireless Ad Hoc Networks
We consider deterministic distributed communication in wireless ad hoc
networks of identical weak devices under the SINR model without predefined
infrastructure. Most algorithmic results in this model rely on various
additional features or capabilities, e.g., randomization, access to geographic
coordinates, power control, carrier sensing with various precision of
measurements, and/or interference cancellation. We study a pure scenario, when
no such properties are available. As a general tool, we develop a deterministic
distributed clustering algorithm. Our solution relies on a new type of
combinatorial structures (selectors), which might be of independent interest.
Using the clustering, we develop a deterministic distributed local broadcast
algorithm accomplishing this task in rounds, where
is the density of the network. To the best of our knowledge, this is
the first solution in pure scenario which is only polylog away from the
universal lower bound , valid also for scenarios with
randomization and other features. Therefore, none of these features
substantially helps in performing the local broadcast task. Using clustering,
we also build a deterministic global broadcast algorithm that terminates within
rounds, where is the diameter of the
network. This result is complemented by a lower bound , where is the path-loss parameter of the
environment. This lower bound shows that randomization or knowledge of own
location substantially help (by a factor polynomial in ) in the global
broadcast. Therefore, unlike in the case of local broadcast, some additional
model features may help in global broadcast
Uniform framework for the objective assessment and optimisation of radiotherapy image quality
Image guidance has rapidly become central to current radiotherapy practice. A
uniform framework is developed for evaluating image quality across all imaging
modalities by modelling the ‘universal phantom’: breaking any phantom
down into its constituent fundamental test objects and applying appropriate
analysis techniques to these through the construction of an automated analysis
tree. This is implemented practically through the new software package
‘IQWorks’ and is applicable to both radiotherapy and diagnostic imaging.
For electronic portal imaging (EPI), excellent agreement was observed with
two commercial solutions: the QC-3V phantom and PIPS Pro software (Standard
Imaging) and EPID QC phantom and epidSoft software (PTW). However,
PIPS Pro’s noise correction strategy appears unnecessary for all but the highest
frequency modulation transfer function (MTF) point and its contrast to noise
ratio (CNR) calculation is not as described. Serious flaws identified in epid-
Soft included erroneous file handling leading to incorrect MTF and signal to
noise ratio (SNR) results, and a sensitivity to phantom alignment resulting in
overestimation of MTF points by up to 150% for alignment errors of only ±1
pixel.
The ‘QEPI1’ is introduced as a new EPI performance phantom. Being a simple
lead square with a central square hole it is inexpensive and straightforward to
manufacture yet enables calculation of a wide range of performance metrics at
multiple locations across the field of view. Measured MTF curves agree with
those of traditional bar pattern phantoms to within the limits of experimental
uncertainty. An intercomparison of the Varian aS1000 and aS500-II detectors
demonstrated an improvement in MTF for the aS1000 of 50–100% over the
clinically relevant range 0.4–1 cycles/mm, yet with a corresponding reduction
in CNR by a factor of
p
2. Both detectors therefore offer advantages for different
clinical applications.
Characterisation of cone-beam CT (CBCT) facilities on two Varian On-Board
Imaging (OBI) units revealed that only two out of six clinical modes had been
calibrated by default, leading to errors of the order of 400 HU for some modes and materials – well outside the ±40 HU tolerance. Following calibration, all
curves agreed sufficiently for dose calculation accuracy within 2%. CNR and
MTF experiments demonstrated that a boost in MTF f50 of 20–30% is achievable
by using a 5122 rather than a 3842 matrix, but with a reduction in CNR of the
order of 30%.
The MTF f50 of the single-pulse half-resolution radiographic mode of the
Varian PaxScan 4030CB detector was measured in the plane of the detector as
1.0±0.1 cycles/mm using both a traditional tungsten edge and the new QEPI1
phantom. For digitally reconstructed radiographs (DRRs), a reduction in CT
slice thickness resulted in an expected improvement in MTF in the patient scanning
direction but a deterioration in the orthogonal direction, with the optimum
slice thickness being 1–2 mm. Two general purposes display devices were
calibrated against the DICOM Greyscale Standard Display Function (GSDF) to
within the ±20% limit for Class 2 review devices.
By providing an approach to image quality evaluation that is uniform across
all radiotherapy imaging modalities this work enables consistent end-to-end
optimisation of this fundamental part of the radiotherapy process, thereby supporting
enhanced use of image-guidance at all relevant stages of radiotherapy
and better supporting the clinical decisions based on it
Lifelines
Harris Solomon takes readers into the trauma ward of one of Mumbai’s busiest public hospitals, narrating the stories of the patients, providers, families, and frontline workers who experience and treat traumatic injury from traffic
Lifelines
Harris Solomon takes readers into the trauma ward of one of Mumbai’s busiest public hospitals, narrating the stories of the patients, providers, families, and frontline workers who experience and treat traumatic injury from traffic