470 research outputs found
Maxwell-compensated design of asymmetric gradient waveforms for tensor-valued diffusion encoding
Purpose: Asymmetric gradient waveforms are attractive for diffusion encoding
due to their superior efficiency, however, the asymmetry may cause a residual
gradient moment at the end of the encoding. Depending on the experiment setup,
this residual moment may cause significant signal bias and image artifacts. The
purpose of this study was to develop an asymmetric gradient waveform design for
tensor-valued diffusion encoding that is not affected by concomitant gradient.
Methods: The Maxwell index was proposed as a scalar invariant that captures the
effect of concomitant gradients and was constrained in the numerical
optimization to 100 (mT/m)ms to yield Maxwell-compensated waveforms. The
efficacy of this design was tested in an oil phantom, and in a healthy human
brain. For reference, waveforms from literature were included in the analysis.
Simulations were performed to investigate if the design was valid for a wide
range of experiments and if it could predict the signal bias. Results:
Maxwell-compensated waveforms showed no signal bias in oil or in the brain. By
contrast, several waveforms from literature showed gross signal bias. In the
brain, the bias was large enough to markedly affect both signal and parameter
maps, and the bias could be accurately predicted by theory. Conclusion:
Constraining the Maxwell index in the optimization of asymmetric gradient
waveforms yields efficient tensor-valued encoding with concomitant gradients
that have a negligible effect on the signal. This waveform design is especially
relevant in combination with strong gradients, long encoding times, thick
slices, simultaneous multi-slice acquisition and large/oblique FOVs
Real-time screen space reflections and refractions using sparse voxel octrees
This thesis explores the data structure known as sparse voxel octree and how it can improve the performance of real-time ray tracing. While ray tracing is an excellent way of producing realistic effects in computer graphics it is also very computationally heavy. Its use in real-time applications such as games and simulators is therefore limited since the hardware must be able to render enough frames per second to satisfy the users. The purpose of an octree is to reduce the amount of intersection tests each ray needs significantly. This thesis will explain the many challenges when implementing and using an octree, and also how to solve them. This includes both how to build the tree using various tests and then also how to use it with a ray tracer to produce reflections and refractions in real time
Intermittent Connectivity for Exploration in Communication-Constrained Multi-Agent Systems
Motivated by exploration of communication-constrained underground environments using robot teams, we study the problem of planning for intermittent connectivity in multi-agent systems. We propose a novel concept of information-consistency to handle situations where the plan is not initially known by all agents, and suggest an integer linear program for synthesizing information-consistent plans that also achieve auxiliary goals. Furthermore, inspired by network flow problems we propose a novel way to pose connectivity constraints that scales much better than previous methods. In the second part of the paper we apply these results in an exploration setting, and propose a clustering method that separates a large exploration problem into smaller problems that can be solved independently. We demonstrate how the resulting exploration algorithm is able to coordinate a team of ten agents to explore a large environment
Diffusional exchange versus microscopic kurtosis from CTI: two conflicting interpretations of the same data
Correlation tensor imaging (CTI) is a new diffusion MRI framework that
utilises double diffusion encoding (DDE) to resolve isotropic, anisotropic and
microscopic kurtosis sources. Microscopic kurtosis in CTI is provided by the
contrast between SDE and parallel DDE signals at the same b-value.
Multi-Gaussian exchange (MGE) is a diffusion MRI framework that utilises DDE to
measure exchange. The highest exchange sensitivity in MGE is obtained by
contrasting SDE and DDE signals at the same b-value. CTI and MGE can thus be
applied to analyse the same data but provide conflicting interpretations of
that data. We perform Monte Carlo simulations in different geometries with
varying levels of exchange to determine which approach is more compatible with
the data. Simulations reveal that in all microstructures considered, CTI
microscopic kurtosis drastically increases when exchange is introduced.
Furthermore, in microstructures that are well-described by the multi-Gaussian
assumption, CTI-estimated microscopic kurtosis increases with both the exchange
rate and the mixing time, despite fulfilment of the long-mixing-time condition
of CTI. Increasing the exchange rate by a factor of 2 positively biases CTI
microscopic kurtosis by approximately the same factor. At a modest exchange
rate of 10 /s, varying the mixing time from 12 to 100 ms increases CTI
microscopic kurtosis by at least a factor of 3. To address this problem, we
propose a heuristic approach to combine CTI and MGE to estimate
intra-compartmental kurtosis unconfounded by exchange and demonstrate its
feasibility using numerical simulations
Particle size and dry matter content in total mixed rations to cattle : how does it affect dry matter intake and sorting behaviour?
Fullfoderblandningar Àr det vanligaste utfodringssystemet utöver bete som anvÀnds till mjölkkor i vÀrlden idag. Systemet bygger pÄ att man blandar allt foder i en mixer tills det blir en homogen blandning, vilket gör att man i teorin kan erbjuda korna en balanserad foderstat i varje tugga. I praktiken sker oftast en sortering av fodret vilket gör att korna inte fÄr i sig den foderstat man har berÀknat. Syftet med denna littera-turstudie var att undersöka hur torrsubstanshalt (ts) och partikelstorlek pÄverkar fo-derintag och sorteringsbeteende hos nötkreatur. Sortering sker nÀstan alltid av korna i nÄgon utstrÀckning, men detta beteende kan minskas genom att justera ts-halten och partikelstorleken. En ts-halt pÄ över 60% har visat sig öka sorteringen vilket kan bero pÄ att det Àr lÀttare för korna att trycka bort de lÄnga partiklarna med tungan för att Àta de smÄ partiklarna. Om ts-halten Àr lÀgre Àn 50% sÄ minskar ts-intaget, detta beror förmodligen pÄ den större mÀngden foder som mÄste konsumeras med den ökade vattenmÀngden. För att fÄ sÄ liten sortering som möjligt behövs en jÀmn blandning av olika partikelstorlekar. Torrsubstansintaget minskar om det Àr för mycket lÄnga partiklar, förmodligen pÄ grund av att de stannar kvar i vÄmmen lÀngre Àn korta partiklar. Torrsubstansintaget minskar ocksÄ om korna fÄr för mycket kort-hackat foder, vilket kan bero pÄ att de behöver en del lÄngstrÄigt foder som stimu-lerar idissling och Àr positivt för vÄmmiljön.Total mixed rations are the main feeding system besides grazing that is being used to dairy cows worldwide. The principle is that you mix all feed compartments in a mixer to get a homogenous mix. By doing this the cows can, in theory, get a well balanced feed ration in each mouthful of feed. In practice the cows almost always sort to some extent, which means that they do not consume the feed ration that is calculated for them. The aim of this literature study was to investigate how dry mat-ter (DM) content and particle size affect feed intake and sorting behaviour in cattle. Sorting occurs almost all of the time by the cows to some extent, but this behaviour can be reduced by adjusting the DM content and the particle size. At a DM content of 60% or above the sorting increases, probably because it is easier for the cows to push the long feed away to eat the short particles. With a feed of 50% DM-content or less the DM-intake decreases and the sorting increases, probably due to the in-creased amount of fresh weight that the cows need to consume to obtain the same amount of feed DM. To get as little sorting as possible you need an even distribution of all particle sizes. The DM-intake decreases if the feed contains too much long particles, probably because those particles stay in the rumen longer. Dry matter in-take decreases also if the feed contains too much short particles. This might be be-cause the cows need some long particles to stimulate rumination and maintain a good rumen function
Dystocia in Swedish beef cattle
In Swedish beef cattle breeding, the genetic evaluation comprises calving traits, carcass traits and growth traits. For the calving traits stillbirth, birth weight and dystocia severity grade are recorded. The genetic evaluation for dystocia is divided into four traits; maternal and direct effect when the mother is a primiparous cow, and maternal and direct effect when the mother is a multiparous cow.
The system for reporting the grade of dystocia changed in May 2012 from a three-graded scale to a four-graded scale. The distribution of the dystocia grades became mark-edly different, which affected the breeding values in a way that several extremely low values have occurred.
This report examines different genetic evaluations with alternative normal scores in-stead of the old scores corrected for heterogeneous variance. Also some genetic correla-tions that had been put to zero to avoid extremely low breeding values were again added to the genetic evaluation to examine their effect on the breeding values.
In the current reporting system there are examples of negative breeding values, while in the old reporting system the breeding values were not that extreme. The breeding values seem to be more normally distributed when using the transformed normal scores com-pared to the old scores. There are also less extremely low breeding values. When adding the genetic correlations between the traits, the breeding values changed a little bit. Overall the results indicate that the genetic evaluation is more influenced by changes in the scores system than by the genetic correlation.I köttaveln pÄ nötkreatur i Sverige avelsvÀrderas djuren för kalvningsegenskaper, slakt-kroppsegenskaper och tillvÀxtegenskaper. För kalvningsegenskaper Àr det tvÄ egenskaper som ingÄr i avelsvÀrderingen; födelsevikt och kalvningssvÄrighet. AvelsvÀrderingen för kalvningssvÄrigheter Àr uppdelad pÄ fyra egenskaper, maternell effekt dÄ modern Àr förs-takalvare, direkt effekt dÄ modern Àr förstakalvare, maternell effekt dÄ modern Àr en Àldre ko, och direkt effekt dÄ modern Àr en Àldre ko.
Rapporteringssystemet för kalvningssvÄrigheter Àndrades i maj 2012 frÄn en tregradig skala till en fyrgradig. Fördelningen mellan de rapporterade klasserna för kalvningssvÄrig-het Àndrades markant, vilket pÄverkade avelsvÀrdena sÄ att avelsvÀrdena för en del djur blev extremt lÄga.
Denna rapport utvĂ€rderar olika sĂ€tt att hantera olika grader av kalvningssvĂ„righet i avelsvĂ€rderingen med alternativa sĂ„ kallade ânormal scoresâ istĂ€llet för de vĂ€rden som anvĂ€nds idag tillsammans med korrektion för heterogen varians. NĂ„gra genetiska korrelat-ioner som tidigare har satts till noll, pĂ„ grund av de extremt lĂ„ga avelsvĂ€rdena, Ă„terinför-des i denna rapport för att utvĂ€rdera deras pĂ„verkan pĂ„ avelsvĂ€rdena.
Dagens rapporteringssystem och avelsvĂ€rdering ger upphov till en del negativa avels-vĂ€rden, medan det med det gamla rapporteringssystemet inte fanns lika extrema avels-vĂ€rden. AvelsvĂ€rdena verkar vara mer normalfördelade nĂ€r man anvĂ€nder de transfor-merade ânormal scoresâ jĂ€mfört med de vĂ€rden som anvĂ€nds idag. Det ger dessutom mindre extremt lĂ„ga avelsvĂ€rden. NĂ€r korrelationerna Ă„terinfördes sĂ„ Ă€ndrades avelsvĂ€r-dena nĂ„got. Ăverlag indikerar resultaten att avelsvĂ€rderingen pĂ„verkas mer av om ânor-mal scoresâ anvĂ€nds Ă€n av om dessa korrelationer utelĂ€mnas eller inte
Direktimport av traktorer
Att direktimportera traktorer var tidigare enkelt. Oavsett om nödvÀndiga dokument medföljde vid köpet eller inte, kunde det alltid lösas nÀr traktorn vÀl var i Sverige. Efter att en nytolkning av importlagen har blivit antagen Àr det numera inte möjligt att godkÀnna traktorer som redan finns i landet utan de rÀtta dokumenten mÄste finnas vid importen. Det dilemma importörerna stÀlls inför Àr att endast ersÀttningsdokument för redan godkÀnda traktorer kan bli utskrivna.
Syftet med denna uppsattas Àr att beskriva och lÀra andra hur man importerar traktorer. Baserat pÄ den stora prisskillnaden mellan Sverige och andra lÀnder Àr utlÀndska traktorer prismÀssigt ett intressant alternativ.
Dessutom kommer denna uppsatts att undersöka lantbrukares kunskaper och attityder till att importera traktorer pÄ egen hand.
Att ha goda relationer med ÄterförsÀljare utomlands Àr avgörande för att undvika att bli lurad i en affÀr. TillvÀgagÄngssÀttet om hur man importerar en traktor Àr tagna frÄn en fallstudie som följt processen pÄ en importerad traktor frÄn Danmark till Sverige. PÄ detta sÀtt Àr mÄnga av de potentiella problem som uppstÄr nÀr man importerar en traktor behandlade. Dessutom har intervjuer genomförts med lantbrukare om deras generella attityd till begagnade importerade traktorer, lantbrukarna var selekterade ur demografiskt och geografiskt olika omrÄden.
Intervjuerna visade att mÄnga lantbrukare var villiga att importera en traktor, men ingen hade valt att göra det. Lantbrukarna sÄg inte regelverket som ett hinder för att importera traktorer, de valde helt enkelt att inte göra det. Detta antas leda till högre traktorpriser i Sverige pÄ grund av bristande konkurens.Importing tractors used to be easy. Whether or not it was accompanied with documents or
not, the problem could always be solved once it arrived in Sweden. Since a new
interpretation of import laws has been applied, this is no longer possible. Because of the
stricter interpretation, tractorâs already in country cannot be approved without proper
paperwork. The quandary for importers and manufacturers is that only replacement
documents for tractors that are already approved can be printed.
The purpose of this thesis is to describe (and teach others) how to import used tractors.
Based on the steep price differences in other countries, often much lower than in Sweden,
foreign tractors are an interesting import option. Consequently, this thesis will also
investigate farmersâ knowledge and attitudes towards importing tractors.
Having good relations with dealers abroad is essential to avoiding subterfuge.
Conclusions on how to import a used tractor are drawn from a field case study that
followed the process on an imported tractor from Denmark to Sweden. In this way, many
of the potential problems encountered when importing a tractor are highlighted. In
addition, interviews were conducted with Swedish farmers concerning their general
attitudes towards used imported tractors. The selection of cases were drawn from a
demographically and geographically diverse area.
The interviews showed that many farmers were willing to import a tractor, but none had
actually done so. The farmersâ did not seem to believe the regulations or other factors
limited their ability to import. Rather, they simply chose not to do so. It is suggested that
this leads to higher prices in Sweden due to a lack of competitio
Gradient waveform design for tensor-valued encoding in diffusion MRI
Diffusion encoding along multiple spatial directions per signal acquisition
can be described in terms of a b-tensor. The benefit of tensor-valued diffusion
encoding is that it unlocks the "shape of the b-tensor" as a new encoding
dimension. By modulating the b-tensor shape, we can control the sensitivity to
microscopic diffusion anisotropy which can be used as a contrast mechanism; a
feature that is inaccessible by conventional diffusion encoding. Since imaging
methods based on tensor-valued diffusion encoding are finding an increasing
number of applications we are prompted to highlight the challenge of designing
the optimal gradient waveforms for any given application. In this review, we
first establish the basic design objectives in creating field gradient
waveforms for tensor-valued diffusion MRI. We also survey additional design
considerations related to limitations imposed by hardware and physiology,
potential confounding effects that cannot be captured by the b-tensor, and
artifacts related to the diffusion encoding waveform. Throughout, we discuss
the expected compromises and tradeoffs with an aim to establish a more complete
understanding of gradient waveform design and its impact on accurate
measurements and interpretations of data.Comment: Invited review, submitted in May 2020 to the Journal of Neuroscience
Methods. 46 pages, 9 figures, 35 equation
Neuroblastoma cells with overexpressed MYCN retain their capacity to undergo neuronal differentiation
The dot-compartment revealed? Diffusion MRI with ultra-strong gradients and spherical tensor encoding in the living human brain
The so-called âdot-compartmentâ is conjectured in diffusion MRI to represent small spherical spaces, such as cell bodies, in which the diffusion is restricted in all directions. Previous investigations inferred its existence from data acquired with directional diffusion encoding which does not permit a straightforward separation of signals from âsticksâ (axons) and signals from âdotsâ. Here we combine isotropic diffusion encoding with ultra-strong diffusion gradients (240âŻmT/m) to achieve high diffusion-weightings with high signal to noise ratio, while suppressing signal arising from anisotropic water compartments with significant mobility along at least one axis (e.g., axons). A dot-compartment, defined to have apparent diffusion coefficient equal to zero and no exchange, would result in a non-decaying signal at very high b-values (b 7000âŻs/mm2). With this unique experimental setup, a residual yet slowly decaying, signal above the noise floor for b-values as high as 15âŻ000âŻs/mm2 was seen clearly in the cerebellar grey matter (GM), and in several white matter (WM) regions to some extent. Upper limits of the dot-signal-fraction were estimated to be 1.8% in cerebellar GM and 0.2% in WM. By relaxing the assumption of zero diffusivity, the signal at high b-values in cerebellar GM could be represented more accurately by an isotropic water pool with a low apparent diffusivity of 0.12 and a substantial signal fraction of 9.7%. The T2 of this component was estimated to be around 61âŻmâŻs. This remaining signal at high b-values has potential to serve as a novel and simple marker for isotropically-restricted water compartments in cerebellar GM
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