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)$^2$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 är det vanligaste utfodringssystemet utöver bete som används till mjölkkor i världen idag. Systemet bygger på 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 får i sig den foderstat man har beräknat. Syftet med denna littera-turstudie var att undersöka hur torrsubstanshalt (ts) och partikelstorlek påverkar fo-derintag och sorteringsbeteende hos nötkreatur. Sortering sker nästan alltid av korna i någon utsträckning, men detta beteende kan minskas genom att justera ts-halten och partikelstorleken. En ts-halt på över 60% har visat sig öka sorteringen vilket kan bero på att det är lättare för korna att trycka bort de långa partiklarna med tungan för att äta de små partiklarna. Om ts-halten är lägre än 50% så minskar ts-intaget, detta beror förmodligen på den större mängden foder som måste konsumeras med den ökade vattenmängden. För att få så liten sortering som möjligt behövs en jämn blandning av olika partikelstorlekar. Torrsubstansintaget minskar om det är för mycket långa partiklar, förmodligen på grund av att de stannar kvar i våmmen längre än korta partiklar. Torrsubstansintaget minskar också om korna får för mycket kort-hackat foder, vilket kan bero på att de behöver en del långstråigt foder som stimu-lerar idissling och är positivt för våmmiljön.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 på nötkreatur i Sverige avelsvärderas djuren för kalvningsegenskaper, slakt-kroppsegenskaper och tillväxtegenskaper. För kalvningsegenskaper är det två egenskaper som ingår i avelsvärderingen; födelsevikt och kalvningssvårighet. Avelsvärderingen för kalvningssvårigheter är uppdelad på fyra egenskaper, maternell effekt då modern är förs-takalvare, direkt effekt då modern är förstakalvare, maternell effekt då modern är en äldre ko, och direkt effekt då modern är en äldre ko. Rapporteringssystemet för kalvningssvårigheter ändrades i maj 2012 från en tregradig skala till en fyrgradig. Fördelningen mellan de rapporterade klasserna för kalvningssvårig-het ändrades markant, vilket påverkade avelsvärdena så att avelsvärdena för en del djur blev extremt låga. Denna rapport utvärderar olika sätt att hantera olika grader av kalvningssvårighet i avelsvärderingen med alternativa så kallade ’normal scores’ istället för de värden som används idag tillsammans med korrektion för heterogen varians. Några genetiska korrelat-ioner som tidigare har satts till noll, på grund av de extremt låga avelsvärdena, återinför-des i denna rapport för att utvärdera deras påverkan på avelsvärdena. Dagens rapporteringssystem och avelsvärdering ger upphov till en del negativa avels-värden, medan det med det gamla rapporteringssystemet inte fanns lika extrema avels-värden. Avelsvärdena verkar vara mer normalfördelade när man använder de transfor-merade ’normal scores’ jämfört med de värden som används idag. Det ger dessutom mindre extremt låga avelsvärden. När korrelationerna återinfördes så ändrades avelsvär-dena något. Överlag indikerar resultaten att avelsvärderingen påverkas mer av om ’nor-mal scores’ används än av om dessa korrelationer utelämnas eller inte

Direktimport av traktorer

Att direktimportera traktorer var tidigare enkelt. Oavsett om nödvändiga dokument medföljde vid köpet eller inte, kunde det alltid lösas när traktorn väl var i Sverige. Efter att en nytolkning av importlagen har blivit antagen är det numera inte möjligt att godkänna traktorer som redan finns i landet utan de rätta dokumenten måste finnas vid importen. Det dilemma importörerna ställs inför är att endast ersättningsdokument för redan godkända traktorer kan bli utskrivna. Syftet med denna uppsattas är att beskriva och lära andra hur man importerar traktorer. Baserat på den stora prisskillnaden mellan Sverige och andra länder är utländska traktorer prismässigt ett intressant alternativ. Dessutom kommer denna uppsatts att undersöka lantbrukares kunskaper och attityder till att importera traktorer på egen hand. Att ha goda relationer med återförsäljare utomlands är avgörande för att undvika att bli lurad i en affär. Tillvägagångssättet om hur man importerar en traktor är tagna från en fallstudie som följt processen på en importerad traktor från Danmark till Sverige. På detta sätt är många av de potentiella problem som uppstår när 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 områden. Intervjuerna visade att många lantbrukare var villiga att importera en traktor, men ingen hade valt att göra det. Lantbrukarna såg 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 på 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, tractor’s 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 farmers’ 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 farmers’ 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

The dot-compartment revealed? Diffusion MRI with ultra-strong gradients and spherical tensor encoding in the living human brain

The so-called “dot-compartment” 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 ‘sticks’ (axons) and signals from ‘dots’. Here we combine isotropic diffusion encoding with ultra-strong diffusion gradients (240 mT/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 7000 s/mm2). With this unique experimental setup, a residual yet slowly decaying, signal above the noise floor for b-values as high as 15 000 s/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 61 m s. 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