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Dive into the research topics where Thorarin A. Bjarnason is active.

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Featured researches published by Thorarin A. Bjarnason.


Magnetic Resonance in Medicine | 2005

Characterization of the NMR behavior of white matter in bovine brain.

Thorarin A. Bjarnason; Irene M. Vavasour; C.L.L. Chia; Alex L. MacKay

In vitro experiments on 15 white matter samples from five bovine brains were performed on a 1H‐NMR spectrometer at 24°C and 37°C. The average myelin water fractions (MWFs) were 10.9% and 11.8% for samples at 24°C and 37°C, respectively. The T1 relaxation time at 37°C was found to be 830 ms, exhibiting monoexponential behavior. A four‐pool model including intra/extracellular (IE) water, myelin water, nonmyelin tissue, and myelin tissue was proposed to simulate the NMR behavior of bovine white matter. A cross‐relaxation correction was introduced to compensate for shifting of the measured data points and T2 times over the duration of the Carr‐Purcell‐Meiboom‐Gill (CPMG) measurement due to cross relaxation. This correction was found to be slight, providing evidence that MWFs measured using a multiecho technique are near physical values. At 24°C the cross‐relaxation times between myelin tissue and myelin water, myelin water and IE water, and IE water and nonmyelin tissue were found to be approximately 227, 2064, and 402 ms, respectively. At 37°C these same cross‐relaxation times were 158, 1021, and 170 ms, respectively. The exchange rate between myelin water and myelin was found to be 11.8 s−1 at 37°C, while the exchange rate between IE water and nonmyelin tissue was found to be 6.8 s−1. These exchange rates are of similar magnitude, which indicates that the interaction between IE water and nonmyelin tissue cannot be ignored. Magn Reson Med, 2005.


NeuroImage | 2009

Multiexponential T2 and magnetization transfer MRI of demyelination and remyelination in murine spinal cord.

Cheryl R. McCreary; Thorarin A. Bjarnason; Viktor Skihar; J. Ross Mitchell; V. Wee Yong; Jeff F. Dunn

Identification of remyelination is important in the evaluation of potential treatments of demyelinating diseases such as multiple sclerosis. Local injection of lysolecithin into the brain or spinal cord provides a murine model of demyelination with spontaneous remyelination. The aim of this study was to determine if quantitative, multicomponent T(2) (qT(2)) analysis and magnetization transfer ratio (MTR), both indicative of myelin content, could detect changes in myelination, particularly remyelination, of the cervical spinal cord in mice treated with lysolecithin. We found that the myelin water fraction and geometric mean T(2) value of the intra/extracellular water significantly decreased at 14 days then returned to control levels by 28 days after injury, corresponding to clearance of myelin debris and remyelination which was shown by eriochrome cyanine and oil red O staining of histological sections. The MTR was significantly decreased 14 days after lysolecithin injection, and remained low over the time course studied. Evidence of demyelination shown by both qT(2) and MTR lagged behind the histological evidence of demyelination. Myelin water fraction increased with remyelination, however MTR remained lower after 28 days. The difference between qT(2) and MTR may identify early remyelination.


Magnetic Resonance in Medicine | 2009

Myelin water measurement in the spinal cord

Evan P. Minty; Thorarin A. Bjarnason; Cornelia Laule; Alex L. MacKay

The desire to monitor the spatial–temporal characteristics of myelination in the spinal cord (SC), in the context of pathological change in demyelinating diseases or proposed neuroregenerative protocols, has led to an interest in noninvasive image‐based myelin measurement methods. We present one strategy: a magnetic resonance‐based measure that capitalizes on the characteristics of T2 relaxation of water compartmentalized within tissue. In this study, 32‐echo relaxation studies for measuring the myelin water fraction (MWF) were applied in healthy control SC in vivo using a sagittal inversion recovery multiecho sequence, and findings were supported with supplemental studies in bovine SC samples in vitro. Mean human MWF varied according the level of the SC examined: cervical, thoracic, and lumbar MWF was found to be 21.8 (SD = 2.1)%, 24.3 (3.6)%, and 11.4 (6.4)%, respectively. Noteworthy reductions were observed in areas consistent with the expected locations of the cervical and lumbar enlargements. Average bovine MWF was 30.0 (2.7)% in white matter and 8.2 (0.4)% in gray matter. The potential applications of T2 measurement in SC, both in characterizing disease processes like multiple sclerosis and in monitoring neuroregenerative therapies, should encourage future research in this area. Magn Reson Med, 2009.


Magnetic Resonance in Medicine | 2009

Quantitative T2 analysis: the effects of noise, regularization, and multivoxel approaches.

Thorarin A. Bjarnason; Cheryl R. McCreary; Jeff F. Dunn; J. Ross Mitchell

Typical quantitative T2 (qT2) analysis involves creating T2 distributions using a regularized algorithm from region‐of‐interest averaged decay data. This study uses qT2 analysis of simulated and experimental decay signals to determine how (a) noise‐type, (b) regularization, and (c) region‐of‐interest versus multivoxel analyses affect T2 distributions. Our simulations indicate that regularization causes myelin water fraction and intra/extracellular water geometric mean T2 underestimation that worsens as the signal‐to‐noise ratio decreases. The underestimation was greater for intra/extracellular water geometric mean T2 measures using Rician noise. Simulations showed significant differences between myelin water fractions determined using region‐of‐interest and multivoxel approaches compared to the true value. The nonregularized voxel‐based approach gave the most accurate measure of myelin water fraction and intra/extracellular water geometric mean T2 for a given signal‐to‐noise ratio and noise type. Additionally, multivoxel analysis provides important information about the variability of the analysis. Results obtained from in vivo rat data were similar to our simulation results. In each case, a nonregularized, multivoxel analysis provided myelin water fractions significantly different from the regularized approaches and obtained the largest myelin water fraction. We conclude that quantitative T2 analysis is best performed using a nonregularized, multivoxel approach. Magn Reson Med, 2010.


Magnetic Resonance in Medicine | 2011

Insight into in vivo magnetization exchange in human white matter regions.

Saeed Kalantari; Cornelia Laule; Thorarin A. Bjarnason; Irene M. Vavasour; Alex L. MacKay

Water exchange can play an important role in interpreting compartment‐specific magnetic resonance imaging data in brain. For example, an MR method of myelin measurement, known as myelin water fraction imaging, assumes that water exchange processes are slow compared with the measurement time scale. In this article, we examined whether water exchange processes have an effect on myelin water fraction values. A previously established four pool model of white matter was used to simulate the interactions between two aqueous compartments (myelin water and intra/extracellular water) and nonaqueous compartments (myelin and nonmyelin tissues). To extract the water exchange cross relaxation times, the Bloch equations were solved analytically. As the water exchange time scales are dependent on the spin‐lattice T1 relaxation of each of these four pools and due to the current uncertainties regarding the T1 associated with each pool, exchange cross relaxation times for three different T1 scenarios were calculated. The corrections that need to be considered in order for myelin water fraction to be an accurate marker for myelin were found to be less than 15%. This work indicates that regional variations in white matter myelin water fraction values are most likely due to variations in myelin content rather than regional differences in exchange rates. Magn Reson Med, 2011.


Annals of Neurology | 2013

Pathological correlates of magnetic resonance imaging texture heterogeneity in multiple sclerosis.

Yunyan Zhang; G. R. Wayne Moore; Cornelia Laule; Thorarin A. Bjarnason; Piotr Kozlowski; Anthony Traboulsee; David Li

To analyze the texture of T2‐weighted magnetic resonance imaging (MRI) of postmortem multiple sclerosis (MS) brain, and to determine whether and how MRI texture correlates with tissue pathology.


Journal of Computer Assisted Tomography | 2012

Organ-based computed tomographic (CT) radiation dose reduction to the lenses: impact on image quality for CT of the head.

Reimann Aj; Davison C; Thorarin A. Bjarnason; Yogesh Thakur; Kryzmyk K; John R. Mayo; Savvas Nicolaou

Purpose Recently, a new specific organ dose adaption and reduction protocol, or SODAR tool (X-CARE, Siemens Healthcare), which reduces dose to the anterior aspect of the body of patients, was installed on our computed tomographic scanner. The purpose of this pilot project was to evaluate image quality and dose distribution in the acquired data with the new protocol. Materials and Methods Sixteen consecutive patients were scanned with the new SODAR head protocol. The findings were compared with 16 matched patients who were imaged with the standard computed tomographic head trauma protocol. Image quality was assessed qualitatively using a scale of 1 to 4 (1, excellent; 2, good; 3, fair; 4, nondiagnostic). Additionally, 1-cm2 regions of interest were placed in the white matter of the cerebral hemispheres, the cerebellar hemispheres, and the brain stem at the level of the pons for a quantitative analysis. The standard deviation of each measurement was recorded as an indicator for image noise. Dose measurement trials were performed using optically stimulated luminescence dosimeters on head phantoms and then on patients. Results Subjective image quality ranged between 1 and 3; no scan areas were considered nondiagnostic. Overall image quality of the posterior fossa averaged at 1.656 was slightly reduced compared to the cerebral hemispheres (mean, 1.141). The mean standard protocol brain stem image quality was 1.604, with only minimal deterioration to 1.708 in the SODAR group. No significant difference in image noise could be found between the SODAR group with a mean noise of 4.515 and standard images with a mean of 4.721 (P > 0.05). The dose to the anterior aspect of the patient was lowered to 3.2 mGy compared to 4.5 mGy on the lateral aspect of the scan (P > 0.05). To compensate for the photon loss in the posterior aspect, the dose has to be slightly increased to a mean of 6 mGy, but overall, a significant dose reduction with stable image quality could be achieved by reducing the dose length product from 1489 to 1347 mGy·cm using SODAR (P < 0.0001). Conclusion Using the SODAR protocol resulted not only in an impressive 46% to 59% frontal dose reduction but also in the overall dose reduction. This dose reduction was obtained without sacrificing image quality, providing diagnostic images of the brain while protecting radiosensitive structures like the eye lenses in trauma brain imaging. Future applications will be reducing dose to other radiosensitive structures such as the thyroid gland and breast tissue from potentially harmful low-energy radiation without compromising image quality.


Journal of Magnetic Resonance | 2010

AnalyzeNNLS: magnetic resonance multiexponential decay image analysis.

Thorarin A. Bjarnason; J. Ross Mitchell

Exponential decays are fundamental to magnetic resonance imaging, yet adequately sampling and analyzing multiexponential decays is rarely attempted. The advantage of multiexponential analysis is the quantification of sub-voxel structure caused by water compartmentalization, with application as a non-invasive imaging biomarker for myelin. We have developed AnalyzeNNLS, software designed specifically for multiexponential decay image analysis that has a user-friendly graphical user interface and can analyze data from many MR manufacturers. AnalyzeNNLS is a simple, platform independent analysis tool that was created using the extensive mathematical and visualization libraries in Matlab, and released as open source code allowing scientists to evaluate, scrutinize, improve, and expand.


Magnetic Resonance in Medicine | 2011

The functional microstructure of tendon collagen revealed by high-field MRI.

Kelsey M. Mountain; Thorarin A. Bjarnason; Jeff F. Dunn; John R. Matyas

T2 was used in this study to assess tendon microstructure. Two unloaded digital extensor tendons were bent such that their long axes were imaged throughout 180° with respect to B0. T2‐weighted images reveal periodic banding (∼200 μm) when tendons were oriented at ±55° with respect to B0. Five pairs of tendons were used to study the influence of load on T2W MRI: one tendon of each pair was loaded with a 7.8‐N mass, and both tendons were fixed in formalin then imaged at 55° to B0. MRI banding was present in the unloaded, but not loaded, tendons. In unloaded tendons, polarized‐light microscopy revealed collagen crimp with a periodicity similar to MRI. In loaded tendons, there was a strain‐induced extinction of periodicity on both MRI and polarized‐light microscopy. These studies confirm that crimp is detectable by high‐field MRI and could serve as an in vivo index of physiological strains in collagenous tissues. Magn Reson Med, 2011.


Journal of Magnetic Resonance | 2013

Temporal phase correction of multiple echo T2 magnetic resonance images

Thorarin A. Bjarnason; Cornelia Laule; Joel Bluman; Piotr Kozlowski

Typically, magnetic resonance imaging (MRI) analysis is performed on magnitude data, and multiple echo T2 data consist of numerous images of the same slice taken with different echo spacing, giving voxel-wise temporal sampling of the noise as the signals decay according to T2 relaxation. Magnitude T2 decay data has Rician distributed noise which is characterized by a change in the noise distribution from Gaussian, through a transitional region, to Rayleigh as the signal to noise ratio decreases with increasing echo time. Non-Gaussian noise distributions may produce errors in the commonly applied non-negative least squares (NNLS) algorithm that is used to assess multiple echo decays for compartmentalized water environments through the creation of T2 distributions. Typically, Gaussian noise is sought by performing spatial-based phase correction on the MRI data however, these methods cannot capitalize on the temporal information available from multiple echo T2 acquisitions. Here we describe a temporal phase correction (TPC) algorithm that utilizes the temporal noise information available in multiple echo T2 acquisitions to put the relevant decay information in the Real portion of the decay data and leave only noise in the Imaginary portion. We apply this TPC algorithm to create real-valued multiple echo T2 data from human subjects measured at 1.5 T. We show that applying TPC causes changes in the T2 distribution estimates; notably the possible resolution of separate extracellular and intracellular water environments, and the disappearance of the commonly labeled cerebrospinal fluid peak, which might be an artefact observed in many previously published multiple echo T2 analyses.

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Yogesh Thakur

University of British Columbia

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Cornelia Laule

University of British Columbia

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Alex L. MacKay

University of British Columbia

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Irene M. Vavasour

University of British Columbia

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John R. Mayo

University of British Columbia

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David Li

University of British Columbia

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John E. Aldrich

University of British Columbia

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