Lise Vejby Søgaard

Validation of in vitro probabilistic tractography

Diffusion weighted imaging (DWI) and tractography allow the non-invasive study of anatomical brain connectivity. However, a gold standard for validating tractography of complex connections is lacking. Using the porcine brain as a highly gyrated brain model, we quantitatively and qualitatively assessed the anatomical validity and reproducibility of in vitro multi-fiber probabilistic tractography against two invasive tracers: the histochemically detectable biotinylated dextran amine and manganese enhanced magnetic resonance imaging. Post mortem DWI was used to ensure that most of the sources known to degrade the anatomical accuracy of in vivo DWI did not influence the tracking results. We demonstrate that probabilistic tractography reliably detected specific pathways. Moreover, the applied model allowed identification of the limitations that are likely to appear in many of the current tractography methods. Nevertheless, we conclude that DWI tractography can be a precise tool in studying anatomical brain connectivity.

An ex vivo imaging pipeline for producing high-quality and high-resolution diffusion-weighted imaging datasets.

Diffusion tensor (DT) imaging and related multifiber reconstruction algorithms allow the study of in vivo microstructure and, by means of tractography, structural connectivity. Although reconstruction algorithms are promising imaging tools, high‐quality diffusion‐weighted imaging (DWI) datasets for verification and validation of postprocessing and analysis methods are lacking. Clinical in vivo DWI is limited by, for example, physiological noise and low signal‐to‐noise ratio. Here, we performed a series of DWI measurements on postmortem pig brains, which resemble the human brain in neuroanatomical complexity, to establish an ex vivo imaging pipeline for generating high‐quality DWI datasets. Perfusion fixation ensured that tissue characteristics were comparable to in vivo conditions. There were three main results: (i) heat conduction and unstable tissue mechanics accounted for time‐varying artefacts in the DWI dataset, which were present for up to 15 h after positioning brain tissue in the scanner; (ii) using fitted DT, q‐ball, and persistent angular structure magnetic resonance imaging algorithms, any b‐value between ∼2,000 and ∼8,000 s/mm2, with an optimal value around 4,000 s/mm2, allowed for consistent reconstruction of fiber directions; (iii) diffusivity measures in the postmortem brain tissue were stable over a 3‐year period. On the basis of these results, we established an optimized ex vivo pipeline for high‐quality and high‐resolution DWI. The pipeline produces DWI data sets with a high level of tissue structure detail showing for example two parallel horizontal rims in the cerebral cortex and multiple rims in the hippocampus. We conclude that high‐quality ex vivo DWI can be used to validate fiber reconstruction algorithms and to complement histological studies. Hum Brain Mapp, 2011.

NORdic trial of oral Methylprednisolone as add-on therapy to Interferon beta-1a for treatment of relapsing-remitting Multiple Sclerosis (NORMIMS study): a randomised, placebo-controlled trial

BACKGROUND Treatment of relapsing-remitting multiple sclerosis with interferon beta is only partly effective, and new more effective and safe strategies are needed. Our aim was to assess the efficacy of oral methylprednisolone as an add-on therapy to subcutaneous interferon beta-1a to reduce the yearly relapse rate in patients with relapsing-remitting multiple sclerosis. METHODS NORMIMS (NORdic trial of oral Methylprednisolone as add-on therapy to Interferon beta-1a for treatment of relapsing-remitting Multiple Sclerosis) was a randomised, placebo-controlled trial done in 29 neurology departments in Denmark, Norway, Sweden, and Finland. We enrolled outpatients with relapsing-remitting multiple sclerosis who had had at least one relapse within the previous 12 months despite subcutaneous interferon beta-1a treatment (44 microg three times per week). We randomly allocated patients by computer to add-on therapy of either 200 mg methylprednisolone or matching placebo, both given orally on 5 consecutive days every 4 weeks for at least 96 weeks. The primary outcome measure was mean yearly relapse rate. Primary analyses were by intention to treat. This trial is registered, number ISRCTN16202527. FINDINGS 66 patients were assigned to interferon beta and oral methylprednisolone and 64 were assigned to interferon beta and placebo. A high proportion of patients withdrew from the study before week 96 (26% [17 of 66] on methylprednisolone vs 17% [11 of 64] on placebo). The mean yearly relapse rate was 0.22 for methylprednisolone compared with 0.59 for placebo (62% reduction, 95% CI 39-77%; p<0.0001). Sleep disturbance and neurological and psychiatric symptoms were the most frequent adverse events recorded in the methylprednisolone group. Bone mineral density had not changed after 96 weeks. INTERPRETATION Oral methylprednisolone given in pulses every 4 weeks as an add-on therapy to subcutaneous interferon beta-1a in patients with relapsing-remitting multiple sclerosis leads to a significant reduction in relapse rate. However, because of the small number of patients and the high dropout rate, these findings need to be corroborated in larger cohorts.

Hyperpolarised 3He MRI versus HRCT in COPD and normal volunteers: PHIL trial

The aim of the present study was to apply hyperpolarised (HP) 3He magnetic resonance imaging (MRI) to identify patients with chronic obstructive pulmonary disease (COPD) and α1-antitrypsin deficiency (α1-ATD) from healthy volunteers and compare HP 3He MRI findings with high-resolution computed tomography (HRCT) in a multicentre study. Quantitative measurements of HP 3He MRI (apparent diffusion coefficient (ADC)) and HRCT (mean lung density (MLD)) were correlated with pulmonary function tests. A prospective three centre study enrolled 122 subjects with COPD (either acquired or genetic) and age-matched never-smokers. All diagnostic studies were completed in 94 subjects (52 with COPD; 13 with α1-ATD; 29 healthy subjects; 63 males; and 31 females; median age 62 yrs). The consensus assessment of radiologists, blinded for other test results, estimated nonventilated lung volume (HP 3He MRI) and percentage diseased lung (HRCT). Quantitative evaluation of all data for each centre consisted of ADC (HP 3He MRI) and MLD measurements (HRCT), and correlation with forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) indicating airway obstruction, and the diffusing capacity of the lung for carbon monoxide (DL,CO) indicating alveolar destruction. Using lung function tests as a reference, regional analysis of HP 3He MRI and HRCT correctly categorised normal volunteers in 100% and 97%, COPD in 42% and 69% and α1-ATD in 69% and 85% of cases, respectively. Direct comparison of HP 3He MRI and CT revealed 23% of subjects with moderate/severe structural abnormalities had only mild ventilation defects. In comparison with lung function tests, ADC was more effective in separating COPD patients from healthy subjects than MLD (p<0.001 versus 0.038). ADC measurements showed better correlation with DL,CO than MLD (r = 0.59 versus 0.29). Hyperpolarised 3He MRI correctly categorised patients with COPD and normal volunteers. It offers additional functional information, without the use of ionising radiation whereas HRCT gives better morphological information. We showed the feasibility of a multicentre study using different magnetic resonance systems.

Assessment of lung microstructure with magnetic resonance imaging of hyperpolarized Helium-3☆

Magnetic resonance imaging of the apparent diffusion coefficient (ADC) of hyperpolarized Helium-3 is a new technique for probing pulmonary microstructure in vivo. The aim of this study was the assessment of potential sources of systematic errors of the ADC measurement. The influence of macroscopic motion was determined by measurements at two different delays after initiating the breath-hold, and before and after cardiac arrest. An intercentre comparison was performed in two age- and lung function-matched groups of lung-healthy volunteers at two research sites. Moreover, measurements of diffusion anisotropy were performed. We found no dependency of the ADC as a function of the delay after stop of inspiration. The influence of cardiac motion was less than 10%. In the intercentre comparison study, an excellent agreement between the two sites was found. First measurements of the diffusion tensor of intrapulmonary Helium-3 are shown.

Hyperpolarised 3He MRI and 81mKr SPECT in chronic obstructive pulmonary disease

PurposeDuring recent years, magnetic resonance imaging (MRI) using hyperpolarised (HP) 3He gas has emerged as a promising new method for the imaging of lung ventilation. However, systematic comparisons with nuclear medicine techniques have not yet been performed. The aim of this study was to compare ventilation imaging methods in 26 patients with chronic obstructive pulmonary disease (COPD) and nine lung healthy volunteers.MethodsHP 3He MRI, 81mKr single-photon emission computed tomography (SPECT), high-resolution computed tomography (HRCT) and pulmonary function tests were performed. The three scans were scored visually as percentage of non-ventilated/diseased lung, and a computer-based objective measure of the ventilated volume in HP 3He MRI and 81mKr SPECT and an emphysema index in HRCT were calculated.ResultsWe found a good correlation between HP 3He MRI and 81mKr SPECT for both visual defect score (r=0.80, p<0.0001) and objective estimate of ventilation (r=0.45, p=0.0157). In addition, both scans were well correlated with reference methods for the diagnosis of emphysema (pulmonary function test and HRCT). The defect scores were largest on 81mKr SPECT (the score on HP 3He MRI was one-third less than that on 81mKr SPECT), but the difference was reduced after normalisation for different breathing depths (HP 3He MRI at total lung capacity; 81mKr SPECT at tidal breathing at functional residual capacity).ConclusionHP 3He MRI provides detailed ventilation distribution images and defect scores are comparable on HP 3He MRI and 81mKr SPECT. Additionally, new insights into the regional pulmonary microstructure via the apparent diffusion coefficient measurements are provided by HP 3He MRI. HP 3He MRI is a promising new diagnostic tool for the assessment of ventilation distribution.

Assessment of early diabetic renal changes with hyperpolarized [1‐13C]pyruvate

This experimental study explores a novel magnetic resonance imaging/spectroscopic (MRI/MRS) method that measures changes in renal metabolism in a diabetic rat model. This hyperpolarized metabolic MRI/MRS method allows monitoring of metabolic processes in seconds by >10 000‐fold enhancement of the MR signal. The method has shown that the conversion of pyruvate to bicarbonate, i.e. pyruvate dehydrogenase (PDH) activity, is significantly altered in the myocardium already at the onset of diabetes, and the predominant Warburg effect is a valuable cancer maker via the lactate dehydrogenase (LDH) activity. We hypothesize that a similar change in PDH and LDH could be found in the early diabetic kidney.

Imaging cerebral 2-ketoisocaproate metabolism with hyperpolarized 13C Magnetic Resonance Spectroscopic Imaging

The branched chain amino acid transaminase (BCAT) has an important role in nitrogen shuttling and glutamate metabolism in the brain. The purpose of this study was to describe the cerebral distribution and metabolism of hyperpolarized 2-keto[1-13C]isocaproate (KIC) in the normal rat using magnetic resonance modalities. Hyperpolarized KIC is metabolized to [1-13C]leucine (leucine) by BCAT. The results show that KIC and its metabolic product, leucine, are present at imageable quantities 20 seconds after end of KIC administration throughout the brain. Further, significantly higher metabolism was observed in hippocampal regions compared with the muscle tissue. In conclusion, the cerebral metabolism of hyperpolarized KIC is imaged and hyperpolarized KIC may be a promising substrate for evaluation of cerebral BCAT activity in conjunction with neurodegenerative disease.

Hydrocephalus induces dynamic spatiotemporal regulation of aquaporin-4 expression in the rat brain

BackgroundThe water channel protein aquaporin-4 (AQP4) is reported to be of possible major importance for accessory cerebrospinal fluid (CSF) circulation pathways. We hypothesized that changes in AQP4 expression in specific brain regions correspond to the severity and duration of hydrocephalus.MethodsHydrocephalus was induced in adult rats (~8 weeks) by intracisternal kaolin injection and evaluated after two days, one week and two weeks. Using magnetic resonance imaging (MRI) we quantified lateral ventricular volume, water diffusion and blood-brain barrier properties in hydrocephalic and control animals. The brains were analysed for AQP4 density by western blotting and localisation by immunohistochemistry. Double fluorescence labelling was used to study cell specific origin of AQP4.ResultsLateral ventricular volume was significantly increased over control at all time points after induction and the periventricular apparent diffusion coefficient (ADC) value significantly increased after one and two weeks of hydrocephalus. Relative AQP4 density was significantly decreased in both cortex and periventricular region after two days and normalized after one week. After two weeks, periventricular AQP4 expression was significantly increased. Relative periventricular AQP4 density was significantly correlated to lateral ventricular volume. AQP4 immunohistochemical analysis demonstrated the morphological expression pattern of AQP4 in hydrocephalus in astrocytes and ventricular ependyma. AQP4 co-localized with astrocytic glial fibrillary acidic protein (GFAP) in glia limitans. In vascular structures, AQP4 co-localized to astroglia but not to microglia or endothelial cells.ConclusionsAQP4 levels are significantly altered in a time and region dependent manner in kaolin-induced hydrocephalus. The presented data suggest that AQP4 could play an important neurodefensive role, and may be a promising future pharmaceutical target in hydrocephalus and CSF disorders.

In vivo measurement of apparent diffusion coefficients of hyperpolarized 13C‐labeled metabolites

The combination of hyperpolarized MRS with diffusion weighting (dw) allows for determination of the apparent diffusion coefficient (ADC), which is indicative of the intra‐ or extracellular localization of the metabolite. Here, a slice‐selective pulsed‐gradient spin echo sequence was implemented to acquire a series of dw spectra from rat muscle in vivo to determine the ADCs of multiple metabolites after a single injection of hyperpolarized [1‐13C]pyruvate. An optimal control optimized universal‐rotation pulse was used for refocusing to minimize signal loss caused by B1 imperfections.