Chronic hypoxia aggravates monocrotaline-induced pulmonary arterial hypertension: a rodent relevant model to the human severe form of the disease

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Front Cardiovasc Med

IntroductionInterventional cardiac MRI in the context of the treatment of cardiac arrhythmia requires submillimeter image resolution to precisely characterize the cardiac substrate and guide the catheter-based ablation procedure in real-time. Conventional MRI receiver coils positioned on the thorax provide insufficient signal-to-noise ratio (SNR) and spatial selectivity to satisfy these constraints.MethodsA small circular MRI receiver coil was developed and evaluated under different experimental conditions, including high-resolution MRI anatomical and thermometric imaging at 1.5 T. From the perspective of developing a therapeutic MR-compatible catheter equipped with a receiver coil, we also propose alternative remote active detuning techniques of the receiver coil using one or two cables. Theoretical details are presented, as well as simulations and experimental validation.ResultsAnatomical images of the left ventricle at 170 µm in-plane resolution are provided on ex vivo beating heart from swine using a 2 cm circular receiver coil. Taking advantage of the increase of SNR at its vicinity (up to 35 fold compared to conventional receiver coils), real-time MR-temperature imaging can reach an uncertainty below 0.1°C at the submillimetric spatial resolution. Remote active detuning using two cables has similar decoupling efficiency to conventional on-site decoupling, at the cost of an acceptable decrease in the resulting SNR.DiscussionThis study shows the potential of small dimension surface coils for minimally invasive therapy of cardiac arrhythmia intraoperatively guided by MRI. The proposed remote decoupling approaches may simplify the construction process and reduce the cost of such single-use devices.Thermometrie cardiaque haute résolution sur une IRM clinique en utilisant des antennes intracardiaquesL'Institut de Rythmologie et modélisation CardiaqueFrance Life Imagin

Sustainable livelihoods to adaptive capabilities: a global learning journey in a small state, Zanzibar

This thesis takes global learning out of the formal setting of a Northern classroom to a rural community setting in the Global South as a social learning process. It begins with a critical reflection of a large EU project to develop a global learning programme as a Global North South initiative. The focus narrows to Zanzibar, a small island state, to critically reflect on the delivery of the programme. And then further to focus on the global social learning and change that occurred in a rural community setting in the north of the island. Through participatory action research, I investigate the relevance of global learning as a social learning process, how norms and rules are shaped within a community setting and how these enable social change towards sustainable livelihoods. The thesis splices the intersection between critical and social theories of learning and engagement, to include critical social theories of Habermas (1984) and Wals (2007); critical race theories of Giroux (1997) and Said (1994) and distributive justice and entitlements theories of Sen (1997) and Moser (1998). It demonstrates the importance of dissonance and a safe space for deliberative dialogue, to be able to consider the global pressures and forces on local realities as the precursor to social change towards sustainability. I conclude by relating the learning from this small island state to the wider world and the current discourse on quality of education in a community development context

Magnetic Resonance Temperature Imaging

International audienceContinuous, real-time 3D temperature mapping during a hyperthermic procedure may provide enhanced safety by visualizing temperature maps in and around the treated region, improved efficiency by adapting local energy deposition with feedback coupling algorithms, and therapy endpoints based on the accumulated thermal dose. Noninvasive mapping of temperature changes can be achieved with MRI, and may be based on temperature dependent MRI parameters. The excellent linearity of the temperature dependency of the proton resonance frequency (PRF) and its near-independence with respect to tissue type make the PRF-based methods the preferred choice for many applications, in particular at mid to highfield strength (≥ 0.5 T). The PRF methods employ RF-spoiled gradient echo imaging methods, and incorporate fat suppression techniques for most organs. A standard deviation of less than 1 o C, for a temporal resolution below 1 s and a spatial resolution of about 2 mm, is feasible for immobile tissues. Special attention is paid to methods for reduction of artifacts in MR temperature mapping caused by intra-scan and inter-scan motion, and motion and temperature-induced susceptibility effects in mobile tissues. Real-time image processing and visualization techniques, together with accelerated MRI acquisition techniques, are described because of their primary importance for real-time, image guided, therapy guidance

Fiber Bragg Grating Sensors for Performance Evaluation of Fast Magnetic Resonance Thermometry on Synthetic Phantom

The increasing recognition of minimally invasive thermal treatment of tumors motivate the development of accurate thermometry approaches for guaranteeing the therapeutic efficacy and safety. Magnetic Resonance Thermometry Imaging (MRTI) is nowadays considered the gold-standard in thermometry for tumor thermal therapy, and assessment of its performances is required for clinical applications. This study evaluates the accuracy of fast MRTI on a synthetic phantom, using dense ultra-short Fiber Bragg Grating (FBG) array, as a reference. Fast MRTI is achieved with a multi-slice gradient-echo echo-planar imaging (GRE-EPI) sequence, allowing monitoring the temperature increase induced with a 980 nm laser source. The temperature distributions measured with 1 mm-spatial resolution with both FBGs and MRTI were compared. The root mean squared error (RMSE) value obtained by comparing temperature profiles showed a maximum error of 1.2 °C. The Bland-Altman analysis revealed a mean of difference of 0.1 °C and limits of agreement 1.5/−1.3 °C. FBG sensors allowed to extensively assess the performances of the GRE-EPI sequence, in addition to the information on the MRTI precision estimated by considering the signal-to-noise ratio of the images (0.4 °C). Overall, the results obtained for the GRE-EPI fully satisfy the accuracy (~2 °C) required for proper temperature monitoring during thermal therapies

Adaptive rejection of outliers for robust motion compensation in cardiac MR-thermometry

International audienceNew Magnetic Resonance (MR) imaging applications include real time monitoring of temperature changes during cardiac radiofrequency ablations. MR-thermometry requires online robust motion compensation to cope with the complex motion of the heart resulting from respiratory activity and cardiac contraction (potentially in presence of arrhythmia), together with the presence of noise in MR images. We propose a method to adaptively and automatically tune parameters of motion compensation algorithms that use robustness function. The core of the method is the estimation of the probability density function (pdf) of the error for each pixel in a reference frame using the Rician noise pdf model in MRI. Then parameter map is derived from estimated pdf. The proposed method leads to better results than using a fixed control parameter of the robustness function, which should facilitate the use of such methods for clinical purpose

3D motion strategy for online volumetric thermometry using simultaneous multi-slice EPI at 1.5T: an evaluation study

AbstractPurpose In presence of respiratory motion, temperature mapping is altered by in-plane and through-plane displacements between successive acquisitions together with periodic phase variations. Fast 2D Echo Planar Imaging (EPI) sequence can accommodate intra-scan motion, but limited volume coverage and inter-scan motion remain a challenge during free-breathing acquisition since position offsets can arise between the different slices.Method To address this limitation, we evaluated a 2D simultaneous multi-slice EPI sequence with multiband (MB) acceleration during radiofrequency ablation on a mobile gel and in the liver of a volunteer (no heating). The sequence was evaluated in terms of resulting inter-scan motion, temperature uncertainty and elevation, potential false-positive heating and repeatability. Lastly, to account for potential through-plane motion, a 3D motion compensation pipeline was implemented and evaluated.Results In-plane motion was compensated whatever the MB factor and temperature distribution was found in agreement during both the heating and cooling periods. No obvious false-positive temperature was observed under the conditions being investigated. Repeatability of measurements results in a 95% uncertainty below 2 °C for MB1 and MB2. Uncertainty up to 4.5 °C was reported with MB3 together with the presence of aliasing artifacts. Lastly, fast simultaneous multi-slice EPI combined with 3D motion compensation reduce residual out-of-plane motion.Conclusion Volumetric temperature imaging (12 slices/700 ms) could be performed with 2 °C accuracy or less, and offer tradeoffs in acquisition time or volume coverage. Such a strategy is expected to increase procedure safety by monitoring large volumes more rapidly for MR-guided thermotherapy on mobile organs

A fast MR-thermometry method for quantitative assessment of temperature increase near an implanted wire.

PurposeTo propose a MR-thermometry method and associated data processing technique to predict the maximal RF-induced temperature increase near an implanted wire for any other MRI sequence.MethodsA dynamic single shot echo planar imaging sequence was implemented that interleaves acquisition of several slices every second and an energy deposition module with adjustable parameters. Temperature images were processed in real time and compared to invasive fiber-optic measurements to assess accuracy of the method. The standard deviation of temperature was measured in gel and in vivo in the human brain of a volunteer. Temperature increases were measured for different RF exposure levels in a phantom containing an inserted wire and then a MR-conditional pacemaker lead. These calibration data set were fitted to a semi-empirical model allowing estimation of temperature increase of other acquisition sequences.ResultsThe precision of the measurement obtained after filtering with a 1.6x1.6 mm2 in plane resolution was 0.2°C in gel, as well as in the human brain. A high correspondence was observed with invasive temperature measurements during RF-induced heating (0.5°C RMSE for a 11.5°C temperature increase). Temperature rises of 32.4°C and 6.5°C were reached at the tip of a wire and of a pacemaker lead, respectively. After successful fitting of temperature curves of the calibration data set, temperature rise predicted by the model was in good agreement (around 5% difference) with measured temperature by a fiber optic probe, for three other MRI sequences.ConclusionThis method proposes a rapid and reliable quantification of the temperature rise near an implanted wire. Calibration data set and resulting fitting coefficients can be used to estimate temperature increase for any MRI sequence as function of its power and duration

MR-ARFI-based method for the quantitative measurement of tissue elasticity: application for monitoring HIFU therapy

International audienceMonitoring thermal therapies through medical imaging is essential in order to ensure that they are safe, efficient and reliable. In this paper, we propose a new approach, halfway between MR acoustic radiation force imaging (MR-ARFI) and MR elastography (MRE), allowing for the quantitative measurement of the elastic modulus of tissue in a highly localized manner. It relies on the simulation of the MR-ARFI profile, which depends on tissue biomechanical properties, and on the identification of tissue elasticity through the fitting of experimental displacement images measured using rapid MR-ARFI. This method was specifically developed to monitor MR-guided high intensity focused ultrasound (MRgHIFU) therapy. Elasticity changes were followed during HIFU ablations (N  =  6) performed ex vivo in porcine muscle samples, and were compared to temperature changes measured by MR-thermometry. Shear modulus was found to increase consistently and steadily a few seconds after the heating started, and such changes were found to be irreversible. The shear modulus was found to increase from 1.49  ±  0.48 kPa (before ablation) to 3.69  ±  0.93 kPa (after ablation and cooling). Thanks to its ability to perform quantitative elasticity measurements in a highly localized manner around the focal spot, this method proved to be particularly attractive for monitoring HIFU ablations