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Marxen M.,TU Dresden | Marxen M.,Rotman Research Institute | Marxen M.,Heart and Stroke Foundation Center for Stroke Recovery | Cassidy R.J.,Rotman Research Institute | And 9 more authors.
Magnetic Resonance Imaging

Blood oxygenation level-dependent (BOLD) signal time courses in functional magnetic resonance imaging are estimated within the framework of general linear modeling by convolving an input function, that represents neural activity, with a canonical hemodynamic response function (HRF). Here we investigate the performance of different neural input functions and latency-optimized HRFs for modeling BOLD signals in response to vibrotactile somatosensory stimuli of variable durations (0.5, 1, 4, 7 s) in 14 young, healthy adults who were required to make button press responses at each stimulus cessation. Informed by electrophysiology and the behavioral task, three nested models with an increasing number of parameters were considered: a boxcar; boxcar and offset transient; and onset transient, boxcar and offset transient (TBT). The TBT model provided the best fit of the group-averaged BOLD time courses based on χ 2 and F statistics. Only the TBT model was capable of fitting the bimodal shape of the BOLD response to the 7-s stimulus and the relative peak amplitudes for all stimulus lengths in key somatosensory and motor areas. This suggests that the TBT model provides a more comprehensive description of brain sensorimotor responses in this experiment than provided by the simple boxcar model. Work comparing the activation maps obtained with the TBT model with magnetoencephalography data is under way. © 2012 Elsevier Inc. Source

Lau A.Z.,University of Toronto | Lau A.Z.,Imaging Research Sunnybrook Health science Center | Chen A.P.,General Electric | Hurd R.E.,General Electric | And 2 more authors.
NMR in Biomedicine

Dynamic nuclear polarization and dissolution offer the exciting possibility of imaging biochemical reactions in vivo, including some of the key enzymatic reactions involved in cellular metabolism. The development of new pulse sequence strategies has been motivated by demanding applications, such as the imaging of hyperpolarized metabolite distributions in the heart. In this article, the key considerations surrounding the application of spectral-spatial imaging pulse sequences for hyperpolarized 13C metabolic imaging in cardiac and cancer applications are explored. Spiral pulse sequences for multislice imaging of [1- 13C]pyruvate in the heart were developed, as well as time-resolved, three-dimensional, echo-planar imaging sequences for the imaging of [1- 13C]pyruvate-lactate exchange in cancer. The advantages and challenges associated with these sequences were determined by testing in pig and rat models. © 2011 John Wiley & Sons, Ltd. Source

Faez T.,Erasmus Medical Center | Goertz D.,Imaging Research Sunnybrook Health science Center | De Jong N.,Erasmus Medical Center | De Jong N.,University of Twente | De Jong N.,Interuniversity Cardiology Institute of the Netherlands
Ultrasound in Medicine and Biology

The status of vasa vasorum, which can be imaged using ultrasound contrast agents, is an indication for the progression of atherosclerosis. The preferred ultrasound frequency for this purpose is between 5 and 15 MHz. Therefore, it is essential to have knowledge about the acoustic properties of microbubbles such as elasticity and viscosity to be able to implement the current models for lipid encapsulated microbubbles developed for frequencies used in precordial imaging. In this study, the shell parameters, stiffness Sp and friction Sf, of Definity™ microbubbles have been calculated at frequency range of 5-15 MHz by comparing the theoretical modeling of acoustic bubble response and experimental measurements. Derived parameters are in good agreement with previous estimations on SonoVue™ and Sonazoid™ contrast agent. However, the value of Sf is higher than previously estimated for Definity™ between 12-28 MHz. © 2011 World Federation for Ultrasound in Medicine & Biology. Source

Chen A.P.,General Electric | Hurd R.E.,General Electric | Schroeder M.A.,University of Oxford | Schroeder M.A.,Imaging Research Sunnybrook Health science Center | And 8 more authors.
NMR in Biomedicine

13C MR spectroscopy studies performed on hearts ex vivo and in vivo following perfusion of prepolarized [1- 13C]pyruvate have shown that changes in pyruvate dehydrogenase (PDH) flux may be monitored non-invasively. However, to allow investigation of Krebs cycle metabolism, the 13C label must be placed on the C2 position of pyruvate. Thus, the utilization of either C1 or C2 labeled prepolarized pyruvate as a tracer can only afford a partial view of cardiac pyruvate metabolism in health and disease. If the prepolarized pyruvate molecules were labeled at both C1 and C2 positions, then it would be possible to observe the downstream metabolites that were the results of both PDH flux ( 13CO 2 and H 13CO 3 -) and Krebs cycle flux ([5- 13C]glutamate) with a single dose of the agent. Cardiac pH could also be monitored in the same experiment, but adequate SNR of the 13CO 2 resonance may be difficult to obtain in vivo. Using an interleaved selective RF pulse acquisition scheme to improve 13CO 2 detection, the feasibility of using dual-labeled hyperpolarized [1,2- 13C 2]pyruvate as a substrate for dynamic cardiac metabolic MRS studies to allow simultaneous investigation of PDH flux, Krebs cycle flux and pH, was demonstrated in vivo. © 2011 John Wiley & Sons, Ltd. Source

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