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بسم الله الرحمن الرحيم

بسم الله الرحمن الرحيم. MRI Instrumentation. MR Spectroscopy (NMRS). Statistical Maps & Time Courses. Use stat maps to pick regions. Then extract the time course. 2D  3D. Design Jargon: Runs. condition : one set of stimuli or one task. 4 stimulus conditions

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بسم الله الرحمن الرحيم

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  1. بسم الله الرحمن الرحيم

  2. MRI Instrumentation

  3. MR Spectroscopy (NMRS)

  4. Statistical Maps & Time Courses Use stat maps to pick regions Then extract the time course

  5. 2D  3D

  6. Design Jargon: Runs condition: one set of stimuli or one task 4 stimulus conditions + 1 baseline condition (fixation) session: all of the scans collected from one subject in one day run (or scan): one continuous period of fMRI scanning (~5-7 min) experiment: a set of conditions you want to compare to each other A session consists of one or more experiments. Each experiment consists of several (e.g., 1-8) runs More runs/expt are needed when SNR is low or the effect is weak. Thus each session consists of numerous (e.g., 5-20) runs (e.g., 0.5 – 3 hours)

  7. Introduction to fMRI physics

  8. Vascular Network Arterioles Y=95% at rest. Y=100% during activation. 25 mm diameter. <15% blood volume of cortical tissue. Venules Y=60% at rest. Y=90% during activation. 25-50 mm diameter. 40% blood volume of cortical tissue. Red blood cell 6 mm wide and 1-2 mm thick. Delivers O2 in form of oxyhemoglobin. Capillaries Y=80% at rest. Y=90% during activation. 8 mm diameter. 40% blood volume of cortical tissue. Primary site of O2 exchange with tissue. Transit Time = 2-3 s Source: Chris Thomas’ Slides

  9. fMRI Experiment Stages: Functionals first volume (2 sec to acquire) • 5) Take functional (T2*) images • images are indirectly related to neural activity • usually low resolution images (3x3x5 mm) • all slices at one time = a volume (sometimes also called an image) • sample many volumes (time points) (e.g., 1 volume every 2 seconds for 150 volumes = 300 sec = 5 minutes) • 4D data: 3 spatial, 1 temporal …

  10. Vascular network and BOLD Source: Buxton book Ch 2

  11. Susceptibility and BOLD fMRI Magnetic susceptibility (c) refers to magnetic response of a material when placed in B0. Red blood cells exhibit a change in c during ‘activation’ Basically, oxyhaemoglobin in the RBC (HbO2) becomes deoxyhaemoglobin (Hb): Becomes paramagnetic. Susceptibility difference between venous vasculature and surroundings (susceptibility induced field shifts).

  12. fMRI Setup

  13. BOLD signal Blood Oxygen Level Dependent signal Source: Buxton book Ch 17

  14. BOLD signal Blood Oxygen Level Dependent signal • CBF, CBV, and CMRO2 have different effects on HbO2 concentration: • Interaction of these 3 produce BOLD response • They change [Hb] which affects magnetic environment. (delivery of more HbO2 -> less Hb on venous side if excess O2 not used) Local Hb Content CBF Local Hb Content (extraction of O2-> HbO2 becomes Hb) CMRO2 Local Hb Content (more Hb in a given imaging voxel) CBV

  15. BOLD signal Source: Doug Noll’s primer

  16. First Functional Images Source: Kwong et al., 1992

  17. Hemodynamic Response Function • % signal change • = (point – baseline)/baseline • usually 0.5-3% • initial dip • -more focal • -somewhat elusive so far • time to rise • signal begins to rise soon after stimulus begins • time to peak • signal peaks 4-6 sec after stimulus begins • post stimulus undershoot • signal suppressed after stimulation ends

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