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Related Concept Videos

Impulse Response01:17

Impulse Response

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The impulse response is the system's reaction to an input impulse. In an RC circuit, the voltage source is the input, and the capacitor's voltage is the output. The system's state and output response before and after input excitation are distinctly defined.
Kirchhoff's law forms an input signal equation, with the capacitor's current and voltage providing the output. Substituting the current and dividing by RC yields a differential equation. The output for an impulse input is the impulse...
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According to Newton’s second law of motion, the rate of change of the momentum of an object is the net external force acting on it. The total change in momentum between two timepoints thus depends on both the external force acting on it and the time over which it acts. Describing this mathematically, the total change of an object’s motion is proportional to the force vector and the time over which it is applied. This product is called impulse.
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While taking the arithmetic, geometric, or harmonic mean of a sample data set, equal importance is assigned to all the data points. However, all the values may not always be equally important in some data sets. An intrinsic bias might make it more important to give more weightage to specific values over others.
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Principle of Impulse and Moment01:15

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When one considers a rigid body undergoing a plane motion, which is essentially a blend of translational and rotational movement, the application of Newton's second law gives the formula for the translational movement of such a body. If this equation is multiplied by a time interval, dt, and then integrated over the limits of integration, it results in an equation that embodies the principle of linear impulse.
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Impulse-Momentum Theorem00:49

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The total change in the motion of an object is proportional to the total force vector acting on it and the time over which it acts. This product is called impulse, a vector quantity with the same direction as the total force acting on the object.
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Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists arbitrarily define this amount of mass as one atomic mass unit (amu) or one Dalton. Electrons are much smaller in mass than protons, weighing only 9.11 × 10-28 grams, or about 1/1800 of an atomic mass unit. As a result, they do not contribute much to an element's overall atomic mass. This means that, when considering atomic mass, it is customary to ignore the mass of any electrons and...
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Related Experiment Video

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Real-Time fMRI Brain Mapping in Animals
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Impulse response timing differences in BOLD and CBV weighted fMRI.

Jacco A de Zwart1, Peter van Gelderen1, Matthew K Schindler2

  • 1Advanced MRI Section, Laboratory of Functional and Molecular Imaging, NINDS, National Institutes of Health, Bethesda, MD, United States.

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|July 9, 2018
PubMed
Summary
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Cerebral blood volume (CBV)-weighted fMRI shows modest temporal improvements over BOLD fMRI for neural activity, with CBV impulse response peaking slightly faster. These small gains may not offer practical advantages for temporal analysis in human brain imaging.

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Area of Science:

  • Neuroimaging
  • Functional Magnetic Resonance Imaging (fMRI)

Background:

  • BOLD fMRI offers high resolution but is limited by physiological constraints of hemodynamic responses.
  • Cerebral blood volume (CBV)-weighted fMRI may overcome BOLD limitations by directly measuring blood volume changes.
  • Quantitative temporal comparisons between CBV and BOLD fMRI in humans are limited.

Purpose of the Study:

  • To quantitatively compare the temporal characteristics of CBV-weighted fMRI versus BOLD fMRI in the human brain.
  • To investigate the potential of CBV fMRI for improved temporal resolution in neuroimaging.

Main Methods:

  • Ferumoxytol, an iron-oxide contrast agent, was administered to subjects for enhanced CBV contrast.
  • Event-related visual stimuli were used to elicit responses.
  • Impulse response (IR) for both CBV and BOLD contrast was measured at 7T with 1.2-1.5 mm spatial and 1s temporal resolution.

Main Results:

  • CBV fMRI demonstrated a faster onset time (0.8s) and time-to-peak (3.5s) compared to BOLD fMRI.
  • These temporal improvements were approximately 0.6s shorter than BOLD, considered relatively small.
  • Nonlinearities in the impulse response were similar for both CBV and BOLD, suggesting vascular compliance influences rather than a CBO ceiling effect.

Conclusions:

  • While CBV-weighted fMRI offers slight temporal advantages over BOLD fMRI, the improvements are not substantial enough for significant practical benefits in temporal analysis.
  • The findings support models of vascular compliance affecting hemodynamic responses, rather than a saturation of the cerebral blood oxygenation response.