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Gradient echo time dependence and quantitative parameter maps for somatosensory activation in rats at 7 T
M Grüne1, F Pillekamp, W Schwindt
1Department of Experimental Neurology, Max-Planck-Institute for Neurological Research, Cologne, Germany. mathias@mpin-koeln.mpg.de
Magnetic Resonance in Medicine
|July 10, 1999
Summary
This study investigated functional MRI (fMRI) contrast in rats, finding that echo time significantly impacts blood oxygenation level-dependent (BOLD) signal intensity and spatial patterns. Quantitative T2* mapping is crucial for accurate interpretation of fMRI activation maps.
Area of Science:
- Neuroimaging
- Magnetic Resonance Imaging
- Physiology
Background:
- Functional magnetic resonance imaging (fMRI) relies on blood oxygenation level-dependent (BOLD) contrast, which is influenced by magnetic field properties and imaging parameters.
- Understanding the dependence of BOLD signal on echo time (TE) is critical for accurate interpretation of brain activity, especially at high magnetic fields like 7 Tesla (7T).
- Regional variations in T2* relaxation times can complicate the interpretation of BOLD signals, necessitating methods to differentiate true BOLD changes from other signal contributions.
Purpose of the Study:
- To investigate the dependence of fMRI contrast on gradient echo time (TE) in T2*-weighted BOLD fast low-angle shot (FLASH) imaging at 7T.
- To assess the impact of regional T2* heterogeneity on BOLD signal intensity and spatial patterns during forepaw stimulation in rats.
- To develop and apply quantitative methods for separating true BOLD signals from inflow effects in functional activation studies.
Main Methods:
- Electrical forepaw stimulation was applied to alpha-chloralose anesthetized rats during 7T fMRI scanning using T2*-weighted FLASH sequences.
- Quantitative T2* maps and signal intensity at echo time zero (STE=0) maps were calculated to characterize tissue properties and signal behavior.
- Pixelwise analysis of T2* and STE=0 activation maps was performed to differentiate BOLD and inflow contributions to the observed signal changes.
Main Results:
- The study observed significant variations in both activation signal intensity and spatial patterns with changing echo time (TE).
- Quantitative T2* and STE=0 maps enabled the separation of true BOLD signal changes from inflow effects in the somatosensory cortex.
- Regional heterogeneity of T2* was identified as a key factor influencing the observed BOLD signal variations.
Conclusions:
- The dependence of BOLD contrast on echo time is significant and influenced by regional T2* heterogeneity.
- Quantitative T2* and STE=0 mapping are essential for accurate interpretation of fMRI activation maps, particularly at high field strengths.
- For functional activation experiments using a single echo time, prior measurement of a quantitative T2* map is recommended to reliably assess activation intensity and spatial patterns.