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Spin-echo MRI underestimates functional changes in microvascular cerebral blood plasma volume using exogenous
Joseph B Mandeville1, Francisca P Leite, John J A Marota
1Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital (MGH), Massachusetts Institute of Technology (MIT), Harvard Medical School (HMS), Boston, Massachusetts, USA. jbm@nmr.mgh.harvard.edu
Abstract:
While most functional MRI studies using exogenous contrast agent employ gradient-echo (GE) signal, spin echo (SE) imaging would represent an attractive alternative if its detection power were more comparable with GE imaging. This study demonstrates that SE methods systematically underestimate functional changes in microvascular cerebral blood plasma volume (CBV), so that SE detection power in brain tissue cannot match that provided by GE signal. Empirically, the in vivo response of SE-CBV was about 40% smaller than that of GE-CBV in rat brain at low basal values of CBV, a result that is consistent with physics predictions under the simplifying assumption of uniform vessel dilation. However, increasing values of basal CBV were associated with monotonically increasing mean vessel sizes and monotonically decreasing GE to SE ratios of functional changes in CBV (fCBV). This result suggests the presence of large but weakly reactive conduit vessels at high basal values of CBV. Hence, we conclude that GE imaging is the method of choice for functional MRI (fMRI) using exogenous contrast agent in most cases, although SE methods may represent a more spatially linear representation of underlying neural activity that becomes most apparent in regions with high basal CBV, such as the cortical surface.
Insights
Spin echo (SE) methods underestimate functional changes in cerebral blood plasma volume (CBV) compared to gradient-echo (GE) imaging in functional MRI (fMRI). GE imaging is generally preferred for contrast-enhanced fMRI due to superior detection power.
Area of Science:
- Neuroimaging
- Biophysics
- Vascular Biology
Background:
- Functional MRI (fMRI) commonly uses gradient-echo (GE) signals for detecting brain activity.
- Spin-echo (SE) imaging is an alternative but often shows lower sensitivity.
- Understanding the comparative detection power of GE and SE imaging with exogenous contrast agents is crucial for optimizing fMRI studies.
Purpose of the Study:
- To compare the detection power of spin-echo (SE) and gradient-echo (GE) imaging for functional changes in microvascular cerebral blood plasma volume (CBV) using exogenous contrast agents in fMRI.
- To investigate the influence of basal CBV levels on the ratio of functional changes between GE and SE methods.
- To determine the optimal imaging method for contrast-enhanced fMRI.
Main Methods:
- In vivo functional MRI experiments were conducted on rat brains using both GE and SE sequences with an exogenous contrast agent.
- Measurements of functional changes in cerebral blood plasma volume (fCBV) were compared between GE and SE signals.
- The relationship between basal CBV values and the GE to SE ratio of fCBV was analyzed.
Main Results:
- SE methods systematically underestimated functional CBV changes compared to GE methods.
- In vivo SE-CBV responses were approximately 40% smaller than GE-CBV responses at low basal CBV.
- Higher basal CBV correlated with larger vessel sizes and a decreased GE to SE ratio of fCBV, suggesting the presence of large, less reactive vessels.
Conclusions:
- Gradient-echo (GE) imaging is generally the preferred method for contrast-enhanced fMRI due to its higher sensitivity in detecting functional CBV changes.
- Spin-echo (SE) methods may offer a more spatially linear representation of neural activity, particularly in regions with high basal CBV like the cortical surface.
- The choice between GE and SE imaging in fMRI depends on the specific research question and the characteristics of the brain regions being studied.
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