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Dynamics of changes in blood flow, volume, and oxygenation: implications for dynamic functional magnetic resonance imaging calibration - PubMed

Dynamics of changes in blood flow, volume, and oxygenation: implications for dynamic functional magnetic resonance imaging calibration

Ikuhiro Kida et al. J Cereb Blood Flow Metab. 2007 Apr.

Abstract

Changes in cerebral blood flow (CBF), volume (CBV), and oxygenation (blood-oxygenation level dependent (BOLD)) during functional activation are important for calculating changes in cerebral metabolic rate of oxygen consumption (CMRo2) from calibrated functional MRI (fMRI). An important part of this process is the CBF/CBV relationship, which is signified by a power-law parameter: gamma=ln (1+DeltaCBV/CBV)/ln (1+DeltaCBF/CBF). Because of difficulty in measuring CBF and CBV with MRI, the value of gamma is therefore assumed to be approximately 0.4 from a prior primate study under hypercapnia. For dynamic fMRI calibration, it is important to know if the value of gamma varies after stimulation onset. We measured transient relationships between DeltaCBF, DeltaCBV, and DeltaBOLD by multimodal MRI with temporal resolution of 500 ms (at 7.0 T) from the rat somatosensory cortex during forepaw stimulation, where the stimulus duration ranged from 4 to 32 secs. Changes in CBF and BOLD were measured before the administration of the contrast agent for CBV measurements in the same subjects. We observed that the relationship between DeltaCBF and DeltaCBV varied dynamically from stimulation onset for all stimulus durations. Typically after stimulation onset and at the peak or plateau of the DeltaCBF, the value of gamma ranged between 0.1 and 0.2. However, after stimulation offset, the value of gamma increased to 0.4 primarily because of rapid and slow decays in DeltaCBF and DeltaCBV, respectively. These results suggest caution in using dynamic measurements of DeltaCBF and DeltaBOLD required for calculating DeltaCMRo2 for functional stimulation, when either DeltaCBV has not been accurately measured or a fixed value of gamma during hypercapnia perturbation is used.

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Figures

Figure 1
Figure 1

Time courses of (a) ΔCBF, (b) ΔCBV and (c) ΔBOLD during forepaw stimulation from a single rat. Time courses were averaged from three significant voxels in the middle layers of the somatosensory cortex. The different colors indicate the different stimulus durations (—— 4 s, —■— 8 s, —▲— 16 s, and —●— 32 s).

Figure 2
Figure 2

Detailed comparison between ΔCBF and ΔCBV for all stimulus durations in terms of the γ value. The relationships are shown for (a) 0 to 4 s, (b) 4 to 12 s, (c) 12 to 24 s, and (d) 24 to 44 s after stimulation onset. Data were averaged over all animals (n = 12 from 8 animals). The dotted contour lines indicate the theoretical relationship between ΔCBF and ΔCBV for different γ values (i.e., 0.1, 0.2, 0.3, and 0.4). The different colors indicate the different stimulus durations (—— 4 s, —■— 8 s, —▲— 16 s, and —●— 32 s).

Figure 3
Figure 3

The averaged values of γ in three periods following stimulation onset (n = 12 from 8 animals). Periods I, II, and III represent the phases of CBF rise, CBF plateau, and CBF decline, respectively. Note there were no γ values reported period II for the 4 s duration stimulation because of rapid rise and fall of the signal. The different colors indicate the different stimulus durations (filled 4 s, checkered 8 s, horizontal lines 16 s, and vertical lines 32 s).

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