Systematic Review: Anaesthetic Protocols and Management as Confounders in Rodent Blood Oxygen Level Dependent Functional Magnetic Resonance Imaging (BOLD fMRI)-Part A: Effects of Changes in Physiological Parameters

Abstract

Background: To understand brain function in health and disease, functional magnetic resonance imaging (fMRI) is widely used in rodent models. Because animals need to be immobilised for image acquisition, fMRI is commonly performed under anaesthesia. The choice of anaesthetic protocols and may affect fMRI readouts, either directly or via changing physiological balance, and thereby threaten the scientific validity of fMRI in rodents. Methods: The present study systematically reviewed the literature investigating the influence of different anaesthesia regimes and changes in physiological parameters as confounders of blood oxygen level dependent (BOLD) fMRI in rats and mice. Four databases were searched, studies selected according to pre-defined criteria, and risk of bias assessed for each study. Results are reported in two separate articles; this part of the review focuses on effects of changes in physiological parameters. Results: A total of 121 publications was included, of which 49 addressed effects of changes in physiological parameters. Risk of bias was high in all included studies. Blood oxygenation [arterial partial pressure of oxygen (p_aO₂)], ventilation [arterial partial pressure of carbon dioxide (p_aCO₂)] and arterial blood pressure affected BOLD fMRI readouts across various experimental paradigms. Conclusions: Blood oxygenation, ventilation and arterial blood pressure should be monitored and maintained at stable physiological levels throughout experiments. Appropriate anaesthetic management and monitoring are crucial to obtain scientifically valid, reproducible results from fMRI studies in rodent models.

Keywords: BOLD fMRI; anaesthetic management; anaesthetic monitoring; mouse; rat; validity.

Figure 1

Prisma flow chart representing workflow from reference identification to definitive inclusion.

Figure 2

Study designs used in included references. Reference counts are presented, because some references re-analyzed only parts of the original dataset. Single session simple = study design can be described by one of the following categories: between group = one group of animals per condition, one condition per animal; multiple conditions - not specified = multiple conditions per animal in not specified or fixed order; multiple conditions - crossover = multiple conditions per animal in a crossover design; timepoints = one condition per animal, multiple measurements at different timepoints; observational = studies measuring and analyzing the effect of naturally occurring signal fluctuations on BOLD signal. Single session nested = elements of simple study design are combined; multiple sessions = animals underwent several experimental sessions on different days; not clear = the study design was not clear from the provided information.

Figure 3

Numbers of references using controlled or spontaneous ventilation in at least one experimental group. Numbers in overlapping areas indicate studies which used both types in different experimental groups. Not clear = the ventilatory management was not explicitly described.

Figure 4

Magnetic field strength in tesla of MRI scanners used in the included studies. The number of included references as well as the number of datasets those publications are based on are shown. Studies using 2.35 T or 7.1 T scanners were categorised as 2.4 and 7.0 T, respectively.

Figure 5

Effects of decreases and increases in p_aO₂ compared to respective baseline condition, on baseline BOLD signal, responses to stimulation and fc in rats and mice. Datapoints displayed on the BOLD signal axis represent studies which have investigated associations between p_aO₂ values and the respective BOLD outcome (in the absence of specific changes in p_aO₂). stronger = higher BOLD signal or lower R2*, higher signal intensity and/or spatial extent of activated area upon stimulation, or higher fc strength and/or spatial extent of connectivities; (stronger) = BOLD signal, response to stimulation or fc stronger in some, but not all aspects; unchanged = no significant difference to respective baseline condition or no association with p_aO₂ fluctuations found. “weaker” and “(weaker)” analogous to “stronger” and “(stronger).” One data point per experimental paradigm and per dataset. If no statement on the significance of reported changes was available, “(stronger)” or “(weaker)” were selected.

Figure 6

Effects of increases in p_aCO₂ compared to respective baseline condition, while p_aO₂ was decreased, unchanged or increased, on baseline BOLD signal, responses to stimulation and fc in rats and mice. Datapoints displayed on the BOLD signal axis represent studies which have investigated just associations between p_aCO₂ values and the respective BOLD outcome (in the absence of specific changes in p_aCO₂). stronger = higher BOLD signal or lower R2*, higher signal intensity and/or spatial extent of activated area upon stimulation, or higher fc strength and/or spatial extent of connectivities; (stronger) = BOLD signal, response to stimulation or fc stronger in some, but not all aspects; unchanged = no significant difference to respective baseline condition or no association with p_aCO₂ fluctuations found. “weaker” and “(weaker)” analogous to “stronger” and “(stronger).” One data point per experimental paradigm and per dataset. If no statement on the significance of reported changes was available, “(stronger)” or “(weaker)” were selected.

Figure 7

Effects of decreases and increases in arterial blood pressure compared to respective baseline, on baseline BOLD signal, responses to stimulation and fc in rats (no studies available in mice). Datapoints displayed on the BOLD signal axis represent studies which have investigated associations between arterial blood pressure values and the respective BOLD outcome (in the absence of specific changes in arterial blood pressure). stronger = higher BOLD signal or lower R2*, higher signal intensity and/or spatial extent of activated area upon stimulation, or higher fc strength and/or spatial extent of connectivities; (stronger) = BOLD signal, response to stimulation or fc stronger in some, but not all aspects; unchanged = no significant difference to respective baseline condition or no association with p_aO₂ fluctuations found. “weaker” and “(weaker)” analogous to “stronger” and “(stronger).” One data point per experimental paradigm and per dataset. If no statement on the significance of reported changes was available, “(stronger)” or “(weaker)” were selected.