Haemoglobin levels affect the results of brain connectivity studies


In a nutshell: Natural variations in haemoglobin levels should be considered when using functional MRI to study brain connectivity.

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Haemoglobin levels affect the results of brain connectivity studies

The functional connectome is a map of all the connections used in the brain to communicate between cells. To determine the functional connectome and link it to brain activity, researchers often use functional magnetic resonance imaging (fMRI).

When brain cells become active, they need more oxygen. This increases blood flow to that part of the brain. It also changes the ratio of oxygenated to deoxygenated haemoglobin in the blood. The fMRI technique measures brain activity by detecting these changes.

However, haemoglobin levels are influenced by many factors other than brain activity – such as a person’s sex, age, race or stress levels. New research shows that these variations can affect fMRI-based studies of functional connectivity.

The team of researchers from Monash University, led by Brain Function CoE investigators Philip Ward and Sharna Jamadar, looked at individual differences in haemoglobin levels in a group of healthy older adults. The researchers split data from 518 participants into four groups: males and females with either a high or a low haemoglobin level. Then they compared the participants’ fMRI measurements to see if group differences influenced how the functional connectome was determined.

In males, differences in haemoglobin levels affected how functional connectivity was measured across the whole brain. In females, however, the effect was weaker and more varied.

Compared with high-haemoglobin females, low-haemoglobin females had higher functional connectivity in regions of the brain at the rear of the cortex. But they had lower connectivity in regions in the middle of the brain.

These results show that if researchers do not control for the variability in people’s haemoglobin in their analyses, they may come to the wrong conclusions when studying the functional connectome using fMRI.

Next steps:
Blood haemoglobin levels are currently estimated from samples obtained by pricking a finger. The researchers are considering ways to estimate haemoglobin from the fMRI images themselves.

Ward, P. G. D., Orchard, E. R., Oldham, S., Arnatkeviciute, A., Sforazzini, F., Fornito, A., Storey, E., Egan, G. F., & Jamadar, S. D. (2020). Individual differences in haemoglobin concentration influence BOLD fMRI functional connectivity and its correlation with cognition. Neuroimage, 221, 117196. doi: 10.1016/j.neuroimage.2020.117196

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