Measuring the Difference Between the Magnetic Susceptibility of Oxygenated and Deoxygenated Blood

Michael Wang, Mark Conradi and Jason Woods

Chemistry Department, Washington University in St. Louis

 

Functional Magnetic Resonance Imaging (fMRI) is a leading tool that scientists use to study the human brain. It has long been known that areas of the brain that are stimulated experience an increase in blood flow. One way to monitor areas of brain activity is to measure a change in the ratio of oxygenated to deoxygenated blood (the hallmark of fMRI). What is unknown, however, is the difference in susceptibility between the two. Since quantitative fMRI relies upon an understanding of the difference in the magnetic susceptibility of oxygenated and deoxygenated red blood cells (RBCs), the goal of this project is to quantitate that difference. Two methods were used to approximate the susceptibility. The first method involves frequency matching of RBCs and saline, which have different local magnetizations and as a result, different

NMR frequencies. This is seen as two peaks on a Nuclear Magnetic Resonance (NMR) spectrum. We attempted to find a gadolinium concentration where the two peaks could not be resolved, meaning that the susceptibilities matched. By adding a paramagnetic material, gadolinium, to the saline, the line shifted up by 544.8Hz. The second method involves measuring linewidth. Differences in magnetic susceptibility within the sample region affect the homogeneity of the local magnetic fields, which causes a broader NMR line. As increasing

gadolinium causes the susceptibility of the plasma to approach that of the blood, the linewidth will be minimized. By combining the two methods, the experiments hoped to find a consistent value where blood linewidth was at a minimum and RBCs and saline were at the same frequency. From this, we are determining the difference of the susceptibility of oxygenated and deoxygenated blood, relative to water.