Optimization of Myocardial FirstPass Perfusion MR Imaging using Gadolinium and Differences in FiO2

The correct perfusion of the myocardium, the heart’s muscle, is a key parameter that has been proven to be a powerful tool for the diagnosis of cardiovascular diseases. In general, perfusion is defined as the delivery of oxygen and nutrients by the circulatory system to any tissue or organ. When an organ like the heart receives insufficient blood supply, an ischemic disease is likely to take place.

So far, several image techniques have been used for the study of myocardial perfusion such as CT, PET, SPECT or MRI. Due to its high spatial resolution and the avoidance of limitations offered by ionizing radiation the following project specifically addresses the MRI technique. The use of MRI as diagnostic readouts in patients suffering from cardiovascular diseases is currently growing in the clinics. Therefore, having this protocol optimize in a research lab will be of great interest.

This work was carried out using a 7 Tesla MRI scanner. This scanner is made for preclinical research with small animal models which is the first step performed in every research project before it can be translated into humans. For the study of perfusion in MRI a contrast agent (commonly gadolinium-based) must be intravenously administered to the patient. However, this approach offers many drawbacks regarding secondary effects with gadolinium posing health risk in patients.

In this context, this project investigates the possibility of avoiding such contrast agent by using intrinsic blood oxygenation level instead. This idea relies on the Blood Oxygenated Level Dependent (BOLD) principle that describes the magnetic characteristics of oxyhemoglobin and deoxyhemoglobin. To this end, the first step was the implementation of a first-pass perfusion in rat’s myocardium using gadolinium as contrast agent. The change produced by the passage of the gadolinium bolus can be screen with T1 weighted images. A Graphic User Interface was developed using MATLAB so as to easily asses the results and furtherly compared both contrast agents.

The paramagnetic characteristics of deoxyhemoglobin locally alters the main magnetic field of the MRI scanner if its concentration is increased. For this purpose, a gas mixer with two different mixtures, one with 100% Oxygen and the other with 100% Nitrogen, was used so as to produce changes in the fractions of inspired oxygen (FiO2). Therefore, this method studies the relationship between the MRI signal created by the deoxyhemoglobin bolus and the changes of FiO2 recorded by a pulse-oximeter.

After several MRI sequence adjustments, the scanner was capable of recording the signal produced by the deoxyhemoglobin bolus. Even though the gadolinium signal is more precise and accurate, our new proposal is clearly a focal point for further research since it overcomes health inconvenience offered by the contrasts in used.