Dependency of R(2) and R(2)* relaxation on Gd‐DTPA concentration in arterial blood: Influence of hematocrit and magnetic field strength

Dynamic susceptibility contrast (DSC) MRI is clinically used to measure brain perfusion by monitoring the dynamic passage of a bolus of contrast agent through the brain. For quantitative analysis of the DSC images, the arterial input function is required. It is known that the original assumption of a linear relation between the R(2) ((*)) relaxation and the arterial contrast agent concentration is invalid, although the exact relation is as of yet unknown. Studying this relation in vitro is time‐consuming, because of the widespread variations in field strengths, MRI sequences, contrast agents, and physiological conditions. This study aims to simulate the R(2) ((*)) versus contrast concentration relation under varying physiological and technical conditions using an adapted version of an open‐source simulation tool. The approach was validated with previously acquired data in human whole blood at 1.5 T by means of a gradient‐echo sequence (proof‐of‐concept). Subsequently, the impact of hematocrit, field strength, and oxygen saturation on this relation was studied for both gradient‐echo and spin‐echo sequences. The results show that for both gradient‐echo and spin‐echo sequences, the relaxivity increases with hematocrit and field strength, while the hematocrit dependency was nonlinear for both types of MRI sequences. By contrast, oxygen saturation has only a minor effect. In conclusion, the simulation setup has proven to be an efficient method to rapidly calibrate and estimate the relation between R(2) ((*)) and gadolinium concentration in whole blood. This knowledge will be useful in future clinical work to more accurately retrieve quantitative information on brain perfusion.