Inflow effects were studied for T 1 -weighted, fast gradient-echo, contrast-enhanced MRI. This was done on the basis of realistic simulations (e.g., taking slice profiles into account) for unsteady flow. The area under the point spread function (PSF) was used to estimate the flow-related enhancement. A simple analytical model that accurately describes the inflow effects was derived and validated. This model was used to correct the experimental perfusion calibration curves (signal intensity vs. relaxation rate) for inflow effects. Hepatic perfusion measurements, performed on patients, were analyzed in terms of a dual-input, first-order linear model. It was shown that inflow causes incorrect perfusion input functions. Perfusion is an important determinant of liver function in health and disease (1). Previous studies performed with CT and MRI have shown that the hepatic perfusion parameters are correlated with liver function in patients with chronic liver diseases, including cirrhosis (2,3). In MRI, perfusion can be measured by using a T 1 -relaxivity contrast agent (such as Gd-DTPA or -DOTA) in combination with a fast gradient-echo sequence (1,2). One can then measure the passage of the bolus at the location of interest by performing a dynamic acquisition. In order to convert the measured signal intensity vs. time curves into (bolus) concentration vs. time curves, the sequence should be calibrated. Therefore, the exact relationship between signal intensity, relaxation rate R 1 Ï 1/T 1 , and concentration should be accurately known. This relationship is usually estimated on the basis of phantom measurements and/or theoretical models (3-6).For the estimation of perfusion parameters, the arterial input function should be known. For the liver, the portal venous input function should also be measured. Therefore, the signal intensity vs. time curves should be measured in the artery (and vein) of interest and converted to concentration vs. time curves. Unfortunately, gradientecho sequences exhibit strong inflow effects. Since the calibration is performed in the absence of flow, these inflow effects lead to overestimation of the (concentration) input functions. It is clear that these inflow effects should be corrected for, if accurate perfusion parameters are desired.In this work we present a method to correct the calibration curves for inflow effects. The method is based on phantom measurements (without flow) and an analytical model that is valid for both steady and pulsatile flows. Such an approach allows one to study the importance of the hemodynamic and sequence-related parameters. The method is validated on a flow phantom and demonstrated for in vivo hepatic perfusion measurements together with its consequences for the estimated perfusion parameters obtained from a dual-input compartmental model.
MATERIALS AND METHODS
Calibration Without Inflow EffectsPerfusion measurements using T 1 -relaxivity agents rely upon the linear relationship between the relaxation rate R 1 and the concentration C (7):where T 10 and T 1 are...