Measurement of the arterial input function using changes in T2 * in the aorta for dynamic contrast-enhanced MR imaging of cancerous lesions in the rat

Dynamic contrast-enhanced MR imaging (DCE-MRI) of the breast involves rapid (~10 s), repeated imaging after intravenous administration of contrast agent (e.g., Gd-DTPA). This technique provides information about tissue perfusion, capillary permeability, vascular volume fraction, or tissue cell fraction. This is helpful for characterizing breast lesions and monitoring the response of breast cancer to therapy. Knowledge of the arterial input function ([C]_a (t)), which is the concentration-versus-time of Gd-DTPA in arterial blood, is necessary in order to extract pharmacokinetic parameters from DCE-MRI data. The purpose of this work was to explore the usefulness of a non-invasive MRI technique for estimating [C]_a (t) for DCE-MRI of rat tumors. This technique involves rapid gradient-echo imaging to monitor changes in T₂* in the aorta. Simultaneous blood sampling and DCE-MRI were performed on 9 rats in order to determine the relationship between the observed changes in T ₂* and Gd-DTPA concentration in the aorta. DCE-MRI was then performed on one tumor-bearing rat with different injection protocols to determine if the non-invasive [C]_a(t) estimation technique would help to correct for the resultant variations in the DCE-MRI data. It was found that the T₂* method is too imprecise to be useful for DCE-MRI of the rat tumor model used. However, these experiments also showed that measurement of the late (t>50 s) clearance of [C]_a(t) (which can be measured relatively easily with minimally invasive blood sampling or T₁ MRI techniques) is most important