Applications of Dynamic Contrast Enhanced MRI in Oncology: Measurement of Tumor Oxygen Tension

Abstract

A new model based on an extension of the Krogh's cylindrical model was developed to calculate tumor oxygen tension ( pO2) from the H-1 dynamic contrast enhanced MRI (DCE-MRI) measurements. The model enables one to calculate the tumor pO2 using the vascular volume fraction (fb) obtained by the DCE-MRI. The proposed model has three parameters. For small values of fb one assumes that there exists a linear relationship between and fb. The constant of proportionality in this case is given by C1 — the oxygen tension per vascular volume fraction. For larger values of fb a modified version of Krogh model using two parameters is developed and here C2 — is the integrated blood oxygen tension, and C3 -given by the combination of the oxygen diffusion coefficient, solubility of oxygen in the tissue, capillary radius, and tissue metabolic consumption rate. The parameters of the model can be determined by performing simultaneous in-vivo F-19 MRI oxygen tension measurement and dynamic Gd-DTPA enhanced MRI on the same tumor. Dynamic MRI data can be used with a compartmental model to calculate tumor vascular volume fraction on a pixel by pixel basis. Then tumor oxygen tension map can be calculated from the vascular volume fraction by the extended Krogh model as described above. In the present work, the model parameters were determined using three rats bearing Walker-256 tumors and performing simultaneous F-19 and DCE MRI on the same tumor. The parameters obtained by fitting the model equation to the experimental data were: C1 = 983.2+ 133.2torr, C2 = 58.20 + 2.4 torr, and C3 = 1.7 + 0.1 torr. The performance of the extended Krogh model was then tested on two additional rats by performing both F-19 and DCE-MRI studies and calculating the pO2 (H-1) using the model and comparing it with the pO2 (F-19) obtained from the F-19 MRI. It was found that the measurements obtained by both techniques had a high degree of correlation [ pO2 (H-1) = (1.01 + 0.07) pO2 (F-19) + (0.91 + 0.05) and r=0.96], indicating the applicability of the proposed model in determining pO2 from the DCE-MRI.