Oxygen sensitive MRI: Comparison of TOLD and pO2 measurements in experimental rat tumors during inspiratory hypoxic challenge

Zielsetzung:

Establishment of TOLD (“Tissue Oxygenation Level Dependent”) measurements in rats with implanted syngeneic mammary carcinoma. TOLD: T1-VIBE, dependent on molecular O2-concentration in tissue. The relative change of tumor oxygenation obtained by TOLD MRI during variation of the breathing gas mixture was compared to simultaneous quantitative measurements by Oxylite, an invasive fluoro-optic oximetry method.

Material und Methodik:

Tumor models: Walker-256 mammary carcinoma in Wister rats and prostate carcinoma AT1 in Copenhagen rats, subcutaneously implanted on the hind foot. MRI: 3T Magnetom Skyra (Siemens). Animal coil: Quadrature volume coil, Ø70 mm (Stark Contrast). Pulse sequences: T1-VIBE (TOLD). Tissue oxygenation monitoring: Oxylite 2000 (Oxford Optronics Ltd., Oxford, UK), fluoro-optical E-Series sensor placed in the center of tumor. The O2-concentration of the inspiratory gas was changed sequentially: From 21% (normoxia) to 8% (hypoxia) to 100% (hyperoxia) to 21%. Tumor segmentation and calculation of intensity mean values were performed with MATLAB.

Ergebnisse:

Both TOLD signal and Oxylite data showed dependence on oxygen concentration, but they are not strictly correlated. Partly this can be attributed to the heterogeneity of tumor tissue and the local character of data collection by the Oxylite probe. Furthermore, the Oxylite measurements show prominent signal increase when breathing pure oxygen compared to normoxic gas. The dynamics of MR measurements were faster than those of the Oxylite system. Comparing both tumor models, the AT1 model showed a diminished response on change of inspiratory O2 content, and oxygenation in the AT1 model was lower than with the W256 model.

Schlussfolgerungen:

Oxygenation levels of two tumor models under normoxia, hypoxia and hyperoxia were assessed non-invasively by TOLD MRI. Comparison with fluorometric oximetry measurements proved TOLD MRI to be a robust and simple method appropriate to estimate oxygenation of tumor tissue.