BOLD contrast fMRI of whole rodent tumour during air or carbogen breathingusing echo-planar imaging at 1.5 T

The aim of this study was to evaluate the feasibility of functional MR imag ing (fMRI) at 1.5 T, exploiting blood oxygenation level-dependent (BOLD) co ntrast, for detecting changes in whole-tumour oxygenation induced by carbog en (5% CO2+95% O-2) inhalation of the host. Adult WAG/Rij rats with rhabdom yosarcomas growing subcutaneously in the lower flank were imaged when tumou rs reached sizes between 1 and 11 cm(3) (n = 12). Air and carbogen were alt ernatively supplied at 2 l/min using a snout mask. Imaging was done on a 1. 5-T MR scanner using a T2*-weighted gradient-echo, echo-planar imaging (GE- EPI) sequence. Analysis of the whole-tumour EPI images was based on statist ical parametric maps. Voxels with and without signal intensity changes (SIC ) were recorded. Significance thresholds were set at p < 0.05, corrected fo r multiple comparisons. In continuous air breathing condition, 3 of 12 tumo urs showed significant negative SIC and 1 tumour had a clear-cut positive S IC. The remaining tumours showed very little or no change. When switching t o carbogen breathing, the SIC were significantly positive in 10 of 12 tumou rs. Negative SIC were present in 4 tumours, of which three were simultaneou sly characterised by positive SIC. The overall analysis indicated that 6 of the 12 tumours could be considered as strong positive responders to carbog en. Our research demonstrates the applicability of fMRI GE-EPI at 1.5 T to study whole-tumour oxygenation non-invasively. The observed negative SIC du ring air condition may reflect the presence of transient hypoxia during the se measurements. Selection of tumours on the basis of their individual resp onse to carbogen is possible, indicating a role of such non-invasive measur ements for using tailor-made treatments.