Modification of tumour perfusion and oxygenation monitored by gradient recalled echo MRI and 31P MRS

Gradient recalled echo (GRE) 1H images can be used to monitor changes in blood oxygenation via the dephasing effects of paramagnetic deoxyhaemoglobin (Hb). We have modulated the blood flow/oxygenation of GH3 rat tumours by i.v. calcitonin gene-related peptide (CGRP) or carbogen (95% O2, 5% CO2) inhalation, and obtained GRE 1H images interleaved with 31P spectra before, during and after the insult. With CGRP the GRE image intensity decreased (6/10) by > 10% with a concomitant 40% decrease (4/4) in beta NTP/P1 and a small decrease in pH. Both the image intensity and 31P spectra returned to near their pre-CGRP levels after 50 min, consistent with a transient episode of hypoxia. Carbogen breathing (5/5) caused > 40% increases in average GRE image intensity, with no significant changes in the 31P spectra (4/4). Three-dimensional GRE images were obtained to confirm that a T2* increase, rather than just an 'in-flow' effect due to increased blood flow, was responsible for the GRE enhancement. Increases in average image intensity > 40% were observed for the three-dimensional GRE images (2/2), indicating a T2* increase. Using Hb as an endogenous contrast agent, the high sensitivity of the GRE technique may provide a method of monitoring heterogeneous tumour perfusion and oxygenation, both in the laboratory and the clinic.