Abstract
A variety of treatments that modulate tumor oxygen tension are used clinically to improve the outcome of radiotherapy. High resolution, noninvasive measurements of the effects of these treatments would greatly facilitate the development of improved therapies and could guide treatment of cancer patients. Previous work demonstrated that magnetic resonance (MR) gradient echo imaging of the water proton resonance detects changes in T~2~* and T~1~ in tumors during hyperoxia that may reflect increased tumor oxygenation. This report describes the use of high resolution MR spectroscopic imaging with short repetition time (TR = 0.2 s) to improve the accuracy with which changes in T~2~* and T~1~ are measured. Mammary adenocarcinomas grown in the hind limbs of rats were studied. Carbogen inhalation was used to induce hyperoxia. A single 2‐mm slice through the center of tumors and underlying muscle was imaged at 4.7 Tesla with in‐plane resolution of approximately 1.2 mm and frequency resolution of 5.8 Hz. The peak integral increased by an average of 6% in tumors during carbogen inhalation suggesting a decrease in T~1~ (n = 8, P <0.001). Peak height increased by an average of 15% in tumors during carbogen inhalation (n = 8, P <0.001). The large difference between increases in peak height and peak integral demonstrates that the width of the water resonance decreased. Assuming a Lorentzian lineshape, an average increase of 12% in T~2~* was observed in tumors. In muscle, peak integral and peak height increased slightly (about 1.2% and 3%. respectively; P <0.02) during carbogen inhalation but no significant change in T~2~*was observed. Spectroscopic imaging detects changes in the water proton resonance in tumors during hyperoxia accurately and reproducibly with high signal‐to‐noise ratio and allows clear separation of T~1~ and T~2~* effects. Increases in T~2~* may be due to decreased deoxyhemoglobin in tumor blood vessels (i.e., the BOLD effect) and may provide a clinically useful index of increases in tumor oxygenation.