SPECTROSCOPIC IMAGING OF THE WATER RESONANCE WITH SHORT REPETITION TIME TO STUDY TUMOR RESPONSE TO HYPEROXIA

A variety of treatments that modulate tumor oxygen tension are used cl inically to improve the outcome of radiotherapy, High resolution, noni nvasive measurements of the effects of these treatments would greatly facilitate the development of improved therapies and could guide treat ment of cancer patients, Previous work demonstrated that magnetic reso nance (MR) gradient echo imaging of the water proton resonance detects changes in T-2 and T-1 in tumors during hyperoxia that may reflect i ncreased tumor oxygenation, This report describes the use of high reso lution 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 measu red, Mammary adenocarcinomas grown in the hind limbs of rats were stud ied, Carbogen inhalation was used to induce hyperoxia, A single 2-mm s lice through the center of tumors and underlying muscle was imaged at 4.7 Tesla with in-plane resolution of approximately 1,2 mm and frequen cy 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 tumor s during carbogen inhalation (n = 8, P < 0.001), The large difference between increases in peak height and peak integral demonstrates that t he width of the water resonance decreased, Assuming a Lorentzian lines hape, an average increase of 12% in T-2 was observed in tumors, In mu scle, peak integral and peak height increased slightly (about 1.2% and 3%, respectively; P < 0.02) during carbogen inhalation but no signifi cant change in T-2 was observed, Spectroscopic imaging detects change s in the water proton resonance in tumors during hyperoxia accurately and reproducibly with high signal-to-noise ratio and allows clear sepa ration of T-1 and T-2 effects, Increases in T-1* may be due to decrea sed deoxyhemoglobin in tumor blood vessels (i,e,, the BOLD effect) and may provide a clinically useful index of increases in tumor oxygenati on.