31P magnetic resonance spectroscopy (MRS) of experimental orbital myositis.

We described a model of orbital myositis that was induced by 12-0-tetradecanoyl-phorbol-13-acetate (TPA) injection into the superior rectus muscle of New Zealand white rabbits. In this study, in vivo 31P magnetic resonance spectroscopy (MRS) was performed with a 4.7 Tesla Oxford (Oxford Instruments, Oxford, England) magnet to monitor the evolution of muscle inflammation in control animals and the response of this model to external beam radiation. 31P MRS showed a dramatic increase in high-energy phosphorus and phospholipid metabolites 48 hr after TPA injection. These spectra were similar to those from implanted allogenic fibroblasts. Within 18-48 hr after a single dose (400 cGy) or sequential doses (3 days at 400 cGy) of orbital irradiation, reduced extraocular muscle swelling and a significant decrease of all 31P metabolites occurred. A decrease (63 +/- 6%) in the signal-to-noise (S/N) ratio of the control inflamed muscle 31P MR spectra increased by 28 days after inflammation. Two days after single-dose radiation, 31P MR metabolites were significantly lower (58 +/- 5%, P less than 0.012) than control spectra. These postradiation spectra mirror the 28-day control spectra and are consistent with previous histologic data that show decreased fibroblastic activity. Change in 31P MRS was a sensitive indicator of treatment response latency.