Evaluation of the effects of high dose irradiation on canine thigh muscle by follow-up magnetic resonance imaging and phosphorus-31 magnetic resonance spectroscopy.

RATIONALE AND OBJECTIVES: The authors investigate alterations of proton T1 and T2 relaxation times and phosphorus metabolites of canine thigh muscle tissue after high dose x-ray irradiation by follow-up magnetic resonance imaging (MRI) and phosphorus-31 (31P) magnetic resonance spectroscopy (MRS). METHODS: A group of 20 dogs was used for MRI and in vivo 31P MRS. Single doses of 5,000 and 10,000 cGy were delivered to the right thigh muscle of groups of 10 dogs each. All MRI and 31P MRS examinations were performed before irradiation and 1, 7, 14, 28, 42, and 56 days after irradiation. For measurement of T1, repetition time (TR) was measured at 300, 500, 1000, 1500, 2000 msec and echo time (TE) was fixed at 12 msec. Also, for measurement of T2, TE was measured at 20, 40, 60, and 80 msec and TR was fixed at 2000 msec. Image selected in vivo spectroscopy (ISIS) pulse sequence was used to obtain 31P MR spectra. Peak areas for each phosphorus metabolite were measured using a Marquart algorithm. RESULTS: Magnetic resonance imaging signal began to change at 28 days after a single dose of 10,000 cGy, whereas there was no significant MRI signal change until 56 days after a single dose of 5,000 cGy. Also, extensive MRI signal changes were observed at 42 days after a single dose of 10,000 cGy. Significant correlation was established between T2 and a lapse of time although there was no correlation between T1 and a lapse of time. T2 value increased substantially corresponding to the time period after x-ray irradiation. Although MR spectral change was not observed until 42 days after a single dose of 5,000 cGy, it began at 14 days after a single dose of 10,000 cGy. And, significant MR spectral changes were observed at 28 and 42 days. Inorganic phosphate and phosphodiesters signal intensities increased while phosphocreatine signal intensity decreased. The pH value was 7.22 +/- 0.05 at control, and 6.98 +/- 0.04 at 42 days after a single dose of 10,000 cGy. CONCLUSIONS: The postirradiation follow-up MRI and 31P MRS studies demonstrated that morphologic and metabolic changes were dependent upon the x-ray dose and a lapse of time.