T1-Based MR Temperature Monitoring with RF Field Change Correction at 7.0T

PURPOSE: The objective of this study is to determine the effect of physical changes on MR temperature imaging at 7.0T and to examine proton-resonance-frequency related changes of MR phase images and T1 related changes of MR magnitude images, which are obtained for MR thermometry at various magnetic field strengths. MATERIALS AND METHODS: An MR-compatible capacitive-coupled radio-frequency hyperthermia system was implemented for heating a phantom and swine muscle tissue, which can be used for both 7.0T and 3.0T MRI. To determine the effect of flip angle correction on T1-based MR thermometry, proton resonance frequency, apparent T1, actual flip angle, and T1 images were obtained. For this purpose, three types of imaging sequences are used, namely, T1-weighted fast field echo with variable flip angle method, dual repetition time method, and variable flip angle method with radio-frequency field nonuniformity correction. RESULTS: Signal-to-noise ratio of the proton resonance frequency shift-based temperature images obtained at 7.0T was five-fold higher than that at 3.0T. The T1 value increases with increasing temperature at both 3.0T and 7.0T. However, temperature measurement using apparent T1-based MR thermometry results in bias and error because B1 varies with temperature. After correcting for the effect of B1 changes, our experimental results confirmed that the calculated T1 increases with increasing temperature both at 3.0T and 7.0T. CONCLUSION: This study suggests that the temperature-induced flip angle variations need to be considered for accurate temperature measurements in T1-based MR thermometry.

Formation of Nuclear Isopeptide in the Process of Neuronal Cell Death following Interstitial Hyperthermia in Normal Rat Brain

BACKGROUND: Radiofrequency (RF) interstitial hyperthermia has recently been shown to be a beneficial treatment modality for human malignant gliomas. It has also been shown that the thermal threshold dose for histopathological damage in the rat brain was heating at 41 degreesCfor 30 min. In the present study, we investigated apoptosis and necrosis of the neuronal cells in the brains of Fischer rats at different times after interstitial heating with lower than the thermal threshold dose. We also measured the isopeptide bond formation in neuronal cells showing apoptosis or necrosis. METHODS: The applicator needles of the RF interstitial heating device, connected to the Thermotron IV was applied to heat the brain at 39, 40, and 41 degreesCfor 30 minutes. Sham-heated control rats were treated the same as the heated rats. The sham-heated animals and those heated at 40 degreesCfor 30 min were sacrificed at 4, 72, 120, and 168 hours after heating, respectively, and the animals heated at 39 and 41 degreesCfor 30 minutes were sacrificed at 168 hours after heating. Coronally sectioned brain tissue, encompassing the heated lesions, were studied immunohistochemically for the expression of TGase1, TGase2, and TGase3, isopeptide. TUNEL assay was performed to examine apoptosis. RESULTS: Immunohistochemical studies showed that in the brains heated at 40 degreesCfor 30 minutes, necrosis with the maximal nuclear isopeptide-positive neuronal cells of the cerebral cortex were seen at 4 hours; The maximal number of isopep-tide- positive neuronal cells showing apoptosis was at 168 hours. CONCLUSIONS: Necrosis of neuronal cells following mild interstitial hyperthermia was maximal at 4 hours and apoptosis was maximal at 168 hours. Neuronal cells showing necrosis or apoptosis formed isopeptide bonds in their nuclei.