Читать книгу Intracranial Gliomas Part III - Innovative Treatment Modalities - Группа авторов - Страница 67

The most well established quantitative monitoring for thermal ultrasound therapies exploits the proton resonance frequency shift detected with MRI to measure temperature changes. In essence, MRI exploits differences in the proton environment to give image contrast and, in the case of thermal ultrasound therapies, this change is temperature. As the tissue temperature increases, so does Brownian motion, leading to the stretching and disruption of hydrogen bonds between water molecules. This net decrease in the hydrogen bond strength leads to an increased strength of the covalent bond between hydrogen and oxygen, with a resultant increase in the shielding of the proton from the external magnetic field of the MRI. The increase in shielding and subsequent change in resonance frequency, as a function of increasing temperature, have been shown to be approximately linear. In practice, most MR thermometry sequences consist of a gradient-echo sequence where the accumulated phase change over time in each voxel, due to the local proton resonance frequency shift, is compared to a reference image in order to generate a spatiotemporal temperature map. MR thermometry is sufficiently fast that it can be used for real-time feedback of thermal ultrasound treatments. Furthermore, treating within the MRI environment offers advantages including structural imaging with high soft-tissue contrast, and the ability to co-register the ultrasound focus and the MRI coordinate systems for image-guidance; confirmation of the focal spot in MRI coordinates can be done by heating a small volume of tissue to sublethal temperatures.