Introduction: Hyperthermia is an appealing oncological treatment since the significant regions of hypoxia contained in most solid tumours are known to be sensitive to the cytotoxic effect of heat. However, due to the seemingly insurmountable technical difficulties associated with delivering thermal doses sufficient to induce cellular deactivation thermotherapy is still regarded as an experimental treatment. In contrast to other thermo-therapeutic modalities Focused Ultrasound (FUS) may be employed non-invasively to deliver a highly localized thermal build-up in deep seated regions of the body, while avoiding damage to healthy tissue. Coupled with the unique ability to deliver real-time temperature maps afforded by MRI, the combined modality MRI-FUS offers great potential not only for exclusive hyperthermia, but in such diverse areas as localized transgene expression using thermo-sensitive promoters and localized drug delivery using thermo-sensitive micro-carriers. Subjects Here we will present some of the recent advances in MRI-FUS, and their technical background. This will include: 1) Real-time MRI-thermometry. 2) FUS-technology. 3) Temporal and Spatial temperature control using MRI-based temperature maps. Discussion MRI-thermometry: Of the various MRI-based thermometers the temperature dependent chemical shift of the proton resonance frequency (PRF) is the most widely used providing accurate and high resolution temperature maps. The primary weaknesses of PRF-based thermometry is the vulnerability to motion-artifacts, baseline drift and the fact that the PRF in lipids is independent of temperature. FUS-technology: At moderate intensities absorption of ultrasound (US) in tissue results in a local increase in temperature. As in other wave phenomena the extent of the focal point and penetration depth are governed by the wavelength. Hence for US it is possible to body non-invasively position sub-millimeter focal points in deep seated regions of the. Temperature Control: Most solid tumours cover volumes larger than that of the focal region. This problem may be reduced somewhat by deconstructing the tumour volume into a series of parallel planar regions of interest (ROI), which are treated sequentially. By employing continuous sonication while moving the focal point along an inside-out spiral trajectory, the thermal transport may be exploited to promote a homogenous temperature rise within the plane ROI with steep temperature gradients at its boundaries. Covering the ROI by several consecutive spirals individually modified with respect to applied FUS power and local speed of the focal point based on the MRI temperature maps, the thermal profile may be controlled towards a pre-defined spatio-temporal distribution.
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Annual Meeting of the European Society for Magnetic Resonance in Medicine and Biology, 2004