FAT- WATER SEPARATION BASED MR IMAGING FOR HIGH-INTENSITY FOCUSED ULTRASOUND (HIFU) BASED THERAPY (18-Jan-2010)

RP13288

FAT- WATER SEPARATION BASED MR IMAGING FOR HIGH-INTENSITY FOCUSED ULTRASOUND (HIFU) BASED THERAPY

BRIEF ABSTRACT

    The present invention discloses a technique that provides an optimal Magnetic Resonance Imaging (MRI) sequence for robust delineation of bone- surface while providing the bone surface contrast similar to that provided by Computed Tomography (CT). Further, the technique offers a protocol for specialized MRI data acquisition and image processing techniques to segment the bone with high specificity and sensitivity. Therefore, this technique is a segmentation of specific sequences.

KEYWORDS

    MRI, Magnetic Resonance Imaging, data acquisition, image processing technique(s), segmentation specific sequences, bone-surface delineation, High- Intensity Focused Ultrasound, HIFU, periosteum, bone metastases, water-fat separation, MEDAL and IDEAL.

DETAILED DESCRIPTION

    The bone is made up of outer cortex (typically mineral deposits, solid material) and the inner trabaculae (porous structure with vasculature). Most of the pain related to bone metastases occurs in the region of periosteum i.e. the outer surface of the bone cortex. Where, HIFU (High-Intensity Focused Ultrasound) energy can potentially denervate the region and result in pain relief for the patients.

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RP13288

    For accurate HIFU treatment planning, it is necessary to determine the location of the bone surface with respect to a surrounding soft tissue. Currently, CT provides a very good contrast between the bone cortex and the surrounding tissue and has been conventionally used to derive the bone surface for treatment planning. Apart from dose concerns, the requirement for CT hinders the smooth workflow for the MR guided Focused UltraSound (MRgFUS) therapy planning.

    While, in context of 1H MRI (1H-Magnetic Resonance Imaging), water present in cortical bone has a very short T₂ (relaxation time) of about 200 to 500 milliseconds (ms). Thus, on clinical MRI systems, with TE (referred to as the echo time) typically in the range of several milliseconds or longer, the bone manifests as a bright structure (signal coming from marrow fat in trabecular structure) surrounded by a dark structure (the cortex).

    Water-fat separation based MR techniques are optimally suited to delineate the surface of bone to provide a mask similar to that obtained from a CT image. Figure 1A illustrates a chemical shift of fat in a bone marrow. One of the issues associated with such a technique called standard 3D T1-weighted SPGR acquisition is having fat chemical shift artifacts, which can be severe at lower spectral bandwidth (for example, ~15.6 KHz). Figure 1B illustrates minimized chemical shift of fat by increasing the bandwidth. Increasing the spectral bandwidth (for example, ~166.6 KHz) ameliorates the problem, but only at degraded image SNR (Signal-to-Noise ratio...