The Research Of Positive Contrast Magnetic Resonance Imaging Based On Susceptibility Mapping

As the increased requirement of therapeutic effects and the life quality, the interventional therapy is popular for its smaller trauma, simple, safe, effective and faster recovery. Interventional therapy relies on the guide of medical imaging and real-time intraoperative monitoring. Magnetic resonance imaging(MRI) has many advantages compared with other imaging modalities, such as non-invasive, no ionizing radiation, excellent soft tissue contrast, arbitrary orientation imaging. MRI has become one of the most ideal imaging modality of the interventional therapy. A strong magnetic is used and a radio field is used to resonate with the spined protons in MR imaging. However, the metal devices can induce magnetic field inhomogeneity and arise susceptibility artifacts which devices neighbourhood manifest as dark dots. It is hard to accurately locate the interventional devices because of the high susceptibility artifacts. Therefore, it is significant to develop the positive contrast MRI of metal interventional devices.To accurately locate and evaluate these metals interventional devices,several positive contrast techniques have been developed which can be divided into two groups. The first is frequency sensitive techniques,which generate positive contrast imaging by modifying pulse sequence. For example, white marker be proposed in 2003 and Gradient echo acquisition for super-paramagnetic particles with positive contrast(GRASP) be proposed in 2003. These techniques is easy to implement by changing amplitude of rephasing gradient, but the area of positive contrast is much larger than its true size. The second is post-processing techniques, they get the data through the gradient echo(GRE), and using the unique reconstruction algorithm to generate positive contrast imaging, such as Quantitative Susceptibility Mapping, however most of the existing QSM method are applicable for superparamagnetic iron oxide, as well as the organization's internal paramagnetic substance quantitative assessment, and does not apply to locate the high magnetic metallic implants.SE-based technology shows excellent performance on visualization and localization of the MR compatible metallic devices based on susceptibility mapping and spin echo sequence. The positive visualization and accurate localization can be realized by SE-based technology which can break through the bottlenecks that the traditional MR methods cannot imaging the metal devices. A spin-echo(SE) sequence with shifted 180o pulse is used to acquire the data. We can get the phase differences induced only by susceptibility gradients and the field map achived. The field map can be used to derive the susceptibility of the devices at their exact locations based on a postprocessing step, i.e., a kernel deconvolution algorithm with a regularized ?1 minimization. Therefore, the susceptibility-based technique can highlight the devices due to their high susceptibility values.The SE-based technique has made some progress in positive contrast MRI, however the data acquisition of the SE sequence is rather slow, leading to time consuming. It is critical to solve the problem of the long time consuming for realizing the clinical application. To solve this problem, a new accelerated method based on a modified Fast spin echo(FSE) sequence was developed to positive visualization. The SE sequence can only acquire a single echo within one TR, and the FSE sequence can realize multiple echos. So the data acquisition of the proposed method is much faster than SE-based technique. The experiments of brachytherapy and biopsy needle respectively were successfully conducted. Compared to the SE-based technique, data sampling speed of the proposed method was improved by 2~3 times without losing its ability to accurately visualize and localize the biopsy needle and brachytherapy seeds. It also improves the feasibility of the susceptibility-based positive contrast MR technique on clinical applications.