Study Of The SAR Safety Of Ultra-high Field MRI For MRIgHIFU

Magnetic resonance imaging is a imaging tomographic technique. The basic principle of MRI is that the nucleus system contain both magnetic moment and angular momentums placed in a static magnetic field, when the nucleus system interact with the specific frequency RF electromagnetic signal there will generated nuclear magnetic resonance effect. After RF signal, the receiver coil receives the magnetic resonance signal which contains the encoded spatial information generated by the excited proton. By using the spectrometer and computer system process the magnetic resonance signal there can be imaging the structure of biological sample. As this character, MRI is widely used in the fields of bioscience research and clinical medical diagnosis.Compared to other clinical medical imaging technologies, MRI has many advantages:First, MRI with the ability to image anatomical structures in any scan orientation; Secondly, MRI image has good resolution of soft tissue; Third, MRI image has a broad view of the imaging and have good spatial resolution; Fourth, many parameters can be imaged in MRI, so MRI can provide a lot of valuable information to help doctor diagnose; Fifth, there will be no bone artifacts in MRI image; Last, the RF electromagnetic field was used to excite hydrogen proton exchange in different energy states does not cause ionizing radiation damage, more safe for people.High Intensity Focused Ultrasound is the rise of a noninvasive local thermal ablation technology, in recent years. It can be used for the treatment of benign and malignant tumors. The basic principle of HIFU is that focused the low energy ultrasound in human body, in the focal area, tissue temperature increases rapidly, within seconds, up to60℃or higher. Consequently, the target tissue is destroyed by protein coagulation due to the deposition of a large quantity of thermal energy.Magnetic resonance imaging based on temperature-sensitive MR parameters such as the proton resonance frequency (PRF), the diffusion coefficient (D), T1and T2relaxation times as well as temperature-sensitive contrast agents, can be used to monitor temperature distribution in vivo. The ability of MRI to construct maps of in vivo body temperature that make it particularly well suited for guiding and monitoring minimally invasive thermal therapy.MRI-guided high intensity focused ultrasound combines both MRI and HIFU merits, MRI provides excellent soft tissue contrast and is thus superior to any other imaging methods for guiding treatments, HIFU is noninvasive local thermal ablation technology. During the process of the thermal ablation, magnetic resonance imaging and thermometry can provide noninvasive image and temperature map guide to ensure the proper implementation and safety of the thermal surgery, MRI can also detect tissue damage induced by ultrasound, thus providing a method for post treatment verification. The Food and Drug Administration (FDA) approved the first commercial system developed by InSightec for uterine fibroid treatments in2004. MRIgHIFU system has been used in the surgery of non-breath effect body such as fibroid, prostate, brain, etc, for the abdomen and other body parts affected by breath movement of tumor ablation also need deep research.As high field MRI have relative high spatial and temporal resolution and higher signal noise ratio (SNR), high field MRI are progressively widely used in clinic, high field MRI showing significant advantages in the clinical applications of nerves, blood vessels etc. High field MRI systems are relatively common used in clinic at present like1.5T and3T MRI systems.7T MRI and even utra-high field MRI imaging technology is certainly an important topic.As coil has two sides, high and ultrahigh field MRI systems also have some disadvantages. Human organs consist of different kinds of tissues which have different dielectric constants and conductivities, when RF electromagnetic wave interacting with human tissues will conduct anti-electric effect in the human body. Anti-electric effect causes electromagnetic wave attenuation at different parts of the body. With the increase of the main magnetic field the corresponding Larmor resonance frequency is raised, that means the wavelength of the incident and reflected electromagnetic waves in the human is shorter, there will produce a standing wave effect in some tissues. Combining anti-electric wave effect with the standing effect, the distribution of electromagnetic wave in human body is complex. Therefore, in high field and ultra-high field MRI the B1field distribution will be seriously deteriorated when it propagates in human body.The inhomogeneous distribution of B1field lead to the image signal-to-noise ratio, image contrast will get worse; Secondly, the interaction between human tissue and RF field caused by the anti-electric effect and standing wave effects will result in the human tissues absorbed more RF energy and maybe cause human tissues thermal damage. The specific absorption rate is used to evaluate human tissues absorb the RF energy, and according to International Electrotechnical Commission standard in different conditions the threshold values of SAR are different. Thus, in high and ultrahigh field MRI systems the electromagnetic effects between RF electromagnetic wave and human body will deteriorate the image quality and even maybe cause human body potential safety issue. When study of the interaction between RF field and human body it is necessary to combine with the clinical application.The study of B1field and SAR distribution in MRI field related to solve the engineering electromagnetic distribution in human body. Actually, this problem usually is very complex, such as the structure of issue is complex, the materials are difference. The basis method of Finite Difference Time Domain (FDTD) is that by using the finite difference equations replace the Maxwell differential equations FDTD can easily modeling the complex media voxel to voxel as well as setting electromagnetic parameters voxel to voxel. As different kinds of human tissues have different electromagnetic parameters thus FDTD is very powerful for calculation the electromagnetic field when RF electromagnetic wave propagating in human body. FDTD method is also powerful for simulation the tuning of the RF coil because of its ability for calculating frequency response. The FDTD method is widely used in the engineering electromagnetic simulation from it comes out. The RF B1field and SAR within the human body were calculated by FDTD algorithm in this paper,Using the FDTD algorithm to solve B1field and SAR in RF System load with human body also need to establish the electromagnetic human model. Developing high field and ultra-high field (>=7T) MRI is a new trend due to the significantly improved SNR with the increase of the main magnetic field and combing with the clinical application of the uterine fibroid can be treated by MRIgHIFU. In this paper, we established the gradient temperature model combining the adult female pelvis electromagnetic model with a single ablation model. Using this gradient model to mimics the temperature of pelvic during the HIFU. According to Clare’s research the highest temperature in the center of ablation is about35℃elevated, in the ablation area is about25℃elevated, the temperature field near the ablation area can’t be neglected, it appears about8℃elevated seen from the temperature map. So we establish two coaxial ellipsoids to simulate the temperature field in the area of thermal ablation, one has the long axis of13mm and short axis11mm and the other ellipsoid is27mm and25mm respectively. Then a sphere with the radius of45mm and the center of the ellipsoids mentioned before to simulate the temperature gradient field around the area of the thermal ablation. The two ellipsoids inside respect to the temperature field of the center of the thermal ablation and the peripheral area respectively and the outer sphere means the temperature field of tissues around. The ball and ellipsoids were modeled by ProE. Then import these models into mimics combine with the normal pelvis model. Assigning the different kinds of tissues with the related electromagnetic parameters, then get the electromagnetic gradient temperature model to mimics the temperature distribution of HIFU was established.To study the efficacy of the B1field shimming technique and evaluate the SAR safety issue at ultra-high field Magnetic resonance imaging (MRI) for MRI-guided high-intensity focused ultrasound (MRIgHIFU). The electromagnetic model of female pelvic with the temperature gradient was established. B1field homogeneity and local SAR were simulated and calculated, using regular and optimized B1shimming coefficients separately. The maximum local SAR reached10.24W/kg which exceeded the safe threshold10W/kg of the maximum local SAR set by IEC, using regular B1shimming coefficients with respect to the normal model. In contrast, the maximum local SAR value of the tissue was9.65W/kg which was below the threshold of the safe standard, when using the optimized B1shimming coefficients. In conclusion, the temperature distributions in the human generated by the ultrasound energy need to be considered under the MRI-guided high-intensity focused ultrasound surgery at ultrahigh field MRI. The proposed optimized B1shimming strategy based on temperature gradient can be used to control the local SAR values.