| Magnetic resonance imaging (MRI) is a kind of tomography and three-dimensional imaging technology, which based on Nuclear Magnetic Resonance (NMR).Compared with other modalities of medical imaging,such as X-ray, Computer Tomography(CT) and ultrasound, MRI has many advantages, such as no ionizing radiation, imaging with multiple parameters, and imaging with an arbitrary plane.moreover, MRI can produce images of structure and function of internal tissues in living body noninvasively. so it is widely used in clinics and become an one of the most important method in the field of medical diagnosis and scientific research.In recent years, with the advance of medical technology, minimally invasive and noninvasive surgery technologies are used for cancer therapy, minimally invasive thermal therapy as local treatment of benign and malignant diseases has received increasing interest. In the process of thermal ablation, we need to real-time monitoring the temperature of the target to kill tumor cells and protect the normal tissue. Therefore, how to quickly and accurately measure the temperature changes of diseased tissue during the process of treatment become a focus. With the development of modern science and technology, especially the development of physical heating technology. Thermal treatment has entered a new period.First of all, the diversification of the heating technology, such as radio frequency, microwave, ultrasonic, laser, infrared and magnetic induction, so that tumor thermal therapy technology has a rapid development. Secondly, tumor thermal ablation has minimally invasive and low recurrence rate, which is widely applied in prostate tumor and uterine fibroids treatment. Currently, in the process of thermal therapy, there are three methods to monitor temperature, such as CT imaging, ultrasonic imaging, and MR imaging. Compared with the other two technologies,MR temperature measuring technique can not only accurately locate position of target area, but also quantitatively measure the temperature change and even forecast the necrosis area in the process of treatment. Temperature changes can cause changes of multiple magnetic resonance imaging parameters, such as proton resonance frequency (PRF) and spin lattice relaxation time (Ti), the diffusion coefficient of water molecule(D),3D-MRSI, etc.Although superconducting system has the advantages of high imaging quality and high speed, but the costs of production and maintenance is high, and the system of magnet is openness. However, permanent MRI system has wide space to facilitate doctors involved in operation, so the study of temperature measurement on the low-field MR systems has very important significance.The diffusion coefficient of water molecules not only need higher gradient field, but also the MR signal is low, which will cause errors of temperature measurement are relatively large.MR spectrum imaging can get the absolute temperature and the error is small, but the collected data need a long time, which is not feasible to the real-time temperature measurement. Except for adipose tissue, the PRF thermal coefficient is tissue type-independent and exhibits only a small dependence on thermal history even when tissue has been coagulated, PRF method is the most commonly used MRI temperature measuring method. However, PRF is mainly used at high field since its sensitivity is proportional to field strength.T1method is not sensitive to the magnetic field strength, even its coefficient of temperature sensitivity is also higher in the permanent magnet, and within a narrow range of temperatures, a linear relationship appear between T1and the variation of temperature, but the T1values of measurement influenced by many factors lead to temperature error. Above all, on the permanent magnet MRI system, due to the influence of the limitation of acceptor magnetic field strength and drift of magnetic field, the above methods can not well meet the needs of temperature measurement, but from the above we also see the PRF method using phase information of an image for measuring temperature, T1method using amplitude information of an image for measuring temperature, if we combine these two methods together as comprehensive utilization of the amplitude and phase information of the image, so that it could achieve a better temperature measurement result, it will well meet the needs of clinical.Recently, Nick Tood etc. have put forward a hybrid PRF/T1method, the method is used to measure temperature in tissue with adipose, a collected data realizes PRF and T1method by using the phase and amplitude information of images, at the same time. In the superconducting magnetic resonance imaging (MRI) system this method has obtained very good temperature measuring results. This article wants to study hybrid PRF/T1method to overcome shortcomings of PRF alone or T1in the permanent magnet MRI system.In this paper, The phantom, consists mainly of AGAR to simulate liver tissue, was heated to65℃in water bath and naturally cooled to36℃inside a0.3T permanent MRI scanner. due to MRI compatibility requirements, the alcohol thermometer with the precision of0.5℃measured the temperature of the phantom. The four phantoms were placed around the heating phantom used for magnetic drift correction in shielding room. During cooling process, the phantom was scanned by gradient echo sequences with flip angles of40°and75°once the decrease of1℃. Then we extracted the phase and amplitude information of the data respectively, the phase information was used for PRF method, and VFA method is used to calculate the T1values for the amplitude information. Finally, average results of the two methods were as the final measurement results.In this paper,we studied the temperature measurement based hybrid PRF/T1average method on permanent magnet MRI system, hybrid PRF/T1average method was used to study the effect of temperature measurement, which used amplitude and phase informations of the images at the same time, and the measurement average results of PRF and T1is as the final measurement results. The experimental results show that the temperature measurement errors of the hybrid PRF/T1method was below±3℃,and its COV was2.63%shows that the repeatability is high.For permanent MRI system, the hybrid PRF/T1method is more accurate and reproducible than the PRF and T1only methods, and offers a promising tool for temperature monitoring in tumor thermometry. |
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