| MRI can produce images of structure and function of internal tissues in living body noninvasively. Compared with others imaging modalities, MRI has advantages of multi-parameters imaging and having no ionizing radiation, so it has been widely used in clinics and become an important diagnostic tool in medicine.In recent years, minimally invasive and noninvasive surgery technologies are constantly developed well. Thermal ablation has received much attention in the treatment of the prostate and uterine fibroids cancer. The aim of this minimally invasive technique is to treat the malignant lesions while causing minimum damage to the normal surrounding tissue. Anticipated advantage are reduction in morbidity and mortality and the ability to perform the treatment on outpatients with reduction of cost.The basic principle of thermal ablation is to rapidly coagulate the diseased tissue by heating it. At the same time, we must ensure that the ablation won't damage the healthy surrounding tissue. Therefore, how to rapidly and accurately measure the changes of the temperature while treating diseased tissue is the key point. Only through temperature measurement, the doctors can ascertain the treatment effect, forecast the necrotic area, and modify the treatment method to avoid damaging the normal surrounding tissue.In the thermal ablation, there are three methods to monitor this process: computer tomography (CT), ultrasonography and magnetic resonance imaging (MRI). Compared with the other two methods, MR temperature mapping can not only accurately locate the lesion, but also quantitatively measure the changes of temperature during thermal ablation. MR temperature measurement is noninvasive, which makes use of the temperature sensitivity of some MR parameters. According to the different temperature-sensitive parameters, MR temperature mapping methods include:proton resonance frequency (PRF), T1 relaxation times, the diffusion coefficient of water molecule,3D-MRSI, etc.Currently the method based T1 is used mostly at low field. But because the temperature-sensitive coefficients are different in different tissue, which is difficult to correct, the method is not suit for clinical application. PRF has received much attention because of the instantaneity of acquisition and the linearity between frequency changes and temperature changes. However, PRF is mainly used at high field since its sensitivity is proportional to field strength.Sparse MRI is a method which reduces data acquisition time by undersampling K-space data. If K space data is undersampled, the measured data do not meet the sampling theory any more, and images reconstructed from them by FFT suffer from severe artifacts. However, sparse MRI theory shows that if the sparse sampling in k-space is random and artifacts caused by it appear as noise in some transform domain such as wavelet transform and the desired image is sparse in that transform domain, we can also reconstruct the image accurately by an appropriate nonlinear reconstruction algorithm.In the paper, we studied the temperature measurement based chemical shifting of proton resonance frequency at low field:(1) We chose the method based on PRF, and carried out the experiment on XGY Oper-0.4T equipment using FLASH to acquire data which records the cooling process of agar gel. The MR thermometry results were compared with that of thermometer. The temperature uncertainty was±1.5℃for head coil and±2℃for body coil, and the scan time was 6s. The results showed that the precision and scan time of the method based on PRF can basically satisfy clinical requirements(2) The theory of sparse MRI was introduced into MR thermometry. A set of sparse data was generated from agar gel cooling experiment data to verify it. The results showed that the temperature changes information of the simulative sparse data was still accurate. |
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