摘要
磁共振成像可在无创条件下对人体或生物体的内部组织的结构和功能进行成像,与CT等成像技术相比,它具有多参数成像、对人体没有电离辐射等优点,因此其在临床上得到广泛应用,并成为医学诊断领域中最重要的方法之一。
近几年,微创和无创手术技术不断发展,针对前列腺癌和子宫肌瘤等的热消融术也受到人们极大地关注。微创技术的目的是在处理恶性病灶的同时不会引起或者最小化对病灶周围正常组织的损伤。实现在病人体外进行手术,降低手术成本,减少发病率和死亡率。
热消融术的基本原理是：通过加热,使病变组织温度升高并将其杀死。于此同时,必须保证其周围健康组织细胞的温度维持在安全范围内。因此,如何快速、精确地测量病变组织在治疗过程中温度的变化成为人们研究的重点。只有通过温度测量反馈回来的信息,医生才能了解治疗效果、预测坏死区域、及时修改治疗方案以及避免损伤病变区域周围的健康组织。
在热消融过程中,用与监控的成像方法主要有三种：CT成像、超声成像技术以及MR成像。与其他两种技术相比,MR温度测量技术不仅能对目标区域进行精确定位,而且还能对治疗过程中的温度变化进行定量测量,甚至还可以对坏死区域进行预测。MR温度测量属于无创温度测量技术,它利用的是某些成像参数的温度敏感性。根据使用的温度敏感参数的不同,MR测量温度的方法主要有质子共振频率(Proton Resonance Frequency, PRF)、自旋晶格弛豫时间T1、水分子扩散系数,3D-MRSI等几种。
目前在低场环境中应用最多的是自旋晶格弛豫时间T1的方法,但是由于不同组织T1对温度敏感系数不同,T1温度敏感校正较为困难,所以此方法的临床应用性不好。PRF方法因其共振频率变化与温度变化在一定范围内成线性关系,其温度敏感系数对不同的组织(脂肪除外)来说差别非常小,接近于纯水的PRF温度敏感系数(-0.01ppm/℃),且可以实现准实时温度测量,受到人们的关注。但是由于PRF方法的温度灵敏度和场强成正比,所以其多是应用于场强较高的超导环境中,而低场应用很少。
稀疏磁共振成像技术是通过减少K空间数据采集量这一途径来缩短数据采集时间的。减少数据采集量,使得采集的数据不再满足抽样定理,用这些数据重建出的图像存在严重的伪影。但是,稀疏磁共振成像理论表明,如果采用的是随机降采样,并且由降采样引起的伪影在图像的某一变换域内(如小波变换)的表现类似于噪声,只要待重建图像在该变换域内具有稀疏性,并且采用一种合适的非线性重建方法进行重建,仍可精确地重建出图像。稀疏磁共振采集方式可以进一步提高测温的时间分辨率。
本文中尝试了在低场永磁环境中,使用基于质子共振频率(PRF)化学位移的测温方法进行实验研究：
(1)针对在低场永磁磁共振环境,使用基于质子共振频率化学位移的方法进行测温的可行性进行了研究。在本文中,选用基于质子共振频率化学位移的测温方法,在XGY Oper-0.4T永磁设备上采用FLASH序列采集琼脂凝胶降温过程的数据,并与玻璃温度计的测量结果进行对比。MRI测温实验结果达到：头线圈±1.5℃,体线圈±2℃的精度；每次扫描时间,即MRI测温的时间分辨率为6s。由此证明,在低场中基于质子共振频率化学位移测温方法的精度和时间分辨率能够满足临床要求。
(2)将磁共振图像的稀疏重建思想引入到测温中,并将一套全矩阵扫描的琼脂降温实验数据模拟生成一套稀疏采集的数据加以验证。实验结果表明,这套生成的稀疏数据的温度变化信息依然比较准确。
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.
引文
[1]Zhi-Pei Liang and Paul C.Lauterbur. Principles of Magnetic Resonance Imaging[M]. IEEE Press, New York,1999.
[2]高元桂,磁共振成像诊断学[M].北京：人民军医出版社,2004.
[3]Paul C. Lauterbur. Image formation by induced local interaction:Examples employing nuclear magnetic resonance[J]. Nature, Vol.8, pp.190-191,1973.
[4]D.C. Noll and W. Schneider. Theory, simulation, and compensation of physiological motion artifacts in functional MRI[J]. IEEE Int. Conf. on Image Proc, Austin, TX, pp.40-44,1994.
[5]R.L. Buckner, P.A. Bandettini and K.M. O'Craven, et al. Detection of cortical activation during averaged single trials of a cognitive task using functional magnetic resonance imaging[J]. Proc. Natl. Acad. Sci., Vol.93, pp.14878-14883,1996.
[6]Basser PJ, Mattiello J, LeBihan D. MR diffusion tensor spectroscopy and imaging[J]. Biophysical Journal, Vol.66, pp.259-267,1994.
[7]Sensky PR, Samani NJ, Reek C, et al. Magnetic resonance perfusion imaging in patients with coronary artery disease:a qualitative approach[J]. Int J Cardiovasc Imaging, Vol.18, pp.373-383,2002.
[8]Rosen BR, Belliveau JW and Chien D. Perfusion imaging by nuclear magnetic resonance[J]. Magn.Reson., Vol.5, pp.263-281,1989.
[9]Pekar J, Jezzard P, Roberts DA,et al. Perfusion imaging with magnetization transfer frequency offset compensation [J]. Magn.Reson.Med., Vol.35, pp.70-79,1996.
[10]赵喜平.磁共振成像[M].北京：科学出版社,2004.
[11]Weiger M, Pruessmann KP and Boesiger P. Cardiac real time imaging using SENSE sensitivity encoding scheme [J]. Magn.Reson.Med., Vol.43, pp.177-184,2000.
[12]Silverman SG, Tuncali K, Adams DF, et al. MR imaging guided percutaneous cryotherapy of liver tumors:initial experience [J]. Radiology, Vol.217, pp.657-664,2000.
[13]Saeed M, Martin AJ, Lee RJ, et al. MR guidance of targeted injections into border and core of scarred myocardium in pigs[J]. Radiology, Vol.240, pp.419-426,2006.
[14]Lewin JS. Interventional MR imaging:concepts, systems and application in neuroradiology[J]. AJNR Am JNeuroradiol, Vol.20, pp.735-748,1999.
[15]Quick HH, Zenge MO, Kuehl,et al. Interventional magnetic resonance angiography with no strings attached:wireless active catheter visualization[J]. Magn. Reson.Med., Vol.53, pp.446-455,2005.
[16]Omary RA, Green JD, Schirf BE, et al. Real time magnetic resonance imaging-guided coronary catheterization in swine[J].Circulation, Vol.107, pp.2656-2659,2003.
[17]卢延,张雪哲.磁共振水成像[M].北京：科学出版社,2000.
[18]Vogl TJ, Muller PK, Mack MG, et al. Liver metastases:interventional therapeutic techniques and results, state of the art. Eur Radiol 1999; 9(4):675-84.
[19]Malone DE, Wyman DR, DeNardi FG, McGrath FP, De Gara C J, et al. Hepatic interstitial laser photocoagulation. An investigation of the relationship between acute thermal lesions and their sonographic images Invest Radiol 1994; 29 (10):915-21.
[20]Heisterkamp J, van Hillegersberg R, Ijzermans JN. Interstitial laser coagulation for hepatic tumours Br J Surg 1999; 86 (3):293-304.
[21]Solbiati L. New applications of ultrasonography:interventional ultrasound. Eur J Radiol 1998;27(Suppl.2):$200-6.
[22]Boetes C, Mus RD, Holland R, Barentsz JO, Strijk SP, et al. Breast tumors:comparative accuracy of MR imaging relative o mammography and US for demonstrating extent. Radiology 1995; 197(3):743-7.
[23]Vogl TJ, Mack MG, Muller PK, Straub R, Engelmann K, et al. Interventional MR: interstitial therapy. Eur Radiol 1999;9(8):1479-87.
[24]Hall-Craggs MA. Interventional MRI of the breast:minimally invasive therapy. Eur Radiol 2000; 10 (1):59-62.
[25]Le Bihan D, Delannoy J, Levin RL. Temperature mapping with MR imaging of molecular diffusion:application to hyperthermia. Radiology 1989; 171 (3):853-7.58
[26]Germain D, Chevallier P, Laurent A, Savart M, Wassef M. et al. MR monitoring of laser-induced lesions of the liver in a low field open magnet:temperature mapping and lesion size prediction. J Magn Reson Imaging 2001, in press.
[27]Abragam A. The principles of nuclear magnetism Oxford:Clarendon Press; 1983.
[28]Johnson F, Eyring H, Stover B. Theory of rate processes in biology and medicine. New York:John Wiley & Sons; 1974.
[29]Chen J, Daniel BL, Pauly KB. Investigation of proton density for measuring tissue temperature J Magn Reson Imaging 2006; 23:430-434.
[30]Parker DL. Applications of NMR imaging in hyperthermia:an evaluation of the potential for localized tissue heating and noninvasive temperature monitoring. IEEE Trans Biomed Eng1984;31:161-167.
[31]Parker DL, Smith V, Sheldon P, Crooks LE, Fussell L. Temperature distribution measurements in two-dimensional NMR imaging. Med Phys 1983; 10:321-325.
[32]Cline HE, Hynynen K, Schneider E, et al. Simultaneous magnetic resonance phase and magnitude temperature maps in muscle. Magn Reson Med 1996; 35:309-315.
[33]Graham SJ, Bronskill MJ, Henkelman RM. Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle. Magn Reson Med 1998; 39:198-203.
[34]Le Bihan D, Delannoy J, Levin RL. Temperature mapping with MR imaging of molecular diffusion:application to hyperthermia. Radiology 1989; 171:853-857.
[35]Bleier AR, Jolesz FA, Cohen MS, et al. Real-time magnetic resonance imaging of laser heat deposition in tissue. Magn Reson Med 1991; 21:132-137.
[36]Hindman JC. Proton resonance shift of water in gas and liquid states. J Chem Phys 1966; 44:4582-4592.
[37]Ishihara Y, Calderon A, Watanabe H, et al. A precise and fast temperature mapping using water proton chemical shift Magn Reson Med 1995; 34:814-823.
[38]De Poorter J, De Wagter C, De Deene Y, Thomsen C, Stahlberg F, Achten E. Noninvasive MRI thermometry with the proton resonance frequency (PRF) method:in vivo results in human muscle. Magn Reson Med 1995; 33:74-81.44.
[39]Kuroda K, Mulkern RV, Oshio K, et al. Temperature mapping using the water proton chemical shift:self-referenced method with echo-planar spectroscopic imaging. Magn Reson Med 2000; 43:220-225.
[40]Kuroda K. Non-invasive MR thermography using the water proton chemical shift. Int J Hyperthermia 2005; 21:547-560.
[41]McDannold N. Quantitative MRI-based temperature mapping based on the proton resonant frequency shift:review of validation studies. Int J Hyperthermia 2005;21:533-546.
[42]El-Sharkawy AM, Schar M, Bottomley PA, Atalar E. Monitoring and correcting spatio-temporal variations of the MR scanner's static magnetic field. MAGMA 2006; 19: 223-236.
[43]De Poorter J, De Wagter C, De Deene Y, Thomsen C, Stahlberg F, Achten E. The proton-resonance-frequency-shift method compared with molecular-diffusion for quantitative measurement of 2-dimensional time-dependent temperature distribution in a phantom. J Magn Reson Ser B 1994; 103:234-241. John De Poorter, Carlos De Wagter, Yves De Deene, Carsten Thomsen,Freddy StAhlberg,Eric Achten.
[44]Noninvasive MRI thermometry with the proton resonance frequency (PRF) method:in vivo results in human muscle. Magn. Reson. Med.33(1),74-81 (1995).
[45]Yiu-Cho Chung, MPhil, Jeffrey L. Duerk, PhD, Ajit Shankaranarayanan, BTech, et al. Temperature Measurement Using Echo-Shifted FLASH at Low Field for Interventional MRI. J Magn Reson Imaging 1999; 9:138-145.
[46]DONOHO D L. Compressed sensing[J]. Information Theory, IEEE Transactions on Informaion Theory,2006,52(4):1289-1306.
[47]OLSHAUSEN B A, FIELD D J. Emergence of simple-cell receptive field properties by learning a sparse code for natural images[J]. Nature,1996,381(6583):607-609.