背景抑制MR全身扩散成像的临床应用(类PET成像).ppt

背景抑制 MR全身扩散成像的临床应用 (类PET成像),DWIBS: Diffusion-weighted whole-body imaging with background body signal suppression,DWIBS: Diffusion-weighted whole-body imaging with background body signal suppression,PET原理,正电子核素或其标记化合物注入人体后，质子衰变为中子，同时发射出1个正电子+，正电子在组织中飞行极短的距离后便与周围组织内的电子相遇并发生湮没辐射，正、负电子消失，其物质转变为2个方向相反（互成180）、能量皆为511 keV的光子，穿透人体并被环绕人体的PET扫描仪探测到。,PET原理,PET扫描仪利用光子对的直线性和同时性两个特性来进行符合探测，当成对光子被2个互成180的探测器在符合时间窗宽（1020ns）内同时被探测到，便得到1个符合电脉冲（计算机记录为1次湮没辐射事件），计算机对这些符合电脉冲进行甄别、分析，然后通过图像重建处理，得到人体断层图像。,PET缺点,所有葡萄糖代谢旺盛的组织（如心肌、神经、肾脏和膀胱）都会有显像剂浓聚，因而特异性较低。 由于探测器密度低，PET空间分辨率要远低于CT和MR，需要图像融合才能对病变进行定位。 费用高，由于PET显像剂使用的是短半衰期核素，其成本非常高，加之PET扫描仪价格不菲，PET检查费用每例近万元。,DWI的基本原理,弥散是指分子的随机侧向运动，即布朗运动。DWI是在常规SE序列基础上，在180度聚焦射频脉冲前后各加上一个位置对称极性相反的梯度场。在梯度场作用下水分子弥散时其中的质子横向磁化发生相位分散，不能完全重聚，导致信号衰减，故形成了DWI上的异常信号。,扩散加权成像是通过一种特殊设计的序列，对于水的扩散（或弥散，布朗运动）非常敏感；扩散速度越快，则信号越弱；扩散速度越慢，则信号越强，因而在图像上形成对比。 自由水（如脑脊液、尿液）具有最快的扩散速度，而正常组织由于有细胞膜的限制，细胞内水的扩散受限，因而扩散速度慢于自由水。正常组织之间扩散速度有一定的差异，最突出的是神经组织，因为其独有的轴突结构使水的扩散速度明显慢于其它组织。,？,蛛网膜囊肿,表皮样囊肿,DWI作为一种功能成像技术已被广泛应用于脑部疾病诊断中，由于它能提供独特的水分子运动信息，近年来它在体部的应用也开始增多。而 WBDWI是最新的磁共振成像技术之一，全身扩散成像的研究中最具潜力的是背景信号抑制的全身扩散成像方法，简称DWIBS。 DWIBS在传统DWI序列的基础上，添加翻转恢复脉冲（IR）以抑制背景，通过分段扫描和拼接组合，再经图像黑白反转得到大范围的“类PET”效果。,Diffusion-weighted magnetic resonance imaging (DWI) provides functional information and can be used for the detection and characterization of pathologic processes, including malignant tumors; it may therefore be of value in staging and follow-up imaging of malignant tumors.,DWI using single-shot echo-planar imaging (EPI) is a well-established method to examine the brain. Extracranial DWI, however, did not become a clinical standard because the use of EPI was complicated by magnetic susceptibility artifacts and severe image distortion in the body.,In 2004, Takahara et al reported a unique concept of whole-body DWI, called “DWIBS”. This technique intentionally uses free breathing scanning rather than breathholding or respiratory triggering to visualize (moving) visceral organs and their lesions.,DWIBS在对人体信号如脂肪、肌肉、自由水等进行抑制的同时，保留了可疑病灶的信号，从而能对全身的可疑病灶进行筛选。 肿瘤组织由于失去接触抑制，致使细胞间隙变窄，水分子弥散受限，ADC值下降，在DWI上出现异常高信号。,一般一次采集240mm覆盖范围的体积。轴位采集结束后，提取出弥散图像，并用三维最大密度投影法重建获得矢状、冠状或三维图像并反色。 类PET成像技术可以抑制肌肉、脂肪、肝脏、肾脏产生的磁共振信号，突出病变区域的对比度。,病理情况：水的扩散受限,细胞水肿细胞水肿后，细胞间液减少，细胞内水增多，因而扩散受限的水增多，水的整体扩散速度慢于正常组织，主要见于脑急性梗死 恶性肿瘤恶性肿瘤细胞较正常细胞体积大，恶性肿瘤的细胞间液较正常少，二者结合，水的整体扩散速度慢于正常组织,DWIBS主要特征,Image acquisition during free breathing multiple signal averaging background body signal suppression by means of a fat suppression pre-pulse heavy diffusion weighting,healthy 60-year-old male volunteer Normal high signal intensity can be seen brain spinal cord nerves of the brachial plexus prostate testis penis spleen Cervical,axillary,pelvic and inguinal lymph nodes,healthy 53-year-old female volunteer Normal high signal intensity in the brain spinal cord nerves of the brachial and lumbosacral plexus vertebral bone marrow endometrium spleen Cervical,axillary,pelvic and inguinal lymph nodes,正常腰丛神经,Cancer is the second leading cause of death in developed countries, is among the three leading causes of death for adults in developing countries, and is responsible for 12.5% of all deaths worldwide.,Once a malignant tumor is detected, determination of disease extent (staging) is important for appropriate treatment planning and determining prognosis. Imaging plays a pivotal role in cancer staging. Furthermore, imaging is of great importance in monitoring response to therapy and in the detection of tumor recurrence.,临床上对肿瘤患者放、化疗前后的全身多部位、多器官受累情况，尤其是远处转移情况的了解还受到一些限制:如 CT辐射损伤及对比剂过敏风险、 B超空间分辨率较低、活检有创伤等，此外，这些检查均缺乏大范围的部位覆盖。 PET-CT作为一种全身检查方法，其临床应用受到经济状况、辐射损害及设备资源稀少等不利因素影响。,Staging,Primary tumors and distant metastases exhibiting restricted diffusion can be identified with DWIBS. DWIBS may especially be useful to detect relatively small lesions because of its high CNR. That evaluation of areas close to the heart and the diaphragm may be hampered in DWIBS because of signal loss and artifacts due to incoherent tissue motion. DWIBS may be a very useful adjunct to anatomical MRI or CT, where small lesions may be obscured or overlooked because of the large amount of image data, and lesions in normal-sized organs may be missed.,下端食管癌,Monitoring response to therapy,Another potential application of DWIBS is in the assessment of radiation and/or chemotherapy efficacy. CT and anatomical MRI are used for the follow-up of cancer patients. Serial measurements of tumor dimensions are made DWIBS can similarly be applied and its ability to obtain (automated) total volume measurements may especially be useful,a 44-year-old man with diffuse large B-cell lymphoma Coronal maximum intensity projection FDG-PET and DWIBS images before initiation of chemotherapy. FDG-PET and DWIBS images at the end of treatment show resolvement of all pre-existing lesions.,治疗前 治疗后,A limitation of DWIBS is that discrimination between normal and metastatic lymph nodes is still based on size criteria (箭头1); the FDG-PET positive left cervical lymph node cannot conclusively be identified as malignant on DWIBS (arrowhead 1). DWIBS also shows prominent bilateral inguinal lymph nodes (encircled) which are normal according to FDG-PET. On the other hand, thanks to its higher spatial resolution (虚线箭头), DWIBS visualizes two separate cardiac lesions whereas FDG-PET shows only one large cardiac lesion (dashed arrow). DWIBS also allows better evaluation of the urinary tract (箭头2) than FDG-PET, where potential lesions can be obscured because of FDG accumulation. Note the physiological FDG uptake in the large intestine (箭), which should not be confused with persistent malignant lymphoma.,Detection of tumor persistence or recurrence,Differentiating persistent or recurrent tumor tissue from posttherapeutic change is important for correct patient management, but is often difficult with CT or anatomical MRI. Functional imaging techniques, such as PET may allow better differentiation. The less expensive DWIBS may also be of value. It can be hypothesized that persistent or recurrent tumor tissue will show a more restricted diffusion than treatment-related changes, mainly because of higher cellular density.,recurrence of rectal cancer,Drawbacks and limitations of DWIBS,First, DWIBS does not exclusively visualize malignant tumors. Benign pathologies with restricted diffusion, such as abscesses, will also exhibit high signal on DWIBS source images. Furthermore, DWIBS not only visualizes pathological areas of restricted diffusion, but also several normal structures：脑、唾液腺、扁桃体、脾脏、胆囊、小肠、大肠、肾上腺、前列腺、睾丸、阴茎、子宫、卵巢、脊髓、周围神经、淋巴结、骨髓。,A 78-year-old man with metastatic lung cancer and two benign lesions lung cancer (arrow) ipsilateral hilar lymph node metastasis (1) retroperitoneal/adrenal metastasis (2 ) vertebral metastasis (3) thoracic vertebra cavernous hemangioma(A) epigastric region sebaceous cyst (B ),Drawbacks and limitations of DWIBS,DWIBS suppression of background body signals, as a result of which sufficient anatomical information is lacking. DWIBS and anatomical MRI can be viewed side-by side, or fused with commercially available software. DWIBS can be post-processed to create MIPs and PET-like images, which are very attractive for demonstration purposes, but source images should always b