1、背景抑制MR全身擴散成像的臨床應用 (類PET成像)DWIBS:Diffusion-weighted whole-body imaging with background body signal suppressionDWIBS:Diffusion-weighted whole-body imaging with background body signal suppressionPET原理正電子核素或其標記化合物注入人體后，質子衰變?yōu)橹凶樱瑫r發(fā)射出1個正電子+，正電子在組織中飛行極短的距離后便與周圍組織內的電子相遇并發(fā)生湮沒輻射，正、負電子消失，其物質轉變?yōu)?個方向相反（互成180）、能量

2、皆為511 keV的光子，穿透人體并被環(huán)繞人體的PET掃描儀探測到。PET原理PET掃描儀利用光子對的直線性和同時性兩個特性來進行符合探測，當成對光子被2個互成180的探測器在符合時間窗寬（1020ns）內同時被探測到，便得到1個符合電脈沖（計算機記錄為1次湮沒輻射事件），計算機對這些符合電脈沖進行甄別、分析，然后通過圖像重建處理，得到人體斷層圖像。PET缺點所有葡萄糖代謝旺盛的組織（如心肌、神經、腎臟和膀胱）都會有顯像劑濃聚，因而特異性較低。由于探測器密度低，PET空間分辨率要遠低于CT和MR，需要圖像融合才能對病變進行定位。費用高，由于PET顯像劑使用的是短半衰期核素，其成本非常高，加之P

3、ET掃描儀價格不菲，PET檢查費用每例近萬元。DWI的基本原理彌散是指分子的隨機側向運動，即布朗運動。DWI是在常規(guī)SE序列基礎上，在180度聚焦射頻脈沖前后各加上一個位置對稱極性相反的梯度場。在梯度場作用下水分子彌散時其中的質子橫向磁化發(fā)生相位分散，不能完全重聚，導致信號衰減，故形成了DWI上的異常信號。擴散加權成像是通過一種特殊設計的序列，對于水的擴散（或彌散，布朗運動）非常敏感；擴散速度越快，則信號越弱；擴散速度越慢，則信號越強，因而在圖像上形成對比。自由水（如腦脊液、尿液）具有最快的擴散速度，而正常組織由于有細胞膜的限制，細胞內水的擴散受限，因而擴散速度慢于自由水。正常組織之間擴散速度

4、有一定的差異，最突出的是神經組織，因為其獨有的軸突結構使水的擴散速度明顯慢于其它組織。？蛛網膜囊腫表皮樣囊腫DWI作為一種功能成像技術已被廣泛應用于腦部疾病診斷中，由于它能提供獨特的水分子運動信息，近年來它在體部的應用也開始增多。而 WBDWI是最新的磁共振成像技術之一，全身擴散成像的研究中最具潛力的是背景信號抑制的全身擴散成像方法，簡稱DWIBS。DWIBS在傳統DWI序列的基礎上，添加翻轉恢復脈沖（IR）以抑制背景，通過分段掃描和拼接組合，再經圖像黑白反轉得到大范圍的“類PET”效果。Diffusion-weighted magnetic resonance imaging (DWI) p

5、rovides 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 t

6、o 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 i

7、ntentionally uses free breathing scanning rather than breathholding or respiratory triggering to visualize (moving) visceral organs and their lesions. DWIBS在對人體信號如脂肪、肌肉、自由水等進行抑制的同時，保留了可疑病灶的信號，從而能對全身的可疑病灶進行篩選。腫瘤組織由于失去接觸抑制，致使細胞間隙變窄，水分子彌散受限，ADC值下降，在DWI上出現異常高信號。一般一次采集240mm覆蓋范圍的體積。軸位采集結束后，提取出彌散圖像，并用三維最大密

8、度投影法重建獲得矢狀、冠狀或三維圖像并反色。類PET成像技術可以抑制肌肉、脂肪、肝臟、腎臟產生的磁共振信號，突出病變區(qū)域的對比度。病理情況：水的擴散受限細胞水腫細胞水腫后，細胞間液減少，細胞內水增多，因而擴散受限的水增多，水的整體擴散速度慢于正常組織，主要見于腦急性梗死惡性腫瘤惡性腫瘤細胞較正常細胞體積大，惡性腫瘤的細胞間液較正常少，二者結合，水的整體擴散速度慢于正常組織DWIBS主要特征Image acquisition during free breathingmultiple signal averagingbackground body signal suppression by me

9、ans of a fat suppression pre-pulse heavy diffusion weightinghealthy 60-year-old male volunteer Normal high signal intensity can be seenbrainspinal cordnerves of the brachial plexusprostatetestispenisspleenCervical,axillary,pelvic and inguinal lymph nodes healthy 53-year-old female volunteerNormal hi

10、gh signal intensity in thebrainspinal cordnerves of the brachial and lumbosacral plexusvertebral bone marrowendometriumspleenCervical,axillary,pelvic and inguinal lymph nodes正常腰叢神經Cancer is the second leading cause of death in developed countries, is among the three leading causes of death for adult

11、s 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, imagi

12、ng is of great importance in monitoring response to therapy and in the detection of tumor recurrence. 臨床上對腫瘤患者放、化療前后的全身多部位、多器官受累情況，尤其是遠處轉移情況的了解還受到一些限制:如 CT輻射損傷及對比劑過敏風險、 B超空間分辨率較低、活檢有創(chuàng)傷等，此外，這些檢查均缺乏大范圍的部位覆蓋。PET-CT作為一種全身檢查方法，其臨床應用受到經濟狀況、輻射損害及設備資源稀少等不利因素影響。StagingPrimary tumors and distant metastases ex

13、hibiting 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

14、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 therapyAnother potential application of DWIBS is in the assess

15、ment 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 madeDWIBS can similarly be applied and its ability to obtain (automated) total volume measurements may especially be useful a 44-year-old m

16、an with diffuse large B-cell lymphomaCoronal 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. 治療前 治療后A limitation of DWIBS is that discrimination between normal and

17、metastatic lymph nodes is still based on size criteria (箭頭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

18、hand, thanks to its higher spatial resolution (虛線箭頭), 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 (箭頭2) than FDG-PET, where potential lesions can be obscured because of FDG

19、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 recurrenceDifferentiating persistent or recurrent tumor tissue from posttherapeutic change is important for correct patient man

20、agement, 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

21、-related changes, mainly because of higher cellular density. recurrence of rectal cancerDrawbacks 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 imag

22、es. Furthermore, DWIBS not only visualizes pathological areas of restricted diffusion, but also several normal structures：腦、唾液腺、扁桃體、脾臟、膽囊、小腸、大腸、腎上腺、前列腺、睪丸、陰莖、子宮、卵巢、脊髓、周圍神經、淋巴結、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