https://scholars.lib.ntu.edu.tw/handle/123456789/144427
標題: | 運用磁振造影技術測定大腦氧氣被利用率 Measurement of Cerebral Oxygen Extraction Fraction Using Magnetic Resonance Imaging Technique |
作者: | 莫元亨 Mo, Yuan-Heng |
關鍵字: | 磁振造影;大腦;氧氣被利用率;MRI;brain;oxygen extraction fraction | 公開日期: | 2005 | 摘要: | 腦血管疾病長年以來高居十大死亡原因的第二名,僅次於癌症。也是導致成人殘障的主要原因,造成社會成本相當大的負擔。腦血管疾病依其成因又可以區分為缺血性中風和出血性中風。 缺血時大腦的血流動力學會發生許多改變:當大腦灌流壓力(CPP)逐漸減少時,腦中的小動脈會擴張,使腦血管阻力(CVR)減小,以維持有效的大腦血液流速(CBF)。但當CPP的減少超過自我調控代償的能力範圍之後,血中氧氣被利用率(OEF)會逐漸提升。如果OEF上升到100%之後,CPP仍然繼續下降,大腦的功能將因為能量不足而出現異常。如果CPP降低的情形仍然持續或更加惡化,終將造成永久性的腦組織傷害。如果在缺血性中風病程初期,可以區別出哪個範圍的腦組織受損但仍然存活、哪個範圍的腦組織已經死亡,相信就可以在合適的病人身上早期地給予適當的治療。OEF的增加,就是一個重要的生理指標。OEF增加的區域,代表了氧氣需求量大於減少血流所能供應的,也意味著這些細胞是具有生理活性的。 目前降低體溫已經被當作是一種治療的方法,可被用在像是重大手術進行當中來減低器官術後的併發症、大腦創傷、或是缺血性中風的患者身上。其所帶來有利的治療效果,是因為會減少組織氧氣的代謝需求,使得組織可以忍受更長時間的無氧代謝狀態。另外的原因則包括:可以減少腦內壓力的上升,維持粒線體的功能,抑制興奮性神經傳導物質像是麩胺酸或甘胺酸的釋放。但過去文獻關於降低體溫究竟會對OEF帶來什麼樣的影響,並沒有明確的結論。 目前公認可以測量CBF、大腦氧氣代謝率(CMRO2),甚至於OEF的影像工具是正子發射斷層掃描(PET)。但此檢查的缺點包括:因為設備和放射源取得不易而不容易執行;耗費時間;空間解像力不高;具有放射性。過去十多年來,隨著電腦技術的進步和超磁導研究的突破,磁振造影(MRI)已經被廣泛運用在中風這個疾病的診斷和預後治療追蹤上,成為重要的影像工具。除了可以從傳統的脈衝序列,如T1WI和T2 WI得到大腦解剖構造的訊息,更可使用DWI和PWI之間異常範圍的不一致性來做到預測缺血半陰影區域的目的,也已被使用在新治療方法臨床試驗中,作為篩選病人的方法。近年來開始有人利用對偵測去氧血紅素含量有敏感性的新脈衝序列-BOLD對比技術,企圖來測量大腦的血氧飽和度,甚至是OEF。 此研究希望運用此對磁場變化敏感的特殊MRI脈衝序列,以兔子為動物模型,測量降低體溫前後之真實生理數值,並與MRI所測得降低體溫前後兔子大腦的OEF相比較,來證明兩者之間的相關性。研究的結果顯示,此MRI脈衝技術所測得的兔子大腦OEF,在降溫之後,有統計顯著的下降,符合降溫會使大腦氧氣被利用率減低的觀念。降溫之後,頸動靜脈中二氧化碳分壓的差值也會隨之減小,亦間接證明氧氣代謝率的下降。而以此MRI脈衝技術所測得年輕正常志願受試者的大腦氧氣利用率,與文獻值相似。 Cerebrovascular disease is the second most common cause of death among adults in Taiwan, just behind the cancer for many years. It also contributes to most case of morbidity and brings a great burden to society. According its etiology, cerebrovascular disease can be subcategorized as ischemic and hemorrhagic stroke. Many hemodynamic changes will consequently happens while ischemia. As cerebral perfusion pressure falls, cerebral blood flow (CBF) is initially maintained by arteriolar dilation. When vasodilatory capacity has been exceeded, cerebral autoregulation fails, and CBF begins to decrease rapidly. A progressive increase in oxygen extraction fraction (OEF) preserves cerebral oxygen metabolism. When the increase in OEF is no longer able to maintain the energy needs of the brain, cortical dysfunction may be observed. If there is no improvement in perfusion, permanent injury is the destiny. If we can differentiate irreversibly damaged tissue from functionally impaired but viable tissue, beneficial treatment for select patients may be possible. Increase in OEF seems to be a good physiologic indicator for possible area of ischemic penumbra. Hypothermia is regarded as a therapy to protect brain tissue during surgery and appears to improve the clinical course of patients with acute ischemic stroke and traumatic brain injury. The beneficial effects owing to reduced aerobic demand and decline of cerebral metabolic rate of oxygen make tissue more tolerable to ischemia. Other protective mechanisms include reduce the increased intracranial pressure, maintenance of mitochondrial function, inhibition of neuroexcitatory transmitter release. However, the relationship between hypothermia and OEF is unclear. Positron emission tomography (PET) is the imaging modality of choice for measurement of CBF, CMRO2 and even OEF. But it has great disadvantage in its availability, time-consuming, poor spatial resolution and radiation. During the last 10 years, major advances in Magnetic Resonance Imaging (MRI) have made it possible to visualize early changes in patients suffering from ischemic stroke. The combination of DWI and PWI has promising future in identifying ischemic areas potentially salvageable. Recently, a novel MRI sequence based on the BOLD contrast is used to assess the extent of deoxygenation in ischemic tissue and to derive the OEF. Our preliminary results show that significantly decreased cerebral oxygen extraction fraction and CO2 production of rabbits after hypothermia and normal volunteers have similar OEF value as previous reports using noninvasive MR technique. We believe this technique provide a possible immediate imaging modality to monitor the physiological change in study of ischemic stroke. |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/55472 | 其他識別: | zh-TW |
顯示於: | 臨床醫學研究所 |
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ntu-94-P92421015-1.pdf | 23.31 kB | Adobe PDF | 檢視/開啟 |
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