Perfusion Methods 灌注方法

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网站： https://www.mriquestions.com/ 

原著：Allen D. Elster, MD 

译注：蒋强盛 

How is perfusion defined and measured? 灌注是如何定义与测量的？

       Perfusion refers to the rate of blood flow through the capillary circulation of an organ or tissue. To account for different sized organs, perfusion is commonly normalized as bulk blood flow rate (mL/min) divided by organ mass (per 100 g). Perfusion is thus typically reported in units of mL/min/100g tissue.  

       灌注 指的是血流通过某一器官或组织毛细血管循环的速率。考虑到不同大小的器官，灌注通常归一化为容积血流量（mL/min）除以器官质量（每 100g）。因此灌注的单位通常为  mL/min/100g 组织 。 

       Perfusion differs among organs and may change in response to pathology or metabolic needs. For example, the perfusion of cerebral gray and white matter are about 60 and 20 mL/min/100g respectively. Myocardial perfusion is much greater, ranging between about 100 mL/min/100g at rest to 400 mL/min/100g during maximal exercise.  不同器官的灌注不同，并可能因病理或代谢的改变而改变。例如，脑灰白质的灌注分别约为 60 和 20 mL/min/100g。心肌的灌注更高，静息时约为 100 mL/min/100g，最大运动量时能达到 400 mL/min/100g。 Measurement of perfusion requires the use of tracers --   molecules, molecular aggregates, or small particles that distribute in tissues commensurate with blood flow and can be detected. Some remain in the intravascular space, but others have a wider distribution. 

       灌注的测量需要使用能够通过血流分布于组织中，并且能被探测到的示踪剂——分子，分子聚合物或小颗粒。其中一些种类的示踪剂只能驻留在血管腔内，而其他一些种类的示踪剂可以有更广泛的分布。 

1. Intravascular tracers remain confined to blood vessels. MR examples include the (now out-of-production) contrast agent gadofosveset (Ablavar®) which bound to serum albumin and ultra-small superparamagnetic iron oxide (USPIO) particles.

1. 血管内示踪剂 局限存留于血管内。MR 血管内示踪剂包括与血清白蛋白相结合的 gadofosveset (Ablavar®) 对比剂（现已停产）和 超小超顺磁性氧化铁（USPIO）颗粒 。

2. 2. Extracellular tracers do not enter cells but freely pass through vessel walls to distribute in the extracellular spaces of tissues. Most gadolinium-based contrast agents are in this category.

3. 细胞外示踪剂 不进入细胞内，但能自由通过血管壁分布于组织的细胞外间隙。大多数钆对比剂就属于此类。

4. 3. Diffusible tracers distribute throughout all tissue compartments including the interior of cells. A common example in MRI is magnetically tagged water molecules used for arterial spin labelling (ASL) .

5. 扩散示踪剂 分布于所有组织内，包括细胞内。MRI 中一个常见的例子是磁化标记的水分子用于 动脉自旋标记（ASL） 成像。

       Arterial spin labelling (ASL) perfusion map of brain with hypervascular glioma Advanced Discussion (show/hide)» Intravascular Tracers Although no method is perfect, the traditional laboratory "gold standard" for measuring tissue perfusion uses ~15 μm diameter microspheres injected intra-arterially. These particles are trapped by capillary beds in proportion to regional blood flow. Anatomic specimens are then harvested, and the particles visually counted by light or electron microscopy. Microspheres can also be radioactively or fluorescently labelled allowing semiautomated counting methods to be used.

       ~15 μm diameter injectable microspheres used for laboratory perfusion measurements Because they require arterial catheterization and tissue harvesting for measurement, laboratory microspheres are not practical for routine clinical perfusion measurements. However, several particulate agents detectable by noninvasive radiologic means have been developed for clinical use. The most commonly recognized ones include radiolabeled blood cells and macroaggregated albumin (for nuclear medicine) and encapsulated microbubbles (for ultrasound). No truly particulate agent is commercially available for MR perfusion imaging, but a close analog might be  gadofosveset (Ablavar®) , which binds to serum albumin and remains largely confined to the intravascular space for several hours after injection. Ultra -small superparamagnetic iron oxide (USPIO) particles have been used "off-label" in MR for this purpose as well. Extracellular Tracers Iodine- and gadolinium-based contrast agents are widely used as tracers to measure perfusion by CT and MRI respectively. These low molecular weight (500-1000 Da) compounds initially distribute in plasma after injection but then rapidly diffuse into the extracellular spaces of most tissues. The only exception is within the central nervous system where tight endothelial junctions along the blood-brain-barrier preclude extracellular passage into normal brain and spinal cord. Although some newer specialized MR contrast agents (e.g. gadoxetate/Eovist®) have hepatic uptake, most do not enter normal cells and remain confined to the extracellular space.

       Cine images from 1st pass myocardial perfusion study using a saturation recovery spoiled-GRE method Diffusible Tracers Freely diffusible tracers are radioactively or magnetically labeled ions and small molecules that easily move between intracellular and extracellular compartments. These are well known in nuclear medicine/PET and include 15O-labelled O 2 , H 2 O, and CO 2 ;  13 NH 3 ; 82 Rb; and  201 Tl. Inhaled Xe gas, another diffusible tracer, achieved minor interest for CT perfusion imaging in the 1990's but has now largely been abandoned. In MRI magnetically tagged water molecules are widely used as diffusible tracers, forming the basis of the arterial spin labelling (ASL) technique.

       PET using 15O-water is generally considered the noninvasive gold standard for measuring perfusion in most organs. However, it is a cumbersome technique requiring on-line tracer production and delivery to the patient with continuous arterial blood sampling. And it is not perfect, being subject to partial volume effects especially around small, convoluted brain structures. Compared to microspheres, 15O-water PET tends to underestimate perfusion at high flow rates and overestimate perfusion at low flow rates.

15O-water PET is considered to be the most accurate method for measuring blood flow noninvasively.

       Relative few side-by-side comparisons of 15O-water PET and MR perfusion using dynamic susceptibilty contrast (DSC) or ASL exist. Studies that have been performed do demonstrate MR perfusion methods provide reproducible and reliable qualitative information about blood flow. Accurate quantitative measurements are possible but challenging, being dependent on the MR method (DSC v ASL) and sophistication of the mathematical model used.

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