To put it simply, cerebral perfusion is *not* the same as "cerebral [blood] circulation" – the latter is not a physiologically viable mechanism. The brain must maintain a very stable internal pressure level (for neuronal survival), and thus it needs to be shielded off from the rest of the circulatory system. No blood vessels (including capillaries) can enter the brain itself. The circulatory functions are instead taken over by the ventricular system (ventricles and cisterns filled with CSF – cerebrospinal fluid) inside the brain. Nutrients and wastes seep or get transported across the blood-brain barrier (see the last reference link) which roughly corresponds to the meninges (membranes wrapping up the brain).

“Cerebral perfusion” (CP) is typically used in the context of “cerebral perfusion pressure” (see definition in 2nd ref. source).

Perfusion itself means “blood flow” (as in first ref. source) here. When CP pressure drops too low, exchange of nutrients and wastes at the blood-brain barrier cannot be sustained, potentially leading to risks of “cerebral ischemia" (insufficient blood supply to the brain – still, this is only an indirect supply).

FYI:

http://www.heartsurgery-usa.com/glossary_c.html
Cerebral Perfusion
Blood flow to the brain. Cerebral perfusion is directly related to mean arterial pressure and inversely related to cerebral vascular resistance.

Cerebral perfusion pressure, or CPP, is the net pressure of blood flow to the brain. It must be maintained within narrow limits because too little presure could cause brain tissue to become ischemic (having inadequate blood flow), and too much could raise intracranial pressure (ICP).
CPP can be defined as: CPP = MAP − ICP
CPP is regulated by two balanced, opposing forces: Mean arterial pressure, or MAP, the arithmetic mean of the body's blood pressure, is the force that pushes blood into the brain, and intracranial pressure is the force that keeps it out. Thus raising MAP raises CPP and raising ICP lowers it (this is one reason that increasing ICP in traumatic brain injury is potentially deadly). CPP, or MAP minus ICP, is normally between 70 and 90 mmHg in an adult human…

[PDF] Perfusion computed tomography:stroke applications
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Glossary. Perfusion. The steady-state delivery of blood to cerebral tissue through the capillaries (normal. cerebral perfusion = 50–60 ml/100g/min). ...
www.medical.philips.com/main/news/ assets/docs/medicamundi/mm_vol46_no1/46-1i.pdf - Similar pages


Blood-brain barrier (meninges):

The blood-brain barrier (BBB) is a membrane that controls the passage of substances from the blood into the central nervous system. It is a physical barrier between the local blood vessels and most parts of the central nervous system itself, and stops many substances from travelling across it. The BBB is permeable to alcohol, and some heavy metals can cross the blood-brain barrier as well.

http://en.wikipedia.org/wiki/Pia_mater


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Note added at 3 hrs (2006-06-15 15:54:18 GMT)
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correction: Sorry, the cisterns are located "around the brain" (not within)

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Note added at 5 hrs (2006-06-15 17:59:12 GMT)
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An extra link for your reference:

Cerebral circulation - Wikipedia, the free encyclopediaCerebral circulation refers to the blood vessels in the brain. Arteries carry blood to the brain, and veins carry it away, back to the heart. ...
en.wikipedia.org/wiki/Cerebral_circulation - 15k - Cached - Similar pages


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Note added at 5 hrs (2006-06-15 18:01:08 GMT)
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The correct wording for the above source should in fact be "cerebral circulation refers to... vascular system *around* the brain"