Basic knowledge of CT perfusion of cerebral ischemia

Perfusion is an important function
of blood flow through the capillary network to transport carried oxygen and nutrients to tissue cells.
Perfusion imaging is an imaging method
based on flow effects.

CT perfusion imaging (CTP) refers to the simultaneous injection of intravenous contrast medium, through multiple consecutive isolayer scans of the selected level to obtain the time-density curve (TDC) curve of each pixel of the plane, which reflects the change of the concentration of contrast agent in the organ and the change of
the perfusion amount of indirect anti-woven organ.

According to the curve, different mathematical models are used to calculate blood flow (BF), blood volume (BV), mean transit time (MTT), and transit time to the peak (TTP), and the above parameters are reconstructed and pseudo-colored staining to obtain the above parameter graphs
.

CTP parameter definition:
cerebral blood flow (CBF): in milliliters of blood flow per minute of brain tissue [ml/(100g.
min)].


Cerebral blood volume (CBV): blood content per 100g of brain tissue [ml/100g].


Peak time (TTP): The time from when the contrast agent reaches the major arteries in the imaging brain area to the maximum amount of contrast agent (s).


Average Passage Time (MTT): The time it takes for the contrast agent to travel from the arterial side of the intracranial to the venous side, the average of all passage times (s).

Normal values for CBF and CBV:
Normal cerebral blood flow is automatically regulated and fluctuates
in a narrow range.
In cerebral ischemia, CBV decreases or rises, depending on the brain's ability to
automatically regulate.

CBF: human gray matter is about: 80ml/(100g.
min); white matter is: 20ml/(100g.
min); the total CBF and average CBF of the cortical area, that is, gray and white matter each account for half of the brain area, is about: 50ml/(100g.
min).


CBV: is the amount of blood in a certain amount of brain tissue, about 4-5ml/100g of tissue
.

When CBF drops below 35ml/(100g.
min) tissue, protein synthesis in neurons stops; when it drops to 20ml/(100g.
min) tissue, neuronal function is lost
.

Interpretation of CTP data: CBF:
Compared with the CBF value of the corresponding non-ischemic side brain area, the CBF decrease of more than 60% in the normal corresponding area can accurately diagnose the cerebral ischemic area
.

CBV: CBV< 2.
5ml/100g indicates cerebral infarction; A drop in CBV of more than 60% can confirm cerebral ischemia<b13>.

MTT: in the ischemic brain region, MTT is prolonged (the area of decreased perfusion is defined as MTT>6s, and the mean MTT of normal MCA is 3.
6s).


TTP: normal tissue, TTP<8s because there is undisturbed forward blood flow; In the ischemic zone, TTP is prolonged, reflecting collateral perfusion; TTP >8s in areas of poor perfusion, indicating suspicious ischemia; TTP plots can have false-positive results
.

Both MTT and TTP plots can be used to confirm cerebral ischemia
.
MTT plots are superior to CBF plots and CBV plots; TTP and MTT are more consistent in showing normal brain tissue, making it easier to identify areas of
haemorrhagic fluid dysplasia.

Staging of cerebral ischemia: From the perspective of CBF change process, the decline of cerebral blood flow to the occurrence of acute cerebral infarction has experienced three change periods:
first, it is due to abnormal local hemodynamic changes in the brain caused by the decrease in cerebral perfusion pressure; Secondly, neuronal function changes caused by cerebral circulation reserve (cerebral circulation reserve) decompensated hypoperfusion; Finally, irreversible neuronal morphological changes, known as cerebral infarction
, occur due to the decline in CBF exceeding the metabolic reserve capacity of the brain.

Some scholars refer to the first two periods as pre-cerebral
infarction.
In stage I in the pre-cerebral infarction period, due to the role of CCR, the patient has almost no obvious clinical symptoms; In phase II, local cerebral blood flow (rCBF) decreases and CCR is decompensated
.
This state can last for several years, with clinical symptoms such as headache, weak limbs, slight shaking of the limbs and poor speech, and transient ischemic attack (TIA)
in severe cases.

Pre-infarction staging: According to the degree of change of local microcirculation of the brain and the performance of dynamic CT perfusion imaging, the pre-cerebral infarction stage is divided into 2 stages and 4 subtypes: stage I.
:


When abnormal changes in cerebral hemodynamics occur and the perfusion pressure of cerebral blood flow fluctuates within a certain range, the body can maintain the relative dynamic stability
of cerebral blood flow through compensatory expansion or contraction of arterioles and capillary smooth muscle.

Stage I.
1: The cerebral blood flow velocity changes, and there is no compensatory expansion
of local microvessels in the brain.
Perfusion imaging showed prolonged time to reach bee (TTP), and mean time to pass (MTT), rCBF, and local cerebral blood volume (rCBV) were normal
.

Stage I.
2: Compensatory expansion
of local microvessels in the brain.
Perfusion imaging shows prolonged TTP and MTT, mild decrease in rCBF, and elevated
rCBV.


Stage II.
: cerebral circulatory reserve decompensation, abnormal neuronal function, the body through brain metabolic reserve to maintain the stability
of neuronal metabolism.

Stage II.
1: CBF decreases, swells local brain cells due to ischemia, and begins to compress local microvessels
.
Perfusion imaging shows TTP, MTT prolongation, and decreased rCBF, with rCBV basically normal or mildly reduced
.

Stage II.
2: Brain cells are significantly swollen and cause compression and narrowing or occlusion of local microvessels in the brain, and local microcirculation disorders
.
Perfusion imaging shows TTP, MTT prolongation, and decreased
rCBF and rCBV.

Clinical application: insufficient perfusion:
MTT, TTP significantly prolonged, rCBF significantly reduced, rCBV decreased
.

Collateral circulation formation: MTT, TTP prolonged, rCBF increased or acceptable
.

Blood flow reperfusion: shortened or normal MTT, TTP shortened or normal, normal or mildly increased rCBF, increased
rCBV.

Overperfusion: rCBF and rCBV were significantly increased
.

Case 1:

Case 2:

Case 3:

Case 4:

Case 5: