ASPECTS evaluation on unenhanced CT
The ASPECTS score is used for quantitative assessment of early ischemic changes in the middle cerebral artery (MCA / СМА) territory on non-contrast CT (NECT). The advantages of this method are its high speed, simplicity, and safety, which are critical within a narrow therapeutic window.
Two axial brain slices are analyzed for evaluation:
- At the level of the basal ganglia and internal capsule.
- At the level of the superior margin of the lateral ventricle bodies.
There are 10 anatomical regions. Of these, 4 are deep: the caudate nucleus (C), lentiform nucleus (L), internal capsule (IC), and insula (I). The remaining 6 regions are cortical areas in the MCA territory: M1, M2, and M3 (at the ganglionic level) and M4, M5, and M6 (above the ganglionic level).
Initially, a normal CT scan is scored as 10 points. One point is subtracted for each zone showing signs of ischemia. If the final score is less than 7 points, the infarct is classified as extensive (involving more than 1/3 of the MCA territory).
Penumbra concept and mismatch visualization
The main objective of advanced imaging (CT perfusion or MRI) is to differentiate between irreversibly damaged tissue (infarct core, core) and surrounding ischemic yet still viable tissue (penumbra, penumbra).
Depending on the ratio of these zones, several patterns are distinguished:
- Mismatch (mismatch): The volume of the penumbra exceeds the size of the core by at least 20%. In this case, the patient will benefit significantly from recanalization of the occluded vessel.
- Matched defect (matched defect): A large infarct core is present without a significant penumbra. The benefit of reperfusion is questionable.
- Benign oligemia: A zone of very mild ischemia that is capable of spontaneous recovery without active treatment.
An extensive infarct core (>70 ml) is associated with a poor clinical prognosis even with successful blood flow restoration. However, identifying the penumbra significantly extends the therapy window: beyond 4.5 hours for thrombolysis and up to 24 hours for mechanical thrombectomy (proven in the DAWN and DEFUSE-3 trials).
Key perfusion imaging parameters
Perfusion data are obtained by deconvolution (mathematical analysis) of time-density curves during the passage of an intravenous contrast bolus. Three classical parameters are distinguished:
- rCBV (relative cerebral blood volume) — relative cerebral blood volume.
- rCBF (relative cerebral blood flow) — relative cerebral blood flow (rate).
- MTT (mean transit time) — mean transit time of blood through the tissue.
Additional metrics are used to identify critically hypoperfused tissue:
- Tmax (time to maximum of the residue function). A value greater than 6 seconds is considered the most accurate marker of a penumbra that will inevitably progress to infarction without treatment. However, the method is sensitive to the arterial input function (AIF).
- TTP (time to peak). Does not require a deconvolution algorithm. With proper normalization, values in the 3–5 second range reliably indicate an ischemic penumbra.
How to differentiate core from penumbra: threshold values
Accurate determination of lesion volumes depends on the chosen imaging method. PET is considered the gold standard for core identification, but in emergency situations, MRI or CT is used (in descending order of accuracy: DWI $\rightarrow$ CT-CBF $\rightarrow$ CT-CBV).
| Zone | CT perfusion (CTP) | MR perfusion (PWI / MRI) |
|---|---|---|
| Infarct core | CBF reduction by 70–85% | Diffusion restriction on DWI |
| Penumbra | Tmax delay > 6 s | Threshold values for MTT, CBF, TTP |
Note: The CBV parameter has limitations. In experiments, it rarely decreases in the infarct zone, and with rapid reperfusion, the true volume of dead tissue may be masked on CBV maps and appear normal.
Briefly: answers to topical questions
Which middle cerebral artery branches supply the deep structures assessed by the ASPECTS score?
Deep brain structures are supplied by deep perforating arteries, which include lenticulostriate perforators originating from the M1 segment of the middle cerebral artery. The ASPECTS score defines 4 deep regions:
- Caudate nucleus (caudate nucleus) — evaluated at the level of the basal ganglia (designated as C).
- Lentiform nucleus (L) — evaluated at the level of the basal ganglia.
- Internal capsule (internal capsule) — connects the globus pallidus and the caudate nucleus (designated as IC).
- Insula (I) — evaluated at the level of the basal ganglia.
The mathematical relationship of basic perfusion parameters is expressed by a simple formula: MTT = CBV / CBF. Mean transit time is the blood volume divided by the cerebral blood flow velocity.
Class and exam questions
What is a "matched defect" on perfusion maps?
This is a pattern in which the size of the irreversible infarct core matches the hypoperfusion zone. There is virtually no penumbra, so the benefit of reperfusion therapy is minimal.
Which imaging parameter most accurately reflects the ischemic penumbra?
Currently, a Tmax delay of more than 6 seconds is considered the most reliable biomarker of the penumbra on CT perfusion.
Why is the CBV parameter not always reliable for assessing the infarct core?
The volume of infarcted tissue may be masked and appear normal on CBV maps under conditions of rapid reperfusion. Furthermore, it has been experimentally proven that CBV rarely decreases within the core.
What else to study on this topic
- Diffusion restriction on MRI: combined analysis of DWI and ADC maps
- Assessment of leptomeningeal collateral blood flow using CTA
- Differential diagnosis of edema and perfusion abnormalities on unenhanced CT
- Indications and therapeutic window for mechanical thrombectomy (up to 24 hours)
- The phenomenon of reversibility of changes on diffusion-weighted MRI (DWI)
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