An acute ischemic stroke, caused by a blood clot blocking an artery to the brain, is a time-sensitive medical emergency. To make effective treatment decisions, neurologists use advanced imaging like the CT perfusion scan. This method goes beyond a standard CT by measuring blood flow dynamics throughout the brain, which helps clinicians visualize the extent of the stroke’s impact.
From this data, a value known as the mismatch ratio is calculated. This ratio compares damaged brain tissue to tissue that is at risk but potentially salvageable, giving medical teams a snapshot of the stroke’s evolution. Understanding this measurement is important for appreciating how modern stroke care is personalized to a patient’s condition, guiding interventions that can affect outcomes.
Identifying Salvageable and Damaged Brain Tissue
A CT perfusion scan helps differentiate between two zones in the affected brain area: the infarct core and the ischemic penumbra. The infarct core is brain tissue that has suffered irreversible damage due to a severe lack of blood flow. The cells in this central zone have died, and this tissue cannot be recovered.
Surrounding the core is the ischemic penumbra. This region has reduced blood flow, causing its cells to be functionally impaired but not yet dead. The tissue in the penumbra is at risk of dying if blood flow is not restored and is considered salvageable. This salvageable tissue is the primary target of acute stroke therapies.
A helpful analogy is a forest fire. The infarct core is the section of the forest already burned to ash and permanently lost. The ischemic penumbra is the surrounding forest that is intensely hot, filled with smoke, and on the verge of igniting. If the firefighters can extinguish the fire quickly, this surrounding area can be saved from destruction. The goal of stroke treatment is to restore blood supply to the penumbra before it becomes part of the infarct core.
The penumbra’s size is not static. Without intervention, the infarct core expands into the penumbral region as more brain cells die from a lack of blood flow. This progression highlights the urgency of stroke treatment. Automated software analyzes the perfusion scan to quantify the volume of both the core and the penumbra, providing the data for treatment decisions.
Calculating the Mismatch Ratio
Once imaging software determines the volumes of the infarct core and penumbra, calculating the mismatch ratio is straightforward. The ratio is derived by dividing the volume of the penumbra (salvageable tissue) by the volume of the infarct core (permanently damaged tissue). This calculation provides insight into the potential benefit of an intervention.
A high, or “favorable,” mismatch ratio indicates the volume of salvageable tissue is much larger than the lost tissue. For example, a ratio of 8 means the penumbra is eight times larger than the core. This suggests that restoring blood flow could have a substantial positive impact by rescuing a large amount of brain tissue.
Conversely, a low, or “unfavorable,” mismatch ratio, such as a value near 1, indicates the core and penumbra are nearly the same size. This implies most of the affected brain tissue has already suffered irreversible damage. In these cases, the potential benefits of an intervention are lower, and the risks might outweigh the small amount of tissue that could be saved. While the threshold can vary, values greater than 1.8 are often considered favorable.
Guiding Acute Stroke Treatment Decisions
The mismatch ratio is a central element in guiding treatment for acute ischemic stroke, particularly for mechanical thrombectomy. This procedure involves physically removing a blood clot from a cerebral artery using a catheter. The mismatch ratio helps physicians select patients most likely to benefit, especially those outside the traditional treatment window.
Historically, thrombectomy was limited to the first six hours after symptom onset, based on the belief that at-risk tissue would be permanently damaged after this point. CT perfusion imaging and the mismatch ratio have challenged this time-based approach. This is because imaging can show that a large penumbra can persist for much longer in some individuals.
The DAWN and DEFUSE 3 clinical trials provided evidence to change stroke treatment guidelines. These studies showed that using perfusion imaging to find a favorable mismatch could identify patients who would benefit from thrombectomy up to 24 hours after symptoms began. Patients in these trials with a small core and large penumbra had better functional outcomes after the procedure compared to those receiving medical management alone.
The criteria used in these trials were specific. For instance, the DEFUSE 3 trial defined a favorable profile by setting a maximum volume for the infarct core, requiring a mismatch ratio of 1.8 or greater, and a minimum volume for the penumbra. The DAWN trial used slightly different criteria that also incorporated the patient’s age and clinical stroke severity. Adopting these principles has expanded the number of patients eligible for treatment, shifting the focus from a strict time window to individual brain physiology.
Interpreting Perfusion Maps
The volumes of the infarct core and penumbra are determined by interpreting color-coded maps generated from the CT perfusion data. Radiologists analyze these maps to understand the tissue’s status by looking at four primary parameters: Cerebral Blood Flow (CBF), Cerebral Blood Volume (CBV), Mean Transit Time (MTT), and Time to Peak (TTP).
The infarct core, where tissue death has occurred, is identified by a severe, matched reduction in both Cerebral Blood Flow (CBF), the rate of blood delivery, and Cerebral Blood Volume (CBV), the total volume of blood in the area. This signature indicates that very little blood is flowing into or resides within that region, confirming a significant loss of circulation.
The ischemic penumbra presents a different signature on these maps. In this zone, time-based parameters like Mean Transit Time (MTT) or Time to Peak (TTP) are significantly delayed. This delay shows that blood is struggling to reach the area. However, the Cerebral Blood Volume in the penumbra is often relatively preserved, suggesting the underlying vascular structure is intact and the tissue is still viable.
This distinct pattern of delayed transit time but preserved blood volume is the classic sign of salvageable tissue. Automated software uses predefined thresholds for these parameters to automatically segment the brain into core, penumbra, and healthy tissue. For instance, the core might be defined as tissue with a relative CBF below 30% of that in healthy brain tissue. This detailed analysis provides the objective data needed to calculate the mismatch ratio.