The CT Perfusion (CTP) mismatch ratio is a measurement used in emergency medicine to assess patients with acute stroke symptoms. This advanced imaging technique tracks blood flow through the brain, creating maps that reveal areas of compromised blood supply. CTP distinguishes between irreversibly damaged tissue and viable tissue that could be saved through urgent medical intervention. By quantifying this difference, the mismatch ratio helps physicians determine whether a patient will benefit from aggressive reperfusion therapies. This metric shifts the decision-making process from a strict time-based timeline to one based on the physiological state of the patient’s brain.
Understanding the Metrics of CT Perfusion
CT Perfusion imaging analyzes how a contrast dye passes through the brain’s vasculature, allowing the calculation of three fundamental metrics of blood flow.
Cerebral Blood Flow (CBF)
CBF represents the total volume of blood moving through a specific region of brain tissue each minute.
Cerebral Blood Volume (CBV)
CBV quantifies the total volume of blood within the capillaries and small blood vessels of a given brain volume.
Mean Transit Time (MTT)
MTT is the average time it takes for blood to travel through that tissue. MTT is inversely related to CBF and CBV through the central volume principle (CBF = CBV / MTT).
These three metrics are processed by specialized software to generate color-coded maps. Physicians analyze these maps to visualize areas receiving normal, reduced, or low levels of blood supply.
Defining the Ischemic Core and Penumbra
The interpretation of CTP maps focuses on identifying two distinct areas of brain tissue affected by the stroke: the ischemic core and the ischemic penumbra.
The ischemic core represents the region of brain tissue that has already suffered irreversible damage (infarction) due to a severe and sustained lack of blood flow. This tissue is defined by a marked reduction in Cerebral Blood Flow (CBF) and a significant decrease in Cerebral Blood Volume (CBV). This tissue is considered unsalvageable, meaning that therapeutic interventions cannot restore its function.
The penumbra is the tissue that is severely hypoperfused but remains viable and potentially salvageable if blood flow is restored quickly. The penumbra is characterized by a significant prolongation of the Mean Transit Time (MTT) or another time-based metric like Tmax, which is the time to the peak of the tissue’s contrast concentration curve. Unlike the core, the penumbra often shows reduced CBF but a preserved or only slightly reduced CBV. This preserved CBV is a sign of the brain’s autoregulation mechanism, where blood vessels dilate to keep the tissue alive for a limited time. The penumbra is the primary target of acute stroke treatment, and its volume is the difference between the total area of hypoperfused tissue and the volume of the ischemic core.
Calculation and Interpretation of the Mismatch Ratio
The Mismatch Ratio (MMR) quantifies the relationship between the salvageable tissue and the irreversibly damaged tissue. It is calculated by dividing the volume of the total hypoperfused tissue (core plus penumbra) by the volume of the ischemic core.
A high mismatch ratio indicates a small ischemic core and a large volume of surrounding viable tissue, representing significant potential for salvage. This “favorable mismatch” profile suggests the patient will benefit from reperfusion therapy. Criteria for a favorable mismatch often include an ischemic core volume below 70 milliliters and a Mismatch Ratio of 1.8 or greater.
Conversely, a ratio close to 1.0 means the volume of the ischemic core is nearly the same as the total hypoperfused area. This signifies a “non-target” profile, indicating that most of the affected tissue has already infarcted, and little penumbra remains. The mismatch ratio serves as an objective quantification of the brain’s capacity to recover following a stroke.
Guiding Treatment Decisions in Acute Stroke
The Mismatch Ratio extends the therapeutic window for acute stroke treatment, moving beyond traditional time limits. For patients presenting between 6 and 24 hours after symptom onset, the ratio selects those most likely to benefit from endovascular thrombectomy (the mechanical removal of a large blood clot). The presence of a favorable mismatch profile suggests that the physiological time window remains open for treatment.
Clinical trials have demonstrated that patients selected by this imaging-based criterion have better outcomes after intervention, even when treated outside the standard early time frame. This principle also applies to intravenous thrombolysis, as the ratio helps identify patients with a significant volume of salvageable tissue. By focusing on the amount of viable tissue rather than the time since stroke onset, the CTP mismatch ratio allows doctors to personalize treatment decisions.