The mismatch ratio is a single number from a stroke scan that weighs brain tissue still worth saving against tissue that has already died. A high ratio means a lot of the starving brain can still be rescued if blood flow returns quickly. It has changed stroke care from watching the clock to looking at the brain itself, letting some people be treated up to a day after symptoms start.1PubMed Central. Tissue window, not the time window, will guide acute stroke treatment
The Two Zones of Brain Tissue the Ratio Compares
When a clot blocks a brain artery, the tissue it feeds does not all die at once. The area with almost no blood flow dies first. This is the core, brain that cannot be saved.2professionalradiology.com. CT Perfusion Basics Around it lies the penumbra, tissue that is getting too little blood to work properly but is still alive and able to recover if flow returns.3professionalradiology.com. CT Perfusion Basics The penumbra is what treatment is aimed at. Left alone, it steadily turns into core.4professionalradiology.com. CT Perfusion Basics
The ring does not stay the same size. Each minute without blood pushes more at-risk cells past the point of recovery, and how fast that happens varies a lot from person to person. Side vessels that reroute blood slow the loss. That is why two people scanned at the same hour can have very different ratios.
The scan measures the whole area with poor blood flow, then subtracts the core to find the penumbra.5professionalradiology.com. CT Perfusion Basics The mismatch ratio divides the whole poorly supplied area by the core.6PubMed Central. What Is the “Optimal” Target Mismatch Criteria for Acute Ischemic Stroke? A ratio of 3 means the starved area is three times the size of what is already lost. A ratio close to 1 means the starved area is little bigger than the dead area, so reopening the artery has little left to rescue.
How the Scan Measures Core and Penumbra
CT perfusion is a CT scan taken while contrast dye is injected into a vein. The scanner films the dye repeatedly as it passes through the brain, and software turns that movement into colour maps of blood flow and delay. On a report, Tmax is how many seconds late the dye reaches its peak in each spot.7professionalradiology.com. CT Perfusion Basics Areas where Tmax is more than 6 seconds are usually counted as poorly supplied.8professionalradiology.com. CT Perfusion Basics
A delay signals trouble because of how blood travels. In healthy brain, dye arrives and peaks within a few seconds because blood flows freely through open arteries. When an artery is blocked, blood reaches the area only by slower, roundabout side routes, so the dye peaks late. A long delay therefore points to tissue being starved even if it is not yet dead. That is why delay marks the at-risk area and very low flow marks the core.
The core comes from rCBF, which is blood flow in each spot compared with the healthy side of the brain.9PubMed Central. What Is the “Optimal” Target Mismatch Criteria for Acute Ischemic Stroke? Areas with flow below 30% of normal are counted as core.10professionalradiology.com. CT Perfusion Basics That 30% cutoff predicted the final damage closely in people whose artery was reopened.11PubMed. Thresholds of CT perfusion in predicting ischemic penumbra and infarct core in patients with acute ischemic stroke Some systems use other cutoffs, such as a delay measure over 3 seconds for the at-risk zone.12PubMed. Thresholds of CT perfusion in predicting ischemic penumbra and infarct core in patients with acute ischemic stroke
On a typical report, the summary box often lists a volume for “CBF<30%”, which is the core in millilitres. A separate “Tmax>6s” volume is the whole poorly supplied area. The mismatch volume and mismatch ratio are calculated from those two numbers. Some reports also show “Tmax>10s”, which marks the most severely delayed tissue.
What Counts as a Good Mismatch Ratio
- Ratio: 1.8 or more is the common threshold for a “target mismatch.”
- Mismatch volume: 15 mL or more of savable brain.
- Core: under 70 mL in most standard criteria.
- Low ratio: below about 1.2 means little savable tissue is left.
These cutoffs come from major clot-removal trials and are what most hospitals use. Thresholds do shift with the situation. For clots in smaller branch arteries, one analysis found looser settings, a ratio of 2.2 with at least 10 mL, picked out people who benefited.13Journal of Cerebral Blood Flow & Metabolism. Redefining ischemic core, penumbra, and target mismatch on CT perfusion in acute anterior distal medium vessel occlusion In large strokes, clot removal helped people whose ratio was above 1.2 but not those below it.14PubMed. Perfusion Imaging and Clinical Outcome in Acute Ischemic Stroke with Large Core A ratio just below a cutoff is therefore not an automatic no, and doctors weigh the whole picture.
The ratio alone is not enough. A ratio of 2 can mean 10 mL against 20 mL, or 100 mL against 200 mL. The second case has a far bigger core and a higher risk, and a large core raises the chance of bleeding after treatment. So doctors read three numbers together: the core says how much is lost, the mismatch volume says how much can be gained, and the ratio compares the two. A person can pass on ratio but fail on volume, such as when a small core gives a high ratio but the mismatch volume is still too small to be worth treating.
A report may describe a “malignant” profile. This means a large core, often over 70 to 100 mL, with severe delays. It signals a high risk of poor outcome and bleeding, though it does not always rule out treatment, especially when some mismatch remains.
The ratio also cannot say which part of the brain is affected, and a small area in a speech or movement centre can matter more than a larger area elsewhere. It does not capture age, earlier disability or other illness either, which the team weighs separately.
How the Ratio Decides Treatment
Thrombectomy means pulling the clot out through a thin tube threaded from the groin or wrist. In people arriving 6 to 24 hours after last seen well, clot removal was effective in those found to have a mismatch.15PubMed Central. Tissue window, not the time window, will guide acute stroke treatment Across more than 500 late-arriving patients, those selected by perfusion mismatch were about three times more likely to have less disability at 90 days after clot removal.16MedPage Today. Stroke Thrombectomy: Strict Mismatch Criteria Not Necessary After 16 Hours? For someone found hours after symptoms began, the scan can be the reason treatment goes ahead.
Clot-busting drugs are normally limited to about 4.5 hours after onset. People picked by perfusion scans and given the drug between 4.5 and 24 hours may have better recovery.17PubMed. Intravenous thrombolysis for ischemic stroke in extended time window selected with CT perfusion: a systematic review and meta-analysis In large strokes treated within 6 hours, clot removal raised good outcomes from 28% to 49% in people with mismatch, but did not help those without it.18PubMed. Time-Dependent Impact of Mismatch Profiles on Outcomes Following Endovascular Thrombectomy for Large Ischemic Stroke
In the minutes after the scan, the stroke team checks the artery scan for a large blocked vessel. That scan decides whether the procedure is possible at all, since clot removal only works on a clot in a large artery a tube can reach. The perfusion scan then decides whether it is worthwhile, because it shows whether enough brain is left to save. The team reads the core, mismatch volume and ratio, often from automated software, and combines them with symptom severity, age and prior health. If it fits, the family may be asked for quick consent.
The two treatments are decided separately. The drug is given through a vein and can work on clots in arteries of many sizes, while clot removal is a mechanical procedure for large vessel blockages. Someone can be eligible for one, both or neither, and a person with a large blocked artery often gets the drug and then goes straight on to clot removal.
What the Ratio Says About Recovery
Reopening the artery is what turns mismatch into recovery. In people with a target mismatch, 66% became independent when blood flow was restored versus 29% when it was not.19PubMed Central. CT Perfusion to Predict Response to Recanalization in Ischemic Stroke Recovery rates were similar whether flow returned before or after 6 hours, 70% versus 62%.20PubMed Central. CT Perfusion to Predict Response to Recanalization in Ischemic Stroke Larger cores and smaller ratios were linked with poorer recovery.21PubMed. Perfusion Imaging Mismatch Profiles in the Early Thrombectomy Window: A Single-Center Analysis With a large, unfavourable profile, independence fell from 75% if flow returned within 30 minutes of the scan to under 40% after two hours.22PubMed Central. Time from Imaging to Endovascular Reperfusion Predicts Outcome in Acute Stroke
“Independent” in these studies generally means a person can look after their own daily needs, possibly with some leftover weakness or difficulty. It does not mean a full return to how they were before the stroke, so it should not be read as a cure.
Once treatment is over, the final damage depends on whether the artery stayed open. A follow-up scan or MRI shows the actual area lost, and recovery depends on where in the brain that damage sits, not just its size. Families can ask the team:
- What were the core volume and the mismatch ratio?
- Was blood flow fully restored?
- How large was the final damage area, compared with the first scan’s core?
- What does that mean for rehabilitation?
How Accurate Is the Mismatch Ratio
Scans done very early can make the core look bigger than the damage that ends up happening.23PubMed. Ischemic Core Overestimation on Computed Tomography Perfusion This “ghost core” was seen in 6% to 58% of patients, by typically 4 to 30 mL.24PubMed. Ghost infarct core: A systematic review of the frequency, magnitude, and variables of CT perfusion overestimation It is more common the sooner after onset the scan is done and when side vessels are poor.25PubMed. Ghost infarct core: A systematic review of the frequency, magnitude, and variables of CT perfusion overestimation In the first hour or so, a region may show very low flow simply because the clot has just blocked it, even though the tissue is not yet dead. If blood flow returns quickly, some of that apparent core survives, so early scans can make the ratio look worse than it really is.
Hospitals use automated programs, such as RAPID or Viz.ai, to compute the ratio.26Stroke. Comparison of Three Automated Ct Perfusion Software Packages for Thrombectomy Eligibility and Final Infarct Volume Prediction One program reported average cores of 26 mL versus 18 mL for another on the same patients.27PubMed Central. Multicenter comparison using two AI stroke CT perfusion software packages for determining thrombectomy eligibility Mismatch ratios agreed less well between programs than other measures.28PubMed Central. A Comparison of CT Perfusion Output of RapidAI and Viz.ai Software in the Evaluation of Acute Ischemic Stroke Programs usually still agreed on who should be treated.29Journal of Medical Imaging and Interventional Radiology. Cerebral perfusion software: comparative evaluation Each program has its own methods for processing images and removing noise, and these choices change where the colour boundaries fall. Because a ratio divides one volume by another, small shifts in the boundary have a large effect. Technical steps in setting up the scan can change the maps too.30PubMed Central. Arterial Input Function Placement for Accurate CT Perfusion Map Construction in Acute Stroke
The method works less well in some settings. Automated maps were less accurate than a doctor’s visual reading for strokes in the back of the brain.31PubMed Central. Qualitative versus automatic evaluation of CT perfusion parameters in acute posterior circulation ischaemic stroke In children, meeting mismatch criteria did not predict outcome.32PubMed Central. Hyperacute Perfusion Imaging Before Pediatric Thrombectomy
MRI shows the core more directly, and the core seen on CT perfusion roughly matches the damaged area an MRI would show.33professionalradiology.com. CT Perfusion Basics CT and MRI agreed on mismatch status about 90% of the time, and CT is used more because it is faster and more widely available.