Table 1. Imaging modalities used to identify the ischaemic penumbra and core.
| Imaging modality | Assessed parameters | Definition of penumbra | Advantages | Limitations |
|---|---|---|---|---|
| CT | ||||
| Perfusion | CBF, CBV, MTT, TTP | Relative CBF <66%a or MTT >145%a and CBV >2 ml 100 g−1 | Cheap, available, fast | Limited brain coverage, not sensitive in posterior circulation, no direct visualization of core |
| Xe | CBF | CBF 7–20 ml 100 g−1 min−1 | Quantitative | Technically complex, not validated, pharmacologic effects of Xe |
| MR | ||||
| DWI–PWI | CBF, CBV, MTT, TTP, ADC | Relative TTP (or MTT) delay >4 sa, relative CBF <37%a & relative ADC above 50%a | Practical, fast, increasingly available, no radiation involved, directly visualizes severely ischaemic tissue | Uncertainties regarding validity of mismatch concept, sensitive to head motion |
| Spectroscopy | NAA, lactate | Elevated lactate and normal NAA | Biochemically characterizes tissue | Not widely available, not validated, poor resolution |
| PET | ||||
| Multi-tracer 15O2 | CBF, CBV, MTT, CMRO2, OEF | CBF 7–22 ml 100 g−1 min −1 and CMRO2>39 μmol 100 g−1 min−1 and OEF>70% | Quantitative, validated | Complex, time consuming, not widely available, expensive |
| 11C-FMZ (+H215O) | Tracer binding | Relative binding ratio>3.4b and CBF <14 ml 100 g−1 min−1 | Based on physiological neuronal integrity | As above + only suitable for grey matter, requires additional perfusion imaging |
| 18F-FMISO | Tracer uptake | Uptake ratio>1.3a | Produces a direct positive image of viable hypoxic tissue | As above + validation incomplete, long imaging time, not practical in acute setting |
| SPECT | ||||
| 99mTc-HMPAO | CBF | Relative CBF 40–70%a | Cheap and relatively available | Thresholds still uncertain, limited spatial resolution |
a
Relative to mean contralateral hemisphere value.
b
Relative to contralateral healthy white matter.
Abbreviations: ADC, apparent diffusion coefficient; CBF, cerebral blood flow; CBV, cerebral blood volume; CMRO2, cerebral metabolic rate of oxygen; CT, computed tomography; DWI, diffusion-weighted imaging; FMISO, fluoromisonidazole; FMZ, flumazenil; HMPAO, hexamethylpropyleneamine oxime; MR, magnetic resonance; MTT, mean transit time; NAA, N-acetylaspartate; OEF, oxygen extraction fraction; PET, positron emission tomography; PWI, perfusion-weighted imaging; SPECT, single photon emission computed tomography; TTP, time to peak.