Review question: Comparison of diffusion‐weighted magnetic resonance imaging with conventional computer tomography for the early detection of ischaemic brain lesions in patients suspected of stroke

Patient population: adults suspected of acute stroke

Setting: hospital departments

Geographical location: studies were conducted in Europe (3 studies), the USA (3 studies), and in Australia (1 study)

Index test: diffusion‐weighted magnetic resonance imaging (DWI) performed within 12 hours of stroke onset

Alternative test: computer tomography (CT) performed within 12 hours of stroke onset

Reference standard: clinical assessment and imaging follow up

Included studies: 7 comparative studies that evaluated DWI and CT in the same patients

Total number of patients assessed: 226

Limitations of included studies

• Limited number of included studies (7 studies); small sample sizes; presence of incorporation bias

• DWI and CT were evaluated in highly selected patient samples (patients with high probability of stroke), which therefore are not representative of the typical population of patients presenting with 'suspected acute stroke' to an emergency department (poor generalisability of results)

• The stroke vascular territory was not reported in the majority of included studies although it is likely that they enrolled patients with typical anterior circulation stroke

• Only a minority of the studied patients had severe strokes (in whom DWI might be contraindicated)

• The high proportion of mild strokes and reclassification of TIA cases with a positive DWI lesion as strokes might have inflated the DWI sensitivity estimate

• In most of the studies stroke mimics were not included

• In all but one study CT was performed before DWI (reducing the sensitivity of CT to detect ischaemia)

CT results

TP       73

FP        0

FN       88

TN       65

Total   226

DWI results 

TP       147

FP        5

FN       14

TN       60

Total   226

Summary effect (95% CI)

DWI sensitivity 0.99 (0.23 to 1.00)

DWI specificity 0.92 (0.83 to 0.97)

CT sensitivity 0.39 (0.16 to 0.69)

CT specificity 1.00 (0.94 to 1.00)

Conclusions and comments
In the small cohort of included studies, DWI is more sensitive than CT ‐ but not more specific ‐ for the early detection of ischaemic stroke.

The small amount of data and the presence of methodological biases preclude any reliable calculation ‐ from the sensitivity and specificity estimates of CT and DWI ‐ of a positive or negative stroke diagnosis at different rates of stroke prevalence. 

Applicability of tests in clinical practice
None of the studies addressed practicality. CT is known to be quicker to perform and more readily available in most emergency care settings than magnetic resonance imaging (MRI). MRI is contraindicated in patients with pacemakers and some metal implants. In acutely ill stroke patients it may be difficult to monitor the patient's condition while being MR scanned (and this increases the risk of any respiratory difficulty or cardiovascular compromise that develops during the scan which passes undetected and may have adverse effects for the patient). If the patient is confused or restless as a result of the stroke, the patient may not be able to co‐operate for the longer scan times of MRI. 

Costs
None of the studies included a cost‐effectiveness evaluation. MRI is known to be more expensive than CT.