We report that MRI is more effective than CT for the diagnosis of acute stroke in a typical patient sample. Our sample was representative of the range of patients who are likely to present with a clinical suspicion of acute stroke, including patients who ultimately proved to have a different diagnosis. Therefore, our results are directly applicable to clinical practice.
The earliest comparisons of MRI to CT in the diagnosis of acute stroke, from the early 1990s, before clinical diffusion-weighted imaging and gradient-echo imaging were routine, showed that acute infarcts were visible more frequently on MRI than on CT and that that these modalities were much the same for detection of intracranial haemorrhage.6,7
In the mid 1990s, diffusion-weighted imaging entered the clinic and showed promise of greater sensitivity for stroke diagnosis than conventional MRI, especially in the initial hours after stroke onset, and for the detection of small lesions.5,9
Early reports that compared diffusion-weighted imaging MRI with CT estimated sensitivities of 86-100% for diffusion-weighted imaging and 42-75% for CT, but were limited by potential biases in patient selection and image assessment.8,10-15
The greater overall sensitivity of MRI for acute stroke in this study is attributable to its effectiveness for detection of acute ischaemic stroke. Diagnostic rates for acute intracranial haemorrhage were much the same for MRI and CT. MRI with diffusion-weighted imaging was both more effective within the critical first 3 h and in the entire sample. Acute ischaemic stroke was diagnosed with MRI in 46% of patients but with CT in only 10%. Of the 190 patients with final clinical diagnosis of ischaemic stroke, independent, blinded assessment with MRI diagnosed ischaemic stroke in 83% of patients, and in 16% with CT. This study accords with the reported difference between MRI and CT, but our rates of imaging diagnoses were lower in both modalities than those in previous studies.8,10-15
In our sample, 25% of the patients with suspected acute stroke had final diagnoses other than cerebrovascular; this rate is consistent with other samples of consecutive patients who present to emergency departments with the initial diagnosis of acute stroke.16,17
Because the accuracies of diagnostic tests are overestimated in non-representative samples,18
we would expect that the true accuracies of MRI and CT in acute stroke in this study would be lower than those previously reported. The addition of angio-graphic and perfusion acquisitions to CT might have increased the accuracy of this modality and made the results more similar to those of MRI.
False-negative diffusion-weighted imaging scans in ischaemic stroke do arise. We estimated the false negatives from such MRI scans at 17%. Two of the predictors of false-negative diffusion-weighted imaging—brainstem location and NIHSS of less than 4—could relate to small lesions that escape visual detection, especially in locations such as the brainstem, in which they might be difficult to distinguish from the hyperintensity of incompletely suppressed anisotropic diffusion or susceptibility artifacts. The practitioner must be cognisant of the possibility of false negatives with diffusion-weighted imaging for ischaemic stroke and note the presence of clinical factors that predispose to such stroke.
These results accord with our previous finding that MRI might be as accurate as CT for diagnosis of intracranial haemorrhage.3
This expanded sample showed that MRI was not worse than CT for the detection of acute intracranial haemorrhage. These results are also consistent with previous reports that MRI can accurately detect acute intracranial haemorrhage.1,7,19-23
Thus, clinicians who use MRI as the sole imaging modality in acute stroke can be assured that a negative MRI excludes acute intracranial haemorrhage as effectively as does a negative CT. Since MRI was done before CT in most patients in our study (77% of cases of intracranial haemorrhage), the MRI signal changes associated with intracranial haemorrhage could have been less conspicuous than they would have been at a later stage. Nevertheless, the potential time bias did not seem to affect the rate of detection of intracranial haemorrhage by MRI in this cohort.
In this study neither MRI nor CT achieved 100% sensitivity for the diagnosis of acute intracranial haemorrhage. When compared with the final clinical diagnosis there were four cases of clinically confirmed acute intracranial haemorrhage that were misdiagnosed by the readers on MRI. In two cases readers erroneously classified acute haemorrhages as chronic; in another (in which the gradient-echo imaging scan was not available) readers missed an acute intracranial haemorrhage in their interpretation of the diffusion-weighted imaging MRI; and in a fourth case, a left frontal acute intracranial haemorrhage was not diagnosed by the readers. When detection by CT images was compared with the final clinical diagnosis there were three false-negative cases of acute intracranial haemorrhage: a subdural haematoma, a haemorrhagic metastasis, and a temporal lobe haematoma were not diagnosed by the readers. Previous studies have also noted that cases of acute haemorrhagic transformation could be seen on gradient-echo imaging but not on CT.3
Although CT scanning has been the criterion that is standard for diagnosis of acute stroke, our study shows that use of CT is no longer justifiable on the basis of diagnostic accuracy alone. Logistical and financial arguments in favour of CT as the preferred emergency test can be made—non-contrast CT is generally more accessible for emergency use, even in facilities at which MRI is available, and the fixed and variable costs of CT scanning are less than for the costs of MRI scanning. Would the improvement in diagnostic accuracy offered by MRI enhance patient outcomes and cost-effectiveness enough to justify the necessary increases in expense and effort? A comparison of immediate CT with delayed CT for acute stroke showed that correct early diagnosis by immediate CT scanning increased independent survival, informed subsequent treatment and management decisions, reduced costs, and increased quality-adjusted life-years.24
A similar analysis, comparing immediate CT with immediate MRI, would help to quantify the potential effect of increased early diagnostic accuracy of MRI on health-care costs and quality of stroke outcomes. Since immediate MRI allows more accurate diagnosis than immediate CT, it might increase the cost-effectiveness of stroke care, since definitive treatments and secondary prevention could be initiated sooner than with CT alone.
A potential bias was introduced by our decision not to randomise the order of scanning. However, since abnormalities become more conspicuous over time with both MRI and CT, the probability of detection of stroke was biased in favour of CT, which was done after MRI in our study. Therefore this bias cannot account for our results.
The selection bias against patients who were judged too medically unstable to undergo MRI probably eliminated severe strokes that would be readily detectable on imaging, and thus falsely decreased the sensitivity to some degree. Our study included the typical acute stroke population, and therefore skewed the distribution towards mild cases. This feature of our sample might explain why we recorded lower CT sensitivity and a greater difference between CT and MRI than studies that excluded cases less severe than a minimum criterion according to an established stroke diagnosis.8
This difference between our findings and other studies persisted at later times from onset.
Although the need for urgent management of patients with transient ischaemic attacks and mild stroke has been increasingly recognised,25,26
accurate diagnosis on the basis of clinical presentation and CT scanning can be especially difficult in these patients. MRI is more sensitive than CT for severe stroke, but the difference might not be clinically significant if a systematic method for CT reading is used.27
Nevertheless, because mild stroke and transient ischaemic attack make up most stroke admissions to a general hospital emergency department, our findings are directly applicable to real-world practice.
MRI can be used as the sole modality for the emergency imaging of patients with suspected acute stroke, whether ischaemic or haemorrhagic. The high diagnostic accuracy of MRI was the same for scans within the first 3 h as it was for the entire sample, and thus is relevant to patients who might be eligible for standard thrombolytic treatment of stroke. Many stroke centres use MRI as the basis of thrombolytic treatment decisions,28
and where MRI is immediately available for emergency stroke diagnosis, initiation of thrombolytic treatment will not be substantially delayed.29
Since imaging studies in acute stroke are usually interpreted by non-specialists, the imaging modality with the highest sensitivity and the highest intra-rater and inter-rater reliability for diagnosis of ischaemic stroke by non-specialists—MRI—should be used.8
Because MRI is more effective for detection of acute ischaemia, and can detect acute and chronic haemorrhage, it should be the preferred test for accurate diagnosis of patients with suspected acute stroke.