Assessment of diffusion and perfusion deficits in patients with small subcortical ischemia

Abstract

Background and purpose: Using perfusion- and diffusion-weighted MR imaging in acute ischemic stroke of the middle cerebral artery (MCA), previous studies have shown a typical pathophysiologic pattern that is characterized by a perfusion deficit larger than the diffusion lesion (mismatch), with the final lesion usually comprising the initial diffusion lesion (core) plus parts of the initial mismatch area. Little is known about underlying pathophysiology in small ischemic stroke. In this study, we used perfusion- and diffusion-weighted MR imaging to investigate the underlying pathophysiology of small subcortical ischemia.

Methods: Six consecutive patients (age range, 42-76 years) with small subcortical ischemia were examined by using a 1.5-T MR system 2-5, 22-55, and 144-392 hours after the onset of symptoms. T2-weighted, diffusion-weighted imaging at b=0 s/mm2 and b=1000 s/mm2, and bolus-track perfusion-weighted imaging were performed. Lesion sizes were determined on the basis of T2-weighted findings as well as those of apparent diffusion coefficient (ADC) maps and CBF.

Results: In every patient, the initial CBF lesion was smaller than the initial ADC lesion. Both the CBF lesion and the ADC lesion increased in size from first to second examination. In all instances, however, the CBF lesion remained smaller than the ADC lesion. The CBF lesion observed during the acute phase and the one seen on the following days were both smaller than the final T2 lesion.

Conclusion: Our data suggest that in contrast to previous findings in MCA ischemia in small subcortical infarcts tissue damage may spread beyond the area of the initial perfusion disturbance. In light of the small number of patients, further studies will have to address the relevance of this observation.

Fig 1.

Patient 1. Whole lesion, as assessed by T2-weighted imaging at the third examination, 392 hours after the onset of symptoms. This demonstrates the typical lesion location in our patients, in all cases affecting the posterior limb of the internal capsule and the medial globus pallidus. In addition, the corona radiata is involved.

Fig 2.

CBF maps (top, relative CBF units, threshold 75% of contralateral side) and ADC maps (bottom) are given for patient 6 at 2 hours and 29 hours after symptom onset, respectively. Right, the corresponding T2-weighted image obtained at the final examination after 220 hours. The CBF lesion (arrow) increases in size from first to second examination. At both time points, the CBF lesion is smaller than the ADC lesion (arrow) and smaller than the final T2 lesion.

Fig 3.

A, ADC, CBF (relative CBF units, threshold 75% of contralateral side), MTT, and CBV maps 4 hours after onset of symptoms, for patient 1. None of the three different types of perfusion maps (MTT, CBF, CBV) showed any pathologic changes (on adjacent sections, however, small areas of altered perfusion were detected in this patient), whereas in the ADC map an ischemic area (arrow) was seen. B, ADC, CBF (relative CBF units, threshold 75% of contralateral side), MTT, and CBV maps 55 hours after onset of symptoms, for patient 1. The three different types of perfusion maps indicate an area of altered perfusion (arrows) smaller than the ADC lesion (arrow) at the same time point.

Fig 4.

Each column in this figure indicates the ratio (mean across all patients) of two lesion sizes. CBF lesion volumes are given at the threshold of 75%. The first and second columns (black) give the ratios CBF₁ to ADC₁ and CBF₁ to T2₃. The CBF lesion at first examination is smaller than the ADC lesion at the same time point (P < .006, paired t test) as well as the final T2 lesion (P < .001, paired t test) as indicated by the ratio below unity. The third and fourth column (blue) give the ratios of the CBF lesion at the second time point to the ADC lesion at the same time point as well to the final T2 lesion. The ratio below unity indicates that the CBF lesion in the subacute phase is smaller than the ADC lesion at the same time point and the final T2 lesion (P < .001, P < .003, paired t test).