Multiple Inflow Pulsed Arterial Spin-Labeling Reveals Delays in the Arterial Arrival Time in Minor Stroke and Transient Ischemic Attack

Discussion

Tables 1 and 2 show demographic data for the 15 patients. DWI lesion volumes were small (median volume = 0.9 mL; range = 0–49 mL), consistent with the clinically minor strokes in this group.¹² Mean NIHSS score was 1 ± 2. There was no significant difference in age between cohorts (patients = 74 ± 11 years, controls = 69 ± 5 years; P = .24).

Figure 1 shows the DWI, CBF (milliliters per 100 grams per minute), and AAT (seconds) maps for 3 patients scanned acutely, ordered with decreasing DWI lesion volumes. Patient 1 had a large DWI infarct despite an NIHSS score of 1. The corresponding CBF and AAT maps show regions of hypoperfusion and delayed AAT in the affected hemisphere. Patients 12 and 9 had small DWI infarcts. While Patient 12 has regions of pronounced AAT delay, findings on CBF and AAT maps for patient 9 are relatively normal.

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Fig 1.

Acute images for 3 patients with decreasing infarct volumes. Patient 1 shows a delayed AAT region (hyperintensity) in the affected hemisphere, where CBF is reduced (hypointensity). Patient 12 shows reduced CBF in the affected hemisphere and delayed AAT, while maps for Patient 9 have normal findings. Abnormal CBF and AAT regions are outlined in red.

Figure 2 shows AAT asymmetry data for patients and controls. The control cohort demonstrates a symmetric AAT hemispheric index that is centered near zero with low variance. Patient AAT hemispheric metrics favor the affected hemisphere, suggesting prolonged AAT, and are significantly different compared with those in controls (P < .048).

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Fig 2.

AAT hemispheric asymmetry index is displayed as a bar plot for each patient. Control cohort data are more symmetric about zero with less variance. Patient data are significantly different from those of controls (P < .048). Light gray indicates DWI-positive; dark gray, DWI-negative; diamond datum, control participant.

A whole-brain noninvasive ASL technique capable of mapping CBF and AAT in patients with minor stroke/TIA is presented. CBF data were consistent with previous ASL studies.^2,3 Our PASL acquisition, however, was designed as a bolus-tracking experiment and provided evidence that AAT can be used to characterize perfusion abnormalities.

CBF and AAT maps revealed asymmetric perfusion patterns in some patients, defined by delayed AAT in patients in whom acute symptoms were minor or had resolved. This result is not unexpected because previous studies have shown perfusion deficits in 32% of patients with TIA.¹³ This technical report demonstrates the feasibility of multi-TI PASL in acute stroke and provides support for future efforts to characterize longitudinal changes.

There are 2 main advantages of this multi-TI 3D-GRASE-PASL: 1) whole-brain coverage allowing visualization of perfusion patterns in patients with only a small embolic infarct in superior regions (eg, patient 9), and 2) calculation of AAT, with a temporal resolution of 250 ms. Others have reported previously that AAT artifacts were found in 7 of 15 patients due to an insufficient postlabeling delay in some patients when using continuous ASL.² This issue is minimized in the current study because both CBF and AAT are estimated.

There are some important caveats: 1) Stroke may result in an AAT that is beyond the maximum TI in the current study (ie, AAT_max = 2500 ms), 2) The presence of large-vessel atherosclerosis likely influenced our AAT estimates. In DSC perfusion MR imaging, timing parameters are often used to delineate perfusion deficits, though they tend to overestimate the ischemic region. It is conceivable that the AAT also shares this same issue of overestimating the perfusion deficit. The degree to which extracranial and intracranial stenoses influence AAT is underinvestigated, and further studies are required.