The Utility of Arterial Transit Time Measurement for Evaluating the Hemodynamic Perfusion State of Patients with Chronic Cerebrovascular Stenosis or Occlusive Disease: Correlative Study between MR Imaging and 15O-labeled H2O Positron Emission Tomography

Abstract

Purpose: To verify whether arterial transit time (ATT) mapping can correct arterial spin labeling-cerebral blood flow (ASL-CBF) values and to verify whether ATT is a parameter that correlates with positron emission tomography (PET)-oxygen extraction fraction (OEF) and PET-mean transit time (MTT).

Methods: Eleven patients with unilateral major cerebral artery stenosis or occlusion underwent MRI and PET in the chronic or asymptomatic phase. ASL-MRI acquisitions were conducted with each of two post-label delay (PLD) settings (0.7s and 2.0s) using a pseudo-continuous ASL pulse sequence and 3D-spin echo spiral readout with vascular crusher gradient. ATT maps were obtained using a low-resolution pre-scan approach with five PLD settings. Using the ASL perfusion images and ATT mapping, ATT-corrected ASL-CBF images were obtained. Four kinds of ASL-CBF methods (PLD 0.7s with or without ATT correction and PLD 2.0s with or without ATT correction) were compared to PET-CBF, using vascular territory ROIs. ATT and OEF were compared for all ROIs, unaffected side ROIs, and affected side ROIs, respectively. ATT and MTT were compared by the ratio of the affected side to the unaffected side. Transit time-based ROIs were used for the comparison with ATT.

Results: Comparing ASL-CBF and PET-CBF, the correlation was higher with ATT correction than without correction, and for a PLD of 2.0s compared with 0.7s. The best correlation was for PLD of 2.0s with ATT correction (R² = 0.547). ROIs on the affected side showed a low but significant correlation between ATT and PET-OEF (R² = 0.141). There was a low correlation between the ATT ratio and the MTT ratio (R² = 0.133).

Conclusion: Low-resolution ATT correction may increase the accuracy of ASL-CBF measurements in patients with unilateral major cerebral artery stenosis or occlusion. In addition, ATT itself might have a potential role in detecting compromised hemodynamic state.

Keywords: arterial spin labeling; arterial transit time; cerebral blood flow; magnetic resonance imaging; oxygen extraction fraction.

Fig. 1

Sample images of ATT transformation to normalization space. (a) ATT, (b) normalized ATT, (c) fused transit time-based ROI on normalized ATT. (b) is a standardized image of (a). The ROI is detected by fusing it on a transit time-based ROI. The transit time-based ROI is shown in Fig. 2b. ATT, arterial transit time.

Fig. 2

a) Automated ROI detection in the conventional vascular anatomical territories. The ROIs are generated by NEUROFLEXER and cover the major vascular regions. Dark purple regions, including the PCA and basal ganglia, and the thalamus regions are not measured. b) Automated ROI detection focusing on the difference in transit time among the major vessel territories. ROIs are set to cover the proximal, middle, and distal regions of the major vascular territories. No measurements are obtained for structures located in the deep gray matter (e.g., the basal ganglia and thalamus). ACA, anterior cerebral artery; MCA, middle cerebral artery; PCA, posterior cerebral artery.

Fig. 3

Representative images of right internal carotid artery occlusion (patient no.3). (a) PET-CBF; (b) ASL-CBF without ATT correction, PLD = 0.7s; (c) ASL-CBF with ATT correction, PLD = 0.7s; (d) ASL-CBF without ATT correction, PLD = 2.0s; (e) ASL-CBF with ATT correction, PLD = 2.0s; (f) MRA-MIP; (g) ASL-ATT; (h) PET-OEF.Increased OEF (h) and prolonged ATT (g) areseen in the right MCA territory. Although PET-CBF isslightly decreased in the right MCA territory, the ASL-CBF values are decreased for both PLD times when ATT correction is not used (b and d). The images for ASL-CBF with longer PLD and with ATT correction (e) are the mostsimilar to the PET-CBF images in (a). ASL, arterial spin labeling; ATT, arterial transit time; CBF, cerebral blood flow; MCA, middle cerebral artery; MIP, maximum intensity projection; MRA, magnetic resonance angiography; OEF, oxygen extraction fraction; PET, positron emission tomography; PLD, post-label delay.

Fig. 4

Comparison between cerebral blood flow measured by MRI and PET for all ROIs. (a) PLD = 0.7s without ATT correction, (b) PLD = 0.7s with ATT correction, (c) PLD = 2.0s without ATT correction, and (d) PLD = 2.0s with ATT correction. The regression line and coefficient of determination (R²) are inset on each graph. The reliability of the ASL-CBF values improves with ATT correction. Among the PLD and ATT correction settings, the best R value is obtained for ASL-CBF with PLD 2.0s and ATT correction. ASL-CBF, arterial spin labeling-cerebral blood flow; ATT, arterial transit time; CBF, cerebral blood flow; PET, positron emission tomography; PLD, post-label delay; R², coefficient of determination.

Fig. 5

Correlation between OEF and ATT. (a) Scatterplot showing the correlation of OEF and ATT values from both the affected and unaffected sides; (b) correlation of these values from only the hemisphere of the unaffected side; (c) correlation of these values from only the affected hemisphere. ATT, arterial transit time; OEF, oxygen extraction fraction.