Comparison of simultaneous arterial spin labeling MRI and 15O-H2O PET measurements of regional cerebral blood flow in rest and altered perfusion states

Abstract

Arterial spin labelling (ASL) is a non-invasive magnetic resonance imaging (MRI) technique that may provide fully quantitative regional cerebral blood flow (rCBF) images. However, before its application in clinical routine, ASL needs to be validated against the clinical gold standard, ¹⁵O-H₂O positron emission tomography (PET). We aimed to compare the two techniques by performing simultaneous quantitative ASL-MRI and ¹⁵O-H₂O-PET examinations in a hybrid PET/MRI scanner. Duplicate rCBF measurements were performed in healthy young subjects (n = 14) in rest, during hyperventilation, and after acetazolamide (post-ACZ), yielding 63 combined PET/MRI datasets in total. Average global CBF by ASL-MRI and ¹⁵O-H₂O-PET was not significantly different in any state (40.0 ± 6.5 and 40.6 ± 4.1 mL/100 g/min, respectively in rest, 24.5 ± 5.1 and 23.4 ± 4.8 mL/100 g/min, respectively, during hyperventilation, and 59.1 ± 10.4 and 64.7 ± 10.0 mL/100 g/min, respectively, post-ACZ). Overall, strong correlation between the two methods was found across all states (slope = 1.01, R² = 0.82), while the correlations within individual states and of reactivity measures were weaker, in particular in rest (R² = 0.05, p = 0.03). Regional distribution was similar, although ASL yielded higher perfusion and absolute reactivity in highly vascularized areas. In conclusion, ASL-MRI and ¹⁵O-H₂O-PET measurements of rCBF are highly correlated across different perfusion states, but with variable correlation within and between hemodynamic states, and systematic differences in regional distribution.

Keywords: 15O-H2O-PET; Arterial spin labeling; PET/MRI; cerebral blood flow; perfusion.

Figure 1.

Correlation of gCBF measurements by ¹⁵O-H₂O PET and ASL MRI. Scatter plot of global cerebral blood flow (gCBF) measured by ¹⁵O-H₂O PET and ASL MRI (a) and Bland–Altman plot showing the difference between the two methods against mean of the methods (b). In red, measurements during hyperventilation, in blue measurements in rest and in green measurements after acetazolamide. Orange line shows the modelled linear fit across all states and black lines the linear fit in each state from the model. In the scatter plots, grey line indicates the line of identity and in the Bland–Altman plots, solid black line indicates the average of the differences (bias) and the dashed lines the upper and lower limits of agreement (bias ± 2 SD).

Figure 2.

Average ASL and PET CBF images in different perfusion states and differences between techniques. Average ¹⁵O-H₂O PET and ASL MRI perfusion measurements in the different perfusion states. The six columns on the left show average MRI ASL and ¹⁵O-H₂O PET images during hyperventilation, in resting state and after acetazolamide. The four right columns show significant differences between the two techniques in each state and in all states combined. The t-value color scale is thresholded at a corrected P-value < 0.05 to show only the significantly different regions. In red-yellow ASL MRI > ¹⁵O-H₂O PET and in blue ASL < ¹⁵O-H₂O PET.

Figure 3.

Global CBF reactivity. Scatter plots of (a) absolute (Δ_abs gCBF) and (b) relative (Δ_rel gCBF) between-state changes in global cerebral blood flow measured by ¹⁵O-H₂O PET and ASL MRI, and of (c) relative gCBF change per % change of P_aCO₂ between hyperventilation and resting state. In blue, gCBF changes between hyperventilation and resting state and in green, changes between resting state and after the administration of acetazolamide. Orange line shows the linear fit from the model and the grey line indicates the line of identity.

Figure 4.

Method specific regional cerebrovascular reactivity. Absolute (a) and relative (b) average reactivity maps between hyperventilation and resting state (blue, for CBF decrease) and post-ACZ and resting state (in red and yellow, for CBF increase) measured by ¹⁵O-H₂O PET and ASL MRI.