Phase contrast mapping MRI measurements of global cerebral blood flow across different perfusion states - A direct comparison with 15O-H2O positron emission tomography using a hybrid PET/MR system

Abstract

Phase-contrast mapping (PCM) magnetic resonance imaging (MRI) provides easy-access non-invasive quantification of global cerebral blood flow (gCBF) but its accuracy in altered perfusion states is not established. We aimed to compare paired PCM MRI and ¹⁵O-H₂O positron emission tomography (PET) measurements of gCBF in different perfusion states in a single scanning session. Duplicate combined gCBF PCM-MRI and ¹⁵O-H₂O PET measurements were performed in the resting condition, during hyperventilation and after acetazolamide administration (post-ACZ) using a 3T hybrid PET/MR system. A total of 62 paired gCBF measurements were acquired in 14 healthy young male volunteers. Average gCBF in resting state measured by PCM-MRI and ¹⁵O-H₂O PET were 58.5 ± 10.7 and 38.6 ± 5.7 mL/100 g/min, respectively, during hyperventilation 33 ± 8.6 and 24.7 ± 5.8 mL/100 g/min, respectively, and post-ACZ 89.6 ± 27.1 and 57.3 ± 9.6 mL/100 g/min, respectively. On average, gCBF measured by PCM-MRI was 49% higher compared to ¹⁵O-H₂O PET. A strong correlation between the two methods across all states was observed (R² = 0.72, p < 0.001). Bland-Altman analysis suggested a perfusion dependent relative bias resulting in higher relative difference at higher CBF values. In conclusion, measurements of gCBF by PCM-MRI in healthy volunteers show a strong correlation with ¹⁵O-H₂O PET, but are associated with a large and non-linear perfusion-dependent difference.

Keywords: 15O-H2O; cerebral blood flow; magnetic resonance imaging; phase contrast mapping; positron emission tomography.

Figure 1.

Experiment overview. PCM: phase contrast mapping; ACZ: acetazolamide; TOF: time of flight arterial angiography; CBF: cerebral blood flow; *Other MRI acquisitions.

Figure 2.

Examples CBF measurements. Panels show ¹⁵O-H₂O PET CBF images (top row), PCM MRI average velocity images across the cardiac cycle (middle row) and arterial mean velocity of each vessel over the cardiac cycle (bottom row) in each state from a single volunteer. Colored ROI for each vessel are displayed over the average velocity images and the corresponding arterial flow over the cardiac cycle is shown below. L: left; R: right.

Figure 3.

Correlation of gCBF measurements by ¹⁵O-H₂O PET and PCM MRI. (a) Scatter plot of global cerebral blood flow (gCBF) measured by ¹⁵O-H₂O PET and phase-contrast mapping (PCM) MRI during hyperventilation (red), in rest (blue) and after acetazolamide (green). (b) Bland–Altman plot showing the ratio of the two methods against mean of the methods. Yellow line shows the linear fit from the mixed model. In 3(a), grey line indicates the line of identity. In 3(b), solid grey and dashed lines indicate bias upper and lower limits of agreement (bias ± 2 SD), respectively.