Arterial Transit Time Mapping Obtained by Pulsed Continuous 3D ASL Imaging with Multiple Post-Label Delay Acquisitions: Comparative Study with PET-CBF in Patients with Chronic Occlusive Cerebrovascular Disease

<div><p>Arterial transit time (ATT) is most crucial for measuring absolute cerebral blood flow (CBF) by arterial spin labeling (ASL), a noninvasive magnetic resonance (MR) perfusion assessment technique, in patients with chronic occlusive cerebrovascular disease. We validated ASL-CBF and ASL-ATT maps calculated by pulsed continuous ASL (pCASL) with multiple post-label delay acquisitions in patients with occlusive cerebrovascular disease. Fifteen patients underwent MR scans, including pCASL, and positron emission tomography (PET) scans with ¹⁵O-water to obtain PET-CBF. MR acquisitions with different post-label delays (1.0, 1.5, 2.0, 2.5 and 3.0 sec) were also obtained for ATT correction. The theoretical framework of 2-compartmental model (2CM) was also used for the delay compensation. ASL-CBF and ASL-ATT were calculated based on the proposed 2CM, and the effect on the CBF values and the ATT correction characteristics were discussed. Linear regression analyses were performed both on pixel-by-pixel and region-of-interest bases in the middle cerebral artery (MCA) territory. There were significant correlations between ASL-CBF and PET-CBF both for voxel values (r = 0.74 ± 0.08, slope: 0.87 ± 0.22, intercept: 6.1 ± 4.9) and for the MCA territorial comparison in both affected (R² = 0.67, y = 0.83x + 6.3) and contralateral sides (R² = 0.66, y = 0.74x + 6.3). ASL-ATTs in the affected side were significantly longer than those in the contralateral side (1.51 ± 0.41 sec and 1.12 ± 0.30 sec, respectively, p <0.0005). CBF measurement using pCASL with delay compensation was feasible and fairly accurate even in altered hemodynamic states.</p></div

Comparison between MR and PET in a typical case of left MCA severe stenosis.

<p><b>a)</b> Comparison between PET-CBF and ASL-CBF. From top to bottom rows, PET-CBF, ASL-CBF, ASL-ATT (transit time map) are calculated from the multiple PLD data and ASL-CBF calculated from single PLD (1.5s) data based on the simple single-compartment model (see text). <b>b)</b> MRA reveals that left MCA branches are less bright than those of the contralateral side. <b>c)</b> 2D-plots of PET and ASL-CBF on a pixel-by-pixel basis. The plotted CBF images through the ventricle body level correspond to third column images from the right side in 1st and 2nd rows. Abscissa and ordinate axes represent PET and ASL CBF, respectively.</p

The ASL signal changes for model parameters.

<p><b>a)</b> ASL signal dependency to T_1e. The single-compartment model is constant along with T_1e value, since labeled spins are retained in the vascular space and correspond to the relaxation of T_1b, while the other model assumption with venous outflow has an increasing trend along with the increase of T_1e. <b>b)</b> ASL signal dependency to CBV. ASL signal has an increasing trend along with CBV increase. Changes are quite stable in the normal brain CBV range (2.0 to 5.0 ml/100 g) (see text). <b>c)</b> ASL signal dependency to PS. PS is also constant despite the wide range of 20 to 200 ml/min/g, which adequately covers the physiological cerebral blood flow range.</p

Comparison between MR and PET in a typical case of left ICA obstruction.

<p><b>a)</b> Comparison between PET-CBF and ASL-CBF. From top to bottom rows, PET-CBF, ASL-CBF, ASL-ATT (transit time map) are calculated from the multiple PLD data and ASL-CBF calculated from single PLD (1.5 s) data, based on the simple single-compartment model (see text). The decreased signal in the right frontal cortex corresponds to cystic change after infarction. <b>b)</b> MRA revealing the complete obstruction of left ICA suggests the left MCA territory is fed through collaterals of A-Com and/or left IC-PC. <b>c)</b> 2D-plots of PET and ASL-CBF on pixel-by-pixel basis. The plotted CBF images through ventricle body level correspond to the third column images from the right side in the 1^st and 2^nd rows. Abscissa and ordinate axes represent PET and ASL CBF, respectively. Scale bars and units as indicated.</p

Patient characteristics, clinical data, and MRI findings.

<p>Patient characteristics, clinical data, and MRI findings.</p

ROI selection for ROI-based comparison between PET-CBF and ASL-CBF maps.

<p><b>a)</b> PET-CBF map, <b>b)</b> ASL-CBF map. ROI selections for 12 gray matter and 4 white matter areas were exactly the same for PET-CBF and ASL-CBF maps.</p

Linear regression analysis of PET-CBF and ASL-CBF.

<p>The ROI values from affected and contralateral normal sides are plotted using a diamond shapes (◊) and crosses (x), respectively. <b>a)</b> 2D-plots of PET and ASL CBF using gray and white matter ROIs in both affected and contralateral cerebral hemispheres. The regression lines and the coefficient of determination (R²) are shown as insets on the graph. <b>b)</b> Bland-Altman plots of ROI-based CBF comparison between ASL-CBF and PET-CBF. Bias and mean ± 2 SD precision lines are drawn on the graph as thick lines and dashed lines, respectively. <b>c)</b> Extracted Bland-Altman plots of affected ROI of ASL-CBF data calculated from multi-PLD data. There is no significant regressed line. The regression lines and the coefficient of determination (R²) are shown as insets of the graph, but were not significant statistically.</p

Linear regression analyses of PET-CBF and ASL-CBF.

<p>ROI values from affected and contralateral normal sides are plotted as diamond shapes (◊) and crosses (x), respectively. <b>a)</b> 2D-plots between PET and ASL CBF using gray and white matter ROIs in both affected and contralateral cerebral hemispheres. The regression lines and the coefficient of determination (R²) are shown as insets of the graph. <b>b)</b> Bland-Altman plots of ROI-based CBF comparison between ASL-CBF and PET-CBF. Bias and mean ± 2 SD precision lines are drawn on the graph as thick lines and dashed lines, respectively. <b>c)</b> Extracted Bland-Altman plots of affected ROI of ASL-CBF data calculated from single PLD = 2.0 s data. The regression lines and the coefficient of determination (R²) are shown as insets of the graph, but were not significant statistically.</p

The comparison between PET and ASL CBF on Bland-Altman plots.

<p>The comparison between PET and ASL CBF on Bland-Altman plots.</p

Definitions of parameters and assumed values for ASL signal simulation.

<p>Definitions of parameters and assumed values for ASL signal simulation.</p