Arterial transit time effects in pulsed arterial spin labeling CBF mapping: insight from a PET and MR study in normal human subjects

Abstract

Arterial transit time (ATT), a key parameter required to calculate absolute cerebral blood flow in arterial spin labeling (ASL), is subject to much uncertainty. In this study, ASL ATTs were estimated on a per-voxel basis using data measured by both ASL and positron emission tomography in the same subjects. The mean ATT increased by 260 +/- 20 (standard error of the mean) ms when the imaging slab shifted downwards by 54 mm, and increased from 630 +/- 30 to 1220 +/- 30 ms for the first slice, with an increase of 610 +/- 20 ms over a four-slice slab when the gap between the imaging and labeling slab increased from 20 to 74 mm. When the per-slice ATTs were employed in ASL cerebral blood flow quantification and the in-slice ATT variations ignored, regional cerebral blood flow could be significantly different from the positron emission tomography measures. ATT also decreased with focal activation by the same amount for both visual and motor tasks (approximately 80 ms). These results provide a quantitative relationship between ATT and the ASL imaging geometry and yield an assessment of the assumptions commonly used in ASL imaging. These findings should be considered in the interpretation of, and comparisons between, different ASL-based cerebral blood flow studies. The results also provide spatially specific ATT data that may aid in optimizing the ASL imaging parameters.

FIG. 1

Group average data. Nine slices (resliced) from the upper acquisition slab show the perfusion-induced signal intensity changes in ASL images (top, a.u.), CBF map measured by [¹⁵O]water PET (middle, mL/100 g/min), and the ATT map (bottom, seconds) estimated on a per-voxel basis. As shown in the ATT map, a brain mask was generated by thresholding the group average blood oxygen level dependent (BOLD) images to excluded the ATT values from those regions where apparent image distortion and signal dropout exist, especially in the inferior slices, as shown in Fig. 2.

FIG. 2

Group average data. Nine slices (resliced) from the lower acquisition slab show the perfusion-induced signal intensity changes in ASL images (top, a.u.), CBF map measured by [¹⁵O]water PET (middle, mL/100 g/min), and the ATT map (bottom, seconds) estimated on a per-voxel basis.

FIG. 3

In-slice mean ATT values on a per-slice basis (a) and the in-slice ATT spatial variability (b) for the upper and lower parts of the brain. The linear regression lines and equations of the ATT values are shown. For comparison, the line of ATT values assumed in QUIPSS is also plotted, whose slope is dependent on the data acquisition duration of each slice. Error bars indicate the standard deviations in the group analysis.

FIG. 4

ASL CBF was first calculated using the mean slice-based ATT values estimated in this study and then compared to CBF measured by PET. The voxelwise differences are shown in (a) for the upper part of the brain and in (b) for the lower part. c,d: The differences between CBF quantified using the QUIPSS II model and that measured by PET.

FIG. 5

ROIs defined based on PET CBF activations.