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 6 20 (standard error of the mean) ms when the imaging slab shifted downwards by 54 mm, and increased from 630 6 30 to 1220 6 30 ms for the first slice, with an increase of 610 6 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 (~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. Key words: arterial transit time; cerebral blood flow; positron emission tomography; pulsed arterial spin labeling; brain vasculature Significant efforts have been made to improve the accuracy of absolute cerebral blood flow (CBF) measurements estimated by pulsed arterial spin labeling (ASL) MRI. To improve the inversion profile and the labeling efficiency, adiabatic or hypersecant pulses and other variants have been proposed (1-4). To overcome the signal dampening due to the off-resonance effect of the inversion radiofrequency (RF) pulse, separate coils for labeling and detection have been employed (5-9). Measurement of the arterial blood proton density was suggested to minimize the nonuniformity of the brain tissue-blood partition coefficient (k) (10), and methods have been presented to reduce the sensitivity of CBF quantification to arterial transit time (ATT) effects (contamination from intravascular arterial blood water) in CBF quantification (11-13).In ASL, water in the arteries is magnetically labeled, and when it reaches the capillary bed and exchanges with the extravascular water, the labeled water induces changes in local magnetization relative to the case in which the arterial water is undisturbed. These changes are detectable by MRI and such images of the intensity changes are perfusion weighted. Absolute CBF values can be calculated based on these perfusion-weighted intensity changes. ATTs, however, are one of the key phenomena that affect the calculation of the absolute CBF from the image intensity differences. In ASL, ATT refers to the time it takes the arterial blood to travel from the labeling site to the capillaries in the tissue being imaged. By definitio...

Arterial Spin-Labeling Parameters Influence Signal Variability and Estimated Regional Relative Cerebral Blood Flow in Normal Aging and Mild Cognitive Impairment: FAIR versus PICORE Techniques

“…Along the same lines, a previous investigation assessed the effect of imaging slab positioning and arterial transit time of ASL compared with H 2 [ 15 0]-PET. 27 The spatial distribution of the direct comparison between ASL and water PET partly overlaps the patterns of the current investigation, indicating that systematic ASL sequencerelated differences in arterial transit time might contribute to the observed spatial differences between FAIR and PICORE. The exact origin of these regional differences between both ASL sequences, irrespective of the vascular territory, remains unclear.…”

“…Correspondingly, previous investigations comparing ASL and H 2 [ ]-PET in 6 healthy controls, 6 subjects with MCI, and 1 patient with Alzheimer disease and again found a systematic offset between 2D-versus-3D ASL and between ASL-versus-water PET. 26 Moreover, the placement of the imaging slab significantly changed the rCBF estimation in ASL and the correlation with water PET, 27 indicating the need for rigorous standardization of this parameter. In our study, the imaging slab was positioned by using the same orientation, and the labeling slab was always parallel to the imaging slab.…”

Cerebral Arterial Bolus Arrival Time is Prolonged in Multiple Sclerosis and Associated with Disability

“…The time from ASL to this point is defined here, as previously, as the BAT. 18 Other scan parameters were: TR 4,000 milliseconds, TE 23 …”

“…Knowledge of BAT also enables more accurate quantification of perfusion, particularly where BAT is prolonged by pathology. 23 A previous ASL study revealed increase in CBF in white matter and decrease in gray matter in patients with MS, 24 but could not quantify BAT, as only a single delay time point was used.…”

“…Bolus arrival time provides complementary information to CBF, and is distinct from mean transit time and time to peak, which are typically measured in PET and contrast-based MRI and CT techniques. 23 In particular, the time to peak measurements always include both a BAT component and an MTT component, rendering a direct comparison inappropriate. Knowledge of BAT also enables more accurate quantification of perfusion, particularly where BAT is prolonged by pathology.…”