Modeling the effects of dispersion and pulsatility of blood flow in pulsed arterial spin labeling

Abstract

Arterial spin labeling (ASL) is a method of using MRI to image cerebral perfusion. For the measurement to be calibrated, a model is required describing the kinetics of the flow of the inverted blood from the labeling region to the imaging region. It is common to assume plug-flow, but alternatives such as a Gaussian distribution of arrival times have also been suggested. In this study a physiologically based model for dispersion is developed and compared to existing models when fit to experimental data. The model is based on the assumption of parabolic flow in the major arteries, and also allows inclusion of cardiac pulsatility. It was found that fitting using the proposed model leads to higher perfusion estimates, with the difference becoming more pronounced in regions where the dispersion is greater. This suggests that current models may underestimate perfusion in these areas. However, fitting using the proposed model also leads to high uncertainties in parameter estimates due to non-orthogonality of the parameters. Effects due to pulsatility are expected to be observable, but when no cardiacgating is used the mean curve over several cardiac cycles is predicted to closely match the curve which assumes constant flow. Key words: pulsed arterial spin labeling; ASL; pulsatility; dispersionArterial spin labeling (ASL) involves acquiring images with and without inversion of water proton spins proximal to the area of interest, thereby allowing the measurement of cerebral blood flow (CBF) without requiring administration of an exogenous tracer (1-3). In order for the measurement to be quantitative, a kinetic model of the tag delivery process needs to be assumed and fit to the data to obtain the flow parameters. Buxton et al. (4) presented a general modeling framework for ASL which includes a tag delivery function that can be modeled to incorporate the dispersion of the tag between the tagging region and the imaging voxel. A common choice of tag delivery function model is 'plug-flow' (i.e., constant and without dispersion). However, it is known that blood flow in the major arteries is likely to be poorly approximated by simple plug-flow, as it is both highly pulsatile and has a time-varying nonuniform cross-sectional flowprofile (5). Indeed, Wu et al. (6) recently demonstrated experimentally that both flow dispersion and flow pulsatility have a significant effect on the ASL signal. They also presented a model to estimate the effect of the cardiac cycle on the ASL signal, but that model appears to implicitly assume plug-flow when calculating the variation in tag-volume. The dispersion is accounted for by assuming a Gaussian distribution of transit delays.Hrabe and Lewis (7) presented analytical forms of the kinetic model by assuming various tag delivery functions that included dispersion and concluded by advocating the assumption of a Gaussian distribution of tag arrival times at the imaging voxel. They justified this choice physiologically by assuming that the primary source of dispersion would be in vessels close to ...

Arterial Spin Labeling Characterization of Cerebral Perfusion during Normal Maturation from Late Childhood into Adulthood: Normal ‘Reference Range’ Values and Their Use in Clinical Studies

“…31, 32 A number of models have been suggested to correct for this, for instance by introducing a Gaussian distribution of tag arrival times at the imaging voxel, and assuming parabolic flow in major arteries. 33 As the temporal width of the d M curve seems to increase with age, particularly in the PCA territory, it would appear that the labeled blood bolus is becoming more dispersed with increasing age, and these models would therefore become particularly relevant in this vascular territory. Furthermore, this natural increase in bolus dispersion with increasing age should be accounted for when assessing changes in T in the presence of pathologies in future clinical studies.…”

Intracranial Hemodynamics Is Altered by Carotid Artery Disease and After Endarterectomy

“…Future work could address cardiac effects on this DynAngio because this is an area in the literature that is developing. 19,20 We found significant differences between patients and controls, such as the number of detected voxels in the right and left MCAs. Differences in vascular anatomy and the fact that the cohorts were not equally matched for age and sex may have influenced the results.…”

Multi-TI Arterial Spin Labeling MRI with Variable TR and Bolus Duration for Cerebral Blood Flow and Arterial Transit Time Mapping

“…The effects of bolus dispersion on CBF and ATT estimation with ASL have been studied [29] and modeled [30]; blood dispersion was not included in CBF and ATT in this work. Wu et al [29] concluded that intervoxel flow dispersion dominates intra-voxel flow dispersion, suggesting that modeling laminar flow may not have an appreciable effect on results.…”

“…Wu et al [29] concluded that intervoxel flow dispersion dominates intra-voxel flow dispersion, suggesting that modeling laminar flow may not have an appreciable effect on results. Gallichan et al [30] proposed a model to account for laminar flow of the labeling bolus which leads to a smoother signal curve, free of discontinuities at the onset and the peak of the perfusion signal curve. This approach would be beneficial to our analysis, however, with only six time points (as was used here) the benefit would not be as great as for data with more time points.…”