CBF measurements using multidelay pseudocontinuous and velocity-selective arterial spin labeling in patients with long arterial transit delays: comparison with xenon CT CBF

Abstract

Purpose: To test the theory that velocity-selective arterial spin labeling (VSASL) is insensitive to transit delay.

Materials and methods: Cerebral blood flow (CBF) was measured in ten Moyamoya disease patients using xenon computed tomography (xeCT) and magnetic resonance imaging (MRI), which included multiple pseudo-continuous ASL (pcASL) with different postlabel delays, VSASL, and dynamic susceptibility contrast (DSC) imaging. Correlation coefficient, root-mean-square difference, mean CBF error between ASL, and gold-standard xeCT CBF measurements as well the dependence of this error on transit delay (TD) as estimated by DSC time-to-peak of the residue function (Tmax) were determined.

Results: For pcASL with different postlabel delay time (PLD), CBF measurement with short PLD (1.5-2 sec) had the strongest correlations with xeCT; VSASL had a lower but still significant correlation with a mean coefficient of 0.55. We noted the theoretically predicted dependence of CBF error on Tmax and on PLD for pcASL; VSASL CBF measurements had the least dependence of the error on TD. We also noted effects suggesting that the location of the label decay (blood vs. tissue) impacted the measurement, which was worse for pcASL than for VSASL.

Conclusion: We conclude that VSASL is less sensitive to TD than conventional ASL techniques and holds promise for CBF measurements in cerebrovascular diseases with slow flow.

Figure 1

CBF maps; from left to right: xeCT, VSASL, pcASL without (top) and with (bottom) vessel suppression for PLD of 1s, …, 3s (left to right). CBF is underestimated by pcASL or pcASL(sup) with short post-label delay (PLD) in regions with slow blood flow, and this problem is mitigated with long PLD (arrow).

Figure 2

CBF map of xeCT (a), pcASL(sup) with post-label delay (PLD) of 1s, 2s and 3s (b–d), and VSASL (e), as well as Tmax image (f). Arterial transit artifacts were still visible in some cases with long PLD of 3s (arrows). At long PLDs, regions with long transit delay show apparent higher CBF value compared to contralateral hemisphere as well as xeCT CBF map (arrowhead), suggesting CBF overestimation in this region at long PLD.

Figure 3

CBF images (bottom panel, from left to right: xeCT, pcASL with PLD of 1s, 1.5s, … 3s, VSASL); scatterplots (top panel, the horizontal axes are xeCT CBF value, vertical axes are ASL CBF values).

Figure 4

Mean and standard error of ΔCBF values in the entire imaged brain (WB) and gray matter (GM) regions of the different CBF measurements techniques. Δ CBF represents the difference between the ASL CBF value and the xeCT CBF value in the same ROI. For the same PLD, pcASL(sup) showed closer estimation of CBF to xeCT than pcASL, except for PLD of 1s. For pcASL, PLD of 1s underestimated the mean CBF in WB and GM regions, while larger PLD’s overestimated these values. VSASL also overestimated the mean CBF values, with a mean error of about 2 ml/100g/min.

Figure 5

(a) Tmax image (in units of seconds). (b) shows the gray matter matter ROI, with different Tmax values: blue, 0–2s; green, 2–4s; yellow 4s–6s; red, >6s.

Figure 6

Mean and standard error Δ CBF values (ASL measurement – xeCT measurement) in GM regions as a function of transit delay, estimated from Tmax values. For pcASL with short post-label-delay (PLD) of 1–1.5s, CBF is underestimated in the long Tmax regions, since the label does not have time to reach the tissue; for shorter Tmax, there is an overestimation of CBF. For longer PLD’s (>2.5s), a reverse trend was observed, with overestimation of CBF in regions with longer transit time and underestimation in regions with shorter Tmax; we believe this paradoxical effect is due to differences between tissue and blood T₁, resulting in different decay rates of the label. For VSASL, ΔCBF showed the least dependence on transit delay.