Determination of spin compartment in arterial spin labeling MRI

Abstract

A major difference between arterial-spin-labeling MRI and gold-standard radiotracer blood flow methods is that the compartment localization of the labeled spins in the arterialspin-labeling image is often ambiguous, which may affect the quantification of cerebral blood flow. In this study, we aim to probe whether the spins are located in the vascular system or tissue by using T2 of the arterial-spin-labeling signal as a marker. We combined two recently developed techniques, pseudo-continuous arterial spin labeling and T2-RelaxationUnder-Spin-Tagging, to determine the T2 of the labeled spins at multiple postlabeling delay times. Our data suggest that the labeled spins first showed the T2 of arterial blood followed by gradually approaching and stabilizing at the tissue T2. The T2 values did not decrease further toward the venous T2. By fitting the experimental data to a two-compartment model, we estimated gray matter cerebral blood flow, arterial transit time, and tissue transit time to be 74.0 6 10.7 mL/ 100g/min (mean 6 SD, N 5 10), 938 6 156 msec, and 1901 6 181 msec, respectively. The arterial blood volume was calculated to be 1.18 6 0.21 mL/100 g. A postlabeling delay time of 2 s is sufficient to allow the spins to completely enter the tissue space for gray matter but not for white matter. Magn Reson Med 65:120-127,

Comparison of arterial transit times estimated using arterial spin labeling

“…These results are in good agreement with prior studies using multi-TI PASL, which have reported transit times of 660-960 ms [6,8]. The LL-pCASL 90 mm data yielded transit times of 908 ± 170 ms (uncrushed) and 1463 ± 135 ms (crushed), also within the range of 600-1,600 ms reported by various other groups using multi-TI CASL [2,7,16,17]. Importantly, neither LL-ASL nor FEAST is capable of isolating transit to the tissue compartment.…”

“…Importantly, neither LL-ASL nor FEAST is capable of isolating transit to the tissue compartment. Other techniques such as detecting changes in T 2 of the spin label [16,18] or using stronger diffusion gradients to separate the capillary and tissue compartments [19] are necessary for accurate tissue transit time mapping.…”

Arterial Spin-Labeling Assessment of Normalized Vascular Intratumoral Signal Intensity as a Predictor of Histologic Grade of Astrocytic Neoplasms

“…They inferred that a postlabeling delay time of 2 seconds is sufficient to allow the spins to completely enter the tissue space for gray matter, and even longer inversion times are needed for white matter. 25 Therefore, we suggest that the most significant intratumoral signal intensity in our study depicts tumor circulation but not tumor perfusion and should be more precisely called "normalized vascular intratumoral signal LGA, diffuse astrocytoma; AA, anaplastic astrocytoma; GB, glioblastoma.…”

“…Recently, Liu et al 25 assessed the labeled spin localization at multiple postlabeling delay times by use of T2 of the arterial spin-labeling signal as a marker. Because of the T2 of arterial blood (approximately 152 ms) and the tissue T2 (approximately 90 ms), they proposed that especially at very low inversion times (below the average range of arterial transit time), the labeled spins are primarily located in arterial vessels.…”

Arterial spin labeling measurement of cerebral perfusion in children with sickle cell disease

“…This illustrates that the perfusion asymmetries in children with SCD as described before can at least partly be attributed to transit time effects. Recently, Liu et al (35) studied compartment localization of labeled spins in ASL and concluded that a postlabeling delay of 2000 msec is sufficient to allow the spins to completely enter the gray matter. Their data also suggested that all labeled spins exchange to tissue and that only very few remain in the vasculature to enter the vein.…”